Always properly copy values into bindings when mutating the match dis…

…criminant

The reassignment checker effectively only checks whether the last
assignment in a body affects the discriminant, but it should of course
check all the assignments.

Fixes #23698

src/librustc_trans/trans/_match.rs

@@ -1352,12 +1352,12 @@ impl<'tcx> euv::Delegate<'tcx> for ReassignmentChecker {
     fn mutate(&mut self, _: ast::NodeId, _: Span, cmt: mc::cmt, _: euv::MutateMode) {
         match cmt.cat {
             mc::cat_upvar(mc::Upvar { id: ty::UpvarId { var_id: vid, .. }, .. }) |
-            mc::cat_local(vid) => self.reassigned = self.node == vid,
+            mc::cat_local(vid) => self.reassigned |= self.node == vid,
             mc::cat_interior(ref base_cmt, mc::InteriorField(field)) => {
                 match base_cmt.cat {
                     mc::cat_upvar(mc::Upvar { id: ty::UpvarId { var_id: vid, .. }, .. }) |
                     mc::cat_local(vid) => {
-                        self.reassigned = self.node == vid && Some(field) == self.field
+                        self.reassigned |= self.node == vid && Some(field) == self.field
                     },
                     _ => {}
                 }

src/test/run-pass/match-reassign.rs

@@ -0,0 +1,30 @@
+// Copyright 2015 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.
+
+// Regression test for #23698: The reassignment checker only cared
+// about the last assigment in a match arm body
+
+// Use an extra function to make sure no extra assignments
+// are introduced by macros in the match statement
+fn check_eq(x: i32, y: i32) {
+    assert_eq!(x, y);
+}
+
+#[allow(unused_assignments)]
+fn main() {
+    let mut x = Box::new(1);
+    match x {
+        y => {
+            x = Box::new(2);
+            let _tmp = 1; // This assignment used to throw off the reassignment checker
+            check_eq(*y, 1);
+        }
+    }
+}