Stabilize `std::{rc,sync}::Weak::{weak_count, strong_count}`
* Original PR: #56696
* Tracking issue: #57977Closes: #57977
Supporting comments:
> Although these were added for testing, it is occasionally useful to have a way to probe optimistically for whether a weak pointer has become dangling, without actually taking the overhead of manipulating atomics. Are there any plans to stabilize this?
_Originally posted by @bdonlan in https://github.com/rust-lang/rust/issues/57977#issuecomment-516970921_
> Having this stabilized would help. Currently, the only way to check if a weak pointer has become dangling is to call `upgrade`, which is by far expensive.
_Originally posted by @glebpom in https://github.com/rust-lang/rust/issues/57977#issuecomment-526934709_
Not sure if stabilizing these warrants a full RFC, so throwing this out here as a start for now.
Note: per CONTRIBUTING.md, I ran the tidy checks, but they seem to be failing on unchanged files (primarily in `src/stdsimd`).
weak-into-raw: Clarify some details in Safety
Clarify it is OK to pass a pointer that never owned a weak count (one
from Weak::new) back into it as it was created from it. Relates to
discussion in #60728.
@CAD97 Do you want to have a look at the new docs?
Clarify it is OK to pass a pointer that never owned a weak count (one
from Weak::new) back into it as it was created from it. Relates to
discussion in #60728.
Layout::pad_to_align is infallible
As per [this comment](https://github.com/rust-lang/rust/issues/55724#issuecomment-441421651) (cc @glandium).
> Per eb981a1/src/libcore/alloc.rs (L63-L65), `layout.size()` is always <= `usize::MAX - (layout.align() - 1)`.
>
> Which means:
>
> * The maximum value `layout.size()` can have is already aligned for `layout.align()` (`layout.align()` being a power of two, `usize::MAX - (layout.align() - 1)` is a multiple of `layout.align()`)
> * Incidentally, any value smaller than that maximum value will align at most to that maximum value.
>
> IOW, `pad_to_align` can not return `Err(LayoutErr)`, except for the layout not respecting its invariants, but we shouldn't care about that.
This PR makes `pad_to_align` return `Layout` directly, representing the fact that it cannot fail.
alloc: Add new_zeroed() versions like new_uninit().
MaybeUninit has both uninit() and zeroed(), it seems reasonable to have the same
surface on Box/Rc/Arc.
Needs tests.
cc #63291
Add `impl<T> FromIterator<T> for Arc/Rc<[T]>`
Add implementations of `FromIterator<T> for Arc/Rc<[T]>` with symmetrical logic.
This also takes advantage of specialization in the case of iterators with known length (`TrustedLen`) to elide the final allocation/copying from a `Vec<T>` into `Rc<[T]>` because we can allocate the space for the `Rc<[T]>` directly when the size is known. This is the primary motivation and why this is to be preferred over `iter.collect::<Vec<_>>().into(): Rc<[T]>`.
Moreover, this PR does some refactoring in some places.
r? @RalfJung for the code
cc @alexcrichton from T-libs
