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Change the placement of two functions.
Right now, the order is as follows:
`pop_front()`
`push_front()`
`push_back()`
`pop_back()`
`swap_remove_back()`
`swap_remove_front()`
I believe it would be more natural, and easier to follow, if we place `pop_back()` right after the `pop_front()`, and `swap_remove_back()` after the `swap_remove_front()` like this:
`pop_front()`
`pop_back()`
`push_front()`
`push_back()`
`swap_remove_front()`
`swap_remove_back()`
The rest of the documentation (at least in this module) adheres to the same logic, where the 'front' function always precedes its 'back' equivalent.
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Add APIs for uninitialized Box, Rc, and Arc. (Plus get_mut_unchecked)
Assigning `MaybeUninit::<Foo>::uninit()` to a local variable is usually free, even when `size_of::<Foo>()` is large. However, passing it for example to `Arc::new` [causes at least one copy](https://youtu.be/F1AquroPfcI?t=4116) (from the stack to the newly allocated heap memory) even though there is no meaningful data. It is theoretically possible that a Sufficiently Advanced Compiler could optimize this copy away, but this is [reportedly unlikely to happen soon in LLVM](https://youtu.be/F1AquroPfcI?t=5431).
This PR proposes two sets of features:
* Constructors for containers (`Box`, `Rc`, `Arc`) of `MaybeUninit<T>` or `[MaybeUninit<T>]` that do not initialized the data, and unsafe conversions to the known-initialized types (without `MaybeUninit`). The constructors are guaranteed not to make unnecessary copies.
* On `Rc` and `Arc`, an unsafe `get_mut_unchecked` method that provides `&mut T` access without checking the reference count. `Arc::get_mut` involves multiple atomic operations whose cost can be non-trivial. `Rc::get_mut` is less costly, but we add `Rc::get_mut_unchecked` anyway for symmetry with `Arc`.
These can be useful independently, but they will presumably be typical when the new constructors of `Rc` and `Arc` are used.
An alternative with a safe API would be to introduce `UniqueRc` and `UniqueArc` types that have the same memory layout as `Rc` and `Arc` (and so zero-cost conversion to them) but are guaranteed to have only one reference. But introducing entire new types feels “heavier” than new constructors on existing types, and initialization of `MaybeUninit<T>` typically requires unsafe code anyway.
Summary of new APIs (all unstable in this PR):
```rust
impl<T> Box<T> { pub fn new_uninit() -> Box<MaybeUninit<T>> {…} }
impl<T> Box<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Box<T> {…} }
impl<T> Box<[T]> { pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>]> {…} }
impl<T> Box<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Box<[T]> {…} }
impl<T> Rc<T> { pub fn new_uninit() -> Rc<MaybeUninit<T>> {…} }
impl<T> Rc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Rc<T> {…} }
impl<T> Rc<[T]> { pub fn new_uninit_slice(len: usize) -> Rc<[MaybeUninit<T>]> {…} }
impl<T> Rc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Rc<[T]> {…} }
impl<T> Arc<T> { pub fn new_uninit() -> Arc<MaybeUninit<T>> {…} }
impl<T> Arc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Arc<T> {…} }
impl<T> Arc<[T]> { pub fn new_uninit_slice(len: usize) -> Arc<[MaybeUninit<T>]> {…} }
impl<T> Arc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Arc<[T]> {…} }
impl<T: ?Sized> Rc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} }
impl<T: ?Sized> Arc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} }
```
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Co-Authored-By: Ralf Jung <post@ralfj.de>
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Hygienize use of built-in macros in the standard library
Same as https://github.com/rust-lang/rust/pull/61629, but for built-in macros.
Closes https://github.com/rust-lang/rust/issues/48781
r? @alexcrichton
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… and add a separately-unstable field to force non-exhaustive matching
(`#[non_exhaustive]` is no implemented yet on enum variants)
so that we have the option to later expose the allocator’s error value.
CC https://github.com/rust-lang/wg-allocators/issues/23
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Per https://github.com/rust-lang/rust/pull/62451#discussion_r303197278
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Bump to 1.39
r? @Centril
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Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>
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Use associated_type_bounds where applicable - closes #61738
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reduce some test sizes in Miri
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Merge recent changes into master
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bump rand in libcore/liballoc test suites
This pulls in the fix for https://github.com/rust-random/rand/issues/779, which trips Miri when running these test suites.
`SmallRng` (formerly used by libcore) is no longer built by default, it needs a feature gate. I opted to switch to `StdRng` instead. Or should I enable the feature gate?
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This mirrors the implementations of reference slices into arrays.
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Remove additional libcore-like restrictions from liballoc, turns out the testing works ok if the tests are a part of liballoc itself.
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Turn `INCOMPLETE_FEATURES` into lint
We do this because it is annoying to see the warning when building rustc and because this is better from a "separation of concerns" POV.
The drawback to this change is that this will respect `--cap-lints`.
Also note that this is not a buffered lint so if there are fatal parser errors then the lint will not trigger.
r? @varkor
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Add doc links to liballoc crate page
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rustbuild
Remove some random unnecessary lint `allow`s
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In which we constantly improve the Vec(Deque) array PartialEq impls
Use the same approach as in https://github.com/rust-lang/rust/pull/62435 as sanctioned by https://github.com/rust-lang/rust/issues/61415#issuecomment-504155110.
r? @scottmcm
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Fix few Clippy warnings
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Right now, the order is as follows:
`pop_front()`
`push_front()`
`push_back()`
`pop_back()`
`swap_remove_back()`
`swap_remove_front()`
I believe it would be more natural, and easier to follow, if we place `pop_back()` right after the `pop_front()`, and `swap_remove_back()` after the `swap_remove_front()` like this:
`pop_front()`
`pop_back()`
`push_front()`
`push_back()`
`swap_remove_front()`
`swap_remove_back()`
The rest of the documentation (at least in this module) adheres to the same logic, where the 'front' function always precedes its 'back' equivalent.
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Add missing doc links in boxed module
r? @rust-lang/docs
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Rollup of 15 pull requests
Successful merges:
- #60066 (Stabilize the type_name intrinsic in core::any)
- #60938 (rustdoc: make #[doc(include)] relative to the containing file)
- #61884 (Stablize Euclidean Modulo (feature euclidean_division))
- #61890 (Fix some sanity checks)
- #62528 (Add joining slices of slices with a slice separator, not just a single item)
- #62707 (Add tests for overlapping explicitly dropped locals in generators)
- #62735 (Turn `#[global_allocator]` into a regular attribute macro)
- #62822 (Improve some pointer-related documentation)
- #62887 (Make the parser TokenStream more resilient after mismatched delimiter recovery)
- #62921 (Add method disambiguation help for trait implementation)
- #62930 (Add test for #51559)
- #62942 (Use match ergonomics in Condvar documentation)
- #62977 (Fix inconsistent highlight blocks.)
- #62978 (Remove `cfg(bootstrap)` code for array implementations)
- #62981 (Add note suggesting to borrow a String argument to find)
Failed merges:
- #62964 (clarify and unify some type test names)
r? @ghost
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Add joining slices of slices with a slice separator, not just a single item
https://github.com/rust-lang/rust/issues/27747#issuecomment-294525391
> It's kinda annoying to be able to join strings with a str (which can have multiple chars), but joining a slice of slices, you can only join with a single element.
This turns out to be fixable, with some possible inference regressions.
# TL;DR
Related trait(s) are unstable and tracked at https://github.com/rust-lang/rust/issues/27747, but the `[T]::join` method that is being extended here is already stable.
Example use of the new insta-stable functionality:
```rust
let nested: Vec<Vec<Foo>> = /* … */;
let separator: &[Foo] = /* … */; // Previously: could only be a single &Foo
nested.join(separator)
```
Complete API affected by this PR, after changes:
```rust
impl<T> [T] {
pub fn concat<Item: ?Sized>(&self) -> <Self as Concat<Item>>::Output
where Self: Concat<Item>
{
Concat::concat(self)
}
pub fn join<Separator>(&self, sep: Separator) -> <Self as Join<Separator>>::Output
where Self: Join<Separator>
{
Join::join(self, sep)
}
}
// The `Item` parameter is only useful for the the slice-of-slices impl.
pub trait Concat<Item: ?Sized> {
type Output;
fn concat(slice: &Self) -> Self::Output;
}
pub trait Join<Separator> {
type Output;
fn join(slice: &Self, sep: Separator) -> Self::Output;
}
impl<T: Clone, V: Borrow<[T]>> Concat<T> for [V] {
type Output = Vec<T>;
}
impl<T: Clone, V: Borrow<[T]>> Join<&'_ T> for [V] {
type Output = Vec<T>;
}
// New functionality here!
impl<T: Clone, V: Borrow<[T]>> Join<&'_ [T]> for [V] {
type Output = Vec<T>;
}
impl<S: Borrow<str>> Concat<str> for [S] {
type Output = String;
}
impl<S: Borrow<str>> Join<&'_ str> for [S] {
type Output = String;
}
```
# Details
After https://github.com/rust-lang/rust/pull/62403 but before this PR, the API is:
```rust
impl<T> [T] {
pub fn concat<Separator: ?Sized>(&self) -> T::Output
where T: SliceConcat<Separator>
{
SliceConcat::concat(self)
}
pub fn join<Separator: ?Sized>(&self, sep: &Separator) -> T::Output
where T: SliceConcat<Separator>
{
SliceConcat::join(self, sep)
}
}
pub trait SliceConcat<Separator: ?Sized>: Sized {
type Output;
fn concat(slice: &[Self]) -> Self::Output;
fn join(slice: &[Self], sep: &Separator) -> Self::Output;
}
impl<T: Clone, V: Borrow<[T]>> SliceConcat<T> for V {
type Output = Vec<T>;
}
impl<S: Borrow<str>> SliceConcat<str> for S {
type Output = String;
}
```
By adding a trait impl we should be able to accept a slice of `T` as the separator, as an alternative to a single `T` value.
In a `some_slice.join(some_separator)` call, trait resolution will pick an impl or the other based on the type of `some_separator`. In `some_slice.concat()` however there is no separator, so this call would become ambiguous. Some regression in type inference or trait resolution may be acceptable on principle, but requiring a turbofish for every single call to `concat` isn’t great.
The solution to that is splitting the `SliceConcat` trait into two `Concat` and `Join` traits, one for each eponymous method. Only `Join` would gain a new impl, so that `some_slice.concat()` would not become ambiguous.
Now, at the trait level the `Concat` trait does not need a `Separator` parameter anymore. However, simply removing it causes one of the impls not to be accepted anymore:
```rust
error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
--> src/liballoc/slice.rs:608:6
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608 | impl<T: Clone, V: Borrow<[T]>> Concat for [V] {
| ^ unconstrained type parameter
```
This makes sense: if `[V]::concat` is a method that is itself not generic, then its return type (which is the `Concat::Output` associated type) needs to be determined based on solely `V`. And although there is no such type in the standard library, there is nothing stopping another crate from defining a `V` type that implements both `Borrow<[Foo]>` and `Borrow<[Bar]>`. It might not be a good idea, but it’s possible. Both would apply here, and there would be no way to determine `T`.
This could be a warning sign that this API is too generic. Perhaps we’d be better off having one less type variable, and only implement `Concat for [&'_ [T]]` and `Concat for [Vec<T>]` etc. However this aspect of `[V]::concat` is already stable, so we’re stuck with it.
The solution is to keep a dummy type parameter on the `Concat` trait. That way, if a type has multiple `Borrow<[_]>` impls, it’ll end up with multiple corresponding `Concat<_>` impls.
In `impl<S: Borrow<str>> Concat<str> for [S]`, the second occurrence of `str` is not meaningful. It could be any type. As long as there is only once such type with an applicable impl, trait resolution will be appeased without demanding turbofishes.
# Joining strings with `char`
For symmetry I also tried adding this impl (because why not):
```rust
impl<S: Borrow<str>> Join<char> for [S] {
type Output = String;
}
```
This immediately caused an inference regression in a dependency of rustc:
```rust
error[E0277]: the trait bound `std::string::String: std::borrow::Borrow<[std::string::String]>` is not satisfied
--> /home/simon/.cargo/registry/src/github.com-1ecc6299db9ec823/getopts-0.2.19/src/lib.rs:595:37
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595 | row.push_str(&desc_rows.join(&desc_sep));
| ^^^^ the trait `std::borrow::Borrow<[std::string::String]>` is not implemented for `std::string::String`
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= help: the following implementations were found:
<std::string::String as std::borrow::Borrow<str>>
= note: required because of the requirements on the impl of `std::slice::Join<&std::string::String>` for `[std::string::String]`
```
In the context of this code, two facts are known:
* `desc_rows` is a `Vec<String>`
* `desc_sep` is a `String`
Previously the first fact alone reduces the resolution of `join` to only one solution, where its argument it expected to be `&str`. Then, `&String` is coerced to `&str`.
With the new `Join` impl, the first fact leavs two applicable impls where the separator can be either `&str` or `char`. But `&String` is neither of these things. It appears that possible coercions are not accounted for, in the search for a solution in trait resolution.
I have not included this new impl in this PR. It’s still possible to add later, but the `getopts` breakage does not need to block the rest of the PR. And the functionality easy for end-user to duplicate: `slice_of_strings.join(&*char_separator.encode_utf8(&mut [0_u8, 4]))`
The `&*` part of that last code snippet is another case of the same issue: `encode_utf8` returns `&mut str` which can be coerced to `&str`, but isn’t when trait resolution is ambiguous.
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