three new tests for assigning to various unassignable things

src/test/compile-fail/borrowck-assign-to-constants.rs

@@ -0,0 +1,7 @@
+const foo: int = 5;
+
+fn main() {
+    // assigning to various global constants
+    none = some(3); //! ERROR assigning to static item
+    foo = 6; //! ERROR assigning to static item
+}

src/test/compile-fail/borrowck-assign-to-enum.rs

@@ -0,0 +1,6 @@
+enum foo = int;
+
+fn main() {
+    let x = foo(3);
+    *x = 4; //! ERROR assigning to enum content
+}

src/test/compile-fail/borrowck-assign-to-resource.rs

@@ -0,0 +1,6 @@
+resource r(_r: int) {}
+
+fn main() {
+    let x = r(3);
+    *x = 4; //! ERROR assigning to resource content
+}