Make ops::DerefMut a diagnostic item

Author: relaxcn

compiler/rustc_span/src/symbol.rs

@@ -219,6 +219,7 @@ symbols! {
         Decoder,
         Default,
         Deref,
+        DerefMut,
         DiagMessage,
         Diagnostic,
         DirBuilder,

library/core/src/ops/deref.rs

@@ -266,6 +266,7 @@ impl<T: ?Sized> const Deref for &mut T {
 #[lang = "deref_mut"]
 #[doc(alias = "*")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "DerefMut"]
 #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
 pub const trait DerefMut: [const] Deref + PointeeSized {
     /// Mutably dereferences the value.