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.

1 files changed, 3 insertions, 3 deletions

diff --git a/compiler/rustc_expand/src/build.rs b/compiler/rustc_expand/src/build.rs
index 824df2757ea..1d83ecbfd40 100644
--- a/compiler/rustc_expand/src/build.rs
+++ b/compiler/rustc_expand/src/build.rs
@@ -2,7 +2,7 @@ use crate::base::ExtCtxt;
 
 use rustc_ast::attr;
 use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, AttrVec, BlockCheckMode, Expr, PatKind, UnOp};
+use rustc_ast::{self as ast, AttrVec, BlockCheckMode, Expr, LocalKind, PatKind, UnOp};
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 
@@ -153,8 +153,8 @@ impl<'a> ExtCtxt<'a> {
         let local = P(ast::Local {
             pat,
             ty: None,
-            init: Some(ex),
             id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Init(ex),
             span: sp,
             attrs: AttrVec::new(),
             tokens: None,
@@ -167,8 +167,8 @@ impl<'a> ExtCtxt<'a> {
         let local = P(ast::Local {
             pat: self.pat_wild(span),
             ty: Some(ty),
-            init: None,
             id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Decl,
             span,
             attrs: AttrVec::new(),
             tokens: None,