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.

1 files changed, 9 insertions, 1 deletions

diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index fdb129d9e2a..dcb6e115eda 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/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, .. }) => {