Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Add a doc note about why Chain is not ExactSizeIterator #107962

Merged
merged 1 commit into from
Feb 13, 2023
Merged
Changes from all commits
Commits
File filter

Filter by extension

Filter by extension

Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
10 changes: 10 additions & 0 deletions library/core/src/iter/traits/exact_size.rs
Original file line number Diff line number Diff line change
@@ -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:
11 changes: 11 additions & 0 deletions library/core/src/iter/traits/marker.rs
Original file line number Diff line number Diff line change
@@ -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