Auto merge of #89841 - cormacrelf:let-else-typed, r=nagisa

Implement let-else type annotations natively

Tracking issue: #87335

Fixes #89688, fixes #89807, edit: fixes  #89960 as well

As explained in #89688 (comment), the previous desugaring moved the let-else scrutinee into a dummy variable, which meant if you wanted to refer to it again in the else block, it had moved.

This introduces a new hir type, ~~`hir::LetExpr`~~ `hir::Let`, which takes over all the fields of `hir::ExprKind::Let(...)` and adds an optional type annotation. The `hir::Let` is then treated like a `hir::Local` when type checking a function body, specifically:

* `GatherLocalsVisitor` overrides a new `Visitor::visit_let_expr` and does pretty much exactly what it does for `visit_local`, assigning a local type to the `hir::Let` ~~(they could be deduplicated but they are right next to each other, so at least we know they're the same)~~
* It reuses the code in `check_decl_local` to typecheck the `hir::Let`, simply returning 'bool' for the expression type after doing that.

* ~~`FnCtxt::check_expr_let` passes this local type in to `demand_scrutinee_type`, and then imitates check_decl_local's pattern checking~~
* ~~`demand_scrutinee_type` (the blindest change for me, please give this extra scrutiny) uses this local type instead of of creating a new one~~
    * ~~Just realised the `check_expr_with_needs` was passing NoExpectation further down, need to pass the type there too. And apparently this Expectation API already exists.~~

Some other misc notes:

* ~~Is the clippy code supposed to be autoformatted? I tried not to give huge diffs but maybe some rustfmt changes simply haven't hit it yet.~~
* in `rustc_ast_lowering/src/block.rs`, I noticed some existing `self.alias_attrs()` calls in `LoweringContext::lower_stmts` seem to be copying attributes from the lowered locals/etc to the statements. Is that right? I'm new at this, I don't know.

compiler/rustc_ast_lowering/src/block.rs

@@ -2,7 +2,6 @@ use crate::{ImplTraitContext, ImplTraitPosition, LoweringContext};
 use rustc_ast::{AttrVec, Block, BlockCheckMode, Expr, Local, LocalKind, Stmt, StmtKind};
 use rustc_hir as hir;
 use rustc_session::parse::feature_err;
-use rustc_span::symbol::Ident;
 use rustc_span::{sym, DesugaringKind};
 
 use smallvec::SmallVec;
@@ -39,8 +38,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     let hir_id = self.lower_node_id(s.id);
                     match &local.kind {
                         LocalKind::InitElse(init, els) => {
-                            let (s, e) = self.lower_let_else(hir_id, local, init, els, tail);
-                            stmts.push(s);
+                            let e = self.lower_let_else(hir_id, local, init, els, tail);
                             expr = Some(e);
                             // remaining statements are in let-else expression
                             break;
@@ -125,36 +123,25 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         init: &Expr,
         els: &Block,
         tail: &[Stmt],
-    ) -> (hir::Stmt<'hir>, &'hir hir::Expr<'hir>) {
+    ) -> &'hir hir::Expr<'hir> {
         let ty = local
             .ty
             .as_ref()
             .map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Binding)));
         let span = self.lower_span(local.span);
         let span = self.mark_span_with_reason(DesugaringKind::LetElse, span, None);
-        let init = Some(self.lower_expr(init));
-        let val = Ident::with_dummy_span(sym::val);
-        let (pat, val_id) =
-            self.pat_ident_binding_mode(span, val, hir::BindingAnnotation::Unannotated);
+        let init = self.lower_expr(init);
         let local_hir_id = self.lower_node_id(local.id);
         self.lower_attrs(local_hir_id, &local.attrs);
-        // first statement which basically exists for the type annotation
-        let stmt = {
-            let local = self.arena.alloc(hir::Local {
+        let let_expr = {
+            let lex = self.arena.alloc(hir::Let {
                 hir_id: local_hir_id,
+                pat: self.lower_pat(&local.pat),
                 ty,
-                pat,
                 init,
                 span,
-                source: hir::LocalSource::Normal,
             });
-            let kind = hir::StmtKind::Local(local);
-            hir::Stmt { hir_id: stmt_hir_id, kind, span }
-        };
-        let let_expr = {
-            let scrutinee = self.expr_ident(span, val, val_id);
-            let let_kind = hir::ExprKind::Let(self.lower_pat(&local.pat), scrutinee, span);
-            self.arena.alloc(self.expr(span, let_kind, AttrVec::new()))
+            self.arena.alloc(self.expr(span, hir::ExprKind::Let(lex), AttrVec::new()))
         };
         let then_expr = {
             let (stmts, expr) = self.lower_stmts(tail);
@@ -165,9 +152,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             let block = self.lower_block(els, false);
             self.arena.alloc(self.expr_block(block, AttrVec::new()))
         };
+        self.alias_attrs(let_expr.hir_id, local_hir_id);
         self.alias_attrs(else_expr.hir_id, local_hir_id);
         let if_expr = self.arena.alloc(hir::Expr {
-            hir_id: self.next_id(),
+            hir_id: stmt_hir_id,
             span,
             kind: hir::ExprKind::If(let_expr, then_expr, Some(else_expr)),
         });
@@ -180,6 +168,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             )
             .emit();
         }
-        (stmt, if_expr)
+        if_expr
     }
 }

compiler/rustc_ast_lowering/src/expr.rs

@@ -91,11 +91,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let ohs = self.lower_expr(ohs);
                     hir::ExprKind::AddrOf(k, m, ohs)
                 }
-                ExprKind::Let(ref pat, ref scrutinee, span) => hir::ExprKind::Let(
-                    self.lower_pat(pat),
-                    self.lower_expr(scrutinee),
-                    self.lower_span(span),
-                ),
+                ExprKind::Let(ref pat, ref scrutinee, span) => {
+                    hir::ExprKind::Let(self.arena.alloc(hir::Let {
+                        hir_id: self.next_id(),
+                        span: self.lower_span(span),
+                        pat: self.lower_pat(pat),
+                        ty: None,
+                        init: self.lower_expr(scrutinee),
+                    }))
+                }
                 ExprKind::If(ref cond, ref then, ref else_opt) => {
                     self.lower_expr_if(cond, then, else_opt.as_deref())
                 }

compiler/rustc_hir/src/arena.rs

@@ -20,6 +20,7 @@ macro_rules! arena_types {
             [] generic_bound: rustc_hir::GenericBound<'tcx>,
             [] generic_param: rustc_hir::GenericParam<'tcx>,
             [] expr: rustc_hir::Expr<'tcx>,
+            [] let_expr: rustc_hir::Let<'tcx>,
             [] expr_field: rustc_hir::ExprField<'tcx>,
             [] pat_field: rustc_hir::PatField<'tcx>,
             [] fn_decl: rustc_hir::FnDecl<'tcx>,

compiler/rustc_hir/src/hir.rs

@@ -1160,10 +1160,24 @@ pub struct Arm<'hir> {
     pub body: &'hir Expr<'hir>,
 }
 
+/// Represents a `let <pat>[: <ty>] = <expr>` expression (not a Local), occurring in an `if-let` or
+/// `let-else`, evaluating to a boolean. Typically the pattern is refutable.
+///
+/// In an if-let, imagine it as `if (let <pat> = <expr>) { ... }`; in a let-else, it is part of the
+/// desugaring to if-let. Only let-else supports the type annotation at present.
+#[derive(Debug, HashStable_Generic)]
+pub struct Let<'hir> {
+    pub hir_id: HirId,
+    pub span: Span,
+    pub pat: &'hir Pat<'hir>,
+    pub ty: Option<&'hir Ty<'hir>>,
+    pub init: &'hir Expr<'hir>,
+}
+
 #[derive(Debug, HashStable_Generic)]
 pub enum Guard<'hir> {
     If(&'hir Expr<'hir>),
-    // FIXME use ExprKind::Let for this.
+    // FIXME use hir::Let for this.
     IfLet(&'hir Pat<'hir>, &'hir Expr<'hir>),
 }
 
@@ -1680,7 +1694,7 @@ pub enum ExprKind<'hir> {
     ///
     /// These are not `Local` and only occur as expressions.
     /// The `let Some(x) = foo()` in `if let Some(x) = foo()` is an example of `Let(..)`.
-    Let(&'hir Pat<'hir>, &'hir Expr<'hir>, Span),
+    Let(&'hir Let<'hir>),
     /// An `if` block, with an optional else block.
     ///
     /// I.e., `if <expr> { <expr> } else { <expr> }`.

compiler/rustc_hir/src/intravisit.rs

@@ -389,6 +389,9 @@ pub trait Visitor<'v>: Sized {
     fn visit_expr(&mut self, ex: &'v Expr<'v>) {
         walk_expr(self, ex)
     }
+    fn visit_let_expr(&mut self, lex: &'v Let<'v>) {
+        walk_let_expr(self, lex)
+    }
     fn visit_ty(&mut self, t: &'v Ty<'v>) {
         walk_ty(self, t)
     }
@@ -1126,6 +1129,14 @@ pub fn walk_anon_const<'v, V: Visitor<'v>>(visitor: &mut V, constant: &'v AnonCo
     visitor.visit_nested_body(constant.body);
 }
 
+pub fn walk_let_expr<'v, V: Visitor<'v>>(visitor: &mut V, let_expr: &'v Let<'v>) {
+    // match the visit order in walk_local
+    visitor.visit_expr(let_expr.init);
+    visitor.visit_id(let_expr.hir_id);
+    visitor.visit_pat(let_expr.pat);
+    walk_list!(visitor, visit_ty, let_expr.ty);
+}
+
 pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>) {
     visitor.visit_id(expression.hir_id);
     match expression.kind {
@@ -1172,10 +1183,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
         ExprKind::DropTemps(ref subexpression) => {
             visitor.visit_expr(subexpression);
         }
-        ExprKind::Let(ref pat, ref expr, _) => {
-            visitor.visit_expr(expr);
-            visitor.visit_pat(pat);
-        }
+        ExprKind::Let(ref let_expr) => visitor.visit_let_expr(let_expr),
         ExprKind::If(ref cond, ref then, ref else_opt) => {
             visitor.visit_expr(cond);
             visitor.visit_expr(then);

compiler/rustc_hir_pretty/src/lib.rs

@@ -1101,13 +1101,17 @@ impl<'a> State<'a> {
     }
 
     /// Print a `let pat = expr` expression.
-    fn print_let(&mut self, pat: &hir::Pat<'_>, expr: &hir::Expr<'_>) {
-        self.word("let ");
+    fn print_let(&mut self, pat: &hir::Pat<'_>, ty: Option<&hir::Ty<'_>>, init: &hir::Expr<'_>) {
+        self.word_space("let");
         self.print_pat(pat);
+        if let Some(ty) = ty {
+            self.word_space(":");
+            self.print_type(ty);
+        }
         self.space();
         self.word_space("=");
-        let npals = || parser::needs_par_as_let_scrutinee(expr.precedence().order());
-        self.print_expr_cond_paren(expr, Self::cond_needs_par(expr) || npals())
+        let npals = || parser::needs_par_as_let_scrutinee(init.precedence().order());
+        self.print_expr_cond_paren(init, Self::cond_needs_par(init) || npals())
     }
 
     // Does `expr` need parentheses when printed in a condition position?
@@ -1462,8 +1466,8 @@ impl<'a> State<'a> {
                 // Print `}`:
                 self.bclose_maybe_open(expr.span, true);
             }
-            hir::ExprKind::Let(ref pat, ref scrutinee, _) => {
-                self.print_let(pat, scrutinee);
+            hir::ExprKind::Let(hir::Let { pat, ty, init, .. }) => {
+                self.print_let(pat, *ty, init);
             }
             hir::ExprKind::If(ref test, ref blk, ref elseopt) => {
                 self.print_if(&test, &blk, elseopt.as_ref().map(|e| &**e));

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -605,9 +605,10 @@ impl<'tcx> Cx<'tcx> {
                 },
                 Err(err) => bug!("invalid loop id for continue: {}", err),
             },
-            hir::ExprKind::Let(ref pat, ref expr, _) => {
-                ExprKind::Let { expr: self.mirror_expr(expr), pat: self.pattern_from_hir(pat) }
-            }
+            hir::ExprKind::Let(let_expr) => ExprKind::Let {
+                expr: self.mirror_expr(let_expr.init),
+                pat: self.pattern_from_hir(let_expr.pat),
+            },
             hir::ExprKind::If(cond, then, else_opt) => ExprKind::If {
                 if_then_scope: region::Scope {
                     id: then.hir_id.local_id,

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -64,7 +64,9 @@ impl<'tcx> Visitor<'tcx> for MatchVisitor<'_, '_, 'tcx> {
         intravisit::walk_expr(self, ex);
         match &ex.kind {
             hir::ExprKind::Match(scrut, arms, source) => self.check_match(scrut, arms, *source),
-            hir::ExprKind::Let(pat, scrut, span) => self.check_let(pat, scrut, *span),
+            hir::ExprKind::Let(hir::Let { pat, init, span, .. }) => {
+                self.check_let(pat, init, *span)
+            }
             _ => {}
         }
     }
@@ -148,9 +150,9 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         }
     }
 
-    fn check_let(&mut self, pat: &'tcx hir::Pat<'tcx>, expr: &hir::Expr<'_>, span: Span) {
+    fn check_let(&mut self, pat: &'tcx hir::Pat<'tcx>, scrutinee: &hir::Expr<'_>, span: Span) {
         self.check_patterns(pat, Refutable);
-        let mut cx = self.new_cx(expr.hir_id);
+        let mut cx = self.new_cx(scrutinee.hir_id);
         let tpat = self.lower_pattern(&mut cx, pat, &mut false);
         check_let_reachability(&mut cx, pat.hir_id, tpat, span);
     }

compiler/rustc_passes/src/liveness.rs

@@ -429,8 +429,8 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
                 intravisit::walk_expr(self, expr);
             }
 
-            hir::ExprKind::Let(ref pat, ..) => {
-                self.add_from_pat(pat);
+            hir::ExprKind::Let(let_expr) => {
+                self.add_from_pat(let_expr.pat);
                 intravisit::walk_expr(self, expr);
             }
 
@@ -856,9 +856,9 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 })
             }
 
-            hir::ExprKind::Let(ref pat, ref scrutinee, _) => {
-                let succ = self.propagate_through_expr(scrutinee, succ);
-                self.define_bindings_in_pat(pat, succ)
+            hir::ExprKind::Let(let_expr) => {
+                let succ = self.propagate_through_expr(let_expr.init, succ);
+                self.define_bindings_in_pat(let_expr.pat, succ)
             }
 
             // Note that labels have been resolved, so we don't need to look
@@ -1401,8 +1401,8 @@ fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
             }
         }
 
-        hir::ExprKind::Let(ref pat, ..) => {
-            this.check_unused_vars_in_pat(pat, None, |_, _, _, _| {});
+        hir::ExprKind::Let(let_expr) => {
+            this.check_unused_vars_in_pat(let_expr.pat, None, |_, _, _, _| {});
         }
 
         // no correctness conditions related to liveness

compiler/rustc_typeck/src/check/expr.rs

@@ -300,7 +300,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
             }
             ExprKind::Ret(ref expr_opt) => self.check_expr_return(expr_opt.as_deref(), expr),
-            ExprKind::Let(pat, let_expr, _) => self.check_expr_let(let_expr, pat),
+            ExprKind::Let(let_expr) => self.check_expr_let(let_expr),
             ExprKind::Loop(body, _, source, _) => {
                 self.check_expr_loop(body, source, expected, expr)
             }
@@ -1044,10 +1044,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    fn check_expr_let(&self, expr: &'tcx hir::Expr<'tcx>, pat: &'tcx hir::Pat<'tcx>) -> Ty<'tcx> {
-        self.warn_if_unreachable(expr.hir_id, expr.span, "block in `let` expression");
-        let expr_ty = self.demand_scrutinee_type(expr, pat.contains_explicit_ref_binding(), false);
-        self.check_pat_top(pat, expr_ty, Some(expr.span), true);
+    fn check_expr_let(&self, let_expr: &'tcx hir::Let<'tcx>) -> Ty<'tcx> {
+        // for let statements, this is done in check_stmt
+        let init = let_expr.init;
+        self.warn_if_unreachable(init.hir_id, init.span, "block in `let` expression");
+        // otherwise check exactly as a let statement
+        self.check_decl(let_expr.into());
+        // but return a bool, for this is a boolean expression
         self.tcx.types.bool
     }