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, 4 insertions, 1 deletions

diff --git a/src/libcore/ptr.rs b/src/libcore/ptr.rs
index ed1d4d48110..4921802ba73 100644
--- a/src/libcore/ptr.rs
+++ b/src/libcore/ptr.rs
@@ -90,11 +90,14 @@
 use mem;
 use clone::Clone;
 use intrinsics;
-use iter::{range, Iterator};
+use iter::range;
 use option::{Some, None, Option};
 
 use cmp::{PartialEq, Eq, PartialOrd, Equiv, Ordering, Less, Equal, Greater};
 
+#[cfg(stage0)]
+use iter::Iterator; // NOTE(stage0): Remove after snapshot.
+
 pub use intrinsics::copy_memory;
 pub use intrinsics::copy_nonoverlapping_memory;
 pub use intrinsics::set_memory;