From 475bb383c958962dee984cf4086b4ec341c3d10e Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:04:12 +0200
Subject: [PATCH 01/21] let_chains: Remove ast::ExprKind::{IfLet, WhileLet} and
 introduce ::Let.

---
 src/libsyntax/ast.rs | 20 +++++---------------
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index d12240655e628..61b1f9be8fe72 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1049,10 +1049,9 @@ impl Expr {
             ExprKind::Unary(..) => ExprPrecedence::Unary,
             ExprKind::Lit(_) => ExprPrecedence::Lit,
             ExprKind::Type(..) | ExprKind::Cast(..) => ExprPrecedence::Cast,
+            ExprKind::Let(..) => ExprPrecedence::Let,
             ExprKind::If(..) => ExprPrecedence::If,
-            ExprKind::IfLet(..) => ExprPrecedence::IfLet,
             ExprKind::While(..) => ExprPrecedence::While,
-            ExprKind::WhileLet(..) => ExprPrecedence::WhileLet,
             ExprKind::ForLoop(..) => ExprPrecedence::ForLoop,
             ExprKind::Loop(..) => ExprPrecedence::Loop,
             ExprKind::Match(..) => ExprPrecedence::Match,
@@ -1135,26 +1134,17 @@ pub enum ExprKind {
     Cast(P<Expr>, P<Ty>),
     /// A type ascription (e.g., `42: usize`).
     Type(P<Expr>, P<Ty>),
+    /// A `let pat = expr` pseudo-expression that only occurs in the scrutinee
+    /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
+    Let(Vec<P<Pat>>, P<Expr>),
     /// An `if` block, with an optional `else` block.
     ///
     /// `if expr { block } else { expr }`
     If(P<Expr>, P<Block>, Option<P<Expr>>),
-    /// An `if let` expression with an optional else block
-    ///
-    /// `if let pat = expr { block } else { expr }`
-    ///
-    /// This is desugared to a `match` expression.
-    IfLet(Vec<P<Pat>>, P<Expr>, P<Block>, Option<P<Expr>>),
-    /// A while loop, with an optional label
+    /// A while loop, with an optional label.
     ///
     /// `'label: while expr { block }`
     While(P<Expr>, P<Block>, Option<Label>),
-    /// A `while let` loop, with an optional label.
-    ///
-    /// `'label: while let pat = expr { block }`
-    ///
-    /// This is desugared to a combination of `loop` and `match` expressions.
-    WhileLet(Vec<P<Pat>>, P<Expr>, P<Block>, Option<Label>),
     /// A `for` loop, with an optional label.
     ///
     /// `'label: for pat in expr { block }`

From 875cdd67f48d9af92430cc7506d63bd8569057a9 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:06:58 +0200
Subject: [PATCH 02/21] let_chains: Remove ast::ExprKind::{IfLet, WhileLet}
 from visitors and introduce ::Let.

---
 src/librustc_save_analysis/dump_visitor.rs | 12 ++----------
 src/libsyntax/mut_visit.rs                 | 16 ++++------------
 src/libsyntax/parse/classify.rs            |  2 --
 src/libsyntax/visit.rs                     | 16 ++++------------
 4 files changed, 10 insertions(+), 36 deletions(-)

diff --git a/src/librustc_save_analysis/dump_visitor.rs b/src/librustc_save_analysis/dump_visitor.rs
index 1e65f868ebac0..d2546fd86deb8 100644
--- a/src/librustc_save_analysis/dump_visitor.rs
+++ b/src/librustc_save_analysis/dump_visitor.rs
@@ -1579,17 +1579,9 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> Visitor<'l> for DumpVisitor<'l, 'tc
                 self.visit_expr(subexpression);
                 visit::walk_block(self, block);
             }
-            ast::ExprKind::WhileLet(ref pats, ref subexpression, ref block, _) => {
+            ast::ExprKind::Let(ref pats, ref scrutinee) => {
                 self.process_var_decl_multi(pats);
-                debug!("for loop, walk sub-expr: {:?}", subexpression.node);
-                self.visit_expr(subexpression);
-                visit::walk_block(self, block);
-            }
-            ast::ExprKind::IfLet(ref pats, ref subexpression, ref block, ref opt_else) => {
-                self.process_var_decl_multi(pats);
-                self.visit_expr(subexpression);
-                visit::walk_block(self, block);
-                opt_else.as_ref().map(|el| self.visit_expr(el));
+                self.visit_expr(scrutinee);
             }
             ast::ExprKind::Repeat(ref element, ref count) => {
                 self.visit_expr(element);
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index f587e63e12b94..51447fce08044 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/src/libsyntax/mut_visit.rs
@@ -1131,28 +1131,20 @@ pub fn noop_visit_expr<T: MutVisitor>(Expr { node, id, span, attrs }: &mut Expr,
             vis.visit_ty(ty);
         }
         ExprKind::AddrOf(_m, ohs) => vis.visit_expr(ohs),
+        ExprKind::Let(pats, scrutinee) => {
+            visit_vec(pats, |pat| vis.visit_pat(pat));
+            vis.visit_expr(scrutinee);
+        }
         ExprKind::If(cond, tr, fl) => {
             vis.visit_expr(cond);
             vis.visit_block(tr);
             visit_opt(fl, |fl| vis.visit_expr(fl));
         }
-        ExprKind::IfLet(pats, expr, tr, fl) => {
-            visit_vec(pats, |pat| vis.visit_pat(pat));
-            vis.visit_expr(expr);
-            vis.visit_block(tr);
-            visit_opt(fl, |fl| vis.visit_expr(fl));
-        }
         ExprKind::While(cond, body, label) => {
             vis.visit_expr(cond);
             vis.visit_block(body);
             visit_opt(label, |label| vis.visit_label(label));
         }
-        ExprKind::WhileLet(pats, expr, body, label) => {
-            visit_vec(pats, |pat| vis.visit_pat(pat));
-            vis.visit_expr(expr);
-            vis.visit_block(body);
-            visit_opt(label, |label| vis.visit_label(label));
-        }
         ExprKind::ForLoop(pat, iter, body, label) => {
             vis.visit_pat(pat);
             vis.visit_expr(iter);
diff --git a/src/libsyntax/parse/classify.rs b/src/libsyntax/parse/classify.rs
index dfd6f451c28d7..6ebfab3a133ef 100644
--- a/src/libsyntax/parse/classify.rs
+++ b/src/libsyntax/parse/classify.rs
@@ -14,11 +14,9 @@ use crate::ast;
 pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
     match e.node {
         ast::ExprKind::If(..) |
-        ast::ExprKind::IfLet(..) |
         ast::ExprKind::Match(..) |
         ast::ExprKind::Block(..) |
         ast::ExprKind::While(..) |
-        ast::ExprKind::WhileLet(..) |
         ast::ExprKind::Loop(..) |
         ast::ExprKind::ForLoop(..) |
         ast::ExprKind::TryBlock(..) => false,
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 0503e5644dbc5..7df360149262c 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -719,6 +719,10 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             visitor.visit_expr(subexpression);
             visitor.visit_ty(typ)
         }
+        ExprKind::Let(ref pats, ref scrutinee) => {
+            walk_list!(visitor, visit_pat, pats);
+            visitor.visit_expr(scrutinee);
+        }
         ExprKind::If(ref head_expression, ref if_block, ref optional_else) => {
             visitor.visit_expr(head_expression);
             visitor.visit_block(if_block);
@@ -729,18 +733,6 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             visitor.visit_expr(subexpression);
             visitor.visit_block(block);
         }
-        ExprKind::IfLet(ref pats, ref subexpression, ref if_block, ref optional_else) => {
-            walk_list!(visitor, visit_pat, pats);
-            visitor.visit_expr(subexpression);
-            visitor.visit_block(if_block);
-            walk_list!(visitor, visit_expr, optional_else);
-        }
-        ExprKind::WhileLet(ref pats, ref subexpression, ref block, ref opt_label) => {
-            walk_list!(visitor, visit_label, opt_label);
-            walk_list!(visitor, visit_pat, pats);
-            visitor.visit_expr(subexpression);
-            visitor.visit_block(block);
-        }
         ExprKind::ForLoop(ref pattern, ref subexpression, ref block, ref opt_label) => {
             walk_list!(visitor, visit_label, opt_label);
             visitor.visit_pat(pattern);

From e9a6e5a38b1827700041541d1a6fc61ed733b972 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:05:58 +0200
Subject: [PATCH 03/21] let_chains: Handle it in AST pretty printing.

---
 src/libsyntax/print/pprust.rs | 54 +++++++++--------------------------
 1 file changed, 14 insertions(+), 40 deletions(-)

diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 86db7a673d471..9253fa67159ea 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1709,6 +1709,16 @@ impl<'a> State<'a> {
         self.ann.post(self, AnnNode::Block(blk))
     }
 
+    pub fn print_let(&mut self, pats: &[P<ast::Pat>], scrutinee: &ast::Expr) -> io::Result<()> {
+        self.s.word("let ")?;
+
+        self.print_pats(pats)?;
+        self.s.space()?;
+
+        self.word_space("=")?;
+        self.print_expr_as_cond(scrutinee)
+    }
+
     fn print_else(&mut self, els: Option<&ast::Expr>) -> io::Result<()> {
         match els {
             Some(_else) => {
@@ -1723,19 +1733,6 @@ impl<'a> State<'a> {
                         self.print_block(then)?;
                         self.print_else(e.as_ref().map(|e| &**e))
                     }
-                    // "another else-if-let"
-                    ast::ExprKind::IfLet(ref pats, ref expr, ref then, ref e) => {
-                        self.cbox(INDENT_UNIT - 1)?;
-                        self.ibox(0)?;
-                        self.s.word(" else if let ")?;
-                        self.print_pats(pats)?;
-                        self.s.space()?;
-                        self.word_space("=")?;
-                        self.print_expr_as_cond(expr)?;
-                        self.s.space()?;
-                        self.print_block(then)?;
-                        self.print_else(e.as_ref().map(|e| &**e))
-                    }
                     // "final else"
                     ast::ExprKind::Block(ref b, _) => {
                         self.cbox(INDENT_UNIT - 1)?;
@@ -1756,20 +1753,10 @@ impl<'a> State<'a> {
     pub fn print_if(&mut self, test: &ast::Expr, blk: &ast::Block,
                     elseopt: Option<&ast::Expr>) -> io::Result<()> {
         self.head("if")?;
+
         self.print_expr_as_cond(test)?;
         self.s.space()?;
-        self.print_block(blk)?;
-        self.print_else(elseopt)
-    }
 
-    pub fn print_if_let(&mut self, pats: &[P<ast::Pat>], expr: &ast::Expr, blk: &ast::Block,
-                        elseopt: Option<&ast::Expr>) -> io::Result<()> {
-        self.head("if let")?;
-        self.print_pats(pats)?;
-        self.s.space()?;
-        self.word_space("=")?;
-        self.print_expr_as_cond(expr)?;
-        self.s.space()?;
         self.print_block(blk)?;
         self.print_else(elseopt)
     }
@@ -2053,12 +2040,12 @@ impl<'a> State<'a> {
                 self.word_space(":")?;
                 self.print_type(ty)?;
             }
+            ast::ExprKind::Let(ref pats, ref scrutinee) => {
+                self.print_let(pats, scrutinee)?;
+            }
             ast::ExprKind::If(ref test, ref blk, ref elseopt) => {
                 self.print_if(test, blk, elseopt.as_ref().map(|e| &**e))?;
             }
-            ast::ExprKind::IfLet(ref pats, ref expr, ref blk, ref elseopt) => {
-                self.print_if_let(pats, expr, blk, elseopt.as_ref().map(|e| &**e))?;
-            }
             ast::ExprKind::While(ref test, ref blk, opt_label) => {
                 if let Some(label) = opt_label {
                     self.print_ident(label.ident)?;
@@ -2069,19 +2056,6 @@ impl<'a> State<'a> {
                 self.s.space()?;
                 self.print_block_with_attrs(blk, attrs)?;
             }
-            ast::ExprKind::WhileLet(ref pats, ref expr, ref blk, opt_label) => {
-                if let Some(label) = opt_label {
-                    self.print_ident(label.ident)?;
-                    self.word_space(":")?;
-                }
-                self.head("while let")?;
-                self.print_pats(pats)?;
-                self.s.space()?;
-                self.word_space("=")?;
-                self.print_expr_as_cond(expr)?;
-                self.s.space()?;
-                self.print_block_with_attrs(blk, attrs)?;
-            }
             ast::ExprKind::ForLoop(ref pat, ref iter, ref blk, opt_label) => {
                 if let Some(label) = opt_label {
                     self.print_ident(label.ident)?;

From cfb02950b7f0f89529e173e55bd0db4d552539f9 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:03:22 +0200
Subject: [PATCH 04/21] let_chains: Add support for parsing let expressions.

---
 src/libsyntax/parse/parser.rs | 67 ++++++++++-------------------------
 src/libsyntax/parse/token.rs  |  1 +
 src/libsyntax/util/parser.rs  |  7 ++--
 3 files changed, 22 insertions(+), 53 deletions(-)

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 2d6c8c5407583..660c2ca2ba63a 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2621,13 +2621,8 @@ impl<'a> Parser<'a> {
                     } else {
                         ex = ExprKind::Yield(None);
                     }
-                } else if self.token.is_keyword(keywords::Let) {
-                    // Catch this syntax error here, instead of in `parse_ident`, so
-                    // that we can explicitly mention that let is not to be used as an expression
-                    let mut db = self.fatal("expected expression, found statement (`let`)");
-                    db.span_label(self.span, "expected expression");
-                    db.note("variable declaration using `let` is a statement");
-                    return Err(db);
+                } else if self.eat_keyword(keywords::Let) {
+                    return self.parse_let_expr(attrs);
                 } else if self.span.rust_2018() && self.eat_keyword(keywords::Await) {
                     // FIXME: remove this branch when `await!` is no longer supported
                     // https://github.com/rust-lang/rust/issues/60610
@@ -2886,15 +2881,13 @@ impl<'a> Parser<'a> {
                 attrs.extend::<Vec<_>>(expr.attrs.into());
                 expr.attrs = attrs;
                 match expr.node {
-                    ExprKind::If(..) | ExprKind::IfLet(..) => {
-                        if !expr.attrs.is_empty() {
-                            // Just point to the first attribute in there...
-                            let span = expr.attrs[0].span;
-
-                            self.span_err(span,
-                                "attributes are not yet allowed on `if` \
-                                expressions");
-                        }
+                    ExprKind::If(..) if !expr.attrs.is_empty() => {
+                        // Just point to the first attribute in there...
+                        let span = expr.attrs[0].span;
+
+                        self.span_err(span,
+                            "attributes are not yet allowed on `if` \
+                            expressions");
                     }
                     _ => {}
                 }
@@ -3778,9 +3771,6 @@ impl<'a> Parser<'a> {
 
     /// Parses an `if` or `if let` expression (`if` token already eaten).
     fn parse_if_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
-        if self.check_keyword(keywords::Let) {
-            return self.parse_if_let_expr(attrs);
-        }
         let lo = self.prev_span;
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
 
@@ -3812,22 +3802,18 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(lo.to(hi), ExprKind::If(cond, thn, els), attrs))
     }
 
-    /// Parses an `if let` expression (`if` token already eaten).
-    fn parse_if_let_expr(&mut self, attrs: ThinVec<Attribute>)
-                             -> PResult<'a, P<Expr>> {
+    /// Parses a `let $pats = $expr` pseudo-expression.
+    /// The `let` token has already been eaten.
+    fn parse_let_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
-        self.expect_keyword(keywords::Let)?;
         let pats = self.parse_pats()?;
         self.expect(&token::Eq)?;
-        let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
-        let thn = self.parse_block()?;
-        let (hi, els) = if self.eat_keyword(keywords::Else) {
-            let expr = self.parse_else_expr()?;
-            (expr.span, Some(expr))
-        } else {
-            (thn.span, None)
-        };
-        Ok(self.mk_expr(lo.to(hi), ExprKind::IfLet(pats, expr, thn, els), attrs))
+
+        let expr = self.with_res(
+            Restrictions::NO_STRUCT_LITERAL,
+            |this| this.parse_assoc_expr_with(1 + AssocOp::LAnd.precedence(), None.into())
+        )?;
+        Ok(self.mk_expr(lo.to(expr.span), ExprKind::Let(pats, expr), attrs))
     }
 
     /// Parses `move |args| expr`.
@@ -3930,9 +3916,6 @@ impl<'a> Parser<'a> {
     fn parse_while_expr(&mut self, opt_label: Option<Label>,
                             span_lo: Span,
                             mut attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
-        if self.token.is_keyword(keywords::Let) {
-            return self.parse_while_let_expr(opt_label, span_lo, attrs);
-        }
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
@@ -3940,20 +3923,6 @@ impl<'a> Parser<'a> {
         return Ok(self.mk_expr(span, ExprKind::While(cond, body, opt_label), attrs));
     }
 
-    /// Parses a `while let` expression (`while` token already eaten).
-    fn parse_while_let_expr(&mut self, opt_label: Option<Label>,
-                                span_lo: Span,
-                                mut attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
-        self.expect_keyword(keywords::Let)?;
-        let pats = self.parse_pats()?;
-        self.expect(&token::Eq)?;
-        let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
-        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
-        attrs.extend(iattrs);
-        let span = span_lo.to(body.span);
-        return Ok(self.mk_expr(span, ExprKind::WhileLet(pats, expr, body, opt_label), attrs));
-    }
-
     // parse `loop {...}`, `loop` token already eaten
     fn parse_loop_expr(&mut self, opt_label: Option<Label>,
                            span_lo: Span,
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 049fb6cb78b84..7023a50b553af 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -122,6 +122,7 @@ pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
         keywords::Continue.name(),
         keywords::False.name(),
         keywords::For.name(),
+        keywords::Let.name(),
         keywords::If.name(),
         keywords::Loop.name(),
         keywords::Match.name(),
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 86e89945afe06..83c05c42afc0a 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -265,6 +265,7 @@ pub enum ExprPrecedence {
 
     Box,
     AddrOf,
+    Let,
     Unary,
 
     Call,
@@ -282,9 +283,7 @@ pub enum ExprPrecedence {
     Path,
     Paren,
     If,
-    IfLet,
     While,
-    WhileLet,
     ForLoop,
     Loop,
     Match,
@@ -324,6 +323,8 @@ impl ExprPrecedence {
             // Unary, prefix
             ExprPrecedence::Box |
             ExprPrecedence::AddrOf |
+            // Here `let pats = expr` has `let pats =` as a "unary" prefix of `expr`.
+            ExprPrecedence::Let |
             ExprPrecedence::Unary => PREC_PREFIX,
 
             // Unary, postfix
@@ -344,9 +345,7 @@ impl ExprPrecedence {
             ExprPrecedence::Path |
             ExprPrecedence::Paren |
             ExprPrecedence::If |
-            ExprPrecedence::IfLet |
             ExprPrecedence::While |
-            ExprPrecedence::WhileLet |
             ExprPrecedence::ForLoop |
             ExprPrecedence::Loop |
             ExprPrecedence::Match |

From 7e4e53e7539c812b0369b84fef75bd25204b8fe3 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:05:39 +0200
Subject: [PATCH 05/21] let_chains: Handle in resolve.

---
 src/librustc_resolve/lib.rs | 62 ++++++++++++++-----------------------
 1 file changed, 24 insertions(+), 38 deletions(-)

diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index 678f4c23d3e9e..03236a516a427 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -484,8 +484,8 @@ type BindingMap = FxHashMap<Ident, BindingInfo>;
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum PatternSource {
     Match,
-    IfLet,
-    WhileLet,
+    // FIXME(54883): Consider fusing with `Let` below once let-statements support or-patterns.
+    LetExpr,
     Let,
     For,
     FnParam,
@@ -495,9 +495,7 @@ impl PatternSource {
     fn descr(self) -> &'static str {
         match self {
             PatternSource::Match => "match binding",
-            PatternSource::IfLet => "if let binding",
-            PatternSource::WhileLet => "while let binding",
-            PatternSource::Let => "let binding",
+            PatternSource::Let | PatternSource::LetExpr => "let binding",
             PatternSource::For => "for binding",
             PatternSource::FnParam => "function parameter",
         }
@@ -3093,13 +3091,7 @@ impl<'a> Resolver<'a> {
     fn resolve_arm(&mut self, arm: &Arm) {
         self.ribs[ValueNS].push(Rib::new(NormalRibKind));
 
-        let mut bindings_list = FxHashMap::default();
-        for pattern in &arm.pats {
-            self.resolve_pattern(&pattern, PatternSource::Match, &mut bindings_list);
-        }
-
-        // This has to happen *after* we determine which pat_idents are variants.
-        self.check_consistent_bindings(&arm.pats);
+        self.resolve_pats(&arm.pats, PatternSource::Match);
 
         if let Some(ast::Guard::If(ref expr)) = arm.guard {
             self.visit_expr(expr)
@@ -3109,6 +3101,16 @@ impl<'a> Resolver<'a> {
         self.ribs[ValueNS].pop();
     }
 
+    /// Arising from `source`, resolve a sequence of patterns (top level or-patterns).
+    fn resolve_pats(&mut self, pats: &[P<Pat>], source: PatternSource) {
+        let mut bindings_list = FxHashMap::default();
+        for pat in pats {
+            self.resolve_pattern(pat, source, &mut bindings_list);
+        }
+        // This has to happen *after* we determine which pat_idents are variants
+        self.check_consistent_bindings(pats);
+    }
+
     fn resolve_block(&mut self, block: &Block) {
         debug!("(resolving block) entering block");
         // Move down in the graph, if there's an anonymous module rooted here.
@@ -3187,8 +3189,7 @@ impl<'a> Resolver<'a> {
                 );
             }
             Some(..) if pat_src == PatternSource::Match ||
-                        pat_src == PatternSource::IfLet ||
-                        pat_src == PatternSource::WhileLet => {
+                        pat_src == PatternSource::LetExpr => {
                 // `Variant1(a) | Variant2(a)`, ok
                 // Reuse definition from the first `a`.
                 res = self.ribs[ValueNS].last_mut().unwrap().bindings[&ident];
@@ -4409,41 +4410,26 @@ impl<'a> Resolver<'a> {
                 visit::walk_expr(self, expr);
             }
 
-            ExprKind::IfLet(ref pats, ref subexpression, ref if_block, ref optional_else) => {
-                self.visit_expr(subexpression);
+            ExprKind::Let(ref pats, ref scrutinee) => {
+                self.visit_expr(scrutinee);
+                self.resolve_pats(pats, PatternSource::LetExpr);
+            }
 
+            ExprKind::If(ref cond, ref then, ref opt_else) => {
                 self.ribs[ValueNS].push(Rib::new(NormalRibKind));
-                let mut bindings_list = FxHashMap::default();
-                for pat in pats {
-                    self.resolve_pattern(pat, PatternSource::IfLet, &mut bindings_list);
-                }
-                // This has to happen *after* we determine which pat_idents are variants
-                self.check_consistent_bindings(pats);
-                self.visit_block(if_block);
+                self.visit_expr(cond);
+                self.visit_block(then);
                 self.ribs[ValueNS].pop();
 
-                optional_else.as_ref().map(|expr| self.visit_expr(expr));
+                opt_else.as_ref().map(|expr| self.visit_expr(expr));
             }
 
             ExprKind::Loop(ref block, label) => self.resolve_labeled_block(label, expr.id, &block),
 
             ExprKind::While(ref subexpression, ref block, label) => {
                 self.with_resolved_label(label, expr.id, |this| {
-                    this.visit_expr(subexpression);
-                    this.visit_block(block);
-                });
-            }
-
-            ExprKind::WhileLet(ref pats, ref subexpression, ref block, label) => {
-                self.with_resolved_label(label, expr.id, |this| {
-                    this.visit_expr(subexpression);
                     this.ribs[ValueNS].push(Rib::new(NormalRibKind));
-                    let mut bindings_list = FxHashMap::default();
-                    for pat in pats {
-                        this.resolve_pattern(pat, PatternSource::WhileLet, &mut bindings_list);
-                    }
-                    // This has to happen *after* we determine which pat_idents are variants.
-                    this.check_consistent_bindings(pats);
+                    this.visit_expr(subexpression);
                     this.visit_block(block);
                     this.ribs[ValueNS].pop();
                 });

From 6162a97eed2cc0fb676b15e655f8ec4389783987 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:03:43 +0200
Subject: [PATCH 06/21] let_chains: Handle in unused parenthesis lint.

---
 src/librustc_lint/unused.rs | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index c3dfd44ad8572..d90433fd93c16 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -294,20 +294,28 @@ impl UnusedParens {
                                 value: &ast::Expr,
                                 msg: &str,
                                 followed_by_block: bool) {
-        if let ast::ExprKind::Paren(ref inner) = value.node {
-            let necessary = followed_by_block && match inner.node {
-                ast::ExprKind::Ret(_) | ast::ExprKind::Break(..) => true,
-                _ => parser::contains_exterior_struct_lit(&inner),
-            };
-            if !necessary {
-                let expr_text = if let Ok(snippet) = cx.sess().source_map()
-                    .span_to_snippet(value.span) {
-                        snippet
-                    } else {
-                        pprust::expr_to_string(value)
-                    };
-                Self::remove_outer_parens(cx, value.span, &expr_text, msg);
+        match value.node {
+            ast::ExprKind::Paren(ref inner) => {
+                let necessary = followed_by_block && match inner.node {
+                    ast::ExprKind::Ret(_) | ast::ExprKind::Break(..) => true,
+                    _ => parser::contains_exterior_struct_lit(&inner),
+                };
+                if !necessary {
+                    let expr_text = if let Ok(snippet) = cx.sess().source_map()
+                        .span_to_snippet(value.span) {
+                            snippet
+                        } else {
+                            pprust::expr_to_string(value)
+                        };
+                    Self::remove_outer_parens(cx, value.span, &expr_text, msg);
+                }
+            }
+            ast::ExprKind::Let(_, ref expr) => {
+                // FIXME(#60336): Properly handle `let true = (false && true)`
+                // actually needing the parenthesis.
+                self.check_unused_parens_expr(cx, expr, "`let` scrutinee", followed_by_block);
             }
+            _ => {}
         }
     }
 
@@ -369,8 +377,6 @@ impl EarlyLintPass for UnusedParens {
         let (value, msg, followed_by_block) = match e.node {
             If(ref cond, ..) => (cond, "`if` condition", true),
             While(ref cond, ..) => (cond, "`while` condition", true),
-            IfLet(_, ref cond, ..) => (cond, "`if let` head expression", true),
-            WhileLet(_, ref cond, ..) => (cond, "`while let` head expression", true),
             ForLoop(_, ref cond, ..) => (cond, "`for` head expression", true),
             Match(ref head, _) => (head, "`match` head expression", true),
             Ret(Some(ref value)) => (value, "`return` value", false),

From 550f653c271a7b09e9526718f3dde513e457eb33 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:02:51 +0200
Subject: [PATCH 07/21] let_chains: Handle disallowing of let chains in places
 lowering won't support.

---
 src/librustc_passes/ast_validation.rs | 158 ++++++++++++++------------
 src/librustc_passes/lib.rs            |   1 +
 2 files changed, 86 insertions(+), 73 deletions(-)

diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index 2bea1db841ae9..5e4713468135d 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -16,13 +16,11 @@ use syntax::ast::*;
 use syntax::attr;
 use syntax::source_map::Spanned;
 use syntax::symbol::{keywords, sym};
-use syntax::ptr::P;
 use syntax::visit::{self, Visitor};
 use syntax::{span_err, struct_span_err, walk_list};
 use syntax_ext::proc_macro_decls::is_proc_macro_attr;
 use syntax_pos::{Span, MultiSpan};
 use errors::{Applicability, FatalError};
-use log::debug;
 
 #[derive(Copy, Clone, Debug)]
 struct OuterImplTrait {
@@ -71,26 +69,42 @@ struct AstValidator<'a> {
     /// these booleans.
     warning_period_57979_didnt_record_next_impl_trait: bool,
     warning_period_57979_impl_trait_in_proj: bool,
+
+    /// Used to ban `let` expressions in inappropriate places.
+    is_let_allowed: bool,
+}
+
+/// With the `new` value in `store`,
+/// runs and returns the `scoped` computation,
+/// resetting the old value of `store` after,
+/// and returning the result of `scoped`.
+fn with<C, T, S>(
+    this: &mut C,
+    new: S,
+    store: impl Fn(&mut C) -> &mut S,
+    scoped: impl FnOnce(&mut C) -> T
+) -> T {
+    let old = mem::replace(store(this), new);
+    let ret = scoped(this);
+    *store(this) = old;
+    ret
 }
 
 impl<'a> AstValidator<'a> {
     fn with_impl_trait_in_proj_warning<T>(&mut self, v: bool, f: impl FnOnce(&mut Self) -> T) -> T {
-        let old = mem::replace(&mut self.warning_period_57979_impl_trait_in_proj, v);
-        let ret = f(self);
-        self.warning_period_57979_impl_trait_in_proj = old;
-        ret
+        with(self, v, |this| &mut this.warning_period_57979_impl_trait_in_proj, f)
     }
 
     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
-        let old = mem::replace(&mut self.is_impl_trait_banned, true);
-        f(self);
-        self.is_impl_trait_banned = old;
+        with(self, true, |this| &mut this.is_impl_trait_banned, f)
     }
 
     fn with_impl_trait(&mut self, outer: Option<OuterImplTrait>, f: impl FnOnce(&mut Self)) {
-        let old = mem::replace(&mut self.outer_impl_trait, outer);
-        f(self);
-        self.outer_impl_trait = old;
+        with(self, outer, |this| &mut this.outer_impl_trait, f)
+    }
+
+    fn with_let_allowed(&mut self, v: bool, f: impl FnOnce(&mut Self)) {
+        with(self, v, |this| &mut this.is_let_allowed, f)
     }
 
     fn visit_assoc_type_binding_from_generic_args(&mut self, type_binding: &'a TypeBinding) {
@@ -297,52 +311,60 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    /// With eRFC 2497, we need to check whether an expression is ambiguous and warn or error
-    /// depending on the edition, this function handles that.
-    fn while_if_let_ambiguity(&self, expr: &P<Expr>) {
-        if let Some((span, op_kind)) = self.while_if_let_expr_ambiguity(&expr) {
-            let mut err = self.err_handler().struct_span_err(
-                span, &format!("ambiguous use of `{}`", op_kind.to_string())
-            );
-
-            err.note(
-                "this will be a error until the `let_chains` feature is stabilized"
-            );
-            err.note(
-                "see rust-lang/rust#53668 for more information"
-            );
-
-            if let Ok(snippet) = self.session.source_map().span_to_snippet(span) {
+    fn obsolete_in_place(&self, expr: &Expr, place: &Expr, val: &Expr) {
+        let mut err = self.err_handler().struct_span_err(
+            expr.span,
+            "emplacement syntax is obsolete (for now, anyway)",
+        );
+        err.note(
+            "for more information, see \
+                <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>"
+        );
+        match val.node {
+            ExprKind::Lit(ref v) if v.node.is_numeric() => {
                 err.span_suggestion(
-                    span, "consider adding parentheses", format!("({})", snippet),
-                    Applicability::MachineApplicable,
+                    place.span.between(val.span),
+                    "if you meant to write a comparison against a negative value, add a \
+                        space in between `<` and `-`",
+                    "< -".to_string(),
+                    Applicability::MaybeIncorrect
                 );
             }
-
-            err.emit();
+            _ => {}
         }
+        err.emit();
     }
 
-    /// With eRFC 2497 adding if-let chains, there is a requirement that the parsing of
-    /// `&&` and `||` in a if-let statement be unambiguous. This function returns a span and
-    /// a `BinOpKind` (either `&&` or `||` depending on what was ambiguous) if it is determined
-    /// that the current expression parsed is ambiguous and will break in future.
-    fn while_if_let_expr_ambiguity(&self, expr: &P<Expr>) -> Option<(Span, BinOpKind)> {
-        debug!("while_if_let_expr_ambiguity: expr.node: {:?}", expr.node);
+    /// Visits the `expr` and adjusts whether `let $pat = $expr` is allowed in decendants.
+    /// Returns whether we walked into `expr` or not.
+    /// If we did, walking should not happen again.
+    fn visit_expr_with_let_maybe_allowed(&mut self, expr: &'a Expr) -> bool {
         match &expr.node {
-            ExprKind::Binary(op, _, _) if op.node == BinOpKind::And || op.node == BinOpKind::Or => {
-                Some((expr.span, op.node))
-            },
-            ExprKind::Range(ref lhs, ref rhs, _) => {
-                let lhs_ambiguous = lhs.as_ref()
-                    .and_then(|lhs| self.while_if_let_expr_ambiguity(lhs));
-                let rhs_ambiguous = rhs.as_ref()
-                    .and_then(|rhs| self.while_if_let_expr_ambiguity(rhs));
-
-                lhs_ambiguous.or(rhs_ambiguous)
+            // Assuming the context permits, `($expr)` does not impose additional constraints.
+            ExprKind::Paren(_) => visit::walk_expr(self, expr),
+            // Assuming the context permits,
+            // l && r` allows decendants in `l` and `r` to be `let` expressions.
+            ExprKind::Binary(op, ..) if op.node == BinOpKind::And => visit::walk_expr(self, expr),
+            // However, we do allow it in the condition of the `if` expression.
+            // We do not allow `let` in `then` and `opt_else` directly.
+            ExprKind::If(ref cond, ref then, ref opt_else) => {
+                self.with_let_allowed(false, |this| {
+                    this.visit_block(then);
+                    walk_list!(this, visit_expr, opt_else);
+                });
+                self.with_let_allowed(true, |this| this.visit_expr(cond));
+            }
+            // The same logic applies to `While`.
+            ExprKind::While(ref cond, ref then, ref opt_label) => {
+                walk_list!(self, visit_label, opt_label);
+                self.with_let_allowed(false, |this| this.visit_block(then));
+                self.with_let_allowed(true, |this| this.visit_expr(cond));
             }
-            _ => None,
+            // Don't walk into `expr` and defer further checks to the caller.
+            _ => return false,
         }
+
+        true
     }
 }
 
@@ -449,38 +471,27 @@ fn validate_generics_order<'a>(
 impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_expr(&mut self, expr: &'a Expr) {
         match expr.node {
-            ExprKind::IfLet(_, ref expr, _, _) | ExprKind::WhileLet(_, ref expr, _, _) =>
-                self.while_if_let_ambiguity(&expr),
+            ExprKind::Let(_, _) if !self.is_let_allowed => {
+                self.err_handler()
+                    .struct_span_err(expr.span, "`let` expressions are not supported here")
+                    .note("only supported directly in conditions of `if`- and `while`-expressions")
+                    .note("as well as when nested within `&&` and parenthesis in those conditions")
+                    .emit();
+            }
+            _ if self.visit_expr_with_let_maybe_allowed(&expr) => {
+                // Prevent `walk_expr` to happen since we've already done that.
+                return;
+            }
             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
                 span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
             }
             ExprKind::ObsoleteInPlace(ref place, ref val) => {
-                let mut err = self.err_handler().struct_span_err(
-                    expr.span,
-                    "emplacement syntax is obsolete (for now, anyway)",
-                );
-                err.note(
-                    "for more information, see \
-                     <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>"
-                );
-                match val.node {
-                    ExprKind::Lit(ref v) if v.node.is_numeric() => {
-                        err.span_suggestion(
-                            place.span.between(val.span),
-                            "if you meant to write a comparison against a negative value, add a \
-                             space in between `<` and `-`",
-                            "< -".to_string(),
-                            Applicability::MaybeIncorrect
-                        );
-                    }
-                    _ => {}
-                }
-                err.emit();
+                self.obsolete_in_place(expr, place, val);
             }
             _ => {}
         }
 
-        visit::walk_expr(self, expr)
+        self.with_let_allowed(false, |this| visit::walk_expr(this, expr));
     }
 
     fn visit_ty(&mut self, ty: &'a Ty) {
@@ -862,6 +873,7 @@ pub fn check_crate(session: &Session, krate: &Crate) -> (bool, bool) {
         is_impl_trait_banned: false,
         warning_period_57979_didnt_record_next_impl_trait: false,
         warning_period_57979_impl_trait_in_proj: false,
+        is_let_allowed: false,
     };
     visit::walk_crate(&mut validator, krate);
 
diff --git a/src/librustc_passes/lib.rs b/src/librustc_passes/lib.rs
index 7c48feecb2110..0a13f27737a25 100644
--- a/src/librustc_passes/lib.rs
+++ b/src/librustc_passes/lib.rs
@@ -7,6 +7,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
 
 #![feature(nll)]
+#![feature(bind_by_move_pattern_guards)]
 #![feature(rustc_diagnostic_macros)]
 
 #![recursion_limit="256"]

From 96f09fef5ae0861331005ffb499ebc69180a77f3 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:05:05 +0200
Subject: [PATCH 08/21] let_chains: Adjust lowering logic in lieu of ::Let.

---
 src/librustc/hir/lowering.rs | 250 +++++++++++++++--------------------
 1 file changed, 109 insertions(+), 141 deletions(-)

diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index dd0d13d8f5a6a..0fd846d9fb9e7 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -4114,56 +4114,123 @@ impl<'a> LoweringContext<'a> {
                 let ohs = P(self.lower_expr(ohs));
                 hir::ExprKind::AddrOf(m, ohs)
             }
-            // More complicated than you might expect because the else branch
-            // might be `if let`.
+            ExprKind::Let(..) => {
+                // This should have been caught `ast_validation`!
+                self.sess.span_err(e.span, "`let` expressions only supported in `if`");
+                // ^-- FIXME(53667): Change to `delay_span_bug` when let_chains handled in lowering.
+                self.sess.abort_if_errors();
+                hir::ExprKind::Err
+            }
+            // FIXME(#53667): handle lowering of && and parens.
             ExprKind::If(ref cond, ref then, ref else_opt) => {
-                // `true => then`:
-                let then_pat = self.pat_bool(e.span, true);
-                let then_blk = self.lower_block(then, false);
-                let then_expr = self.expr_block(then_blk, ThinVec::new());
-                let then_arm = self.arm(hir_vec![then_pat], P(then_expr));
-
                 // `_ => else_block` where `else_block` is `{}` if there's `None`:
                 let else_pat = self.pat_wild(e.span);
-                let else_expr = match else_opt {
-                    None => self.expr_block_empty(e.span),
-                    Some(els) => match els.node {
-                        ExprKind::IfLet(..) => {
-                            // Wrap the `if let` expr in a block.
-                            let els = self.lower_expr(els);
-                            let blk = self.block_all(els.span, hir_vec![], Some(P(els)));
-                            self.expr_block(P(blk), ThinVec::new())
-                        }
-                        _ => self.lower_expr(els),
-                    }
+                let (else_expr, contains_else_clause) = match else_opt {
+                    None => (self.expr_block_empty(e.span), false),
+                    Some(els) => (self.lower_expr(els), true),
                 };
                 let else_arm = self.arm(hir_vec![else_pat], P(else_expr));
 
-                // Lower condition:
-                let span_block = self
-                    .sess
-                    .source_map()
-                    .mark_span_with_reason(IfTemporary, cond.span, None);
-                let cond = self.lower_expr(cond);
-                // Wrap in a construct equivalent to `{ let _t = $cond; _t }` to preserve drop
-                // semantics since `if cond { ... }` don't let temporaries live outside of `cond`.
-                let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
+                // Handle then + scrutinee:
+                let then_blk = self.lower_block(then, false);
+                let then_expr = self.expr_block(then_blk, ThinVec::new());
+                let (then_pats, scrutinee, desugar) = match cond.node {
+                    // `<pat> => <then>`
+                    ExprKind::Let(ref pats, ref scrutinee) => {
+                        let scrutinee = self.lower_expr(scrutinee);
+                        let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
+                        let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
+                        (pats, scrutinee, desugar)
+                    }
+                    // `true => then`:
+                    _ => {
+                        // Lower condition:
+                        let cond = self.lower_expr(cond);
+                        // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
+                        // to preserve drop semantics since `if cond { ... }`
+                        // don't let temporaries live outside of `cond`.
+                        let span_block = self
+                            .sess
+                            .source_map()
+                            .mark_span_with_reason(IfTemporary, cond.span, None);
+                        // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
+                        // to preserve drop semantics since `if cond { ... }` does not
+                        // let temporaries live outside of `cond`.
+                        let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
+
+                        let desugar = hir::MatchSource::IfDesugar { contains_else_clause };
+                        let pats = hir_vec![self.pat_bool(e.span, true)];
+                        (pats, cond, desugar)
+                    }
+                };
+                let then_arm = self.arm(then_pats, P(then_expr));
 
-                hir::ExprKind::Match(
-                    P(cond),
-                    vec![then_arm, else_arm].into(),
-                    hir::MatchSource::IfDesugar {
-                        contains_else_clause: else_opt.is_some()
-                    },
-                )
+                hir::ExprKind::Match(P(scrutinee), vec![then_arm, else_arm].into(), desugar)
+            }
+            // FIXME(#53667): handle lowering of && and parens.
+            ExprKind::While(ref cond, ref body, opt_label) => {
+                // Desugar `ExprWhileLet`
+                // from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
+                if let ExprKind::Let(ref pats, ref sub_expr) = cond.node {
+                    // to:
+                    //
+                    //   [opt_ident]: loop {
+                    //     match <sub_expr> {
+                    //       <pat> => <body>,
+                    //       _ => break
+                    //     }
+                    //   }
+
+                    // Note that the block AND the condition are evaluated in the loop scope.
+                    // This is done to allow `break` from inside the condition of the loop.
+                    let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
+                        (
+                            this.lower_block(body, false),
+                            this.expr_break(e.span, ThinVec::new()),
+                            this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
+                        )
+                    });
+
+                    // `<pat> => <body>`
+                    let pat_arm = {
+                        let body_expr = P(self.expr_block(body, ThinVec::new()));
+                        let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
+                        self.arm(pats, body_expr)
+                    };
+
+                    // `_ => break`
+                    let break_arm = {
+                        let pat_under = self.pat_wild(e.span);
+                        self.arm(hir_vec![pat_under], break_expr)
+                    };
+
+                    // `match <sub_expr> { ... }`
+                    let arms = hir_vec![pat_arm, break_arm];
+                    let match_expr = self.expr(
+                        sub_expr.span,
+                        hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
+                        ThinVec::new(),
+                    );
+
+                    // `[opt_ident]: loop { ... }`
+                    let loop_block = P(self.block_expr(P(match_expr)));
+                    let loop_expr = hir::ExprKind::Loop(
+                        loop_block,
+                        self.lower_label(opt_label),
+                        hir::LoopSource::WhileLet,
+                    );
+                    // Add attributes to the outer returned expr node.
+                    loop_expr
+                } else {
+                    self.with_loop_scope(e.id, |this| {
+                        hir::ExprKind::While(
+                            this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
+                            this.lower_block(body, false),
+                            this.lower_label(opt_label),
+                        )
+                    })
+                }
             }
-            ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
-                hir::ExprKind::While(
-                    this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
-                    this.lower_block(body, false),
-                    this.lower_label(opt_label),
-                )
-            }),
             ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
                 hir::ExprKind::Loop(
                     this.lower_block(body, false),
@@ -4482,105 +4549,6 @@ impl<'a> LoweringContext<'a> {
 
             ExprKind::Err => hir::ExprKind::Err,
 
-            // Desugar `ExprIfLet`
-            // from: `if let <pat> = <sub_expr> <body> [<else_opt>]`
-            ExprKind::IfLet(ref pats, ref sub_expr, ref body, ref else_opt) => {
-                // to:
-                //
-                //   match <sub_expr> {
-                //     <pat> => <body>,
-                //     _ => [<else_opt> | ()]
-                //   }
-
-                let mut arms = vec![];
-
-                // `<pat> => <body>`
-                {
-                    let body = self.lower_block(body, false);
-                    let body_expr = P(self.expr_block(body, ThinVec::new()));
-                    let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
-                    arms.push(self.arm(pats, body_expr));
-                }
-
-                // _ => [<else_opt>|{}]
-                {
-                    let wildcard_arm: Option<&Expr> = else_opt.as_ref().map(|p| &**p);
-                    let wildcard_pattern = self.pat_wild(e.span);
-                    let body = if let Some(else_expr) = wildcard_arm {
-                        self.lower_expr(else_expr)
-                    } else {
-                        self.expr_block_empty(e.span)
-                    };
-                    arms.push(self.arm(hir_vec![wildcard_pattern], P(body)));
-                }
-
-                let contains_else_clause = else_opt.is_some();
-
-                let sub_expr = P(self.lower_expr(sub_expr));
-
-                hir::ExprKind::Match(
-                    sub_expr,
-                    arms.into(),
-                    hir::MatchSource::IfLetDesugar {
-                        contains_else_clause,
-                    },
-                )
-            }
-
-            // Desugar `ExprWhileLet`
-            // from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
-            ExprKind::WhileLet(ref pats, ref sub_expr, ref body, opt_label) => {
-                // to:
-                //
-                //   [opt_ident]: loop {
-                //     match <sub_expr> {
-                //       <pat> => <body>,
-                //       _ => break
-                //     }
-                //   }
-
-                // Note that the block AND the condition are evaluated in the loop scope.
-                // This is done to allow `break` from inside the condition of the loop.
-                let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
-                    (
-                        this.lower_block(body, false),
-                        this.expr_break(e.span, ThinVec::new()),
-                        this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
-                    )
-                });
-
-                // `<pat> => <body>`
-                let pat_arm = {
-                    let body_expr = P(self.expr_block(body, ThinVec::new()));
-                    let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
-                    self.arm(pats, body_expr)
-                };
-
-                // `_ => break`
-                let break_arm = {
-                    let pat_under = self.pat_wild(e.span);
-                    self.arm(hir_vec![pat_under], break_expr)
-                };
-
-                // `match <sub_expr> { ... }`
-                let arms = hir_vec![pat_arm, break_arm];
-                let match_expr = self.expr(
-                    sub_expr.span,
-                    hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
-                    ThinVec::new(),
-                );
-
-                // `[opt_ident]: loop { ... }`
-                let loop_block = P(self.block_expr(P(match_expr)));
-                let loop_expr = hir::ExprKind::Loop(
-                    loop_block,
-                    self.lower_label(opt_label),
-                    hir::LoopSource::WhileLet,
-                );
-                // Add attributes to the outer returned expr node.
-                loop_expr
-            }
-
             // Desugar `ExprForLoop`
             // from: `[opt_ident]: for <pat> in <head> <body>`
             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {

From 635021b014e1761c76c03b027b50f06425589566 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:11:04 +0200
Subject: [PATCH 09/21] let_chains: Add feature gate.

---
 src/libsyntax/feature_gate.rs | 31 ++++++++++++++++++++++++++++++-
 src/libsyntax_pos/symbol.rs   |  1 +
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 8a066f3f4a093..202e809c00ae9 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -554,6 +554,9 @@ declare_features! (
     // Allows using C-variadics.
     (active, c_variadic, "1.34.0", Some(44930), None),
 
+    // Allows `if/while p && let q = r && ...` chains.
+    (active, let_chains, "1.36.0", Some(53667), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -565,7 +568,8 @@ declare_features! (
 const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::impl_trait_in_bindings,
     sym::generic_associated_types,
-    sym::const_generics
+    sym::const_generics,
+    sym::let_chains,
 ];
 
 declare_features! (
@@ -1884,6 +1888,27 @@ impl<'a> PostExpansionVisitor<'a> {
             Err(mut err) => err.emit(),
         }
     }
+
+    /// Recurse into all places where a `let` expression would be feature gated
+    /// and emit gate post errors for those.
+    fn find_and_gate_lets(&mut self, e: &'a ast::Expr) {
+        match &e.node {
+            ast::ExprKind::Paren(e) => {
+                self.find_and_gate_lets(e);
+            }
+            ast::ExprKind::Binary(op, lhs, rhs) if op.node == ast::BinOpKind::And => {
+                self.find_and_gate_lets(lhs);
+                self.find_and_gate_lets(rhs);
+            }
+            ast::ExprKind::Let(..) => {
+                gate_feature_post!(
+                    &self, let_chains, e.span,
+                    "`let` expressions in this position are experimental"
+                );
+            }
+            _ => {}
+        }
+    }
 }
 
 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
@@ -2099,6 +2124,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_expr(&mut self, e: &'a ast::Expr) {
         match e.node {
+            ast::ExprKind::If(ref e, ..) | ast::ExprKind::While(ref e, ..) => match e.node {
+                ast::ExprKind::Let(..) => {} // Stable!,
+                _ => self.find_and_gate_lets(e),
+            }
             ast::ExprKind::Box(_) => {
                 gate_feature_post!(&self, box_syntax, e.span, EXPLAIN_BOX_SYNTAX);
             }
diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index ec0ce4253fa2d..687a19cb1feae 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -305,6 +305,7 @@ symbols! {
         label_break_value,
         lang,
         lang_items,
+        let_chains,
         lib,
         link,
         linkage,

From 6b4e833e87ae40406b9f991f4a97feecc26ee604 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:06:36 +0200
Subject: [PATCH 10/21] let_chains: Add tests for places where let expressions
 aren't allowed.

---
 .../disallowed-positions.rs                   | 159 ++++++
 .../disallowed-positions.stderr               | 476 ++++++++++++++++++
 2 files changed, 635 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr

diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
new file mode 100644
index 0000000000000..d871f2fad2358
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
@@ -0,0 +1,159 @@
+// Here we test that `ast_validation` behaves correctly wrt. `let $pats = $expr` expressions.
+//
+// We want to make sure that `let` is banned in situations other than:
+//
+// expr =
+//   | ...
+//   | "if" expr_with_let block {"else" block}?
+//   | {label ":"}? while" expr_with_let block
+//   ;
+//
+// expr_with_let =
+//   | "let" top_pats "=" expr
+//   | expr_with_let "&&" expr_with_let
+//   | "(" expr_with_let ")"
+//   | expr
+//   ;
+//
+// To that end, we check some positions which is not part of the language above.
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn main() {}
+
+fn nested_within_if_expr() {
+    if &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR `let` expressions only supported in `if`
+
+    if !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        if let 0 = 0? {}
+        Ok(())
+    }
+    if (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+
+    if true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if true && (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if true || (true && let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    if x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    if true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+
+    // Binds as `(let ... = true)..true &&/|| false`.
+    if let Range { start: _, end: _ } = true..true && false {}
+    //~^ ERROR `let` expressions are not supported here
+    if let Range { start: _, end: _ } = true..true || false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: F, end } = F)..(|| true)`.
+    const F: fn() -> bool = || true;
+    if let Range { start: F, end } = F..|| true {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: true, end } = t)..(&&false)`.
+    let t = &&true;
+    if let Range { start: true, end } = t..&&false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    if let true = let true = true {} //~ ERROR `let` expressions are not supported here
+}
+
+fn nested_within_while_expr() {
+    while &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    while !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        while let 0 = 0? {}
+        Ok(())
+    }
+    while (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+
+    while true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while true && (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while true || (true && let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    while x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    while true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+
+    // Binds as `(let ... = true)..true &&/|| false`.
+    while let Range { start: _, end: _ } = true..true && false {}
+    //~^ ERROR `let` expressions are not supported here
+    while let Range { start: _, end: _ } = true..true || false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: F, end } = F)..(|| true)`.
+    const F: fn() -> bool = || true;
+    while let Range { start: F, end } = F..|| true {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: true, end } = t)..(&&false)`.
+    let t = &&true;
+    while let Range { start: true, end } = t..&&false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    while let true = let true = true {} //~ ERROR `let` expressions are not supported here
+}
+
+fn not_error_because_clarified_intent() {
+    if let Range { start: _, end: _ } = (true..true || false) { }
+
+    if let Range { start: _, end: _ } = (true..true && false) { }
+
+    while let Range { start: _, end: _ } = (true..true || false) { }
+
+    while let Range { start: _, end: _ } = (true..true && false) { }
+}
+
+fn outside_if_and_while_expr() {
+    &let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    !let 0 = 0; //~ ERROR `let` expressions are not supported here
+    *let 0 = 0; //~ ERROR `let` expressions are not supported here
+    -let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        let 0 = 0?;
+        Ok(())
+    }
+    (let 0 = 0)?; //~ ERROR `let` expressions are not supported here
+
+    true || let 0 = 0; //~ ERROR `let` expressions are not supported here
+    (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
+    true && (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    x = let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    true..(let 0 = 0); //~ ERROR `let` expressions are not supported here
+    ..(let 0 = 0); //~ ERROR `let` expressions are not supported here
+    (let 0 = 0)..; //~ ERROR `let` expressions are not supported here
+
+    (let Range { start: _, end: _ } = true..true || false);
+    //~^ ERROR `let` expressions are not supported here
+
+    (let true = let true = true);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+
+    // Check function tail position.
+    &let 0 = 0
+    //~^ ERROR `let` expressions are not supported here
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
new file mode 100644
index 0000000000000..18665e7933dd4
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
@@ -0,0 +1,476 @@
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:27:9
+   |
+LL |     if &let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:30:9
+   |
+LL |     if !let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:31:9
+   |
+LL |     if *let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:32:9
+   |
+LL |     if -let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:38:9
+   |
+LL |     if (let 0 = 0)? {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:40:16
+   |
+LL |     if true || let 0 = 0 {}
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:41:17
+   |
+LL |     if (true || let 0 = 0) {}
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:42:25
+   |
+LL |     if true && (true || let 0 = 0) {}
+   |                         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:43:25
+   |
+LL |     if true || (true && let 0 = 0) {}
+   |                         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:46:12
+   |
+LL |     if x = let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:48:15
+   |
+LL |     if true..(let 0 = 0) {}
+   |               ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:49:11
+   |
+LL |     if ..(let 0 = 0) {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:50:9
+   |
+LL |     if (let 0 = 0).. {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:53:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:55:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:60:8
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:65:8
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:68:19
+   |
+LL |     if let true = let true = true {}
+   |                   ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:72:12
+   |
+LL |     while &let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:74:12
+   |
+LL |     while !let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:75:12
+   |
+LL |     while *let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:76:12
+   |
+LL |     while -let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:82:12
+   |
+LL |     while (let 0 = 0)? {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:84:19
+   |
+LL |     while true || let 0 = 0 {}
+   |                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:85:20
+   |
+LL |     while (true || let 0 = 0) {}
+   |                    ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:86:28
+   |
+LL |     while true && (true || let 0 = 0) {}
+   |                            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:87:28
+   |
+LL |     while true || (true && let 0 = 0) {}
+   |                            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:90:15
+   |
+LL |     while x = let 0 = 0 {}
+   |               ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:92:18
+   |
+LL |     while true..(let 0 = 0) {}
+   |                  ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:93:14
+   |
+LL |     while ..(let 0 = 0) {}
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:94:12
+   |
+LL |     while (let 0 = 0).. {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:97:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:99:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:104:11
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:109:11
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:112:22
+   |
+LL |     while let true = let true = true {}
+   |                      ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:126:6
+   |
+LL |     &let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:128:6
+   |
+LL |     !let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:129:6
+   |
+LL |     *let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:130:6
+   |
+LL |     -let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:136:6
+   |
+LL |     (let 0 = 0)?;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:138:13
+   |
+LL |     true || let 0 = 0;
+   |             ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:139:14
+   |
+LL |     (true || let 0 = 0);
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:140:22
+   |
+LL |     true && (true || let 0 = 0);
+   |                      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:143:9
+   |
+LL |     x = let 0 = 0;
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:145:12
+   |
+LL |     true..(let 0 = 0);
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:146:8
+   |
+LL |     ..(let 0 = 0);
+   |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:147:6
+   |
+LL |     (let 0 = 0)..;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:149:6
+   |
+LL |     (let Range { start: _, end: _ } = true..true || false);
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:152:6
+   |
+LL |     (let true = let true = true);
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:152:17
+   |
+LL |     (let true = let true = true);
+   |                 ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:157:6
+   |
+LL |     &let 0 = 0
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions only supported in `if`
+  --> $DIR/disallowed-positions.rs:27:9
+   |
+LL |     if &let 0 = 0 {}
+   |         ^^^^^^^^^
+
+error: aborting due to 53 previous errors
+

From cb6c0beb481491884b58b98848c7ed26fec8fbe3 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:11:31 +0200
Subject: [PATCH 11/21] let_chains: Add feature gate tests.

---
 .../ui/rfc-2497-if-let-chains/feature-gate.rs | 101 +++++++
 .../feature-gate.stderr                       | 279 ++++++++++++++++++
 2 files changed, 380 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr

diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
new file mode 100644
index 0000000000000..cf4a44a9ce773
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
@@ -0,0 +1,101 @@
+// gate-test-let_chains
+
+// Here we test feature gating for ´let_chains`.
+// See `disallowed-positions.rs` for the grammar
+// defining the language for gated allowed positions.
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn _if() {
+    if let 0 = 1 {} // Stable!
+
+    if (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions only supported in `if`
+
+    if (((let 0 = 1))) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if (let 0 = 1) && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if true && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if (let 0 = 1) && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    if let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+}
+
+fn _while() {
+    if let 0 = 1 {} // Stable!
+
+    while (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while (((let 0 = 1))) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while (let 0 = 1) && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while true && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while (let 0 = 1) && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    while let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+}
+
+fn _macros() {
+    macro_rules! noop_expr { ($e:expr) => {}; }
+    noop_expr!((let 0 = 1));
+
+    macro_rules! use_expr {
+        ($e:expr) => {
+            if $e {}
+            while $e {}
+        }
+    }
+    use_expr!(let 0 = 1 && 0 == 0);
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    use_expr!((let 0 = 1));
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    use_expr!(let 0 = 1);
+}
+
+fn main() {}
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
new file mode 100644
index 0000000000000..f641429a6bbb9
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
@@ -0,0 +1,279 @@
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:14:9
+   |
+LL |     if (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:18:11
+   |
+LL |     if (((let 0 = 1))) {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:21:16
+   |
+LL |     if true && let 0 = 1 {}
+   |                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:24:8
+   |
+LL |     if let 0 = 1 && true {}
+   |        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:27:9
+   |
+LL |     if (let 0 = 1) && true {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:30:17
+   |
+LL |     if true && (let 0 = 1) {}
+   |                 ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:33:9
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:33:24
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |                        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:8
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:21
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                     ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:35
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:48
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:61
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                             ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:44:8
+   |
+LL |     if let Range { start: _, end: _ } = (true..true) && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:51:12
+   |
+LL |     while (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:54:14
+   |
+LL |     while (((let 0 = 1))) {}
+   |              ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:57:19
+   |
+LL |     while true && let 0 = 1 {}
+   |                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:60:11
+   |
+LL |     while let 0 = 1 && true {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:63:12
+   |
+LL |     while (let 0 = 1) && true {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:66:20
+   |
+LL |     while true && (let 0 = 1) {}
+   |                    ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:69:12
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:69:27
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |                           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:11
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:24
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:38
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                      ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:51
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:64
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:80:11
+   |
+LL |     while let Range { start: _, end: _ } = (true..true) && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:94:15
+   |
+LL |     use_expr!(let 0 = 1 && 0 == 0);
+   |               ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:96:16
+   |
+LL |     use_expr!((let 0 = 1));
+   |                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error: `let` expressions only supported in `if`
+  --> $DIR/feature-gate.rs:14:9
+   |
+LL |     if (let 0 = 1) {}
+   |         ^^^^^^^^^
+
+error: aborting due to 31 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

From 170c8a85c5fb569ba677d3753c4d9ed91c909a9f Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 15:33:34 +0200
Subject: [PATCH 12/21] let_chains: Adjust unnecessary parens tests.

---
 src/test/ui/lint/lint-unnecessary-parens.rs     | 4 ++--
 src/test/ui/lint/lint-unnecessary-parens.stderr | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/test/ui/lint/lint-unnecessary-parens.rs b/src/test/ui/lint/lint-unnecessary-parens.rs
index dc74d69bd8d54..d684616565f8e 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.rs
+++ b/src/test/ui/lint/lint-unnecessary-parens.rs
@@ -22,8 +22,8 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
-    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `if let` head expression
-    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `while let` head expression
+    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `let` scrutinee
+    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `let` scrutinee
     let v = X { y: false };
     // struct lits needs parens, so these shouldn't warn.
     if (v == X { y: true }) {}
diff --git a/src/test/ui/lint/lint-unnecessary-parens.stderr b/src/test/ui/lint/lint-unnecessary-parens.stderr
index fe2ee38eb42a3..eb6ead2a274ce 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.stderr
+++ b/src/test/ui/lint/lint-unnecessary-parens.stderr
@@ -40,13 +40,13 @@ error: unnecessary parentheses around `match` head expression
 LL |     match (true) {
    |           ^^^^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `if let` head expression
+error: unnecessary parentheses around `let` scrutinee
   --> $DIR/lint-unnecessary-parens.rs:25:16
    |
 LL |     if let 1 = (1) {}
    |                ^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `while let` head expression
+error: unnecessary parentheses around `let` scrutinee
   --> $DIR/lint-unnecessary-parens.rs:26:19
    |
 LL |     while let 1 = (2) {}

From f397d55e8771e1bf6084dd3995fd0c7e96f8cf50 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 15:16:04 +0200
Subject: [PATCH 13/21] let_chains: Add test protecting the precedence of && in
 relation to other things.

---
 .../protect-precedences.rs                     | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs

diff --git a/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs
new file mode 100644
index 0000000000000..1de4e5bcebee9
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs
@@ -0,0 +1,18 @@
+// run-pass
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn main() {
+    let x: bool;
+    // This should associate as: `(x = (true && false));`.
+    x = true && false;
+    assert!(!x);
+
+    fn _f1() -> bool {
+        // Should associate as `(let _ = (return (true && false)))`.
+        if let _ = return true && false {};
+    }
+    assert!(!_f1());
+}

From aaaae35d843cc318c04bbd0e935f641a1d264c68 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:11:54 +0200
Subject: [PATCH 14/21] let_chains: Remove redundant tests in
 syntax-ambiguity-*.rs.

---
 .../syntax-ambiguity-2015.rs                  | 38 -------------
 .../syntax-ambiguity-2015.stderr              | 56 -------------------
 .../syntax-ambiguity-2018.rs                  | 38 -------------
 .../syntax-ambiguity-2018.stderr              | 56 -------------------
 4 files changed, 188 deletions(-)
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr

diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
deleted file mode 100644
index d79798d57e820..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-// edition:2015
-
-// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
-// with examples easier.
-
-#[allow(irrefutable_let_patterns)]
-fn main() {
-    use std::ops::Range;
-
-    if let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    if let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    while let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    if let true = false && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let true = (1 == 2) && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    // The following cases are not an error as parenthesis are used to
-    // clarify intent:
-
-    if let Range { start: _, end: _ } = true..(true || false) { }
-
-    if let Range { start: _, end: _ } = true..(true && false) { }
-
-    while let Range { start: _, end: _ } = true..(true || false) { }
-
-    while let Range { start: _, end: _ } = true..(true && false) { }
-}
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
deleted file mode 100644
index 2cd59fe56cf2d..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
+++ /dev/null
@@ -1,56 +0,0 @@
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:10:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true && false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2015.rs:13:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true || false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:16:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true && false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2015.rs:19:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true || false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:22:19
-   |
-LL |     if let true = false && false { }
-   |                   ^^^^^^^^^^^^^^ help: consider adding parentheses: `(false && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:25:22
-   |
-LL |     while let true = (1 == 2) && false { }
-   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parentheses: `((1 == 2) && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: aborting due to 6 previous errors
-
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
deleted file mode 100644
index 687bf659416ab..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-// edition:2018
-
-// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
-// with examples easier.
-
-#[allow(irrefutable_let_patterns)]
-fn main() {
-    use std::ops::Range;
-
-    if let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    if let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    while let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    if let true = false && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let true = (1 == 2) && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    // The following cases are not an error as parenthesis are used to
-    // clarify intent:
-
-    if let Range { start: _, end: _ } = true..(true || false) { }
-
-    if let Range { start: _, end: _ } = true..(true && false) { }
-
-    while let Range { start: _, end: _ } = true..(true || false) { }
-
-    while let Range { start: _, end: _ } = true..(true && false) { }
-}
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr
deleted file mode 100644
index cbba2d7473334..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr
+++ /dev/null
@@ -1,56 +0,0 @@
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:10:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true && false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2018.rs:13:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true || false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:16:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true && false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2018.rs:19:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true || false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:22:19
-   |
-LL |     if let true = false && false { }
-   |                   ^^^^^^^^^^^^^^ help: consider adding parentheses: `(false && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:25:22
-   |
-LL |     while let true = (1 == 2) && false { }
-   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parentheses: `((1 == 2) && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: aborting due to 6 previous errors
-

From d5a16217070a471f0335f49b0a16bfdad2f60a04 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 21:03:44 +0200
Subject: [PATCH 15/21] let_chains: Test pretty output for simple stable
 if-let.

---
 src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs |  6 ++++++
 .../ui/rfc-2497-if-let-chains/ast-pretty-check.stdout  | 10 ++++++++++
 2 files changed, 16 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout

diff --git a/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
new file mode 100644
index 0000000000000..e66d46575664f
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
@@ -0,0 +1,6 @@
+// compile-pass
+// compile-flags: -Z unpretty=expanded
+
+fn main() {
+    if let 0 = 1 {}
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout
new file mode 100644
index 0000000000000..a6b15f9bbf65d
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout
@@ -0,0 +1,10 @@
+#![feature(prelude_import)]
+#![no_std]
+#[prelude_import]
+use ::std::prelude::v1::*;
+#[macro_use]
+extern crate std;
+// compile-pass
+// compile-flags: -Z unpretty=expanded
+
+fn main() { if let 0 = 1 { } }

From b840eb1aa42d5e35a6434db166cd4111684f737b Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Thu, 16 May 2019 14:17:10 +0200
Subject: [PATCH 16/21] let_chains: Comment out Let in ident_can_begin_expr.

---
 src/libsyntax/parse/token.rs                           | 3 ++-
 src/test/ui/rfc-2497-if-let-chains/feature-gate.rs     | 5 +++--
 src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr | 6 +++---
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 7023a50b553af..abe5b8aad7f36 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -122,7 +122,8 @@ pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
         keywords::Continue.name(),
         keywords::False.name(),
         keywords::For.name(),
-        keywords::Let.name(),
+        // FIXME(53667): Consider whether `Let` can be added here.
+        // keywords::Let.name(),
         keywords::If.name(),
         keywords::Loop.name(),
         keywords::Match.name(),
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
index cf4a44a9ce773..b6f70b912acac 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
@@ -91,11 +91,12 @@ fn _macros() {
             while $e {}
         }
     }
-    use_expr!(let 0 = 1 && 0 == 0);
+    use_expr!((let 0 = 1 && 0 == 0));
     //~^ ERROR `let` expressions in this position are experimental [E0658]
     use_expr!((let 0 = 1));
     //~^ ERROR `let` expressions in this position are experimental [E0658]
-    use_expr!(let 0 = 1);
+    // use_expr!(let 0 = 1);
+    // ^--- FIXME(53667): Consider whether `Let` can be added to `ident_can_begin_expr`.
 }
 
 fn main() {}
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
index f641429a6bbb9..fdfbb8fe4d441 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
@@ -251,10 +251,10 @@ LL |     while let Range { start: _, end: _ } = (true..true) && false {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:94:15
+  --> $DIR/feature-gate.rs:94:16
    |
-LL |     use_expr!(let 0 = 1 && 0 == 0);
-   |               ^^^^^^^^^
+LL |     use_expr!((let 0 = 1 && 0 == 0));
+   |                ^^^^^^^^^
    |
    = note: for more information, see https://github.com/rust-lang/rust/issues/53667
    = help: add #![feature(let_chains)] to the crate attributes to enable

From ccb40bce46c10855927f456e607aa553336bc1c2 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 00:27:12 +0200
Subject: [PATCH 17/21] let_chains: Improve documentation for
 ast::ExprKind::Let(..).

---
 src/libsyntax/ast.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 61b1f9be8fe72..abae13cc9f378 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1134,8 +1134,11 @@ pub enum ExprKind {
     Cast(P<Expr>, P<Ty>),
     /// A type ascription (e.g., `42: usize`).
     Type(P<Expr>, P<Ty>),
-    /// A `let pat = expr` pseudo-expression that only occurs in the scrutinee
+    /// A `let pats = expr` pseudo-expression that only occurs in the scrutinee
     /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
+    ///
+    /// The `Vec<P<Pat>>` is for or-patterns at the top level.
+    /// FIXME(54883): Change this to just `P<Pat>`.
     Let(Vec<P<Pat>>, P<Expr>),
     /// An `if` block, with an optional `else` block.
     ///

From e97b0275eb76462f74a64a0fe1a5433dfc869168 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 00:34:55 +0200
Subject: [PATCH 18/21] let_chains: Fix outdated doc-comment re.
 'parse_if_expr'.

---
 src/libsyntax/parse/parser.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 660c2ca2ba63a..7a940f0798a85 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -3769,7 +3769,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// Parses an `if` or `if let` expression (`if` token already eaten).
+    /// Parses an `if` expression (`if` token already eaten).
     fn parse_if_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;

From c887d02faaa33b4d3f66b5a48ac2f010e28d6f46 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 00:37:02 +0200
Subject: [PATCH 19/21] let_chains: reorder Let in 'ident_can_begin_expr' for
 lexical order.

---
 src/libsyntax/parse/token.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index abe5b8aad7f36..da6f0b47ccd32 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -122,9 +122,9 @@ pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
         keywords::Continue.name(),
         keywords::False.name(),
         keywords::For.name(),
+        keywords::If.name(),
         // FIXME(53667): Consider whether `Let` can be added here.
         // keywords::Let.name(),
-        keywords::If.name(),
         keywords::Loop.name(),
         keywords::Match.name(),
         keywords::Move.name(),

From 2178db422ead43732fee1435ead548cf8ec92ed5 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 01:22:43 +0200
Subject: [PATCH 20/21] let_chains: scrutinee -> head expression.

---
 src/librustc_lint/unused.rs                     | 2 +-
 src/test/ui/lint/lint-unnecessary-parens.rs     | 4 ++--
 src/test/ui/lint/lint-unnecessary-parens.stderr | 4 ++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index d90433fd93c16..7975a1042d45a 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -313,7 +313,7 @@ impl UnusedParens {
             ast::ExprKind::Let(_, ref expr) => {
                 // FIXME(#60336): Properly handle `let true = (false && true)`
                 // actually needing the parenthesis.
-                self.check_unused_parens_expr(cx, expr, "`let` scrutinee", followed_by_block);
+                self.check_unused_parens_expr(cx, expr, "`let` head expression", followed_by_block);
             }
             _ => {}
         }
diff --git a/src/test/ui/lint/lint-unnecessary-parens.rs b/src/test/ui/lint/lint-unnecessary-parens.rs
index d684616565f8e..c36101043a7f7 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.rs
+++ b/src/test/ui/lint/lint-unnecessary-parens.rs
@@ -22,8 +22,8 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
-    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `let` scrutinee
-    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `let` scrutinee
+    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `let` head expression
+    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `let` head expression
     let v = X { y: false };
     // struct lits needs parens, so these shouldn't warn.
     if (v == X { y: true }) {}
diff --git a/src/test/ui/lint/lint-unnecessary-parens.stderr b/src/test/ui/lint/lint-unnecessary-parens.stderr
index eb6ead2a274ce..dfbefd7b03ee5 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.stderr
+++ b/src/test/ui/lint/lint-unnecessary-parens.stderr
@@ -40,13 +40,13 @@ error: unnecessary parentheses around `match` head expression
 LL |     match (true) {
    |           ^^^^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `let` scrutinee
+error: unnecessary parentheses around `let` head expression
   --> $DIR/lint-unnecessary-parens.rs:25:16
    |
 LL |     if let 1 = (1) {}
    |                ^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `let` scrutinee
+error: unnecessary parentheses around `let` head expression
   --> $DIR/lint-unnecessary-parens.rs:26:19
    |
 LL |     while let 1 = (2) {}

From f607ff1cd2356b99af2b8bdf461458fb027f4d5a Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 11:49:27 +0200
Subject: [PATCH 21/21] let_chains: Change AST validation strategy slightly.

---
 src/librustc_passes/ast_validation.rs | 79 +++++++++++++++------------
 1 file changed, 45 insertions(+), 34 deletions(-)

diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index 5e4713468135d..844763d6c13eb 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -103,8 +103,10 @@ impl<'a> AstValidator<'a> {
         with(self, outer, |this| &mut this.outer_impl_trait, f)
     }
 
-    fn with_let_allowed(&mut self, v: bool, f: impl FnOnce(&mut Self)) {
-        with(self, v, |this| &mut this.is_let_allowed, f)
+    fn with_let_allowed(&mut self, v: bool, f: impl FnOnce(&mut Self, bool)) {
+        let old = mem::replace(&mut self.is_let_allowed, v);
+        f(self, old);
+        self.is_let_allowed = old;
     }
 
     fn visit_assoc_type_binding_from_generic_args(&mut self, type_binding: &'a TypeBinding) {
@@ -338,27 +340,29 @@ impl<'a> AstValidator<'a> {
     /// Visits the `expr` and adjusts whether `let $pat = $expr` is allowed in decendants.
     /// Returns whether we walked into `expr` or not.
     /// If we did, walking should not happen again.
-    fn visit_expr_with_let_maybe_allowed(&mut self, expr: &'a Expr) -> bool {
+    fn visit_expr_with_let_maybe_allowed(&mut self, expr: &'a Expr, let_allowed: bool) -> bool {
         match &expr.node {
             // Assuming the context permits, `($expr)` does not impose additional constraints.
-            ExprKind::Paren(_) => visit::walk_expr(self, expr),
+            ExprKind::Paren(_) => {
+                self.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
+            }
             // Assuming the context permits,
             // l && r` allows decendants in `l` and `r` to be `let` expressions.
-            ExprKind::Binary(op, ..) if op.node == BinOpKind::And => visit::walk_expr(self, expr),
+            ExprKind::Binary(op, ..) if op.node == BinOpKind::And => {
+                self.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
+            }
             // However, we do allow it in the condition of the `if` expression.
             // We do not allow `let` in `then` and `opt_else` directly.
-            ExprKind::If(ref cond, ref then, ref opt_else) => {
-                self.with_let_allowed(false, |this| {
-                    this.visit_block(then);
-                    walk_list!(this, visit_expr, opt_else);
-                });
-                self.with_let_allowed(true, |this| this.visit_expr(cond));
+            ExprKind::If(cond, then, opt_else) => {
+                self.visit_block(then);
+                walk_list!(self, visit_expr, opt_else);
+                self.with_let_allowed(true, |this, _| this.visit_expr(cond));
             }
             // The same logic applies to `While`.
-            ExprKind::While(ref cond, ref then, ref opt_label) => {
+            ExprKind::While(cond, then, opt_label) => {
                 walk_list!(self, visit_label, opt_label);
-                self.with_let_allowed(false, |this| this.visit_block(then));
-                self.with_let_allowed(true, |this| this.visit_expr(cond));
+                self.visit_block(then);
+                self.with_let_allowed(true, |this, _| this.visit_expr(cond));
             }
             // Don't walk into `expr` and defer further checks to the caller.
             _ => return false,
@@ -366,6 +370,15 @@ impl<'a> AstValidator<'a> {
 
         true
     }
+
+    /// Emits an error banning the `let` expression provided.
+    fn ban_let_expr(&self, expr: &'a Expr) {
+        self.err_handler()
+            .struct_span_err(expr.span, "`let` expressions are not supported here")
+            .note("only supported directly in conditions of `if`- and `while`-expressions")
+            .note("as well as when nested within `&&` and parenthesis in those conditions")
+            .emit();
+    }
 }
 
 enum GenericPosition {
@@ -470,28 +483,26 @@ fn validate_generics_order<'a>(
 
 impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_expr(&mut self, expr: &'a Expr) {
-        match expr.node {
-            ExprKind::Let(_, _) if !self.is_let_allowed => {
-                self.err_handler()
-                    .struct_span_err(expr.span, "`let` expressions are not supported here")
-                    .note("only supported directly in conditions of `if`- and `while`-expressions")
-                    .note("as well as when nested within `&&` and parenthesis in those conditions")
-                    .emit();
-            }
-            _ if self.visit_expr_with_let_maybe_allowed(&expr) => {
-                // Prevent `walk_expr` to happen since we've already done that.
-                return;
-            }
-            ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
-                span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
-            }
-            ExprKind::ObsoleteInPlace(ref place, ref val) => {
-                self.obsolete_in_place(expr, place, val);
+        self.with_let_allowed(false, |this, let_allowed| {
+            match &expr.node {
+                ExprKind::Let(_, _) if !let_allowed => {
+                    this.ban_let_expr(expr);
+                }
+                _ if this.visit_expr_with_let_maybe_allowed(&expr, let_allowed) => {
+                    // Prevent `walk_expr` to happen since we've already done that.
+                    return;
+                }
+                ExprKind::InlineAsm(..) if !this.session.target.target.options.allow_asm => {
+                    span_err!(this.session, expr.span, E0472, "asm! is unsupported on this target");
+                }
+                ExprKind::ObsoleteInPlace(place, val) => {
+                    this.obsolete_in_place(expr, place, val);
+                }
+                _ => {}
             }
-            _ => {}
-        }
 
-        self.with_let_allowed(false, |this| visit::walk_expr(this, expr));
+            visit::walk_expr(this, expr);
+        });
     }
 
     fn visit_ty(&mut self, ty: &'a Ty) {