Auto merge of #43489 - petrochenkov:mutref, r=GuillaumeGomez

Better diagnostics and recovery for `mut ref` in patterns

Fixes https://github.com/rust-lang/rust/issues/43286
Supersedes https://github.com/rust-lang/rust/pull/43451

r? @GuillaumeGomez

src/libsyntax/parse/parser.rs

@@ -3523,8 +3523,18 @@ impl<'a> Parser<'a> {
             }
             // At this point, token != _, &, &&, (, [
             _ => if self.eat_keyword(keywords::Mut) {
-                // Parse mut ident @ pat
-                pat = self.parse_pat_ident(BindingMode::ByValue(Mutability::Mutable))?;
+                // Parse mut ident @ pat / mut ref ident @ pat
+                let mutref_span = self.prev_span.to(self.span);
+                let binding_mode = if self.eat_keyword(keywords::Ref) {
+                    self.diagnostic()
+                        .struct_span_err(mutref_span, "the order of `mut` and `ref` is incorrect")
+                        .span_suggestion(mutref_span, "try switching the order", "ref mut".into())
+                        .emit();
+                    BindingMode::ByRef(Mutability::Mutable)
+                } else {
+                    BindingMode::ByValue(Mutability::Mutable)
+                };
+                pat = self.parse_pat_ident(binding_mode)?;
             } else if self.eat_keyword(keywords::Ref) {
                 // Parse ref ident @ pat / ref mut ident @ pat
                 let mutbl = self.parse_mutability();

src/test/ui/mut-ref.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z parse-only
+
+fn main() {
+    let mut ref x = 10;
+    let ref mut y = 11;
+}

src/test/ui/mut-ref.stderr

@@ -0,0 +1,8 @@
+error: the order of `mut` and `ref` is incorrect
+  --> $DIR/mut-ref.rs:14:9
+   |
+14 |     let mut ref x = 10;
+   |         ^^^^^^^ help: try switching the order: `ref mut`
+
+error: aborting due to previous error
+