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_trait_selection/src/lib.rs

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

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1038,13 +1038,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let hir = self.tcx.hir();
         let parent_node = hir.get_parent_node(obligation.cause.body_id);
         let node = hir.find(parent_node);
-        let (sig, body_id) = if let Some(hir::Node::Item(hir::Item {
+        let Some(hir::Node::Item(hir::Item {
             kind: hir::ItemKind::Fn(sig, _, body_id),
             ..
         })) = node
-        {
-            (sig, body_id)
-        } else {
+        else {
             return false;
         };
         let body = hir.body(*body_id);

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -647,9 +647,7 @@ fn receiver_is_dispatchable<'tcx>(
     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 
     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
-    let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
-        (u, cu)
-    } else {
+    let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
         return false;
     };

compiler/rustc_trait_selection/src/traits/on_unimplemented.rs

@@ -164,9 +164,7 @@ impl<'tcx> OnUnimplementedDirective {
     ) -> Result<Option<Self>, ErrorReported> {
         let attrs = tcx.get_attrs(impl_def_id);
 
-        let attr = if let Some(item) = tcx.sess.find_by_name(&attrs, sym::rustc_on_unimplemented) {
-            item
-        } else {
+        let Some(attr) = tcx.sess.find_by_name(&attrs, sym::rustc_on_unimplemented) else {
             return Ok(None);
         };
 

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -716,9 +716,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             cause.clone(),
         );
 
-        let data = if let ty::Dynamic(ref data, ..) = normalized_ty.kind() {
-            data
-        } else {
+        let ty::Dynamic(data, ..) = normalized_ty.kind() else {
             return None;
         };