In the past, Vec::clone_from was implemented using slice::clone_into.
The code from clone_into was later duplicated into clone_from in
8725e4c337, which is the commit that adds custom allocator support to
Vec. Presumably this was done because the slice::clone_into only works
for vecs with the default allocator so it would have the wrong type to
clone into Vec<T, A>.
Now that the clone_into implementation is moved out into a specializable
trait anyway we might as well use that to share the code between the two
methods.
Remove incorrect comment in `Vec::drain`
r? ``@scottmcm``
Turns out this comment wasn't correct for 6 years, since #34951, which switched from using `slice::IterMut` into using `slice::Iter`.
add Vec::push_within_capacity - fallible, does not allocate
This method can serve several purposes. It
* is fallible
* guarantees that items in Vec aren't moved
* allows loops that do `reserve` and `push` separately to avoid pulling in the allocation machinery a second time in the `push` part which should make things easier on the optimizer
* eases the path towards `ArrayVec` a bit since - compared to `push()` - there are fewer questions around how it should be implemented
I haven't named it `try_push` because that should probably occupy a middle ground that will still try to reserve and only return an error in the unlikely OOM case.
resolves#84649
Fix in-place collection leak when remaining element destructor panic
Fixes#101628
cc `@the8472`
I went for the drop guard route, placing it immediately before the `forget_allocation_drop_remaining` call and after the comment, as to signal they are closely related.
I also updated the test to check for the leak, though the only change really needed was removing the leak clean up for miri since now that's no longer leaked.
docs: be less harsh in wording for Vec::from_raw_parts
In particular, be clear that it is sound to specify memory not
originating from a previous `Vec` allocation. That is already suggested
in other parts of the documentation about zero-alloc conversions to Box<[T]>.
Incorporate a constraint from `slice::from_raw_parts` that was missing
but needs to be fulfilled, since a `Vec` can be converted into a slice.
Fixes https://github.com/rust-lang/rust/issues/98780.
There's a bunch of these checks because of special handing for ZSTs in various unsafe implementations of stuff.
This lets them be `T::IS_ZST` instead of `mem::size_of::<T>() == 0` every time, making them both more readable and more terse.
*Not* proposed for stabilization at this time. Would be `pub(crate)` except `alloc` wants to use it too.
(And while it doesn't matter now, if we ever get something like 85836 making it a const can help codegen be simpler.)
This PR will fix some typos detected by [typos].
I only picked the ones I was sure were spelling errors to fix, mostly in
the comments.
[typos]: https://github.com/crate-ci/typos
Add guarantee that Vec::default() does not alloc
Currently `Vec::new()` is guaranteed to not allocate until elements are pushed onto the `Vec`, but such a guarantee is missing for `Vec`'s implementation of `Default::default`.
This adds such a guarantee for `Vec::default()` to the API reference.
See also [this discussion on URLO](https://users.rust-lang.org/t/guarantee-that-vec-default-does-not-allocate/79903).
Guarantee `try_reserve` preserves the contents on error
Update doc comments to make the guarantee explicit. However, some
implementations does not have the statement though.
* `HashMap`, `HashSet`: require guarantees on hashbrown side.
* `PathBuf`: simply redirecting to `OsString`.
Fixes#99606.
Currently `Vec::new()` is guaranteed to not allocate until elements are
pushed onto the `Vec`, but such a guarantee is missing for `Vec`'s
implementation of `Default::default`. This adds such a guarantee for
`Vec::default()` to the API reference.
Replace most uses of `pointer::offset` with `add` and `sub`
As PR title says, it replaces `pointer::offset` in compiler and standard library with `pointer::add` and `pointer::sub`. This generally makes code cleaner, easier to grasp and removes (or, well, hides) integer casts.
This is generally trivially correct, `.offset(-constant)` is just `.sub(constant)`, `.offset(usized as isize)` is just `.add(usized)`, etc. However in some cases we need to be careful with signs of things.
r? ````@scottmcm````
_split off from #100746_
Update doc comments to make the guarantee explicit. However, some
implementations does not have the statement though.
* `HashMap`, `HashSet`: require guarantees on hashbrown side.
* `PathBuf`: simply redirecting to `OsString`.
Fixes#99606.
In particular, be clear that it is sound to specify memory not
originating from a previous `Vec` allocation. That is already suggested
in other parts of the documentation about zero-alloc conversions to Box<[T]>.
Incorporate a constraint from `slice::from_raw_parts` that was missing
but needs to be fulfilled, since a `Vec` can be converted into a slice.
Optimize `Vec::insert` for the case where `index == len`.
By skipping the call to `copy` with a zero length. This makes it closer
to `push`.
I did this recently for `SmallVec`
(https://github.com/servo/rust-smallvec/pull/282) and it was a big perf win in
one case. Although I don't have a specific use case in mind, it seems
worth doing it for `Vec` as well.
Things to note:
- In the `index < len` case, the number of conditions checked is
unchanged.
- In the `index == len` case, the number of conditions checked increases
by one, but the more expensive zero-length copy is avoided.
- In the `index > len` case the code now reserves space for the extra
element before panicking. This seems like an unimportant change.
r? `@cuviper`
By skipping the call to `copy` with a zero length. This makes it closer
to `push`.
I did this recently for `SmallVec`
(https://github.com/servo/rust-smallvec/pull/282) and it was a big perf win in
one case. Although I don't have a specific use case in mind, it seems
worth doing it for `Vec` as well.
Things to note:
- In the `index < len` case, the number of conditions checked is
unchanged.
- In the `index == len` case, the number of conditions checked increases
by one, but the more expensive zero-length copy is avoided.
- In the `index > len` case the code now reserves space for the extra
element before panicking. This seems like an unimportant change.
Documentation for the following methods
with_capacity
with_capacity_in
with_capacity_and_hasher
reserve
reserve_exact
try_reserve
try_reserve_exact
was inconsistent and often not entirely correct where they existed on the following types
Vec
VecDeque
String
OsString
PathBuf
BinaryHeap
HashSet
HashMap
BufWriter
LineWriter
since the allocator is allowed to allocate more than the requested capacity in all such cases, and will frequently "allocate" much more in the case of zero-sized types (I also checked BufReader, but there the docs appear to be accurate as it appears to actually allocate the exact capacity).
Some effort was made to make the documentation more consistent between types as well.
Fix with_capacity* methods for Vec
Fix *reserve* methods for Vec
Fix docs for *reserve* methods of VecDeque
Fix docs for String::with_capacity
Fix docs for *reserve* methods of String
Fix docs for OsString::with_capacity
Fix docs for *reserve* methods on OsString
Fix docs for with_capacity* methods on HashSet
Fix docs for *reserve methods of HashSet
Fix docs for with_capacity* methods of HashMap
Fix docs for *reserve methods on HashMap
Fix expect messages about OOM in doctests
Fix docs for BinaryHeap::with_capacity
Fix docs for *reserve* methods of BinaryHeap
Fix typos
Fix docs for with_capacity on BufWriter and LineWriter
Fix consistent use of `hasher` between `HashMap` and `HashSet`
Fix warning in doc test
Add test for capacity of vec with ZST
Fix doc test error
Add #[rustc_box] and use it inside alloc
This commit adds an alternative content boxing syntax, and uses it inside alloc.
```Rust
#![feature(box_syntax)]
fn foo() {
let foo = box bar;
}
```
is equivalent to
```Rust
#![feature(rustc_attrs)]
fn foo() {
let foo = #[rustc_box] Box::new(bar);
}
```
The usage inside the very performance relevant code in
liballoc is the only remaining relevant usage of box syntax
in the compiler (outside of tests, which are comparatively easy to port).
box syntax was originally designed to be used by all Rust
developers. This introduces a replacement syntax more tailored
to only being used inside the Rust compiler, and with it,
lays the groundwork for eventually removing box syntax.
[Earlier work](https://github.com/rust-lang/rust/pull/87781#issuecomment-894714878) by `@nbdd0121` to lower `Box::new` to `box` during THIR -> MIR building ran into borrow checker problems, requiring the lowering to be adjusted in a way that led to [performance regressions](https://github.com/rust-lang/rust/pull/87781#issuecomment-894872367). The proposed change in this PR lowers `#[rustc_box] Box::new` -> `box` in the AST -> HIR lowering step, which is way earlier in the compiler, and thus should cause less issues both performance wise as well as regarding type inference/borrow checking/etc. Hopefully, future work can move the lowering further back in the compiler, as long as there are no performance regressions.
alloc: remove repeated word in comment
Linux's `checkpatch.pl` reports:
```txt
#42544: FILE: rust/alloc/vec/mod.rs:2692:
WARNING: Possible repeated word: 'to'
+ // - Elements are :Copy so it's OK to to copy them, without doing
```
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Clarify the guarantees of Vec::as_ptr and Vec::as_mut_ptr when there's no allocation
Currently the documentation says they return a pointer to the vector's buffer, which has the implied precondition that the vector allocated some memory. However `Vec`'s documentation also specifies that it won't always allocate, so it's unclear whether the pointer returned is valid in that case. Of course you won't be able to read/write actual bytes to/from it since the capacity is 0, but there's an exception: zero sized read/writes. They are still valid as long as the pointer is not null and the memory it points to wasn't deallocated, but `Vec::as_ptr` and `Vec::as_mut_ptr` don't specify that's not the case. This PR thus specifies they are actually valid for zero sized reads since `Vec` is implemented to hold a dangling pointer in those cases, which is neither null nor was deallocated.
Linux's `checkpatch.pl` reports:
```txt
#42544: FILE: rust/alloc/vec/mod.rs:2692:
WARNING: Possible repeated word: 'to'
+ // - Elements are :Copy so it's OK to to copy them, without doing
```
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
