Make transpose const and inline
r? `@scottmcm`
- These should have been const from the beginning since we're never going to do more than a transmute.
- Inline these always because that's what every other method in MaybeUninit which simply casts does. :) Ok, but a stronger justification is that because we're taking in arrays by `self`, not inlining would defeat the whole purpose of using `MaybeUninit` due to the copying.
Optimize `slice_iter.copied().next_chunk()`
```
OLD:
test iter::bench_copied_array_chunks ... bench: 371 ns/iter (+/- 7)
NEW:
test iter::bench_copied_array_chunks ... bench: 31 ns/iter (+/- 0)
```
The default `next_chunk` implementation suffers from having to assemble the array byte by byte via `next()`, checking the `Option<&T>` and then dereferencing `&T`. The specialization copies the chunk directly from the slice.
More slice::partition_point examples
After seeing the discussion of `binary_search` vs `partition_point` in #101999, I thought some more example code could be helpful.
doc: rewrite doc for uint::{carrying_add,borrowing_sub}
Reword the documentation for bigint helper methods `uint::{carrying_add,borrowing_sub}` (#85532).
The examples were also rewritten to demonstrate how the methods can be used in bignum arithmetic. No loops are used in the examples, but the variable names were chosen to include indices so that it is clear how this can be used in a loop if required.
Also, previously `carrying_add` had an example to say that if the input carry is false, the method is equivalent to `overflowing_add`. While the note was kept, the example was removed and an extra note was added to make sure this equivalence is not assumed for signed integers as well.
Remove the redundant `Some(try_opt!(..))` in `checked_pow`
The final return value doesn't need to be tried at all -- we can just
return the checked option directly. The optimizer can probably figure
this out anyway, but there's no need to make it work here.
Make diagnostic for unsatisfied `Termination` bounds more precise
Don't blindly emit a diagnostic claiming that “*`main` has an invalid return type*” if we encounter a type that should but doesn't implement `std::process::Termination` and isn't actually the return type of the program entry `main`.
Fixes#103052.
``@rustbot`` label A-diagnostics T-compiler T-libs
r? diagnostics
Add `Box<[T; N]>: TryFrom<Vec<T>>`
We have `[T; N]: TryFrom<Vec<T>>` (#76310) and `Box<[T; N]>: TryFrom<Box<[T]>>`, but not this combination.
`vec.into_boxed_slice().try_into()` isn't quite a replacement for this, as that'll reallocate unnecessarily in the error case.
**Insta-stable, so needs an FCP**
(I tried to make this work with `, A`, but that's disallowed because of `#[fundamental]` https://github.com/rust-lang/rust/issues/29635#issuecomment-1247598385)
The final return value doesn't need to be tried at all -- we can just
return the checked option directly. The optimizer can probably figure
this out anyway, but there's no need to make it work here.
Clarify the possible return values of `len_utf16`
`char::len_utf16` always return 1 or 2. Clarify this in the docs, in the same way as `char::len_utf8`.
Documentation BTreeMap::append's behavior for already existing keys
`BTreeMap::append` overwrites existing values with new ones. This commit adds explicit documentation for that.
Add documentation about the memory layout of `UnsafeCell<T>`
The documentation for `UnsafeCell<T>` currently does not make any promises about its memory layout. This PR adds this documentation, namely that the memory layout of `UnsafeCell<T>` is the same as the memory layout of its inner `T`.
# Use case
Without this layout promise, the following cast would not be legally possible:
```rust
fn example<T>(ptr: *mut T) -> *const UnsafeCell<T> {
ptr as *const UnsafeCell<T>
}
```
A use case where this can come up involves FFI. If Rust receives a pointer over a FFI boundary which provides shared read-write access (with some form of custom synchronization), and this pointer is managed by some Rust struct with lifetime `'a`, then it would greatly simplify its (internal) API and safety contract if a `&'a UnsafeCell<T>` can be created from a raw FFI pointer `*mut T`. A lot of safety checks can be done when receiving the pointer for the first time through FFI (non-nullness, alignment, initialize uninit bytes, etc.) and these properties can then be encoded into the `&UnsafeCell<T>` type. Without this documentation guarantee, this is not legal today outside of the standard library.
# Caveats
Casting in the opposite direction is still not valid, even with this documentation change:
```rust
fn example2<T>(ptr: &UnsafeCell<T>) -> &mut T {
let t = ptr as *const UnsafeCell<T> as *mut T;
unsafe { &mut *t }
}
```
This is because the only legal way to obtain a mutable pointer to the contents of the shared reference is through [`UnsafeCell::get`](https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html#method.get) and [`UnsafeCell::raw_get`](https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html#method.raw_get). Although there might be a desire to also make this legal at some point in the future, that part is outside the scope of this PR. Also see this relevant [Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/transmuting.20.26.20-.3E.20.26mut).
# Alternatives
Instead of adding a new documentation promise, it's also possible to add a new method to `UnsafeCell<T>` with signature `pub fn from_ptr_bikeshed(ptr: *mut T) -> *const UnsafeCell<T>` which indirectly only allows one-way casting to `*const UnsafeCell<T>`.
std: use `sync::Mutex` for internal statics
Since `sync::Mutex` is now `const`-constructible, it can be used for internal statics, removing the need for `sys_common::StaticMutex`. This adds some extra allocations on platforms which need to box their mutexes (currently SGX and some UNIX), but these will become unnecessary with the lock improvements tracked in #93740.
I changed the program argument implementation on Hermit, it does not need `Mutex` but can use atomics like some UNIX systems (ping `@mkroening` `@stlankes).`
Use semaphores for thread parking on Apple platforms
Currently we use a mutex-condvar pair for thread parking on Apple systems. Unfortunately, `pthread_cond_timedwait` uses the real-time clock for measuring time, which causes problems when the system time changes. The parking implementation in this PR uses a semaphore instead, which measures monotonic time by default, avoiding these issues. As a further benefit, this has the potential to improve performance a bit, since `unpark` does not need to wait for a lock to be released.
Since the Mach semaphores are poorly documented (I could not find availability or stability guarantees for instance), this uses a [dispatch semaphore](https://developer.apple.com/documentation/dispatch/dispatch_semaphore?language=objc) instead. While it adds a layer of indirection (it uses Mach semaphores internally), the overhead is probably negligible.
Tested on macOS 12.5.
r? ``````@thomcc``````
Add `IsTerminal` trait to determine if a descriptor or handle is a terminal
The UNIX implementation uses `isatty`. The Windows implementation uses
the same logic the `atty` crate uses, including the hack needed to
detect msys terminals.
Implement this trait for `Stdin`/`Stdout`/`Stderr`/`File` on all
platforms. On Unix, implement it for `BorrowedFd`/`OwnedFd`. On Windows,
implement it for `BorrowedHandle`/`OwnedHandle`.
Based on https://github.com/rust-lang/rust/pull/91121
Co-authored-by: Matt Wilkinson <mattwilki17@gmail.com>
Rather than referencing a slice's pointer and then creating a new slice
with a longer length, offset from the base structure pointer instead.
This makes some choices of Rust semantics happier.
The UNIX and WASI implementations use `isatty`. The Windows
implementation uses the same logic the `atty` crate uses, including the
hack needed to detect msys terminals.
Implement this trait for `File` and for `Stdin`/`Stdout`/`Stderr` and
their locked counterparts on all platforms. On UNIX and WASI, implement
it for `BorrowedFd`/`OwnedFd`. On Windows, implement it for
`BorrowedHandle`/`OwnedHandle`.
Based on https://github.com/rust-lang/rust/pull/91121
Co-authored-by: Matt Wilkinson <mattwilki17@gmail.com>
Fix `Duration::{try_,}from_secs_f{32,64}(-0.0)`
Make `Duration::{try_,}from_secs_f{32,64}(-0.0)` return `Duration::ZERO` (as they did before #90247) instead of erroring/panicking.
I'll update this PR to remove the `#![feature(duration_checked_float)]` if #102271 is merged before this PR.
Tracking issue for `try_from_secs_f{32,64}`: #83400
sync thread_local key conditions exactly with what the macro uses
This makes the `cfg` in `mod.rs` syntactically the same as those in `local.rs`.
I don't think this should actually change anything, but seems better to be consistent?
I looked into this due to https://github.com/rust-lang/rust/issues/102549, but this PR would make it *less* likely that `__OsLocalKeyInner` is going to get provided, so this cannot help with that issue.
r? `@thomcc`
