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]

2 files changed, 12 insertions, 4 deletions

diff --git a/src/libcore/fmt/float.rs b/src/libcore/fmt/float.rs
index ef5b51fd00b..386fc28119a 100644
--- a/src/libcore/fmt/float.rs
+++ b/src/libcore/fmt/float.rs
@@ -13,15 +13,19 @@
 use char;
 use collections::Collection;
 use fmt;
-use iter::{Iterator, range, DoubleEndedIterator};
+use iter::{range, DoubleEndedIterator};
 use num::{Float, FPNaN, FPInfinite, ToPrimitive, Primitive};
 use num::{Zero, One, cast};
-use option::{None, Some};
 use result::Ok;
 use slice::{ImmutableVector, MutableVector};
 use slice;
 use str::StrSlice;
 
+#[cfg(stage0)]
+use iter::Iterator;         // NOTE(stage0): Remove after snapshot.
+#[cfg(stage0)]
+use option::{Some, None};   // NOTE(stage0): Remove after snapshot.
+
 /// A flag that specifies whether to use exponential (scientific) notation.
 pub enum ExponentFormat {
     /// Do not use exponential notation.
diff --git a/src/libcore/fmt/num.rs b/src/libcore/fmt/num.rs
index d52791f6b0e..81e84c447e7 100644
--- a/src/libcore/fmt/num.rs
+++ b/src/libcore/fmt/num.rs
@@ -16,11 +16,15 @@
 
 use collections::Collection;
 use fmt;
-use iter::{Iterator, DoubleEndedIterator};
+use iter::DoubleEndedIterator;
 use num::{Int, cast, zero};
-use option::{Some, None};
 use slice::{ImmutableVector, MutableVector};
 
+#[cfg(stage0)]
+use iter::Iterator;         // NOTE(stage0): Remove after snapshot.
+#[cfg(stage0)]
+use option::{Some, None};   // NOTE(stage0): Remove after snapshot.
+
 /// A type that represents a specific radix
 trait GenericRadix {
     /// The number of digits.