Fix pretty printing an AST representing &(mut ident)

`PatKind::Ref(PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..), ..)`
is an AST representing `&(mut ident)`. It was errorneously printed as
`&mut ident` which reparsed into a syntactically different AST.

This affected help diagnostics in the parser.

Author: Thomas Bahn

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -2420,7 +2420,15 @@ impl<'a> State<'a> {
                 if mutbl == ast::Mutability::Mut {
                     self.s.word("mut ");
                 }
-                self.print_pat(inner);
+                if let PatKind::Ident(ast::BindingMode::ByValue(ast::Mutability::Mut), ..) =
+                    inner.kind
+                {
+                    self.popen();
+                    self.print_pat(inner);
+                    self.pclose();
+                } else {
+                    self.print_pat(inner);
+                }
             }
             PatKind::Lit(ref e) => self.print_expr(&**e),
             PatKind::Range(ref begin, ref end, Spanned { node: ref end_kind, .. }) => {

src/test/ui/pattern/issue-80186-mut-binding-help-suggestion.rs

@@ -0,0 +1,9 @@
+// Regression test for correct pretty-printing of an AST representing `&(mut x)` in help
+// suggestion diagnostic.
+
+fn main() {
+    let mut &x = &0;
+    //~^ ERROR `mut` must be attached to each individual binding
+    //~| HELP add `mut` to each binding
+    //~| SUGGESTION &(mut x)
+}

src/test/ui/pattern/issue-80186-mut-binding-help-suggestion.stderr

@@ -0,0 +1,10 @@
+error: `mut` must be attached to each individual binding
+  --> $DIR/issue-80186-mut-binding-help-suggestion.rs:5:9
+   |
+LL |     let mut &x = &0;
+   |         ^^^^^^ help: add `mut` to each binding: `&(mut x)`
+   |
+   = note: `mut` may be followed by `variable` and `variable @ pattern`
+
+error: aborting due to previous error
+