Gate implicit mutable by-reference bindings behind mut ref

Author: Jules-Bertholet

compiler/rustc_feature/src/removed.rs

@@ -128,6 +128,8 @@ declare_features! (
     /// Allows the use of type alias impl trait in function return positions
     (removed, min_type_alias_impl_trait, "1.56.0", Some(63063),
      Some("removed in favor of full type_alias_impl_trait")),
+    /// Make `mut` not reset the binding mode on edition >= 2024.
+    (removed, mut_preserve_binding_mode_2024, "1.79.0", Some(123076), Some("superseded by `ref_pat_eat_one_layer_2024`")),
     (removed, needs_allocator, "1.4.0", Some(27389),
      Some("subsumed by `#![feature(allocator_internals)]`")),
     /// Allows use of unary negate on unsigned integers, e.g., -e for e: u8

compiler/rustc_feature/src/unstable.rs

@@ -527,8 +527,6 @@ 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.
-    (incomplete, mut_preserve_binding_mode_2024, "1.79.0", Some(123076)),
     /// Allows `mut ref` and `mut ref mut` identifier patterns.
     (incomplete, mut_ref, "1.79.0", Some(123076)),
     /// Allows using `#[naked]` on functions.

compiler/rustc_hir_typeck/src/pat.rs

@@ -12,7 +12,7 @@ use rustc_infer::infer;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 use rustc_middle::{bug, span_bug};
-use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
+use rustc_session::{lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS, parse::feature_err};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::Spanned;
@@ -669,17 +669,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Determine the binding mode...
         let bm = match user_bind_annot {
-            // `mut` resets binding mode on edition <= 2021
-            BindingMode(ByRef::No, Mutability::Mut)
-                if !(pat.span.at_least_rust_2024()
-                    && self.tcx.features().mut_preserve_binding_mode_2024)
-                    && matches!(def_br, ByRef::Yes(_)) =>
-            {
-                self.typeck_results
-                    .borrow_mut()
-                    .rust_2024_migration_desugared_pats_mut()
-                    .insert(pat_info.top_info.hir_id);
-                BindingMode(ByRef::No, Mutability::Mut)
+            BindingMode(ByRef::No, Mutability::Mut) if matches!(def_br, ByRef::Yes(_)) => {
+                if pat.span.at_least_rust_2024() && self.tcx.features().ref_pat_eat_one_layer_2024 {
+                    if !self.tcx.features().mut_ref {
+                        feature_err(
+                            &self.tcx.sess,
+                            sym::mut_ref,
+                            pat.span.until(ident.span),
+                            "binding cannot be both mutable and by-reference",
+                        )
+                        .emit();
+                    }
+
+                    BindingMode(def_br, Mutability::Mut)
+                } else {
+                    // `mut` resets binding mode on edition <= 2021
+                    self.typeck_results
+                        .borrow_mut()
+                        .rust_2024_migration_desugared_pats_mut()
+                        .insert(pat_info.top_info.hir_id);
+                    BindingMode(ByRef::No, Mutability::Mut)
+                }
             }
             BindingMode(ByRef::No, mutbl) => BindingMode(def_br, mutbl),
             BindingMode(ByRef::Yes(_), _) => user_bind_annot,

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024_fail.rs

@@ -49,4 +49,14 @@ pub fn main() {
 
     let &mut &mut &mut &mut _ = &mut &&&&mut &&&mut &mut 0;
     //~^ ERROR: mismatched types
+
+    struct Foo(u8);
+
+    let Foo(mut a) = &Foo(0);
+    //~^ ERROR: binding cannot be both mutable and by-reference
+    a = &42;
+
+    let Foo(mut a) = &mut Foo(0);
+    //~^ ERROR: binding cannot be both mutable and by-reference
+    a = &mut 42;
 }

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024_fail.stderr

@@ -152,6 +152,27 @@ LL |     let &mut &mut &mut &mut _ = &mut &&&&mut &&&mut &mut 0;
    = note:      expected reference `&&&&mut &&&mut &mut {integer}`
            found mutable reference `&mut _`
 
-error: aborting due to 14 previous errors
+error[E0658]: binding cannot be both mutable and by-reference
+  --> $DIR/ref_pat_eat_one_layer_2024_fail.rs:55:13
+   |
+LL |     let Foo(mut a) = &Foo(0);
+   |             ^^^^
+   |
+   = 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]: binding cannot be both mutable and by-reference
+  --> $DIR/ref_pat_eat_one_layer_2024_fail.rs:59:13
+   |
+LL |     let Foo(mut a) = &mut Foo(0);
+   |             ^^^^
+   |
+   = 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 16 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0308, E0658.
+For more information about an error, try `rustc --explain E0308`.

tests/ui/pattern/feature-gate-mut_preserve_binding_mode_2024.rs

@@ -2,7 +2,7 @@
 //@ run-rustfix
 //@ rustfix-only-machine-applicable
 //@ aux-build:match_ergonomics_2024_macros.rs
-#![feature(mut_preserve_binding_mode_2024, ref_pat_eat_one_layer_2024)]
+#![feature(mut_ref, ref_pat_eat_one_layer_2024)]
 #![allow(incomplete_features, unused)]
 #![deny(rust_2024_incompatible_pat)]
 

tests/ui/pattern/feature-gate-mut_preserve_binding_mode_2024.stderr

@@ -2,7 +2,7 @@
 //@ run-rustfix
 //@ rustfix-only-machine-applicable
 //@ aux-build:match_ergonomics_2024_macros.rs
-#![feature(mut_preserve_binding_mode_2024, ref_pat_eat_one_layer_2024)]
+#![feature(mut_ref, ref_pat_eat_one_layer_2024)]
 #![allow(incomplete_features, unused)]
 #![deny(rust_2024_incompatible_pat)]
 

tests/ui/pattern/match_ergonomics_2024.fixed

@@ -1,6 +1,6 @@
 //@ edition: 2021
 //@ compile-flags: -Zunstable-options
-#![feature(mut_preserve_binding_mode_2024)]
+#![feature(ref_pat_eat_one_layer_2024)]
 #![allow(incomplete_features)]
 
 struct Foo(u8);