https://github.com/rust-lang/rust/pull/49496 introduced specialization based on:
```
unsafe impl<T: ?Sized> IsZero for *mut T {
fn is_zero(&self) -> bool {
(*self).is_null()
}
}
```
… to call `RawVec::with_capacity_zeroed` for creating `Vec<*mut T>`,
which is incorrect for fat pointers
since `<*mut T>::is_null` only looks at the data component.
That is, a fat pointer can be “null” without being made entirely of zero bits.
This commit fixes it by removing the `?Sized` bound on this impl
(and the corresponding `*const T` one).
This regresses `vec![x; n]` with `x` a null raw slice of length zero,
but that seems exceptionally uncommon.
(Vtable pointers are never null, so raw trait objects would not take
the fast path anyway.
An alternative to keep the `?Sized` bound
(or even generalize to `impl<U: Copy> IsZero for U`)
would be to cast to `&[u8]` of length `size_of::<U>()`,
but the optimizer seems not to be able to propagate alignment information
and sticks with comparing one byte at a time:
https://rust.godbolt.org/z/xQFkwL
----
Without the library change, the new test fails as follows:
```
---- vec::vec_macro_repeating_null_raw_fat_pointer stdout ----
[src/liballoc/tests/vec.rs:1301] ptr_metadata(raw_dyn) = 0x00005596ef95f9a8
[src/liballoc/tests/vec.rs:1306] ptr_metadata(vec[0]) = 0x0000000000000000
thread 'vec::vec_macro_repeating_null_raw_fat_pointer' panicked at 'assertion failed: vec[0] == null_raw_dyn', src/liballoc/tests/vec.rs:1307:5
```
Improve BTreeSet::Intersection::size_hint
A comment on `IntersectionInner` mentions `small_iter` should be smaller than `other_iter` but this condition is broken while iterating because those two iterators can be consumed at a different rate. I added a test to demonstrate this situation.
<del>I made `small_iter.len() < other_iter.len()` always true by swapping two iterators when that condition became false. This change affects the return value of `size_hint`. The previous result was also correct but this new version always returns smaller upper bound than the previous version.</del>
I changed `size_hint` to taking minimum of both lengths of iterators and renamed fields to `a` and `b` to match `Union` iterator.
The commented invariant that an iterator is smaller than other iterator
was violated after next is called and two iterators are consumed at
different rates.
Fast path for vec.clear/truncate
For trivial types like `u8`, `vec.truncate()`/`vec.clear()` relies on the optimizer to remove the loop. This means more work in debug builds, and more work for the optimizer.
Avoiding this busywork is exactly what `mem::needs_drop::<T>()` is for.
This allows lints and other diagnostics to refer to items
by a unique ID instead of relying on whacky path
resolution schemes that may break when items are
relocated.
Remove recommendation about idiomatic syntax for Arc::clone
I believe we should not make this recommendation. I don't want to argue that `Arc::clone` is less idiomatic than `arc.clone`, but that the choice is not clear cut and that we should not be making this kind of call in the docs.
The `.clone()` form has advantages too: it is more succinct, it is more likely to be understood by beginners, and it is more uniform with other `clone` calls, indeed with most other method calls.
Whichever approach is better, I think that this discussion belongs in a style guide or textbook, rather than the library docs. We don't talk much about idiomatic code in the docs, this place is pretty exceptional.
The recommendation is also not followed in this repo. It is hard to figure out how many calls there are of the `.clone()` form, but there are 1550 uses of `Arc` and only 65 uses of `Arc::clone`. The recommendation has existed for over two years.
The recommendation was added in https://github.com/rust-lang/rust/pull/42137, as a result of https://github.com/rust-lang/rfcs/pull/1954. However, note that that RFC was closed because it was not necessary to change the docs (the original RFC proposed a new function instead). So I don't think an RFC is necessary here (and I'm not trying to re-litigate the discussion on that RFC (which favoured `Arc::clone` as idiomatic) in any case).
cc @nical (who added the docs in the first place; sorry :-) )
r? @alexcrichton (or someone else on @rust-lang/libs )
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.
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 {…} }
```
… 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
Mazdak Farrokhzad