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.

compiler/rustc_borrowck/src/borrow_set.rs

@@ -315,9 +315,7 @@ impl<'a, 'tcx> GatherBorrows<'a, 'tcx> {
         //    TEMP = &foo
         //
         // so extract `temp`.
-        let temp = if let Some(temp) = assigned_place.as_local() {
-            temp
-        } else {
+        let Some(temp) = assigned_place.as_local() else {
             span_bug!(
                 self.body.source_info(start_location).span,
                 "expected 2-phase borrow to assign to a local, not `{:?}`",

compiler/rustc_borrowck/src/diagnostics/outlives_suggestion.rs

@@ -90,9 +90,7 @@ impl OutlivesSuggestionBuilder {
         let mut unified_already = FxHashSet::default();
 
         for (fr, outlived) in &self.constraints_to_add {
-            let fr_name = if let Some(fr_name) = self.region_vid_to_name(mbcx, *fr) {
-                fr_name
-            } else {
+            let Some(fr_name) = self.region_vid_to_name(mbcx, *fr) else {
                 continue;
             };
 

compiler/rustc_borrowck/src/lib.rs

@@ -7,6 +7,7 @@
 #![feature(format_args_capture)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]