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

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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.
This commit is contained in:
bors 2021-12-17 22:12:34 +00:00
commit dde825db46
46 changed files with 900 additions and 142 deletions
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14
compiler/rustc_ast_lowering/src/expr.rs
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@ -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())
}