librustc: Implement write guards for borrowing @mut to & or &mut. r=nmatsakis

src/test/run-fail/write-guard-fail-2.rs

@@ -0,0 +1,11 @@
+// error-pattern:borrowed
+
+struct S {
+    x: int
+}
+
+fn main() {
+    let x = @mut S { x: 3 };
+    let y: &S = x;
+    x.x = 5;
+}

src/test/run-fail/write-guard-fail-3.rs

@@ -0,0 +1,8 @@
+// error-pattern:borrowed
+
+fn main() {
+    let x = @mut 3;
+    let y: &mut int = x;
+    *x = 5;
+}
+

src/test/run-fail/write-guard-fail.rs

@@ -0,0 +1,13 @@
+// error-pattern:borrowed
+
+fn f(x: &int, y: @mut int) {
+    unsafe {
+        *y = 2;
+    }
+}
+
+fn main() {
+    let x = @mut 3;
+    f(x, x);
+}
+

src/test/run-pass/write-guard.rs

@@ -0,0 +1,9 @@
+fn f(x: &int) {
+    io::println(x.to_str());
+}
+
+fn main() {
+    let x = @mut 3;
+    f(x);
+}
+