Replace sort implementations
This PR replaces the sort implementations with tailor-made ones that strike a balance of run-time, compile-time and binary-size, yielding run-time and compile-time improvements. Regressing binary-size for `slice::sort` while improving it for `slice::sort_unstable`. All while upholding the existing soft and hard safety guarantees, and even extending the soft guarantees, detecting strict weak ordering violations with a high chance and reporting it to users via a panic.
* `slice::sort` -> driftsort [design document](https://github.com/Voultapher/sort-research-rs/blob/main/writeup/driftsort_introduction/text.md), includes detailed benchmarks and analysis.
* `slice::sort_unstable` -> ipnsort [design document](https://github.com/Voultapher/sort-research-rs/blob/main/writeup/ipnsort_introduction/text.md), includes detailed benchmarks and analysis.
#### Why should we change the sort implementations?
In the [2023 Rust survey](https://blog.rust-lang.org/2024/02/19/2023-Rust-Annual-Survey-2023-results.html#challenges), one of the questions was: "In your opinion, how should work on the following aspects of Rust be prioritized?". The second place was "Runtime performance" and the third one "Compile Times". This PR aims to improve both.
#### Why is this one big PR and not multiple?
* The current documentation gives performance recommendations for `slice::sort` and `slice::sort_unstable`. If for example only one of them were to be changed, this advice would be misleading for some Rust versions. By replacing them atomically, the advice remains largely unchanged, and users don't have to change their code.
* driftsort and ipnsort share a substantial part of their implementations.
* The implementation of `select_nth_unstable` uses internals of `slice::sort_unstable`, which makes it impractical to split changes.
---
This PR is a collaboration with `@orlp.`
Clean up some comments near `use` declarations
#125443 will reformat all `use` declarations in the repository. There are a few edge cases involving comments on `use` declarations that require care. This PR cleans up some clumsy comment cases, taking us a step closer to #125443 being able to merge.
r? ``@lqd``
Most modules have such a blank line, but some don't. Inserting the blank
line makes it clearer that the `//!` comments are describing the entire
module, rather than the `use` declaration(s) that immediately follows.
The addition of `core::iter::zip` (#82917) set a precedent for adding
plain functions for iterator adaptors. Adding `chain` makes it a little
easier to `chain` two iterators.
```
for (x, y) in chain(xs, ys) {}
// vs.
for (x, y) in xs.into_iter().chain(ys) {}
```
Change f32::midpoint to upcast to f64
This has been verified by kani as a correct optimization
see: https://github.com/rust-lang/rust/issues/110840#issuecomment-1942587398
The new implementation is branchless and only differs in which NaN values are produced (if any are produced at all), which is fine to change. Aside from NaN handling, this implementation produces bitwise identical results to the original implementation.
Question: do we need a codegen test for this? I didn't add one, since the original PR #92048 didn't have any codegen tests.
This has been verified by kani as a correct optimization
see: https://github.com/rust-lang/rust/issues/110840#issuecomment-1942587398
The new implementation is branchless, and only differs in which NaN
values are produced (if any are produced at all). Which is fine to change.
Aside from NaN handling, this implementation produces bitwise identical
results to the original implementation.
The new implementation is gated on targets that have a fast 64-bit
floating point implementation in hardware, and on WASM.
Expand `for_loops_over_fallibles` lint to lint on fallibles behind references.
Extends the scope of the (warn-by-default) lint `for_loops_over_fallibles` from just `for _ in x` where `x: Option<_>/Result<_, _>` to also cover `x: &(mut) Option<_>/Result<_>`
```rs
fn main() {
// Current lints
for _ in Some(42) {}
for _ in Ok::<_, i32>(42) {}
// New lints
for _ in &Some(42) {}
for _ in &mut Some(42) {}
for _ in &Ok::<_, i32>(42) {}
for _ in &mut Ok::<_, i32>(42) {}
// Should not lint
for _ in Some(42).into_iter() {}
for _ in Some(42).iter() {}
for _ in Some(42).iter_mut() {}
for _ in Ok::<_, i32>(42).into_iter() {}
for _ in Ok::<_, i32>(42).iter() {}
for _ in Ok::<_, i32>(42).iter_mut() {}
}
```
<details><summary><code>cargo build</code> diff</summary>
```diff
diff --git a/old.out b/new.out
index 84215aa..ca195a7 100644
--- a/old.out
+++ b/new.out
`@@` -1,33 +1,93 `@@`
warning: for loop over an `Option`. This is more readably written as an `if let` statement
--> src/main.rs:3:14
|
3 | for _ in Some(42) {}
| ^^^^^^^^
|
= note: `#[warn(for_loops_over_fallibles)]` on by default
help: to check pattern in a loop use `while let`
|
3 | while let Some(_) = Some(42) {}
| ~~~~~~~~~~~~~~~ ~~~
help: consider using `if let` to clear intent
|
3 | if let Some(_) = Some(42) {}
| ~~~~~~~~~~~~ ~~~
warning: for loop over a `Result`. This is more readably written as an `if let` statement
--> src/main.rs:4:14
|
4 | for _ in Ok::<_, i32>(42) {}
| ^^^^^^^^^^^^^^^^
|
help: to check pattern in a loop use `while let`
|
4 | while let Ok(_) = Ok::<_, i32>(42) {}
| ~~~~~~~~~~~~~ ~~~
help: consider using `if let` to clear intent
|
4 | if let Ok(_) = Ok::<_, i32>(42) {}
| ~~~~~~~~~~ ~~~
-warning: `for-loops-over-fallibles` (bin "for-loops-over-fallibles") generated 2 warnings
- Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.04s
+warning: for loop over a `&Option`. This is more readably written as an `if let` statement
+ --> src/main.rs:7:14
+ |
+7 | for _ in &Some(42) {}
+ | ^^^^^^^^^
+ |
+help: to check pattern in a loop use `while let`
+ |
+7 | while let Some(_) = &Some(42) {}
+ | ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+ |
+7 | if let Some(_) = &Some(42) {}
+ | ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `&mut Option`. This is more readably written as an `if let` statement
+ --> src/main.rs:8:14
+ |
+8 | for _ in &mut Some(42) {}
+ | ^^^^^^^^^^^^^
+ |
+help: to check pattern in a loop use `while let`
+ |
+8 | while let Some(_) = &mut Some(42) {}
+ | ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+ |
+8 | if let Some(_) = &mut Some(42) {}
+ | ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `&Result`. This is more readably written as an `if let` statement
+ --> src/main.rs:9:14
+ |
+9 | for _ in &Ok::<_, i32>(42) {}
+ | ^^^^^^^^^^^^^^^^^
+ |
+help: to check pattern in a loop use `while let`
+ |
+9 | while let Ok(_) = &Ok::<_, i32>(42) {}
+ | ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+ |
+9 | if let Ok(_) = &Ok::<_, i32>(42) {}
+ | ~~~~~~~~~~ ~~~
+
+warning: for loop over a `&mut Result`. This is more readably written as an `if let` statement
+ --> src/main.rs:10:14
+ |
+10 | for _ in &mut Ok::<_, i32>(42) {}
+ | ^^^^^^^^^^^^^^^^^^^^^
+ |
+help: to check pattern in a loop use `while let`
+ |
+10 | while let Ok(_) = &mut Ok::<_, i32>(42) {}
+ | ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+ |
+10 | if let Ok(_) = &mut Ok::<_, i32>(42) {}
+ | ~~~~~~~~~~ ~~~
+
+warning: `for-loops-over-fallibles` (bin "for-loops-over-fallibles") generated 6 warnings
+ Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s
```
</details>
-----
Question:
* ~~Currently, the article `an` is used for `&Option`, and `&mut Option` in the lint diagnostic, since that's what `Option` uses. Is this okay or should it be changed? (likewise, `a` is used for `&Result` and `&mut Result`)~~ The article `a` is used for `&Option`, `&mut Option`, `&Result`, `&mut Result` and (as before) `Result`. Only `Option` uses `an` (as before).
`@rustbot` label +A-lint
- `slice::sort` -> driftsort
https://github.com/Voultapher/sort-research-rs/blob/main/writeup/driftsort_introduction/text.md
- `slice::sort_unstable` -> ipnsort
https://github.com/Voultapher/sort-research-rs/blob/main/writeup/ipnsort_introduction/text.md
Replaces the sort implementations with tailor made ones that strike a
balance of run-time, compile-time and binary-size, yielding run-time and
compile-time improvements. Regressing binary-size for `slice::sort`
while improving it for `slice::sort_unstable`. All while upholding the
existing soft and hard safety guarantees, and even extending the soft
guarantees, detecting strict weak ordering violations with a high chance
and reporting it to users via a panic.
In addition the implementation of `select_nth_unstable` is also adapted
as it uses `slice::sort_unstable` internals.
Add `Ord::cmp` for primitives as a `BinOp` in MIR
Update: most of this OP was written months ago. See https://github.com/rust-lang/rust/pull/118310#issuecomment-2016940014 below for where we got to recently that made it ready for review.
---
There are dozens of reasonable ways to implement `Ord::cmp` for integers using comparison, bit-ops, and branches. Those differences are irrelevant at the rust level, however, so we can make things better by adding `BinOp::Cmp` at the MIR level:
1. Exactly how to implement it is left up to the backends, so LLVM can use whatever pattern its optimizer best recognizes and cranelift can use whichever pattern codegens the fastest.
2. By not inlining those details for every use of `cmp`, we drastically reduce the amount of MIR generated for `derive`d `PartialOrd`, while also making it more amenable to MIR-level optimizations.
Having extremely careful `if` ordering to μoptimize resource usage on broadwell (#63767) is great, but it really feels to me like libcore is the wrong place to put that logic. Similarly, using subtraction [tricks](https://graphics.stanford.edu/~seander/bithacks.html#CopyIntegerSign) (#105840) is arguably even nicer, but depends on the optimizer understanding it (https://github.com/llvm/llvm-project/issues/73417) to be practical. Or maybe [bitor is better than add](https://discourse.llvm.org/t/representing-in-ir/67369/2?u=scottmcm)? But maybe only on a future version that [has `or disjoint` support](https://discourse.llvm.org/t/rfc-add-or-disjoint-flag/75036?u=scottmcm)? And just because one of those forms happens to be good for LLVM, there's no guarantee that it'd be the same form that GCC or Cranelift would rather see -- especially given their very different optimizers. Not to mention that if LLVM gets a spaceship intrinsic -- [which it should](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Suboptimal.20inlining.20in.20std.20function.20.60binary_search.60/near/404250586) -- we'll need at least a rustc intrinsic to be able to call it.
As for simplifying it in Rust, we now regularly inline `{integer}::partial_cmp`, but it's quite a large amount of IR. The best way to see that is with 8811efa88b (diff-d134c32d028fbe2bf835fef2df9aca9d13332dd82284ff21ee7ebf717bfa4765R113) -- I added a new pre-codegen MIR test for a simple 3-tuple struct, and this PR change it from 36 locals and 26 basic blocks down to 24 locals and 8 basic blocks. Even better, as soon as the construct-`Some`-then-match-it-in-same-BB noise is cleaned up, this'll expose the `Cmp == 0` branches clearly in MIR, so that an InstCombine (#105808) can simplify that to just a `BinOp::Eq` and thus fix some of our generated code perf issues. (Tracking that through today's `if a < b { Less } else if a == b { Equal } else { Greater }` would be *much* harder.)
---
r? `@ghost`
But first I should check that perf is ok with this
~~...and my true nemesis, tidy.~~
Make `io::BorrowedCursor::advance` safe
This also keeps the old `advance` method under `advance_unchecked` name.
This makes pattern like `std::io::default_read_buf` safe to write.
Rename MaybeUninit::write_slice
A step to push #79995 forward.
https://github.com/rust-lang/libs-team/issues/122 also suggested to make them inherent methods, but they can't be — they'd conflict with slice's regular methods.
core: add Duration constructors
Add more `Duration` constructors.
Tracking issue: #120301.
These match similar convenience constructors available on both `chrono::Duration` and `time::Duration`.
What's the best ordering for these with respect to the existing constructors?
stabilise array methods
Closes#76118
Stabilises the remaining array methods
FCP is yet to be carried out for this
There wasn't a clear consensus on the naming, but all the other alternatives had some flaws as discussed in the tracking issue and there was a silence on this issue for a year
Implement iterator specialization traits on more adapters
This adds
* `TrustedLen` to `Skip` and `StepBy`
* `TrustedRandomAccess` to `Skip`
* `InPlaceIterable` and `SourceIter` to `Copied` and `Cloned`
The first two might improve performance in the compiler itself since `skip` is used in several places. Constellations that would exercise the last point are probably rare since it would require an owning iterator that has references as Items somewhere in its iterator pipeline.
Improvements for `Skip`:
```
# old
test iter::bench_skip_trusted_random_access ... bench: 8,335 ns/iter (+/- 90)
# new
test iter::bench_skip_trusted_random_access ... bench: 2,753 ns/iter (+/- 27)
```
Add Ipv6Addr::is_ipv4_mapped
This change consists of cherry-picking the content from the original PR[1], which got closed due to inactivity, and applying the following changes:
* Resolving merge conflicts (obviously)
* Linked to to_ipv4_mapped instead of to_ipv4 in the documentation (seems more appropriate)
* Added the must_use and rustc_const_unstable attributes the original didn't have
I think it's a reasonably useful method to have.
[1] https://github.com/rust-lang/rust/pull/86490
Use `bool` instead of `PartiolOrd` as return value of the comparison closure in `{slice,Iteraotr}::is_sorted_by`
Changes the function signature of the closure given to `{slice,Iteraotr}::is_sorted_by` to return a `bool` instead of a `PartiolOrd` as suggested by the libs-api team here: https://github.com/rust-lang/rust/issues/53485#issuecomment-1766411980.
This means these functions now return true if the closure returns true for all the pairs of values.
Stabilize single-field offset_of
This PR stabilizes offset_of for a single field. There has been some further discussion at https://github.com/rust-lang/rust/issues/106655 about whether this is advisable; I'm opening the PR anyway so that the code is available.
Change return type of unstable `Waker::noop()` from `Waker` to `&Waker`.
The advantage of this is that it does not need to be assigned to a variable to be used in a `Context` creation, which is the most common thing to want to do with a noop waker. It also avoids unnecessarily executing the dynamically dispatched drop function when the noop waker is dropped.
If an owned noop waker is desired, it can be created by cloning, but the reverse is harder to do since it requires declaring a constant. Alternatively, both versions could be provided, like `futures::task::noop_waker()` and `futures::task::noop_waker_ref()`, but that seems to me to be API clutter for a very small benefit, whereas having the `&'static` reference available is a large reduction in boilerplate.
[Previous discussion on the tracking issue starting here](https://github.com/rust-lang/rust/issues/98286#issuecomment-1862159766)
Stabilize `slice_first_last_chunk`
This PR does a few different things based around stabilizing `slice_first_last_chunk`. They are split up so this PR can be by-commit reviewed, I can move parts to a separate PR if desired.
This feature provides a very elegant API to extract arrays from either end of a slice, such as for parsing integers from binary data.
## Stabilize `slice_first_last_chunk`
ACP: https://github.com/rust-lang/libs-team/issues/69
Implementation: https://github.com/rust-lang/rust/issues/90091
Tracking issue: https://github.com/rust-lang/rust/issues/111774
This stabilizes the functionality from https://github.com/rust-lang/rust/issues/111774:
```rust
impl [T] {
pub const fn first_chunk<const N: usize>(&self) -> Option<&[T; N]>;
pub fn first_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>;
pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]>;
pub fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>;
pub const fn split_first_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>;
pub fn split_first_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>;
pub const fn split_last_chunk<const N: usize>(&self) -> Option<(&[T], &[T; N])>;
pub fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T], &mut [T; N])>;
}
```
Const stabilization is included for all non-mut methods, which are blocked on `const_mut_refs`. This change includes marking the trivial function `slice_split_at_unchecked` const-stable for internal use (but not fully stable).
## Remove `split_array` slice methods
Tracking issue: https://github.com/rust-lang/rust/issues/90091
Implementation: https://github.com/rust-lang/rust/pull/83233#pullrequestreview-780315524
This PR also removes the following unstable methods from the `split_array` feature, https://github.com/rust-lang/rust/issues/90091:
```rust
impl<T> [T] {
pub fn split_array_ref<const N: usize>(&self) -> (&[T; N], &[T]);
pub fn split_array_mut<const N: usize>(&mut self) -> (&mut [T; N], &mut [T]);
pub fn rsplit_array_ref<const N: usize>(&self) -> (&[T], &[T; N]);
pub fn rsplit_array_mut<const N: usize>(&mut self) -> (&mut [T], &mut [T; N]);
}
```
This is done because discussion at #90091 and its implementation PR indicate a strong preference for nonpanicking APIs that return `Option`. The only difference between functions under the `split_array` and `slice_first_last_chunk` features is `Option` vs. panic, so remove the duplicates as part of this stabilization.
This does not affect the array methods from `split_array`. We will want to revisit these once `generic_const_exprs` is further along.
## Reverse order of return tuple for `split_last_chunk{,_mut}`
An unresolved question for #111774 is whether to return `(preceding_slice, last_chunk)` (`(&[T], &[T; N])`) or the reverse (`(&[T; N], &[T])`), from `split_last_chunk` and `split_last_chunk_mut`. It is currently implemented as `(last_chunk, preceding_slice)` which matches `split_last -> (&T, &[T])`. The first commit changes these to `(&[T], &[T; N])` for these reasons:
- More consistent with other splitting methods that return multiple values: `str::rsplit_once`, `slice::split_at{,_mut}`, `slice::align_to` all return tuples with the items in order
- More intuitive (arguably opinion, but it is consistent with other language elements like pattern matching `let [a, b, rest @ ..] ...`
- If we ever added a varidic way to obtain multiple chunks, it would likely return something in order: `.split_many_last::<(2, 4)>() -> (&[T], &[T; 2], &[T; 4])`
- It is the ordering used in the `rsplit_array` methods
I think the inconsistency with `split_last` could be acceptable in this case, since for `split_last` the scalar `&T` doesn't have any internal order to maintain with the other items.
## Unresolved questions
Do we want to reserve the same names on `[u8; N]` to avoid inference confusion? https://github.com/rust-lang/rust/pull/117561#issuecomment-1793388647
---
`slice_first_last_chunk` has only been around since early 2023, but `split_array` has been around since 2021.
`@rustbot` label -T-libs +T-libs-api -T-libs +needs-fcp
cc `@rust-lang/wg-const-eval,` `@scottmcm` who raised this topic, `@clarfonthey` implementer of `slice_first_last_chunk` `@jethrogb` implementer of `split_array`
Zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Stabilizing.20array-from-slice.20*something*.3FFixes: #111774
This stabilizes all methods under `slice_first_last_chunk`.
Additionally, it const stabilizes the non-mut functions and moves the `_mut`
functions under `const_slice_first_last_chunk`. These are blocked on
`const_mut_refs`.
As part of this change, `slice_split_at_unchecked` was marked const-stable for
internal use (but not fully stable).
Add `IntoAsyncIterator`
This introduces the `IntoAsyncIterator` trait and uses it in the desugaring of the unstable `for await` loop syntax. This is mostly added for symmetry with `Iterator` and `IntoIterator`.
r? `@compiler-errors`
cc `@rust-lang/libs-api,` `@rust-lang/wg-async`
This change consists of cherry-picking the content from the original
PR[1], which got closed due to inactivity, and applying the following
changes:
* Resolving merge conflicts (obviously)
* Linked to to_ipv4_mapped instead of to_ipv4 in the documentation (seems
more appropriate)
* Added the must_use and rustc_const_unstable attributes the original
didn't have
I think it's a reasonably useful method.
[1] https://github.com/rust-lang/rust/pull/86490
detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
Add support for making lib features internal
We have the notion of an "internal" lang feature: a feature that is never intended to be stabilized, and using which can cause ICEs and other issues without that being considered a bug.
This extends that idea to lib features as well. It is an alternative to https://github.com/rust-lang/rust/pull/115623: instead of using an attribute to declare lib features internal, we simply do this based on the name. Everything ending in `_internals` or `_internal` is considered internal.
Then we rename `core_intrinsics` to `core_intrinsics_internal`, which fixes https://github.com/rust-lang/rust/issues/115597.
The functionality of these methods from `split_array` has been absorbed by the
`slice_first_last_chunk` feature. This only affects the methods on slices,
not those with the same name that are implemented on array types.
Also adjusts testing to reflect this change.
Remove option_payload_ptr; redundant to offset_of
The `option_payload_ptr` intrinsic is no longer required as `offset_of` supports traversing enums (#114208). This PR removes it in order to dogfood offset_of (as suggested at https://github.com/rust-lang/rust/issues/106655#issuecomment-1790907626). However, it will not build until those changes reach beta (which I think is within the next 8 days?) so I've opened it as a draft.
Add `std:#️⃣:{DefaultHasher, RandomState}` exports (needs FCP)
This implements rust-lang/libs-team#267 to move the libstd hasher types to `std::hash` where they belong, instead of `std::collections::hash_map`.
<details><summary>The below no longer applies, but is kept for clarity.</summary>
This is a small refactor for #27242, which moves the definitions of `RandomState` and `DefaultHasher` into `std::hash`, but in a way that won't be noticed in the public API.
I've opened rust-lang/libs-team#267 as a formal ACP to move these directly into the root of `std::hash`, but for now, they're at least separated out from the collections code in a way that will make moving that around easier.
I decided to simply copy the rustdoc for `std::hash` from `core::hash` since I think it would be ideal for the two to diverge longer-term, especially if the ACP is accepted. However, I would be willing to factor them out into a common markdown document if that's preferred.
</details>
