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_mir_transform/src/inline.rs

@@ -673,9 +673,7 @@ impl Inliner<'tcx> {
             assert!(args.next().is_none());
 
             let tuple = Place::from(tuple);
-            let tuple_tys = if let ty::Tuple(s) = tuple.ty(caller_body, tcx).ty.kind() {
-                s
-            } else {
+            let ty::Tuple(tuple_tys) = tuple.ty(caller_body, tcx).ty.kind() else {
                 bug!("Closure arguments are not passed as a tuple");
             };
 

compiler/rustc_mir_transform/src/lib.rs

@@ -4,6 +4,7 @@
 #![cfg_attr(bootstrap, feature(const_panic))]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(option_get_or_insert_default)]

compiler/rustc_mir_transform/src/lower_slice_len.rs

@@ -17,9 +17,7 @@ impl<'tcx> MirPass<'tcx> for LowerSliceLenCalls {
 
 pub fn lower_slice_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let language_items = tcx.lang_items();
-    let slice_len_fn_item_def_id = if let Some(slice_len_fn_item) = language_items.slice_len_fn() {
-        slice_len_fn_item
-    } else {
+    let Some(slice_len_fn_item_def_id) = language_items.slice_len_fn() else {
         // there is no language item to compare to :)
         return;
     };

compiler/rustc_mir_transform/src/normalize_array_len.rs

@@ -208,7 +208,7 @@ fn normalize_array_len_call<'tcx>(
                         operand,
                         cast_ty,
                     ) => {
-                        let local = if let Some(local) = place.as_local() { local } else { return };
+                        let Some(local) = place.as_local() else { return };
                         match operand {
                             Operand::Copy(place) | Operand::Move(place) => {
                                 let operand_local =
@@ -255,9 +255,7 @@ fn normalize_array_len_call<'tcx>(
                         }
                     }
                     Rvalue::Len(place) => {
-                        let local = if let Some(local) = place.local_or_deref_local() {
-                            local
-                        } else {
+                        let Some(local) = place.local_or_deref_local() else {
                             return;
                         };
                         if let Some(cast_statement_idx) = state.get(&local).copied() {

compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -83,12 +83,9 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
             let bb = BasicBlock::from_usize(bb);
             trace!("processing block {:?}", bb);
 
-            let discriminant_ty =
-                if let Some(ty) = get_switched_on_type(&body.basic_blocks()[bb], tcx, body) {
-                    ty
-                } else {
-                    continue;
-                };
+            let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks()[bb], tcx, body) else {
+                continue;
+            };
 
             let layout = tcx.layout_of(tcx.param_env(body.source.def_id()).and(discriminant_ty));