Feature-gate mut ref patterns in struct field shorthand.

compiler/rustc_parse/src/parser/pat.rs

@@ -1750,6 +1750,12 @@ impl<'a> Parser<'a> {
             hi = self.prev_token.span;
             let ann = BindingMode(by_ref, mutability);
             let fieldpat = self.mk_pat_ident(boxed_span.to(hi), ann, fieldname);
+            if matches!(
+                fieldpat.kind,
+                PatKind::Ident(BindingMode(ByRef::Yes(..), Mutability::Mut), ..)
+            ) {
+                self.psess.gated_spans.gate(sym::mut_ref, fieldpat.span);
+            }
             let subpat = if is_box {
                 self.mk_pat(lo.to(hi), PatKind::Box(Box::new(fieldpat)))
             } else {

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

@@ -12,5 +12,5 @@ fn main() {
     let mut ref mut y = 10; //~  ERROR [E0658]
 
     struct Foo { x: i32 }
-    let Foo { mut ref x } = Foo { x: 10 };
+    let Foo { mut ref x } = Foo { x: 10 }; //~  ERROR [E0658]
 }

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

@@ -38,6 +38,16 @@ LL |     let mut ref mut y = 10;
    = 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    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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
+error[E0658]: mutable by-reference bindings are experimental
+  --> $DIR/feature-gate-mut-ref.rs:15:15
+   |
+LL |     let Foo { mut ref x } = Foo { 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+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0658`.