Gate implicit mutable by-reference bindings behind mut ref

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+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-For more information about this error, try `rustc --explain E0308`.
+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+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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
+
+Some errors have detailed explanations: E0308, E0658.
+For more information about an error, try `rustc --explain E0308`.