Don't reset binding mode on mut with edition >= 2024

Author: Jules-Bertholet

compiler/rustc_hir_typeck/src/pat.rs

@@ -605,23 +605,23 @@ 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,
-            BindingAnnotation(ByRef::No, Mutability::Mut) => {
-                if matches!(def_bm.0, ByRef::Yes(_)) {
-                    // `mut x` resets the binding mode.
-                    self.tcx.emit_node_span_lint(
-                        lint::builtin::DEREFERENCING_MUT_BINDING,
-                        pat.hir_id,
-                        pat.span,
-                        errors::DereferencingMutBinding { span: pat.span },
-                    );
-                }
+            BindingAnnotation(ByRef::No, Mutability::Mut)
+                if !pat.span.at_least_rust_2024() && matches!(def_br, ByRef::Yes(_)) =>
+            {
+                // `mut x` resets the binding mode in edition <= 2021.
+                self.tcx.emit_node_span_lint(
+                    lint::builtin::DEREFERENCING_MUT_BINDING,
+                    pat.hir_id,
+                    pat.span,
+                    errors::DereferencingMutBinding { span: pat.span },
+                );
                 BindingAnnotation(ByRef::No, Mutability::Mut)
             }
+            BindingAnnotation(ByRef::No, mutbl) => BindingAnnotation(def_br, mutbl),
             BindingAnnotation(ByRef::Yes(_), _) => ba,
         };
         // ...and store it in a side table:
@@ -731,7 +731,7 @@ 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

tests/ui/match/mut-ref-mut-2024.rs

@@ -0,0 +1,54 @@
+//@ edition: 2024
+//@ compile-flags: -Zunstable-options
+
+struct Foo(u8);
+
+fn main() {
+    let Foo(a) = Foo(0);
+    a = 42; //~ ERROR [E0384]
+
+    let Foo(mut a) = Foo(0);
+    a = 42;
+
+    let Foo(ref a) = Foo(0);
+    a = &42; //~ ERROR [E0384]
+
+    let Foo(mut ref a) = Foo(0);
+    a = &42;
+
+    let Foo(ref mut a) = Foo(0);
+    a = &mut 42; //~ ERROR [E0384]
+
+    let Foo(mut ref mut a) = Foo(0);
+    a = &mut 42;
+
+    let Foo(a) = &Foo(0);
+    a = &42; //~ ERROR [E0384]
+
+    let Foo(mut a) = &Foo(0);
+    a = &42;
+
+    let Foo(ref a) = &Foo(0);
+    a = &42; //~ ERROR [E0384]
+
+    let Foo(mut ref a) = &Foo(0);
+    a = &42;
+
+    let Foo(a) = &mut Foo(0);
+    a = &mut 42; //~ ERROR [E0384]
+
+    let Foo(mut a) = &mut Foo(0);
+    a = &mut 42;
+
+    let Foo(ref a) = &mut Foo(0);
+    a = &42; //~ ERROR [E0384]
+
+    let Foo(mut ref a) = &mut Foo(0);
+    a = &42;
+
+    let Foo(ref mut a) = &mut Foo(0);
+    a = &mut 42; //~ ERROR [E0384]
+
+    let Foo(mut ref mut a) = &mut Foo(0);
+    a = &mut 42;
+}

tests/ui/match/mut-ref-mut-2024.stderr

@@ -0,0 +1,70 @@
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:8:5
+   |
+LL |     let Foo(a) = Foo(0);
+   |             -
+   |             |
+   |             first assignment to `a`
+   |             help: consider making this binding mutable: `mut a`
+LL |     a = 42;
+   |     ^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:14:5
+   |
+LL |     let Foo(ref a) = Foo(0);
+   |             ----- first assignment to `a`
+LL |     a = &42;
+   |     ^^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:20:5
+   |
+LL |     let Foo(ref mut a) = Foo(0);
+   |             --------- first assignment to `a`
+LL |     a = &mut 42;
+   |     ^^^^^^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:26:5
+   |
+LL |     let Foo(a) = &Foo(0);
+   |             - first assignment to `a`
+LL |     a = &42;
+   |     ^^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:32:5
+   |
+LL |     let Foo(ref a) = &Foo(0);
+   |             ----- first assignment to `a`
+LL |     a = &42;
+   |     ^^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:38:5
+   |
+LL |     let Foo(a) = &mut Foo(0);
+   |             - first assignment to `a`
+LL |     a = &mut 42;
+   |     ^^^^^^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:44:5
+   |
+LL |     let Foo(ref a) = &mut Foo(0);
+   |             ----- first assignment to `a`
+LL |     a = &42;
+   |     ^^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign twice to immutable variable `a`
+  --> $DIR/mut-ref-mut-2024.rs:50:5
+   |
+LL |     let Foo(ref mut a) = &mut Foo(0);
+   |             --------- first assignment to `a`
+LL |     a = &mut 42;
+   |     ^^^^^^^^^^^ cannot assign twice to immutable variable
+
+error: aborting due to 8 previous errors
+
+For more information about this error, try `rustc --explain E0384`.