reset default binding mode when we pass through a & pattern

Fixes rust-lang#46688.

Author: nikomatsakis

Diff for: src/librustc_typeck/check/_match.rs

@@ -151,6 +151,19 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     err.emit();
                 }
             }
+        } else if let PatKind::Ref(..) = pat.node {
+            // When you encounter a `&pat` pattern, reset to "by
+            // value". This is so that `x` and `y` here are by value,
+            // as they appear to be:
+            //
+            // ```
+            // match &(&22, &44) {
+            //   (&x, &y) => ...
+            // }
+            // ```
+            //
+            // cc #46688
+            def_bm = ty::BindByValue(hir::MutImmutable);
         }
 
         // Lose mutability now that we know binding mode and discriminant type.

Diff for: src/test/run-pass/rfc-2005-default-binding-mode/reset-mode.rs

@@ -0,0 +1,25 @@
+// 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.
+
+#![feature(match_default_bindings)]
+
+// Test that we "reset" the mode as we pass through a `&` pattern.
+//
+// cc #46688
+
+fn surprise(x: i32) {
+    assert_eq!(x, 2);
+}
+
+fn main() {
+    let x = &(1, &2);
+    let (_, &b) = x;
+    surprise(b);
+}