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_span/src/hygiene.rs

@@ -1097,6 +1097,7 @@ pub enum DesugaringKind {
     Async,
     Await,
     ForLoop(ForLoopLoc),
+    LetElse,
 }
 
 /// A location in the desugaring of a `for` loop
@@ -1117,6 +1118,7 @@ impl DesugaringKind {
             DesugaringKind::TryBlock => "`try` block",
             DesugaringKind::OpaqueTy => "`impl Trait`",
             DesugaringKind::ForLoop(_) => "`for` loop",
+            DesugaringKind::LetElse => "`let...else`",
         }
     }
 }

compiler/rustc_span/src/symbol.rs

@@ -744,6 +744,7 @@ symbols! {
         le,
         len,
         let_chains,
+        let_else,
         lhs,
         lib,
         libc,