Add core::iter::once_with()
Functions `iter::once()` and `iter::repeat()` construct iterators from values. The latter has the lazy variant `iter::repeat_with()`, but the former doesn't. This PR therefore adds `iter::once_with()`.
Another way to think of `iter::once_with()` is that it's a function that converts `FnOnce() -> T` into `Iterator<Item = T>`.
If this seems like a reasonable addition, I'll open a tracking issue and update the `#[feature(...)]` attributes.
Add unstable Iterator::copied()
Initially suggested at https://github.com/bluss/rust-itertools/pull/289, however the maintainers of itertools suggested this may be better of in a standard library.
The intent of `copied` is to avoid accidentally cloning iterator elements after doing a code refactoring which causes a structure to be no longer `Copy`. This is a relatively common pattern, as it can be seen by calling `rg --pcre2 '[.]map[(][|](?:(\w+)[|] [*]\1|&(\w+)[|] \2)[)]'` on Rust main repository. Additionally, many uses of `cloned` actually want to simply `Copy`, and changing something to be no longer copyable may introduce unnoticeable performance penalty.
Also, this makes sense because the standard library includes `[T].copy_from_slice` to pair with `[T].clone_from_slice`.
This also adds `Option::copied`, because it makes sense to pair it with `Iterator::copied`. I don't think this feature is particularly important, but it makes sense to update `Option` along with `Iterator` for consistency.
name old ns/iter new ns/iter diff ns/iter diff % speedup
iter::bench_cycle_take_ref_sum 927,152 927,194 42 0.00% x 1.00
iter::bench_cycle_take_sum 938,129 603,492 -334,637 -35.67% x 1.55
Stabilize Iterator::flatten in 1.29, fixes#48213.
This PR stabilizes [`Iterator::flatten`](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#method.flatten) in *version 1.29* (1.28 goes to beta in 10 days, I don't think there's enough time to land it in that time, but let's see...).
Tracking issue is: #48213.
cc @bluss re. itertools.
r? @SimonSapin
ping @pietroalbini -- let's do a crater run when this passes CI :)
the originally generated code was highly suboptimal
this brings it close to the same code or even exactly the same as a
manual while-loop by eliminating a branch and the
double stepping of n-1 + 1 steps
The intermediate trait lets us circumvent the specialization
type inference bugs
FusedIterator is a marker trait that promises that the implementing
iterator continues to return `None` from `.next()` once it has returned
`None` once (and/or `.next_back()`, if implemented).
The effects of FusedIterator are already widely available through
`.fuse()`, but with stable `FusedIterator`, stable Rust users can
implement this trait for their iterators when appropriate.
