Auto merge of #89916 - the8472:advance_by-avoid-err-0, r=dtolnay

Fix Iterator::advance_by contract inconsistency

The `advance_by(n)` docs state that in the error case `Err(k)` that k is always less than n.
It also states that `advance_by(0)` may return `Err(0)` to indicate an exhausted iterator.
These statements are inconsistent.
Since only one implementation (Skip) actually made use of that I changed it to return Ok(()) in that case too.

While adding some tests I also found a bug in `Take::advance_back_by`.

1 files changed, 3 insertions, 0 deletions

diff --git a/library/alloc/tests/vec.rs b/library/alloc/tests/vec.rs
index 00a878c0794..77314282532 100644
--- a/library/alloc/tests/vec.rs
+++ b/library/alloc/tests/vec.rs
@@ -985,6 +985,9 @@ fn test_into_iter_advance_by() {
 
     assert_eq!(i.advance_by(usize::MAX), Err(0));
 
+    i.advance_by(0).unwrap();
+    i.advance_back_by(0).unwrap();
+
     assert_eq!(i.len(), 0);
 }