From ffb315fd248d07047626b7eda12646a1ff8c5633 Mon Sep 17 00:00:00 2001
From: Edward Wang <edward.yu.wang@gmail.com>
Date: Mon, 24 Feb 2014 07:02:27 +0800
Subject: [PATCH] Match binding is assignment

In its first pass, namely gather_loans, the borrow checker tracks the
initialization sites among other things it does. It does so for let
bindings with initializers but not for bindings in match arms, which are
effectively also assignments. This patch does that for borrow checker.

Closes #12452.
---
 .../middle/borrowck/gather_loans/mod.rs       | 23 +++++----
 .../borrowck-match-binding-is-assignment.rs   | 51 +++++++++++++++++++
 2 files changed, 64 insertions(+), 10 deletions(-)
 create mode 100644 src/test/compile-fail/borrowck-match-binding-is-assignment.rs

diff --git a/src/librustc/middle/borrowck/gather_loans/mod.rs b/src/librustc/middle/borrowck/gather_loans/mod.rs
index 15922d57ba83..263ed47b29e3 100644
--- a/src/librustc/middle/borrowck/gather_loans/mod.rs
+++ b/src/librustc/middle/borrowck/gather_loans/mod.rs
@@ -160,16 +160,8 @@ fn gather_loans_in_local(this: &mut GatherLoanCtxt,
             })
         }
         Some(init) => {
-            // Variable declarations with initializers are considered "assigns":
-            let tcx = this.bccx.tcx;
-            pat_util::pat_bindings(tcx.def_map, local.pat, |_, id, span, _| {
-                gather_moves::gather_assignment(this.bccx,
-                                                &this.move_data,
-                                                id,
-                                                span,
-                                                @LpVar(id),
-                                                id);
-            });
+            // Variable declarations with initializers are considered "assigns",
+            // which is handled by `gather_pat`:
             let init_cmt = this.bccx.cat_expr(init);
             this.gather_pat(init_cmt, local.pat, None);
         }
@@ -811,6 +803,17 @@ impl<'a> GatherLoanCtxt<'a> {
         self.bccx.cat_pattern(discr_cmt, root_pat, |cmt, pat| {
             match pat.node {
               ast::PatIdent(bm, _, _) if self.pat_is_binding(pat) => {
+                // Each match binding is effectively an assignment.
+                let tcx = self.bccx.tcx;
+                pat_util::pat_bindings(tcx.def_map, pat, |_, id, span, _| {
+                    gather_moves::gather_assignment(self.bccx,
+                                                    &self.move_data,
+                                                    id,
+                                                    span,
+                                                    @LpVar(id),
+                                                    id);
+                });
+
                 match bm {
                   ast::BindByRef(mutbl) => {
                     // ref x or ref x @ p --- creates a ptr which must
diff --git a/src/test/compile-fail/borrowck-match-binding-is-assignment.rs b/src/test/compile-fail/borrowck-match-binding-is-assignment.rs
new file mode 100644
index 000000000000..72d9afd4d2b2
--- /dev/null
+++ b/src/test/compile-fail/borrowck-match-binding-is-assignment.rs
@@ -0,0 +1,51 @@
+// Copyright 2012-2014 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.
+
+// Test that immutable pattern bindings cannot be reassigned.
+
+enum E {
+    Foo(int)
+}
+
+struct S {
+    bar: int,
+}
+
+pub fn main() {
+    match 1i {
+        x => {
+            x += 1; //~ ERROR re-assignment of immutable variable `x`
+        }
+    }
+
+    match Foo(1) {
+        Foo(x) => {
+            x += 1; //~ ERROR re-assignment of immutable variable `x`
+        }
+    }
+
+    match S { bar: 1 } {
+        S { bar: x } => {
+            x += 1; //~ ERROR re-assignment of immutable variable `x`
+        }
+    }
+
+    match (1i,) {
+        (x,) => {
+            x += 1; //~ ERROR re-assignment of immutable variable `x`
+        }
+    }
+
+    match [1,2,3] {
+        [x,_,_] => {
+            x += 1; //~ ERROR re-assignment of immutable variable `x`
+        }
+    }
+}