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_expand/src/expand.rs

@@ -447,9 +447,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         let mut undetermined_invocations = Vec::new();
         let (mut progress, mut force) = (false, !self.monotonic);
         loop {
-            let (invoc, ext) = if let Some(invoc) = invocations.pop() {
-                invoc
-            } else {
+            let Some((invoc, ext)) = invocations.pop() else {
                 self.resolve_imports();
                 if undetermined_invocations.is_empty() {
                     break;

compiler/rustc_expand/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(format_args_capture)]
 #![feature(if_let_guard)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(proc_macro_diagnostic)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_span)]

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -116,10 +116,8 @@ pub(super) fn transcribe<'a>(
 
     loop {
         // Look at the last frame on the stack.
-        let tree = if let Some(tree) = stack.last_mut().unwrap().next() {
-            // If it still has a TokenTree we have not looked at yet, use that tree.
-            tree
-        } else {
+        // If it still has a TokenTree we have not looked at yet, use that tree.
+        let Some(tree) = stack.last_mut().unwrap().next() else {
             // This else-case never produces a value for `tree` (it `continue`s or `return`s).
 
             // Otherwise, if we have just reached the end of a sequence and we can keep repeating,
@@ -190,9 +188,7 @@ pub(super) fn transcribe<'a>(
                     LockstepIterSize::Constraint(len, _) => {
                         // We do this to avoid an extra clone above. We know that this is a
                         // sequence already.
-                        let (sp, seq) = if let mbe::TokenTree::Sequence(sp, seq) = seq {
-                            (sp, seq)
-                        } else {
+                        let mbe::TokenTree::Sequence(sp, seq) = seq else {
                             unreachable!()
                         };