diff options
Diffstat (limited to 'library/core/src/iter/traits/exact_size.rs')
| -rw-r--r-- | library/core/src/iter/traits/exact_size.rs | 10 |
1 files changed, 10 insertions, 0 deletions
diff --git a/library/core/src/iter/traits/exact_size.rs b/library/core/src/iter/traits/exact_size.rs index 1757e37ec..908830d8a 100644 --- a/library/core/src/iter/traits/exact_size.rs +++ b/library/core/src/iter/traits/exact_size.rs @@ -21,6 +21,16 @@ /// /// [`len`]: ExactSizeIterator::len /// +/// # When *shouldn't* an adapter be `ExactSizeIterator`? +/// +/// If an adapter makes an iterator *longer*, then it's usually incorrect for +/// that adapter to implement `ExactSizeIterator`. The inner exact-sized +/// iterator might already be `usize::MAX`-long, and thus the length of the +/// longer adapted iterator would no longer be exactly representable in `usize`. +/// +/// This is why [`Chain<A, B>`](crate::iter::Chain) isn't `ExactSizeIterator`, +/// even when `A` and `B` are both `ExactSizeIterator`. +/// /// # Examples /// /// Basic usage: |