Auto merge of #89933 - est31:let_else, r=michaelwoerister

Adopt let_else across the compiler

This performs a substitution of code following the pattern:

```
let <id> = if let <pat> = ... { identity } else { ... : ! };
```

To simplify it to:

```
let <pat> = ... { identity } else { ... : ! };
```

By adopting the `let_else` feature (cc #87335).

The PR also updates the syn crate because the currently used version of the crate doesn't support `let_else` syntax yet.

Note: Generally I'm the person who *removes* usages of unstable features from the compiler, not adds more usages of them, but in this instance I think it hopefully helps the feature get stabilized sooner and in a better state. I have written a [comment](https://github.com/rust-lang/rust/issues/87335#issuecomment-944846205) on the tracking issue about my experience and what I feel could be improved before stabilization of `let_else`.

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -263,9 +263,7 @@ pub fn suggest_new_region_bound(
         match fn_return.kind {
             TyKind::OpaqueDef(item_id, _) => {
                 let item = tcx.hir().item(item_id);
-                let opaque = if let ItemKind::OpaqueTy(opaque) = &item.kind {
-                    opaque
-                } else {
+                let ItemKind::OpaqueTy(opaque) = &item.kind else {
                     return;
                 };
 

compiler/rustc_infer/src/lib.rs

@@ -17,6 +17,7 @@
 #![feature(box_patterns)]
 #![feature(extend_one)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(never_type)]
 #![feature(in_band_lifetimes)]
 #![feature(control_flow_enum)]