Adopt let else in more places

compiler/rustc_monomorphize/src/collector.rs

@@ -847,14 +847,13 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirNeighborCollector<'a, 'tcx> {
                 debug!(?source_info);
                 let lint_root = source_info.scope.lint_root(&self.body.source_scopes);
                 debug!(?lint_root);
-                let lint_root = match lint_root {
-                    Some(lint_root) => lint_root,
+                let Some(lint_root) = lint_root else {
                     // This happens when the issue is in a function from a foreign crate that
                     // we monomorphized in the current crate. We can't get a `HirId` for things
                     // in other crates.
                     // FIXME: Find out where to report the lint on. Maybe simply crate-level lint root
                     // but correct span? This would make the lint at least accept crate-level lint attributes.
-                    None => return,
+                    return;
                 };
                 self.tcx.struct_span_lint_hir(
                     LARGE_ASSIGNMENTS,
@@ -1256,9 +1255,8 @@ impl<'v> RootCollector<'_, 'v> {
     /// the return type of `main`. This is not needed when
     /// the user writes their own `start` manually.
     fn push_extra_entry_roots(&mut self) {
-        let main_def_id = match self.entry_fn {
-            Some((def_id, EntryFnType::Main)) => def_id,
-            _ => return,
+        let Some((main_def_id, EntryFnType::Main)) = self.entry_fn else {
+            return;
         };
 
         let start_def_id = match self.tcx.lang_items().require(LangItem::Start) {