Suggestion for immutable reference -> mutable pointer should be emitted

as `Applicability::Unspecified`

src/test/ui/issues/issue-71676-2.rs

@@ -37,7 +37,6 @@ impl DerefMut for Emm {
     }
 }
 fn main() {
-    // Should not suggest when a is immutable
     let a = Emm(Foo(Bar(0)));
     let _: *mut u8 = &a; //~ ERROR mismatched types
 }

src/test/ui/issues/issue-71676-2.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/issue-71676-2.rs:42:22
+  --> $DIR/issue-71676-2.rs:41:22
    |
 LL |     let _: *mut u8 = &a;
    |            -------   ^^