Improve mutability checking. Closes #118.

src/test/compile-fail/writing-through-read-alias.rs

@@ -1,6 +1,6 @@
 // -*- rust -*-
 
-// error-pattern: writing to non-mutable slot
+// error-pattern: writing to immutable type
 
 type point = rec(int x, int y, int z);
 

src/test/compile-fail/writing-to-immutable-obj.rs

@@ -0,0 +1,8 @@
+// error-pattern: writing to immutable type
+obj objy(int x) {
+    fn foo() -> () {
+        x = 5;
+    }
+}
+fn main() {
+}

src/test/compile-fail/writing-to-immutable-rec.rs

@@ -0,0 +1,5 @@
+// error-pattern: writing to immutable type
+fn main() {
+  let rec(int x) r = rec(x=1);
+  r.x = 6;
+}

src/test/compile-fail/writing-to-immutable-tup.rs

@@ -0,0 +1,5 @@
+// error-pattern: writing to immutable type
+fn main() {
+  let tup(int) t = tup(1);
+  t._0 = 5;
+}

src/test/compile-fail/writing-to-immutable-vec.rs

@@ -0,0 +1,5 @@
+// error-pattern: writing to immutable type
+fn main() {
+  let vec[int] v = vec(1, 2, 3);
+  v.(1) = 4;
+}