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, 3 deletions

diff --git a/src/test/bench/shootout-meteor.rs b/src/test/bench/shootout-meteor.rs
index e13c53407e4..615dd1d69d5 100644
--- a/src/test/bench/shootout-meteor.rs
+++ b/src/test/bench/shootout-meteor.rs
@@ -297,10 +297,10 @@ fn search(
     // for every unused piece
     for id in range(0u, 10).filter(|id| board & (1 << (id + 50)) == 0) {
         // for each mask that fits on the board
-        for &m in masks_at.get(id).iter().filter(|&m| board & *m == 0) {
+        for m in masks_at.get(id).iter().filter(|&m| board & *m == 0) {
             // This check is too costy.
             //if is_board_unfeasible(board | m, masks) {continue;}
-            search(masks, board | m, i + 1, Cons(m, &cur), data);
+            search(masks, board | *m, i + 1, Cons(*m, &cur), data);
         }
     }
 }
@@ -311,9 +311,10 @@ fn par_search(masks: Vec<Vec<Vec<u64>>>) -> Data {
 
     // launching the search in parallel on every masks at minimum
     // coordinate (0,0)
-    for &m in masks.get(0).iter().flat_map(|masks_pos| masks_pos.iter()) {
+    for m in masks.get(0).iter().flat_map(|masks_pos| masks_pos.iter()) {
         let masks = masks.clone();
         let tx = tx.clone();
+        let m = *m;
         spawn(proc() {
             let mut data = Data::new();
             search(&*masks, m, 1, Cons(m, &Nil), &mut data);