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Diffstat (limited to 'library/core/src/iter/traits/marker.rs')
| -rw-r--r-- | library/core/src/iter/traits/marker.rs | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/library/core/src/iter/traits/marker.rs b/library/core/src/iter/traits/marker.rs index da7537457..af0284823 100644 --- a/library/core/src/iter/traits/marker.rs +++ b/library/core/src/iter/traits/marker.rs @@ -31,6 +31,17 @@ impl<I: FusedIterator + ?Sized> FusedIterator for &mut I {} /// The iterator must produce exactly the number of elements it reported /// or diverge before reaching the end. /// +/// # When *shouldn't* an adapter be `TrustedLen`? +/// +/// If an adapter makes an iterator *shorter* by a given amount, then it's +/// usually incorrect for that adapter to implement `TrustedLen`. The inner +/// iterator might return more than `usize::MAX` items, but there's no way to +/// know what `k` elements less than that will be, since the `size_hint` from +/// the inner iterator has already saturated and lost that information. +/// +/// This is why [`Skip<I>`](crate::iter::Skip) isn't `TrustedLen`, even when +/// `I` implements `TrustedLen`. +/// /// # Safety /// /// This trait must only be implemented when the contract is upheld. Consumers |