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Add implementation of Extend for ()
This is useful in some generic code which wants to collect iterators of items into a result.
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Fix incorrect statement about return value for Iterator::zip.
Fixes https://github.com/rust-lang/rust/issues/50225.
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Fixes https://github.com/rust-lang/rust/issues/50225.
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Correct a few stability attributes
* `const_indexing` language feature was stabilized in 1.26.0 by #46882
* `Display` impls for `PanicInfo` and `Location` were stabilized in 1.26.0 by #47687
* `TrustedLen` is still unstable so its impls should be as well even though `RangeInclusive` was stabilized by #47813
* `!Send` and `!Sync` for `Args` and `ArgsOs` were stabilized in 1.26.0 by #48005
* `EscapeDefault` has been stable since 1.0.0 so should continue to show that even though it was moved to core in #48735
This could be backported to beta like #49612
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Fix build error when compiling libcore for 16bit targets
Fixes #49617
cc @SimonSapin
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Stabilize iterator methods in 1.27
- Closes #39480, feature `iter_rfind`
- `DoubleEndedIterator::rfind`
- Closes #44705, feature `iter_rfold`
- `DoubleEndedIterator::rfold`
- Closes #45594, feature `iterator_try_fold`
- `Iterator::try_fold`
- `Iterator::try_for_each`
- `DoubleEndedIterator::try_rfold`
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Add #[must_use] to a few standard library methods
Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
```rust
warning: unused return value of `std::iter::Iterator::collect` which must be used: if you really need to exhaust the iterator, consider `.for_each(drop)` instead
--> $DIR/fn_must_use_stdlib.rs:19:5
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LL | "1 2 3".split_whitespace().collect::<Vec<_>>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::borrow::ToOwned::to_owned` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:21:5
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LL | "hello".to_owned();
| ^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::clone::Clone::clone` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:23:5
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LL | String::from("world").clone();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
cc https://github.com/rust-lang/rust/issues/48926
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Fixes #49617
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Add Iterator::find_map
I'd like to propose to add `find_map` method to the `Iterator`: an occasionally useful utility, which relates to `filter_map` in the same way that `find` relates to `filter`.
`find_map` takes an `Option`-returning function, applies it to the elements of the iterator, and returns the first non-`None` result. In other words, `find_map(f) == filter_map(f).next()`.
Why do we want to add a function to the `Iterator`, which can be trivially expressed as a combination of existing ones? Observe that `find(f) == filter(f).next()`, so, by the same logic, `find` itself is unnecessary!
The more positive argument is that desugaring of `find[_map]` in terms of `filter[_map]().next()` is not super obvious, because the `filter` operation reads as if it is applies to the whole collection, although in reality we are interested only in the first element. That is, the jump from "I need a **single** result" to "let's use a function which maps **many** values to **many** values" is a non-trivial speed-bump, and causes friction when reading and writing code.
Does the need for `find_map` arise in practice? Yes!
* Anecdotally, I've more than once searched the docs for the function with `[T] -> (T -> Option<U>) -> Option<U>` signature.
* The direct cause for this PR was [this](https://github.com/rust-lang/cargo/pull/5187/files/1291c50e86ed4b31db0c76de03a47a5d0074bbd7#r174934173) discussion in Cargo, which boils down to "there's some pattern that we try to express here, but current approaches looks non-pretty" (and the pattern is `filter_map`
* There are several `filter_map().next` combos in Cargo: [[1]](https://github.com/rust-lang/cargo/blob/545a4a2c930916cc9c3dc1716fb7a33299e4062b/src/cargo/ops/cargo_new.rs#L585), [[2]](https://github.com/rust-lang/cargo/blob/545a4a2c930916cc9c3dc1716fb7a33299e4062b/src/cargo/core/resolver/mod.rs#L1130), [[3]](https://github.com/rust-lang/cargo/blob/545a4a2c930916cc9c3dc1716fb7a33299e4062b/src/cargo/ops/cargo_rustc/mod.rs#L1086).
* I've also needed similar functionality in `Kotlin` several times. There, it is expressed as `mapNotNull {}.firstOrNull`, as can be seen [here](https://github.com/intellij-rust/intellij-rust/blob/ee8bdb4e073fd07142fc6e1853ca288c57495e69/src/main/kotlin/org/rust/cargo/project/model/impl/CargoProjectImpl.kt#L154), [here](https://github.com/intellij-rust/intellij-rust/blob/ee8bdb4e073fd07142fc6e1853ca288c57495e69/src/main/kotlin/org/rust/lang/core/resolve/ImplLookup.kt#L444) [here](https://github.com/intellij-rust/intellij-rust/blob/ee8bdb4e073fd07142fc6e1853ca288c57495e69/src/main/kotlin/org/rust/ide/inspections/RsLint.kt#L38) and [here](https://github.com/intellij-rust/intellij-rust/blob/ee8bdb4e073fd07142fc6e1853ca288c57495e69/src/main/kotlin/org/rust/cargo/toolchain/RustToolchain.kt#L74) (and maybe in some other cases as well)
Note that it is definitely not among the most popular functions (it definitely is less popular than `find`), but, for example it (in case of Cargo) seems to be more popular than `rposition` (1 occurrence), `step_by` (zero occurrences) and `nth` (three occurrences as `nth(0)` which probably should be replaced with `next`).
Do we necessary need this function in `std`? Could we move it to itertools? That is possible, but observe that `filter`, `filter_map`, `find` and `find_map` together really form a complete table:
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|-------|---------|
| filter| find|
|filter_map|find_map|
It would be somewhat unsatisfying to have one quarter of this table live elsewhere :) Also, if `Itertools` adds an `find_map` method, it would be more difficult to move it to std due to name collision.
Hm, at this point I've searched for `filter_map` the umpteenth time, and, strangely, this time I do find this RFC: https://github.com/rust-lang/rfcs/issues/1801. I guess this could be an implementation though? :)
To sum up:
Pro:
- complete the symmetry with existing method
- codify a somewhat common non-obvious pattern
Contra:
- niche use case
- we can, and do, live without it
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Implement some trivial size_hints for various iterators
This also implements ExactSizeIterator where applicable.
Addresses most of the Iterator traits mentioned in #23708.
I intend to do more, but I don't want to make the PR too large.
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Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
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portability lint
https://github.com/rust-lang/rust/pull/49305#issuecomment-376293243
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The comment "the value passed on to the next iteration" confused me since it sounded more like what Haskell's [scanl](http://hackage.haskell.org/package/base-4.11.0.0/docs/Prelude.html#v:scanl) does where the closure's return value serves as both the "yielded value" *and* the new value of the "state".
I tried changing the example to make it clear that the closure's return value is decoupled from the state argument.
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This also implements ExactSizeIterator where applicable.
Addresses most of the Iterator traits mentioned in #23708.
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lifetimes)
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Rollup of 17 pull requests
- Successful merges: #48706, #48875, #48892, #48922, #48957, #48959, #48961, #48965, #49007, #49024, #49042, #49050, #48853, #48990, #49037, #49049, #48972
- Failed merges:
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Stabilize std::ops::RangeInclusive and std::ops::RangeInclusiveTo.
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Stabilize FusedIterator
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.
Closes #35602
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doc: no need for the reference
Also, we are well within line length limit
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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.
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Makes the bench asked about on URLO 58x faster :)
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Also:
- apply some rustfmt love
- fix output of one example
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Add Iterator::flatten
This adds the trait method `.flatten()` on `Iterator` which flattens one level of nesting from an iterator or (into)iterators. The method `.flat_fmap(f)` is then redefined as `.map(f).flatten()`. The implementation of `Flatten` is essentially that of what it was for `FlatMap` but removing the call to `f` at various places.
Hopefully the type alias approach should be OK as was indicated / alluded to by @bluss and @eddyb in https://github.com/rust-lang/rfcs/pull/2306#issuecomment-361391370.
cc @scottmcm
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Add Iterator::try_for_each
The fallible version of `for_each` aka the stateless version of `try_fold`. Inspired by @cuviper's comment in https://github.com/rust-lang/rust/pull/45379#issuecomment-338370020 as a more direct and obvious solution than `.map(f).collect::<Result<(), _>>()`.
Like `for_each`, no need for an `r` version thanks to overrides in `Rev`.
`iterator_try_fold` tracking issue: https://github.com/rust-lang/rust/issues/45594
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per @clarcharr's review
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#37653 support `default impl` for specialization
this commit implements the second part of the `default impl` feature:
> - a `default impl` need not include all items from the trait
> - a `default impl` alone does not mean that a type implements the trait
The first point allows rustc to compile and run something like this:
```
trait Foo {
fn foo_one(&self) -> &'static str;
fn foo_two(&self) -> &'static str;
}
default impl<T> Foo for T {
fn foo_one(&self) -> &'static str {
"generic"
}
}
struct MyStruct;
fn main() {
assert!(MyStruct.foo_one() == "generic");
}
```
but it shows a proper error if trying to call `MyStruct.foo_two()`
The second point allows a `default impl` to be considered as not implementing the `Trait` if it doesn't implement all the trait items.
The tests provided (in the compile-fail section) should cover all the possible trait resolutions.
Let me know if some tests is missed.
See [referenced ](https://github.com/rust-lang/rust/issues/37653) issue for further info
r? @nikomatsakis
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Add std/core::iter::repeat_with
Adds an iterator primitive `repeat_with` which is the "lazy" version of `repeat` but also more flexible since you can build up state with the `FnMut`. The design is mostly taken from `repeat`.
r? @rust-lang/libs
cc @withoutboats, @scottmcm
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Add Range[Inclusive]::is_empty
During https://github.com/rust-lang/rfcs/pull/1980, it was discussed that figuring out whether a range is empty was subtle, and thus there should be a clear and obvious way to do it. It can't just be ExactSizeIterator::is_empty (also unstable) because not all ranges are ExactSize -- such as `Range<i64>` and `RangeInclusive<usize>`.
Things to ponder:
- Unless this is stabilized first, this makes stabilizing ExactSizeIterator::is_empty more icky, since this hides that.
- This is only on `Range` and `RangeInclusive`, as those are the only ones where it's interesting. But one could argue that it should be on more for consistency, or on RangeArgument instead.
- The bound on this is PartialOrd, since that works ok (see tests for float examples) and is consistent with `contains`. But ranges like `NAN..=NAN`_are_ kinda weird.
- [x] ~~There's not a real issue number on this yet~~
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