Auto merge of #87688 - camsteffen:let-else, r=cjgillot

Introduce `let...else`

Tracking issue: #87335

The trickiest part for me was enforcing the diverging else block with clear diagnostics. Perhaps the obvious solution is to expand to `let _: ! = ..`, but I decided against this because, when a "mismatched type" error is found in typeck, there is no way to trace where in the HIR the expected type originated, AFAICT. In order to pass down this information, I believe we should introduce `Expectation::LetElseNever(HirId)` or maybe add `HirId` to `Expectation::HasType`, but I left that as a future enhancement. For now, I simply assert that the block is `!` with a custom `ObligationCauseCode`, and I think this is clear enough, at least to start. The downside here is that the error points at the entire block rather than the specific expression with the wrong type. I left a todo to this effect.

Overall, I believe this PR is feature-complete with regard to the RFC.

compiler/rustc_ast/src/ast.rs

@@ -1005,13 +1005,42 @@ pub struct Local {
     pub id: NodeId,
     pub pat: P<Pat>,
     pub ty: Option<P<Ty>>,
-    /// Initializer expression to set the value, if any.
-    pub init: Option<P<Expr>>,
+    pub kind: LocalKind,
     pub span: Span,
     pub attrs: AttrVec,
     pub tokens: Option<LazyTokenStream>,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum LocalKind {
+    /// Local declaration.
+    /// Example: `let x;`
+    Decl,
+    /// Local declaration with an initializer.
+    /// Example: `let x = y;`
+    Init(P<Expr>),
+    /// Local declaration with an initializer and an `else` clause.
+    /// Example: `let Some(x) = y else { return };`
+    InitElse(P<Expr>, P<Block>),
+}
+
+impl LocalKind {
+    pub fn init(&self) -> Option<&Expr> {
+        match self {
+            Self::Decl => None,
+            Self::Init(i) | Self::InitElse(i, _) => Some(i),
+        }
+    }
+
+    pub fn init_else_opt(&self) -> Option<(&Expr, Option<&Block>)> {
+        match self {
+            Self::Decl => None,
+            Self::Init(init) => Some((init, None)),
+            Self::InitElse(init, els) => Some((init, Some(els))),
+        }
+    }
+}
+
 /// An arm of a 'match'.
 ///
 /// E.g., `0..=10 => { println!("match!") }` as in

compiler/rustc_ast/src/mut_visit.rs

@@ -571,11 +571,20 @@ pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(
 }
 
 pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
-    let Local { id, pat, ty, init, span, attrs, tokens } = local.deref_mut();
+    let Local { id, pat, ty, kind, span, attrs, tokens } = local.deref_mut();
     vis.visit_id(id);
     vis.visit_pat(pat);
     visit_opt(ty, |ty| vis.visit_ty(ty));
-    visit_opt(init, |init| vis.visit_expr(init));
+    match kind {
+        LocalKind::Decl => {}
+        LocalKind::Init(init) => {
+            vis.visit_expr(init);
+        }
+        LocalKind::InitElse(init, els) => {
+            vis.visit_expr(init);
+            vis.visit_block(els);
+        }
+    }
     vis.visit_span(span);
     visit_thin_attrs(attrs, vis);
     visit_lazy_tts(tokens, vis);

compiler/rustc_ast/src/util/classify.rs

@@ -23,3 +23,30 @@ pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
             | ast::ExprKind::TryBlock(..)
     )
 }
+
+/// If an expression ends with `}`, returns the innermost expression ending in the `}`
+pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
+    use ast::ExprKind::*;
+
+    loop {
+        match &expr.kind {
+            AddrOf(_, _, e)
+            | Assign(_, e, _)
+            | AssignOp(_, _, e)
+            | Binary(_, _, e)
+            | Box(e)
+            | Break(_, Some(e))
+            | Closure(.., e, _)
+            | Let(_, e, _)
+            | Range(_, Some(e), _)
+            | Ret(Some(e))
+            | Unary(_, e)
+            | Yield(Some(e)) => {
+                expr = e;
+            }
+            Async(..) | Block(..) | ForLoop(..) | If(..) | Loop(..) | Match(..) | Struct(..)
+            | TryBlock(..) | While(..) => break Some(expr),
+            _ => break None,
+        }
+    }
+}

compiler/rustc_ast/src/visit.rs

@@ -242,7 +242,10 @@ pub fn walk_local<'a, V: Visitor<'a>>(visitor: &mut V, local: &'a Local) {
     }
     visitor.visit_pat(&local.pat);
     walk_list!(visitor, visit_ty, &local.ty);
-    walk_list!(visitor, visit_expr, &local.init);
+    if let Some((init, els)) = local.kind.init_else_opt() {
+        visitor.visit_expr(init);
+        walk_list!(visitor, visit_block, els);
+    }
 }
 
 pub fn walk_label<'a, V: Visitor<'a>>(visitor: &mut V, label: &'a Label) {

compiler/rustc_ast_lowering/src/block.rs

@@ -0,0 +1,185 @@
+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;
+
+impl<'a, 'hir> LoweringContext<'a, 'hir> {
+    pub(super) fn lower_block(
+        &mut self,
+        b: &Block,
+        targeted_by_break: bool,
+    ) -> &'hir hir::Block<'hir> {
+        self.arena.alloc(self.lower_block_noalloc(b, targeted_by_break))
+    }
+
+    pub(super) fn lower_block_noalloc(
+        &mut self,
+        b: &Block,
+        targeted_by_break: bool,
+    ) -> hir::Block<'hir> {
+        let (stmts, expr) = self.lower_stmts(&b.stmts);
+        let rules = self.lower_block_check_mode(&b.rules);
+        let hir_id = self.lower_node_id(b.id);
+        hir::Block { hir_id, stmts, expr, rules, span: self.lower_span(b.span), targeted_by_break }
+    }
+
+    fn lower_stmts(
+        &mut self,
+        mut ast_stmts: &[Stmt],
+    ) -> (&'hir [hir::Stmt<'hir>], Option<&'hir hir::Expr<'hir>>) {
+        let mut stmts = SmallVec::<[hir::Stmt<'hir>; 8]>::new();
+        let mut expr = None;
+        while let [s, tail @ ..] = ast_stmts {
+            match s.kind {
+                StmtKind::Local(ref local) => {
+                    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);
+                            expr = Some(e);
+                            // remaining statements are in let-else expression
+                            break;
+                        }
+                        _ => {
+                            let local = self.lower_local(local);
+                            self.alias_attrs(hir_id, local.hir_id);
+                            let kind = hir::StmtKind::Local(local);
+                            let span = self.lower_span(s.span);
+                            stmts.push(hir::Stmt { hir_id, kind, span });
+                        }
+                    }
+                }
+                StmtKind::Item(ref it) => {
+                    stmts.extend(self.lower_item_id(it).into_iter().enumerate().map(
+                        |(i, item_id)| {
+                            let hir_id = match i {
+                                0 => self.lower_node_id(s.id),
+                                _ => self.next_id(),
+                            };
+                            let kind = hir::StmtKind::Item(item_id);
+                            let span = self.lower_span(s.span);
+                            hir::Stmt { hir_id, kind, span }
+                        },
+                    ));
+                }
+                StmtKind::Expr(ref e) => {
+                    let e = self.lower_expr(e);
+                    if tail.is_empty() {
+                        expr = Some(e);
+                    } else {
+                        let hir_id = self.lower_node_id(s.id);
+                        self.alias_attrs(hir_id, e.hir_id);
+                        let kind = hir::StmtKind::Expr(e);
+                        let span = self.lower_span(s.span);
+                        stmts.push(hir::Stmt { hir_id, kind, span });
+                    }
+                }
+                StmtKind::Semi(ref e) => {
+                    let e = self.lower_expr(e);
+                    let hir_id = self.lower_node_id(s.id);
+                    self.alias_attrs(hir_id, e.hir_id);
+                    let kind = hir::StmtKind::Semi(e);
+                    let span = self.lower_span(s.span);
+                    stmts.push(hir::Stmt { hir_id, kind, span });
+                }
+                StmtKind::Empty => {}
+                StmtKind::MacCall(..) => panic!("shouldn't exist here"),
+            }
+            ast_stmts = &ast_stmts[1..];
+        }
+        (self.arena.alloc_from_iter(stmts), expr)
+    }
+
+    fn lower_local(&mut self, l: &Local) -> &'hir hir::Local<'hir> {
+        let ty = l
+            .ty
+            .as_ref()
+            .map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Binding)));
+        let init = l.kind.init().map(|init| self.lower_expr(init));
+        let hir_id = self.lower_node_id(l.id);
+        let pat = self.lower_pat(&l.pat);
+        let span = self.lower_span(l.span);
+        let source = hir::LocalSource::Normal;
+        self.lower_attrs(hir_id, &l.attrs);
+        self.arena.alloc(hir::Local { hir_id, ty, pat, init, span, source })
+    }
+
+    fn lower_block_check_mode(&mut self, b: &BlockCheckMode) -> hir::BlockCheckMode {
+        match *b {
+            BlockCheckMode::Default => hir::BlockCheckMode::DefaultBlock,
+            BlockCheckMode::Unsafe(u) => {
+                hir::BlockCheckMode::UnsafeBlock(self.lower_unsafe_source(u))
+            }
+        }
+    }
+
+    fn lower_let_else(
+        &mut self,
+        stmt_hir_id: hir::HirId,
+        local: &Local,
+        init: &Expr,
+        els: &Block,
+        tail: &[Stmt],
+    ) -> (hir::Stmt<'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 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 {
+                hir_id: local_hir_id,
+                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()))
+        };
+        let then_expr = {
+            let (stmts, expr) = self.lower_stmts(tail);
+            let block = self.block_all(span, stmts, expr);
+            self.arena.alloc(self.expr_block(block, AttrVec::new()))
+        };
+        let else_expr = {
+            let block = self.lower_block(els, false);
+            self.arena.alloc(self.expr_block(block, AttrVec::new()))
+        };
+        self.alias_attrs(else_expr.hir_id, local_hir_id);
+        let if_expr = self.arena.alloc(hir::Expr {
+            hir_id: self.next_id(),
+            span,
+            kind: hir::ExprKind::If(let_expr, then_expr, Some(else_expr)),
+        });
+        if !self.sess.features_untracked().let_else {
+            feature_err(
+                &self.sess.parse_sess,
+                sym::let_else,
+                local.span,
+                "`let...else` statements are unstable",
+            )
+            .emit();
+        }
+        (stmt, if_expr)
+    }
+}