Rollup merge of #122461 - the8472:fix-step-forward-unchecked, r=Amanieu

fix unsoundness in Step::forward_unchecked for signed integers

Fixes #122420

```rust
pub fn foo(a: i8, b: u8) -> i8 {
    unsafe { a.checked_add_unsigned(b).unwrap_unchecked() }
}
```

still compiles down to a single arithmetic instruction ([godbolt](https://rust.godbolt.org/z/qsd3xYWfE)).

But we may be losing some loop optimizations if llvm can no longer easily derive that it's a finite counted loop from the no-wrapping flags.

1 files changed, 5 insertions, 0 deletions

diff --git a/library/core/tests/iter/range.rs b/library/core/tests/iter/range.rs
index 9af07119a89..e31db0732e0 100644
--- a/library/core/tests/iter/range.rs
+++ b/library/core/tests/iter/range.rs
@@ -325,6 +325,11 @@ fn test_range_advance_by() {
     assert_eq!(Ok(()), r.advance_back_by(usize::MAX));
 
     assert_eq!((r.start, r.end), (0u128 + usize::MAX as u128, u128::MAX - usize::MAX as u128));
+
+    // issue 122420, Step::forward_unchecked was unsound for signed integers
+    let mut r = -128i8..127;
+    assert_eq!(Ok(()), r.advance_by(200));
+    assert_eq!(r.next(), Some(72));
 }
 
 #[test]