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 https://github.com/rust-lang/rust/issues/89688#issuecomment-940405082, 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_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);