Feature gate

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -565,6 +565,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
     gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
     gate_all!(postfix_match, "postfix match is experimental");
+    gate_all!(mut_ref, "mutable by-reference bindings are experimental");
 
     if !visitor.features.never_patterns {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_feature/src/unstable.rs

@@ -529,6 +529,8 @@ 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)),
+    /// Allows `mut ref` and `mut ref mut` identifier patterns.
+    (unstable, mut_ref, "CURRENT_RUSTC_VERSION", Some(123076)),
     /// Allows using `#[naked]` on functions.
     (unstable, naked_functions, "1.9.0", Some(90957)),
     /// Allows specifying the as-needed link modifier

compiler/rustc_parse/src/parser/pat.rs

@@ -779,6 +779,10 @@ impl<'a> Parser<'a> {
             self.ban_mut_general_pat(mut_span, &pat, changed_any_binding);
         }
 
+        if matches!(pat.kind, PatKind::Ident(BindingAnnotation(ByRef::Yes(_), Mutability::Mut), ..))
+        {
+            self.psess.gated_spans.gate(sym::mut_ref, pat.span);
+        }
         Ok(pat.into_inner().kind)
     }
 

compiler/rustc_span/src/symbol.rs

@@ -1184,6 +1184,7 @@ symbols! {
         multiple_supertrait_upcastable,
         must_not_suspend,
         must_use,
+        mut_ref,
         naked,
         naked_functions,
         name,

tests/ui/feature-gates/feature-gate-mut-ref.rs

@@ -0,0 +1,13 @@
+fn main() {
+    let mut ref x = 10; //~  ERROR [E0658]
+    x = &11;
+    let ref mut y = 12;
+    *y = 13;
+    let mut ref mut z = 14; //~  ERROR [E0658]
+    z = &mut 15;
+
+    #[cfg(FALSE)]
+    let mut ref x = 10; //~  ERROR [E0658]
+    #[cfg(FALSE)]
+    let mut ref mut y = 10; //~  ERROR [E0658]
+}

tests/ui/feature-gates/feature-gate-mut-ref.stderr

@@ -0,0 +1,43 @@
+error[E0658]: mutable by-reference bindings are experimental
+  --> $DIR/feature-gate-mut-ref.rs:2:17
+   |
+LL |     let mut ref x = 10;
+   |                 ^
+   |
+   = note: see issue #123076 <https://github.com/rust-lang/rust/issues/123076> for more information
+   = help: add `#![feature(mut_ref)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: mutable by-reference bindings are experimental
+  --> $DIR/feature-gate-mut-ref.rs:6:21
+   |
+LL |     let mut ref mut z = 14;
+   |                     ^
+   |
+   = note: see issue #123076 <https://github.com/rust-lang/rust/issues/123076> for more information
+   = help: add `#![feature(mut_ref)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: mutable by-reference bindings are experimental
+  --> $DIR/feature-gate-mut-ref.rs:10:17
+   |
+LL |     let mut ref x = 10;
+   |                 ^
+   |
+   = note: see issue #123076 <https://github.com/rust-lang/rust/issues/123076> for more information
+   = help: add `#![feature(mut_ref)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: mutable by-reference bindings are experimental
+  --> $DIR/feature-gate-mut-ref.rs:12:21
+   |
+LL |     let mut ref mut y = 10;
+   |                     ^
+   |
+   = note: see issue #123076 <https://github.com/rust-lang/rust/issues/123076> for more information
+   = help: add `#![feature(mut_ref)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/match/mut-ref-mut-2021.rs

@@ -1,4 +1,5 @@
 //@ edition: 2021
+#![feature(mut_ref)]
 
 struct Foo(u8);
 

tests/ui/match/mut-ref-mut-2021.stderr

@@ -1,5 +1,5 @@
 error[E0384]: cannot assign twice to immutable variable `a`
-  --> $DIR/mut-ref-mut-2021.rs:7:5
+  --> $DIR/mut-ref-mut-2021.rs:8:5
    |
 LL |     let Foo(a) = Foo(0);
    |             -
@@ -10,55 +10,55 @@ LL |     a = 42;
    |     ^^^^^^ cannot assign twice to immutable variable
 
 error[E0384]: cannot assign twice to immutable variable `a`
-  --> $DIR/mut-ref-mut-2021.rs:13:5
+  --> $DIR/mut-ref-mut-2021.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-2021.rs:19:5
+  --> $DIR/mut-ref-mut-2021.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-2021.rs:25:5
+  --> $DIR/mut-ref-mut-2021.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-2021.rs:31:5
+  --> $DIR/mut-ref-mut-2021.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-2021.rs:37:5
+  --> $DIR/mut-ref-mut-2021.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-2021.rs:43:5
+  --> $DIR/mut-ref-mut-2021.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-2021.rs:49:5
+  --> $DIR/mut-ref-mut-2021.rs:50:5
    |
 LL |     let Foo(ref mut a) = &mut Foo(0);
    |             --------- first assignment to `a`

tests/ui/mut/mut-ref.rs

@@ -1,5 +1,5 @@
 //@ check-pass
-
+#![feature(mut_ref)]
 fn main() {
     let mut ref x = 10;
     x = &11;