Avoid lint to unsized mutable reference

clippy_lints/src/mut_mut.rs

@@ -68,7 +68,8 @@ fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
                     expr.span,
                     "generally you want to avoid `&mut &mut _` if possible",
                 );
-            } else if let ty::Ref(_, _, hir::Mutability::Mut) = self.cx.typeck_results().expr_ty(e).kind() {
+            } else if let ty::Ref(_, ty, hir::Mutability::Mut) = self.cx.typeck_results().expr_ty(e).kind() {
+                if ty.peel_refs().is_sized(self.cx.tcx.at(expr.span), self.cx.param_env) {
                     span_lint(
                         self.cx,
                         MUT_MUT,
@@ -78,6 +79,7 @@ fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
                 }
             }
         }
+    }
 
     fn visit_ty(&mut self, ty: &'tcx hir::Ty<'_>) {
         if in_external_macro(self.cx.sess(), ty.span) {

tests/ui/mut_mut.rs

@@ -57,3 +57,20 @@ fn issue6922() {
     // do not lint from an external macro
     mut_mut!();
 }
+
+mod issue9035 {
+    use std::fmt::Display;
+
+    struct Foo<'a> {
+        inner: &'a mut dyn Display,
+    }
+
+    impl Foo<'_> {
+        fn foo(&mut self) {
+            let hlp = &mut self.inner;
+            bar(hlp);
+        }
+    }
+
+    fn bar(_: &mut impl Display) {}
+}