use places_conflict to handle reassignment

This fixes the handling of reassignment of struct fields.

src/test/compile-fail/immut-function-arguments.rs

@@ -8,14 +8,17 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// revisions: ast mir
+//[mir]compile-flags: -Z borrowck=mir
 
 fn f(y: Box<isize>) {
-    *y = 5; //~ ERROR cannot assign
+    *y = 5; //[ast]~ ERROR cannot assign
+            //[mir]~^ ERROR cannot assign twice
 }
 
 fn g() {
-    let _frob = |q: Box<isize>| { *q = 2; }; //~ ERROR cannot assign
-
+    let _frob = |q: Box<isize>| { *q = 2; }; //[ast]~ ERROR cannot assign
+    //[mir]~^ ERROR cannot assign twice
 }
 
 fn main() {}

src/test/compile-fail/mutable-class-fields.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// revisions: ast mir
+//[mir]compile-flags: -Z borrowck=mir
+
 struct cat {
   meows : usize,
   how_hungry : isize,
@@ -22,5 +25,6 @@ fn cat(in_x : usize, in_y : isize) -> cat {
 
 fn main() {
   let nyan : cat = cat(52, 99);
-  nyan.how_hungry = 0; //~ ERROR cannot assign
+  nyan.how_hungry = 0; //[ast]~ ERROR cannot assign
+  //[mir]~^ ERROR cannot assign
 }