Require DerefMut if deref pattern has nested ref mut binding

Author: compiler-errors

compiler/rustc_hir_typeck/src/pat.rs

@@ -2017,6 +2017,30 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let ty = self.normalize(span, ty);
         let ty = self.try_structurally_resolve_type(span, ty);
         self.check_pat(inner, ty, pat_info);
+
+        // Check if the pattern has any `ref mut` bindings, which would require
+        // `DerefMut` to be emitted in MIR building instead of just `Deref`.
+        let mut needs_mut = false;
+        inner.walk(|pat| {
+            if let hir::PatKind::Binding(_, id, _, _) = pat.kind
+                && let Some(ty::BindByReference(ty::Mutability::Mut)) =
+                    self.typeck_results.borrow().pat_binding_modes().get(id)
+            {
+                needs_mut = true;
+                // No need to continue recursing
+                false
+            } else {
+                true
+            }
+        });
+        if needs_mut {
+            self.register_bound(
+                expected,
+                tcx.require_lang_item(hir::LangItem::DerefMut, Some(span)),
+                self.misc(span),
+            );
+        }
+
         expected
     }
 

tests/ui/pattern/deref-patterns/ref-mut.rs

@@ -0,0 +1,17 @@
+#![feature(deref_patterns)]
+//~^ WARN the feature `deref_patterns` is incomplete
+
+use std::rc::Rc;
+
+fn main() {
+    match &mut vec![1] {
+        deref!(x) => {}
+        _ => {}
+    }
+
+    match &mut Rc::new(1) {
+        deref!(x) => {}
+        //~^ ERROR the trait bound `Rc<{integer}>: DerefMut` is not satisfied
+        _ => {}
+    }
+}

tests/ui/pattern/deref-patterns/ref-mut.stderr

@@ -0,0 +1,20 @@
+warning: the feature `deref_patterns` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/ref-mut.rs:1:12
+   |
+LL | #![feature(deref_patterns)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: see issue #87121 <https://github.com/rust-lang/rust/issues/87121> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0277]: the trait bound `Rc<{integer}>: DerefMut` is not satisfied
+  --> $DIR/ref-mut.rs:13:9
+   |
+LL |         deref!(x) => {}
+   |         ^^^^^^^^^ the trait `DerefMut` is not implemented for `Rc<{integer}>`
+   |
+   = note: this error originates in the macro `deref` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0277`.