specialize StepBy<Range(Inclusive)>

the originally generated code was highly suboptimal
this brings it close to the same code or even exactly the same as a
manual while-loop by eliminating a branch and the
double stepping of n-1 + 1 steps

The intermediate trait lets us circumvent the specialization
type inference bugs

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index 9b8d7031f8e..72b115f8b5f 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1619,6 +1619,14 @@ fn test_range_step() {
 }
 
 #[test]
+fn test_range_inclusive_step() {
+    assert_eq!((0..=50).step_by(10).collect::<Vec<_>>(), [0, 10, 20, 30, 40, 50]);
+    assert_eq!((0..=5).step_by(1).collect::<Vec<_>>(), [0, 1, 2, 3, 4, 5]);
+    assert_eq!((200..=255u8).step_by(10).collect::<Vec<_>>(), [200, 210, 220, 230, 240, 250]);
+    assert_eq!((250..=255u8).step_by(1).collect::<Vec<_>>(), [250, 251, 252, 253, 254, 255]);
+}
+
+#[test]
 fn test_range_last_max() {
     assert_eq!((0..20).last(), Some(19));
     assert_eq!((-20..0).last(), Some(-1));