Ensure inherited reference is never set to &mut behind an &

compiler/rustc_ast/src/ast.rs

@@ -36,6 +36,7 @@ use rustc_macros::HashStable_Generic;
 use rustc_span::source_map::{respan, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{ErrorGuaranteed, Span, DUMMY_SP};
+use std::cmp;
 use std::fmt;
 use std::mem;
 use thin_vec::{thin_vec, ThinVec};
@@ -731,6 +732,13 @@ impl BindingAnnotation {
             Self::MUT_REF_MUT => "mut ref mut ",
         }
     }
+
+    pub fn cap_ref_mutability(mut self, mutbl: Mutability) -> Self {
+        if let ByRef::Yes(old_mutbl) = self.0 {
+            self.0 = ByRef::Yes(cmp::min(old_mutbl, mutbl));
+        }
+        self
+    }
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]