This commit consolidates all changes, including the core logic fix for IntoIter::nth_back and the addition of the Miri regression test in `library/alloctests/tests/vec.rs`, to prevent Undefined Behavior (UB) when dealing with highly-aligned Zero-Sized Types.
In `BTreeMap::eq`, do not compare the elements if the sizes are different.
Reverts rust-lang/rust#147101 in library/alloc/src/btree/
rust-lang/rust#147101 replaced some instances of code like `a.len() == b.len() && a.iter().eq(&b)` with just `a.iter().eq(&b)`, but the optimization that PR introduced only applies for `TrustedLen` iterators, and `BTreeMap`'s itertors are not `TrustedLen`, so this theoretically regressed perf for comparing large `BTreeMap`/`BTreeSet`s with unequal lengths but equal prefixes, (and also made it so that comparing two different-length `BTreeMap`/`BTreeSet`s with elements whose `PartialEq` impls that can panic now can panic, though this is not a "promised" behaviour either way (cc rust-lang/rust#149122))
Given that `TrustedLen` is an unsafe trait, I opted to not implement it for `BTreeMap`'s iterators, and instead just revert the change. If someone else wants to audit `BTreeMap`'s iterators to make sure they always return the right number of items (even in the face of incorrect user `Ord` impls) and then implement `TrustedLen` for them so that the optimization works for them, then this can be closed in favor of that (or if the perf regression is deemed too theoretical, this can be closed outright).
Example of theoretical perf regression: https://play.rust-lang.org/?version=beta&mode=release&edition=2024&gist=a37e3d61e6bf02669b251315c9a44fe2 (very rough estimates, using `Instant::elapsed`).
In release mode on stable the comparison takes ~23.68µs.
In release mode on beta/nightly the comparison takes ~48.351057ms.
alloc: fix `Debug` implementation of `ExtractIf`
I noticed this while reviewing rust-lang/rust#141032. Calling `get` on the inner `Vec` never returns `Some` as the `Vec`'s length is temporarily set to zero while the `ExtractIf` exists.
Turn `Cow::is_borrowed,is_owned` into associated functions.
This is done because `Cow` implements `Deref`. Therefore, to avoid conflicts with an inner type having a method of the same name, we use an associated method, like `Box::into_raw`.
Tracking issue: #65143
Constify conversion traits (part 1)
This is the first part of rust-lang/rust#144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature:
* `From`
* `Into`
* `TryFrom`
* `TryInto`
* `FromStr`
* `AsRef`
* `AsMut`
* `Borrow`
* `BorrowMut`
* `Deref`
* `DerefMut`
There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`:
* `ByteStr::new` (unstable under `bstr` feature)
* `OsStr::new`
* `Path::new`
Those which directly use `Into`:
* `Result::into_ok`
* `Result::into_err`
And those which use `Deref` and `DerefMut`:
* `Pin::as_ref`
* `Pin::as_mut`
* `Pin::as_deref_mut`
* `Option::as_deref`
* `Option::as_deref_mut`
* `Result::as_deref`
* `Result::as_deref_mut`
(note: the `Option` and `Result` methods were suggested by ``@npmccallum`` initially as rust-lang/rust#146101)
The parts which are missing from this PR are:
* Anything that involves heap-allocated types
* Making any method const than the ones listed above
* Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO)
r? ``@tgross35`` (who mostly already reviewed this)
This is done because `Cow` implements `Deref`. Therefore, to
avoid conflicts with an inner type having a method of the same
name, we use an associated method, like `Box::into_raw`.
Use a distinct `ToString` implementation for `u128` and `i128`
Part of https://github.com/rust-lang/rust/issues/135543.
Follow-up of rust-lang/rust#136264.
When working on https://github.com/rust-lang/rust/pull/142098, I realized that `i128` and `u128` could also benefit from a distinct `ToString` implementation so here it.
The last commit is just me realizing that I forgot to add the format tests for `usize` and `isize`.
Here is the bench comparison:
| bench name | last nightly | with this PR | diff |
|-|-|-|-|
| bench_i128 | 29.25 ns/iter (+/- 0.66) | 17.52 ns/iter (+/- 0.7) | -40.1% |
| bench_u128 | 34.06 ns/iter (+/- 0.21) | 16.1 ns/iter (+/- 0.6) | -52.7% |
I used this code to test:
```rust
#![feature(test)]
extern crate test;
use test::{Bencher, black_box};
#[inline(always)]
fn convert_to_string<T: ToString>(n: T) -> String {
n.to_string()
}
macro_rules! decl_benches {
($($name:ident: $ty:ident,)+) => {
$(
#[bench]
fn $name(c: &mut Bencher) {
c.iter(|| convert_to_string(black_box({ let nb: $ty = 20; nb })));
}
)+
}
}
decl_benches! {
bench_u128: u128,
bench_i128: i128,
}
```
Chris Denton