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.

Author: est31

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)]

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -174,9 +174,8 @@ pub fn each_linked_rlib(
             _ => {}
         }
     }
-    let fmts = match fmts {
-        Some(f) => f,
-        None => return Err("could not find formats for rlibs".to_string()),
+    let Some(fmts) = fmts else {
+        return Err("could not find formats for rlibs".to_string());
     };
     for &cnum in crates {
         match fmts.get(cnum.as_usize() - 1) {

compiler/rustc_codegen_ssa/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(box_patterns)]
 #![feature(try_blocks)]
 #![feature(in_band_lifetimes)]
+#![feature(let_else)]
 #![feature(once_cell)]
 #![feature(nll)]
 #![feature(associated_type_bounds)]

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -343,9 +343,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
             .unwrap_or_else(|| bug!("indirect_dest has non-pointer type: {:?}", indirect_dest))
             .ty;
 
-        let (llptr, llextra) = if let OperandValue::Ref(llptr, Some(llextra), _) = self {
-            (llptr, llextra)
-        } else {
+        let OperandValue::Ref(llptr, Some(llextra), _) = self else {
             bug!("store_unsized called with a sized value")
         };
 

compiler/rustc_const_eval/src/lib.rs

@@ -13,6 +13,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(exact_size_is_empty)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(slice_ptr_get)]

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -122,9 +122,7 @@ impl Qualif for NeedsNonConstDrop {
             Ok([..]) => {}
         }
 
-        let drop_trait = if let Some(did) = cx.tcx.lang_items().drop_trait() {
-            did
-        } else {
+        let Some(drop_trait) = cx.tcx.lang_items().drop_trait() else {
             // there is no way to define a type that needs non-const drop
             // without having the lang item present.
             return false;

compiler/rustc_errors/src/emitter.rs

@@ -449,11 +449,7 @@ pub trait Emitter {
         span: &mut MultiSpan,
         children: &mut Vec<SubDiagnostic>,
     ) {
-        let source_map = if let Some(ref sm) = source_map {
-            sm
-        } else {
-            return;
-        };
+        let Some(source_map) = source_map else { return };
         debug!("fix_multispans_in_extern_macros: before: span={:?} children={:?}", span, children);
         self.fix_multispan_in_extern_macros(source_map, span);
         for child in children.iter_mut() {

compiler/rustc_errors/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(if_let_guard)]
 #![feature(format_args_capture)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(nll)]
 
 #[macro_use]