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_query_system/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(core_intrinsics)]
 #![feature(hash_raw_entry)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(thread_local_const_init)]
 

compiler/rustc_query_system/src/query/job.rs

@@ -644,9 +644,7 @@ pub fn print_query_stack<CTX: QueryContext>(
         if Some(i) == num_frames {
             break;
         }
-        let query_info = if let Some(info) = query_map.as_ref().and_then(|map| map.get(&query)) {
-            info
-        } else {
+        let Some(query_info) = query_map.as_ref().and_then(|map| map.get(&query)) else {
             break;
         };
         let mut diag = Diagnostic::new(

compiler/rustc_query_system/src/query/plumbing.rs

@@ -761,11 +761,9 @@ where
         return false;
     }
 
-    let key = if let Some(key) =
+    let Some(key) =
         <Q::Key as DepNodeParams<CTX::DepContext>>::recover(*tcx.dep_context(), &dep_node)
-    {
-        key
-    } else {
+    else {
         return false;
     };