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RangeInclusive internal iteration performance improvement.
Specialize `Iterator::try_fold` and `DoubleEndedIterator::try_rfold` to improve code generation in all internal iteration scenarios.
This changes brings the performance of internal iteration with `RangeInclusive` on par with the performance of iteration with `Range`:
- Single conditional jump in hot loop,
- Unrolling and vectorization,
- And even Closed Form substitution.
Unfortunately, it only applies to internal iteration. Despite various attempts at stream-lining the implementation of `next` and `next_back`, LLVM has stubbornly refused to optimize external iteration appropriately, leaving me with a choice between:
- The current implementation, for which Closed Form substitution is performed, but which uses 2 conditional jumps in the hot loop when optimization fail.
- An implementation using a `is_done` boolean, which uses 1 conditional jump in the hot loop when optimization fail, allowing unrolling and vectorization, but for which Closed Form substitution fails.
In the absence of any conclusive evidence as to which usecase matters most, and with no assurance that the lack of Closed Form substitution is not indicative of other optimizations being foiled, there is no way
to pick one implementation over the other, and thus I defer to the statu quo as far as `next` and `next_back` are concerned.
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FCP: https://github.com/rust-lang/rust/issues/55977#issuecomment-463964234
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FCP: https://github.com/rust-lang/rust/issues/58045#issuecomment-464674773
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The phrase "... or some similar thing." is very vague and contributes nothing to understanding the example. Simply removed.
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Specialize Iterator::try_fold and DoubleEndedIterator::try_rfold to
improve code generation in all internal iteration scenarios.
This changes brings the performance of internal iteration with
RangeInclusive on par with the performance of iteration with Range:
- Single conditional jump in hot loop,
- Unrolling and vectorization,
- And even Closed Form substitution.
Unfortunately, it only applies to internal iteration. Despite various
attempts at stream-lining the implementation of next and next_back,
LLVM has stubbornly refused to optimize external iteration
appropriately, leaving me with a choice between:
- The current implementation, for which Closed Form substitution is
performed, but which uses 2 conditional jumps in the hot loop when
optimization fail.
- An implementation using a "is_done" boolean, which uses 1
conditional jump in the hot loop when optimization fail, allowing
unrolling and vectorization, but for which Closed Form substitution
fails.
In the absence of any conclusive evidence as to which usecase matters
most, and with no assurance that the lack of Closed Form substitution
is not indicative of other optimizations being foiled, there is no way
to pick one implementation over the other, and thus I defer to the
statu quo as far as next and next_back are concerned.
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Update visibility of intermediate use items.
Fixes #57410 and fixes #53925 and fixes #47816.
Currently, the target of a use statement will be updated with
the visibility of the use statement itself (if the use statement was
visible).
This PR ensures that if the path to the target item is via another
use statement then that intermediate use statement will also have the
visibility updated like the target. This silences incorrect
`unreachable_pub` lints with inactionable suggestions.
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Currently, the target of a use statement will be updated with
the visibility of the use statement itself (if the use statement was
visible).
This commit ensures that if the path to the target item is via another
use statement then that intermediate use statement will also have the
visibility updated like the target. This silences incorrect
`unreachable_pub` lints with inactionable suggestions.
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This API is unstable.
CC https://github.com/rust-lang/rust/issues/55977#issuecomment-459657195
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Simplify 'product' factorial example
This simplifies the [`factorial(n: 32)`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#examples-46) implementation as example for the `Iterator::product()` function.
It currently uses unnecessary additional complexity.
Although very minimal, I do not want to include it in some other irrelevant PR.
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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.
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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.
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Add missing link in docs
r? @steveklabnik
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