librustc: Stop desugaring `for` expressions and translate them directly.

This makes edge cases in which the `Iterator` trait was not in scope
and/or `Option` or its variants were not in scope work properly.

This breaks code that looks like:

    struct MyStruct { ... }

    impl MyStruct {
        fn next(&mut self) -> Option<int> { ... }
    }

    for x in MyStruct { ... } { ... }

Change ad-hoc `next` methods like the above to implementations of the
`Iterator` trait. For example:

    impl Iterator<int> for MyStruct {
        fn next(&mut self) -> Option<int> { ... }
    }

Closes #15392.

[breaking-change]

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libunicode/decompose.rs b/src/libunicode/decompose.rs
index 832b65d4739..25e06bf7c13 100644
--- a/src/libunicode/decompose.rs
+++ b/src/libunicode/decompose.rs
@@ -39,6 +39,7 @@ pub fn decompose_canonical(c: char, i: |char|) { d(c, i, false); }
 pub fn decompose_compatible(c: char, i: |char|) { d(c, i, true); }
 
 fn d(c: char, i: |char|, k: bool) {
+    #[cfg(stage0)]
     use core::iter::Iterator;
 
     // 7-bit ASCII never decomposes