Add PeekMut::refresh
I'm not sure if this should go through ACP or not. BinaryHeap is not the most critical data structure in the standard library and it would be understandable if maintainer throughput is thus too limited to accept this PR without a proper design phase that ensures the required understanding of consequence over a longer time period.
This aims to improve the useability of heaps for priority-based work queues. In certain scenarios, modifications on the most relevant or critical items are performed until a condition that determines the work items have been sufficiently addressed. For instance the criticality could be a deadline that is relaxed whenever some part of a work item is completed. Such a loop will repeatedly access the most critical item and put it back in a sorted position when it is complete. Crucially, due to the ordering invariant we know that all necessary work was performed when the completed item remains the most critical. Getting this information from the heap position avoids a (potentially more costly) check on the item state itself.
A customized `drop` with boolean result would avoid up to two more comparisons performed in both the last no-op refresh and Drop code but this occurs once in each execution of the above scenario whereas refresh occurs any number of times. Also note that the comparison overhead of Drop is only taken if the element is mutably inspected to determine the end condition, i.e. not when refresh itself is the break condition.
This improves the useability of heaps for priority-based work queues. In
certain scenarios, modifications on the most relevant or critical items are
performed until a condition that determines the work items have been
sufficiently addressed. The loop will repeatedly access the most critical
item and put it back in a sorted position when it is complete. Crucially,
due to the ordering invariant we know that all work was performed when the
completed item remains the most critical. Getting this information from the
heap position avoids a (potentially more costly) check on the item state
itself.
A customized `drop` with boolean result would avoid up to two more
comparisons performed in both the last no-op refresh and Drop code but this
occurs once in each execution of the above scenario whereas refresh occurs
any number of times. Also note that the comparison overhead of Drop is only
taken if the element is mutably inspected to determine the end condition,
i.e. not when refresh itself is the break condition.
Put the alloc unit tests in a separate alloctests package
Same rationale as https://github.com/rust-lang/rust/pull/135937. This PR has some extra complexity though as a decent amount of tests are testing internal implementation details rather than the public api. As such I opted to include the modules containing the types under test using `#[path]` into the alloctests package. This means that those modules still need `#[cfg(test)]`, but the rest of liballoc no longer need it.
library: Use `size_of` from the prelude instead of imported
Use `std::mem::{size_of, size_of_val, align_of, align_of_val}` from the prelude instead of importing or qualifying them.
These functions were added to all preludes in Rust 1.80.
try-job: test-various
try-job: x86_64-gnu
try-job: x86_64-msvc-1
Use `std::mem::{size_of, size_of_val, align_of, align_of_val}` from the
prelude instead of importing or qualifying them.
These functions were added to all preludes in Rust 1.80.
More precisely document `Global::deallocate()`'s safety.
There is a subtlety which "other conditions must be upheld by the caller" does not capture: `GlobalAlloc`/`alloc::dealloc()` require that the provided layout will be *equal*, not just that it "fits", the layout used to allocate. This is always true here due to how `allocate()`, `grow()`, and `shrink()` are implemented (they never return a larger allocation than requested), but that is a non-local property of the implementation, so it should be documented explicitly.
r? libs
`@rustbot` label A-allocators
There is a subtlety which "other conditions must be upheld by the caller"
does not capture: `GlobalAlloc`/`alloc::dealloc()` require that the
provided layout will be *equal*, not just that it "fits", the layout
used to allocate. This is always true here due to how `allocate()`,
`grow()`, and `shrink()` are implemented (they never return a larger
allocation than requested), but that is a non-local property of the
implementation, so it should be documented explicitly.
Don't doc-comment BTreeMap<K, SetValZST, A>
This otherwise shows up in documentation as an empty impl block (worse, at the *top* of the docs above the public impls).
Update `String::from_raw_parts` safety requirements
These have become out of sync with `Vec::from_raw_part`'s safety requirements, and are likely to diverge again. I think it's safest to just point at `Vec`'s requirements.
https://github.com/rust-lang/rust/issues/119206#issuecomment-2180116680
Reduce `Box::default` stack copies in debug mode
The `Box::new(T::default())` implementation of `Box::default` only
had two stack copies in debug mode, compared to the current version,
which has four. By avoiding creating any `MaybeUninit<T>`'s and just writing
`T` directly to the `Box` pointer, the stack usage in debug mode remains
the same as the old version.
Another option would be to mark `Box::write` as `#[inline(always)]`,
and change it's implementation to to avoid calling `MaybeUninit::write`
(which creates a `MaybeUninit<T>` on the stack) and to use `ptr::write` instead.
Fixes: #136043
Impl TryFrom<Vec<u8>> for String
I think this is useful enough to have :)
As a general question, is there any policy around adding "missing" trait implementations? (like adding `AsRef<T> for T` for std types), I mostly stumble upon them when using a lot of "impl Trait in argument position" like (`foo: impl Into<String>`)
Add `MAX_LEN_UTF8` and `MAX_LEN_UTF16` Constants
This pull request adds the `MAX_LEN_UTF8` and `MAX_LEN_UTF16` constants as per #45795, gated behind the `char_max_len` feature.
The constants are currently applied in the `alloc`, `core` and `std` libraries.
Restrict DerefPure for Cow<T> impl to T = impl Clone, [impl Clone], str.
Fixes#136046
`feature(deref_patterns)` tracking issue: https://github.com/rust-lang/rust/issues/87121
`Cow<'_, T>` should only implement `DerefPure` if its `Deref` impl is pure, which requires `<T::Owned as Borrow<T>>::borrow` to be pure. This PR restricts `impl DerefPure for Cow<'_, T>` to `T: Sized + Clone`, `T = [U: Clone]`, and `T = str` (for all of whom `<T::Owned as Borrow<T>>::borrow` is implemented in the stdlib and is pure).
cc ``@Nadrieril``
------
An alternate approach would be to introduce a new `unsafe trait BorrowPure<T>` analogous to `DerefPure` that could be implemented for `T: Sized`, `&T`, `&mut T`, `String`, `Vec`, `Box`, `PathBuf`, `OsString`, etc. https://github.com/rust-lang/rust/compare/master...zachs18:borrow-pure-trait
Implement Extend<AsciiChar> for String
Implement `Extend<AsciiChar>` for `String` as suggested in https://github.com/rust-lang/rust/issues/110998#issuecomment-2590122968. Also implements `Extend<&AsciiChar>` since there's an analogous impl for `Extend<&char>`, but happy to remove if not thought useful.
r? `@scottmcm`
since you requested it, but no pressure to review!
Prepare standard library for Rust 2024 migration
This includes a variety of commits preparing the standard library for migration to Rust 2024.
The actual migration is blocked on a few things, so I wanted to get this out of the way in a relatively digestable PR.
alloc boxed: docs: use MaybeUninit::write instead of as_mut_ptr
In the deferred initialization pattern, the docs were needlessly going through `as_mut_ptr().write()` to initialize, which is unnecessary use of a pointer, needs to be inside an `unsafe` block, and may weaken alias analysis.
Matthias Krüger