Remove special-case handling of `vec.split_off(0)`
#76682 added special handling to `Vec::split_off` for the case where `at == 0`. Instead of copying the vector's contents into a freshly-allocated vector and returning it, the special-case code steals the old vector's allocation, and replaces it with a new (empty) buffer with the same capacity.
That eliminates the need to copy the existing elements, but comes at a surprising cost, as seen in #119913. The returned vector's capacity is no longer determined by the size of its contents (as would be expected for a freshly-allocated vector), and instead uses the full capacity of the old vector.
In cases where the capacity is large but the size is small, that results in a much larger capacity than would be expected from reading the documentation of `split_off`. This is especially bad when `split_off` is called in a loop (to recycle a buffer), and the returned vectors have a wide variety of lengths.
I believe it's better to remove the special-case code, and treat `at == 0` just like any other value:
- The current documentation states that `split_off` returns a “newly allocated vector”, which is not actually true in the current implementation when `at == 0`.
- If the value of `at` could be non-zero at runtime, then the caller has already agreed to the cost of a full memcpy of the taken elements in the general case. Avoiding that copy would be nice if it were close to free, but the different handling of capacity means that it is not.
- If the caller specifically wants to avoid copying in the case where `at == 0`, they can easily implement that behaviour themselves using `mem::replace`.
Fixes#119913.
Initial implementation of `str::from_raw_parts[_mut]`
ACP (accepted): rust-lang/libs-team#167
Tracking issue: #119206
Thanks to ``@Kixiron`` for previous work on this (#107207)
``@rustbot`` label +T-libs-api -T-libs
r? ``@thomcc``
Closes#107207.
Use `assert_unchecked` instead of `assume` intrinsic in the standard library
Now that a public wrapper for the `assume` intrinsic exists, we can use it in the standard library.
CC #119131
Fix deallocation with wrong allocator in (A)Rc::from_box_in
Deallocate the `Box` with the original allocator (via `&A`), not `Global`.
Fixes#119749
<details> <summary>Example code with error and Miri output</summary>
(Note that this UB is not observable on stable, because the only usable allocator on stable is `Global` anyway.)
Code ([playground link](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=96193c2c6a1912d7f669fbbe39174b09)):
```rs
#![feature(allocator_api)]
use std::alloc::System;
// uncomment one of these
use std::rc::Rc;
//use std::sync::Arc as Rc;
fn main() {
let x: Box<[u32], System> = Box::new_in([1,2,3], System);
let _: Rc<[u32], System> = Rc::from(x);
}
```
Miri output:
```rs
error: Undefined Behavior: deallocating alloc904, which is C heap memory, using Rust heap deallocation operation
--> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:117:14
|
117 | unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ deallocating alloc904, which is C heap memory, using Rust heap deallocation operation
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
= note: BACKTRACE:
= note: inside `std::alloc::dealloc` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:117:14: 117:64
= note: inside `<std::alloc::Global as std::alloc::Allocator>::deallocate` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:254:22: 254:51
= note: inside `<std::boxed::Box<std::mem::ManuallyDrop<[u32]>> as std::ops::Drop>::drop` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1244:17: 1244:66
= note: inside `std::ptr::drop_in_place::<std::boxed::Box<std::mem::ManuallyDrop<[u32]>>> - shim(Some(std::boxed::Box<std::mem::ManuallyDrop<[u32]>>))` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:507:1: 507:56
= note: inside `std::mem::drop::<std::boxed::Box<std::mem::ManuallyDrop<[u32]>>>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:992:24: 992:25
= note: inside `std::rc::Rc::<[u32], std::alloc::System>::from_box_in` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/rc.rs:1928:13: 1928:22
= note: inside `<std::rc::Rc<[u32], std::alloc::System> as std::convert::From<std::boxed::Box<[u32], std::alloc::System>>>::from` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/rc.rs:2504:9: 2504:27
note: inside `main`
--> src/main.rs:10:32
|
10 | let _: Rc<[u32], System> = Rc::from(x);
| ^^^^^^^^^^^
note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
error: aborting due to 1 previous error
```
</details>
Document some alternatives to `Vec::split_off`
One of the discussion points that came up in #119917 is that some people use `Vec::split_off` in cases where they probably shouldn't, because the alternatives (like `mem::take`) are hard to discover.
This PR adds some suggestions to the documentation of `split_off` that should point people towards alternatives that might be more appropriate for their use-case.
I've deliberately tried to keep these changes as simple and uncontroversial as possible, so that they don't depend on how the team decides to handle the concerns raised in #119917. That's why I haven't touched the existing documentation for `split_off`, and haven't added links to `split_off` to the documentation of other methods.
fix: Drop guard was deallocating with the incorrect size
InPlaceDstBufDrop holds onto the allocation before the shrinking happens which means it must deallocate the destination elements but the source allocation.
Thanks `@cuviper` for spotting this.
Remove alignment-changing in-place collect
This removes the alignment-changing in-place collect optimization introduced in #110353
Currently stable users can't benefit from the optimization because GlobaAlloc doesn't support alignment-changing realloc and neither do most posix allocators. So in practice it has a negative impact on performance.
Explanation from https://github.com/rust-lang/rust/issues/120091#issuecomment-1899071681:
> > You mention that in case of alignment mismatch -- when the new alignment is less than the old -- the implementation calls `mremap`.
>
> I was trying to note that this isn't really the case in practice, due to the semantics of Rust's allocator APIs. The only use of the allocator within the `in_place_collect` implementation itself is [a call to `Allocator::shrink()`](db7125f008/library/alloc/src/vec/in_place_collect.rs (L299-L303)), which per its documentation [allows decreasing the required alignment](https://doc.rust-lang.org/1.75.0/core/alloc/trait.Allocator.html). However, in stable Rust, the only available `Allocator` is [`Global`](https://doc.rust-lang.org/1.75.0/alloc/alloc/struct.Global.html), which delegates to the registered `GlobalAlloc`. Since `GlobalAlloc::realloc()` [cannot change the required alignment](https://doc.rust-lang.org/1.75.0/core/alloc/trait.GlobalAlloc.html#method.realloc), the implementation of [`<Global as Allocator>::shrink()`](db7125f008/library/alloc/src/alloc.rs (L280-L321)) must fall back to creating a brand-new allocation, `memcpy`ing the data into it, and freeing the old allocation, whenever the alignment doesn't remain exactly the same.
>
> Therefore, the underlying allocator, provided by libc or some other source, has no opportunity to internally `mremap()` the data when the alignment is changed, since it has no way of knowing that the allocation is the same.
This also removes
* impl From<&Context> for ContextBuilder
* Context::try_waker()
The from implementation is removed because now that
wakers are always supported, there are less incentives
to override the current context. Before, the incentive
was to add Waker support to a reactor that didn't have
any.
InPlaceDstBufDrop holds onto the allocation before the shrinking happens
which means it must deallocate the destination elements but the source
allocation.
Update documentation for Vec::into_boxed_slice to be more clear about excess capacity
Currently, the documentation for Vec::into_boxed_slice says that "if the vector has excess capacity, its items will be moved into a newly-allocated buffer with exactly the right capacity." This is misleading, as copies do not necessarily occur, depending on if the allocator supports in-place shrinking. I copied some of the wording from shrink_to_fit, though it could potentially still be worded better than this.
Currently stable users can't benefit from this because GlobaAlloc doesn't support
alignment-changing realloc and neither do most posix allocators.
So in practice it always results in an extra memcpy.
merge core_panic feature into panic_internals
I don't know why those are two separate features, but it does not seem intentional. This merge is useful because with https://github.com/rust-lang/rust/pull/118123, panic_internals is recognized as an internal feature, but core_panic is not -- but core_panic definitely should be internal.
rc: Take *const T in is_dangling
It is not important which one is used since `is_dangling` does not access memory, but `*const` removes the needs of `*const T` -> `*mut T` casts in `from_raw_in`.
Clean up alloc::sync::Weak Clone implementation
Since both return points (tail and early return) return the same expression and the only difference is whether inner is available, the code that does the atomic operations and checks on inner was moved into the if body and the only return is at the tail. Original comments preserved.
It is not important which one is used since `is_dangling` does not access
memory, but `*const` removes the needs of `*const T` -> `*mut T` casts
in `from_raw_in`.
fix minor mistake in comments describing VecDeque resizing
Avoiding confusion where one of the items in the deque seems to disappear in two of the three cases
Since both return points (tail and early return) return the same
expression and the only difference is whether inner is available, the
code that does the atomic operations and checks on inner was moved into
the if body and the only return is at the tail. Original comments
preserved.
add more niches to rawvec
Previously RawVec only had a single niche in its `NonNull` pointer. With this change it now has `isize::MAX` niches since half the value-space of the capacity field is never needed, we can't have a capacity larger than isize::MAX.
