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_monomorphize/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
 #![feature(control_flow_enum)]
+#![feature(let_else)]
 #![feature(in_band_lifetimes)]
 #![recursion_limit = "256"]
 

compiler/rustc_monomorphize/src/partitioning/default.rs

@@ -451,9 +451,7 @@ fn mono_item_visibility(
     let is_generic = instance.substs.non_erasable_generics().next().is_some();
 
     // Upstream `DefId` instances get different handling than local ones.
-    let def_id = if let Some(def_id) = def_id.as_local() {
-        def_id
-    } else {
+    let Some(def_id) = def_id.as_local() else {
         return if export_generics && is_generic {
             // If it is an upstream monomorphization and we export generics, we must make
             // it available to downstream crates.