Do not suggest ampmut if rhs is already mutable

compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs

@@ -902,9 +902,13 @@ fn suggest_ampmut<'tcx>(
             {
                 let lt_name = &src[1..ws_pos];
                 let ty = &src[ws_pos..];
-                return (assignment_rhs_span, format!("&{} mut {}", lt_name, ty));
+                if !ty.trim_start().starts_with("mut") {
+                    return (assignment_rhs_span, format!("&{} mut {}", lt_name, ty));
+                }
             } else if let Some(stripped) = src.strip_prefix('&') {
-                return (assignment_rhs_span, format!("&mut {}", stripped));
+                if !stripped.trim_start().starts_with("mut") {
+                    return (assignment_rhs_span, format!("&mut {}", stripped));
+                }
             }
         }
     }

src/test/ui/borrowck/issue-85765.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let mut test = Vec::new();
+    let rofl: &Vec<Vec<i32>> = &mut test;
+    //~^ HELP consider changing this to be a mutable reference
+    rofl.push(Vec::new());
+    //~^ ERROR cannot borrow `*rofl` as mutable, as it is behind a `&` reference
+    //~| NOTE `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+}

src/test/ui/borrowck/issue-85765.stderr

@@ -0,0 +1,12 @@
+error[E0596]: cannot borrow `*rofl` as mutable, as it is behind a `&` reference
+  --> $DIR/issue-85765.rs:5:5
+   |
+LL |     let rofl: &Vec<Vec<i32>> = &mut test;
+   |         ---- help: consider changing this to be a mutable reference: `&mut Vec<Vec<i32>>`
+LL |
+LL |     rofl.push(Vec::new());
+   |     ^^^^ `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.