librustc: Remove the fallback to int from typechecking.

This breaks a fair amount of code. The typical patterns are:

* `for _ in range(0, 10)`: change to `for _ in range(0u, 10)`;

* `println!("{}", 3)`: change to `println!("{}", 3i)`;

* `[1, 2, 3].len()`: change to `[1i, 2, 3].len()`.

RFC #30. Closes #6023.

[breaking-change]

src/libtest/lib.rs

@@ -658,11 +658,11 @@ impl<T: Writer> ConsoleTestState<T> {
     }
 
     pub fn write_metric_diff(&mut self, diff: &MetricDiff) -> io::IoResult<()> {
-        let mut noise = 0;
-        let mut improved = 0;
-        let mut regressed = 0;
-        let mut added = 0;
-        let mut removed = 0;
+        let mut noise = 0u;
+        let mut improved = 0u;
+        let mut regressed = 0u;
+        let mut added = 0u;
+        let mut removed = 0u;
 
         for (k, v) in diff.iter() {
             match *v {