Match ergonomics 2024: mut doesn't reset binding mode

compiler/rustc_feature/src/unstable.rs

@@ -533,6 +533,8 @@ declare_features! (
     (unstable, more_qualified_paths, "1.54.0", Some(86935)),
     /// Allows the `#[must_not_suspend]` attribute.
     (unstable, must_not_suspend, "1.57.0", Some(83310)),
+    /// Make `mut` not reset the binding mode on edition >= 2024.
+    (unstable, mut_dont_reset_binding_mode_2024, "CURRENT_RUSTC_VERSION", Some(123076)),
     /// Allows `mut ref` and `mut ref mut` identifier patterns.
     (incomplete, mut_ref, "CURRENT_RUSTC_VERSION", Some(123076)),
     /// Allows using `#[naked]` on functions.

compiler/rustc_hir_typeck/src/pat.rs

@@ -629,12 +629,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Ty<'tcx>,
         pat_info: PatInfo<'tcx, '_>,
     ) -> Ty<'tcx> {
-        let PatInfo { binding_mode: def_bm, top_info: ti, .. } = pat_info;
+        let PatInfo { binding_mode: BindingAnnotation(def_br, _), top_info: ti, .. } = pat_info;
 
         // Determine the binding mode...
         let bm = match ba {
-            BindingAnnotation(ByRef::No, Mutability::Not) => def_bm,
-            _ => ba,
+            BindingAnnotation(ByRef::No, Mutability::Mut)
+                if !(pat.span.at_least_rust_2024()
+                    && self.tcx.features().mut_dont_reset_binding_mode_2024)
+                    && matches!(def_br, ByRef::Yes(_)) =>
+            {
+                // `mut x` resets the binding mode in edition <= 2021.
+                BindingAnnotation(ByRef::No, Mutability::Mut)
+            }
+            BindingAnnotation(ByRef::No, mutbl) => BindingAnnotation(def_br, mutbl),
+            BindingAnnotation(ByRef::Yes(_), _) => ba,
         };
         // ...and store it in a side table:
         self.typeck_results.borrow_mut().pat_binding_modes_mut().insert(pat.hir_id, bm);
@@ -743,7 +751,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    // Precondition: pat is a Ref(_) pattern
+    /// Precondition: pat is a `Ref(_)` pattern
     fn borrow_pat_suggestion(&self, err: &mut Diag<'_>, pat: &Pat<'_>) {
         let tcx = self.tcx;
         if let PatKind::Ref(inner, mutbl) = pat.kind

compiler/rustc_span/src/symbol.rs

@@ -1194,6 +1194,7 @@ symbols! {
         multiple_supertrait_upcastable,
         must_not_suspend,
         must_use,
+        mut_dont_reset_binding_mode_2024,
         mut_ref,
         naked,
         naked_functions,