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_middle/src/lib.rs

@@ -39,6 +39,7 @@
 #![feature(new_uninit)]
 #![feature(nll)]
 #![feature(once_cell)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(trusted_len)]
 #![feature(in_band_lifetimes)]

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -221,9 +221,7 @@ pub fn suggest_constraining_type_param(
 ) -> bool {
     let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);
 
-    let param = if let Some(param) = param {
-        param
-    } else {
+    let Some(param) = param else {
         return false;
     };
 

compiler/rustc_middle/src/ty/layout.rs

@@ -755,17 +755,14 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     }
 
                     // Extract the number of elements from the layout of the array field:
-                    let len = if let Ok(TyAndLayout {
+                    let Ok(TyAndLayout {
                         layout: Layout { fields: FieldsShape::Array { count, .. }, .. },
                         ..
-                    }) = self.layout_of(f0_ty)
-                    {
-                        count
-                    } else {
+                    }) = self.layout_of(f0_ty) else {
                         return Err(LayoutError::Unknown(ty));
                     };
 
-                    (*e_ty, *len, true)
+                    (*e_ty, *count, true)
                 } else {
                     // First ADT field is not an array:
                     (f0_ty, def.non_enum_variant().fields.len() as _, false)
@@ -787,9 +784,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
 
                 // Compute the ABI of the element type:
                 let e_ly = self.layout_of(e_ty)?;
-                let e_abi = if let Abi::Scalar(scalar) = e_ly.abi {
-                    scalar
-                } else {
+                let Abi::Scalar(e_abi) = e_ly.abi else {
                     // This error isn't caught in typeck, e.g., if
                     // the element type of the vector is generic.
                     tcx.sess.fatal(&format!(