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Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>
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Add documentation about the memory layout of `UnsafeCell<T>`
The documentation for `UnsafeCell<T>` currently does not make any promises about its memory layout. This PR adds this documentation, namely that the memory layout of `UnsafeCell<T>` is the same as the memory layout of its inner `T`.
# Use case
Without this layout promise, the following cast would not be legally possible:
```rust
fn example<T>(ptr: *mut T) -> *const UnsafeCell<T> {
ptr as *const UnsafeCell<T>
}
```
A use case where this can come up involves FFI. If Rust receives a pointer over a FFI boundary which provides shared read-write access (with some form of custom synchronization), and this pointer is managed by some Rust struct with lifetime `'a`, then it would greatly simplify its (internal) API and safety contract if a `&'a UnsafeCell<T>` can be created from a raw FFI pointer `*mut T`. A lot of safety checks can be done when receiving the pointer for the first time through FFI (non-nullness, alignment, initialize uninit bytes, etc.) and these properties can then be encoded into the `&UnsafeCell<T>` type. Without this documentation guarantee, this is not legal today outside of the standard library.
# Caveats
Casting in the opposite direction is still not valid, even with this documentation change:
```rust
fn example2<T>(ptr: &UnsafeCell<T>) -> &mut T {
let t = ptr as *const UnsafeCell<T> as *mut T;
unsafe { &mut *t }
}
```
This is because the only legal way to obtain a mutable pointer to the contents of the shared reference is through [`UnsafeCell::get`](https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html#method.get) and [`UnsafeCell::raw_get`](https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html#method.raw_get). Although there might be a desire to also make this legal at some point in the future, that part is outside the scope of this PR. Also see this relevant [Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/transmuting.20.26.20-.3E.20.26mut).
# Alternatives
Instead of adding a new documentation promise, it's also possible to add a new method to `UnsafeCell<T>` with signature `pub fn from_ptr_bikeshed(ptr: *mut T) -> *const UnsafeCell<T>` which indirectly only allows one-way casting to `*const UnsafeCell<T>`.
…r=scottmcm Add MaybeUninit array transpose From impls See discussion in rust-lang#101179 and rust-lang#96097. I believe this solution offers the simplest implementation with minimal future API regret. `@RalfJung` mind doing a correctness review?
Update pkg-config I'd like to be able to cross-compile rustc in a scenario where it'd be really helpful to have rust-lang/pkg-config-rs@cd3ccca. I've done some test builds of the compiler on x86_64 linux, targeting x86_64 linux and aarch64 linux.
…imes, r=cjgillot pretty: fix to print some lifetimes on HIR pretty-print HIR pretty-printer doesn't seem to print some lifetimes in types. This PR fixes that. Closes rust-lang#85089
Surround type with backticks Very smol PR. :)
…r=notriddle Fix settings page Thanks to rust-lang#103060, I discovered that the settings page was badly rendered. This PR fixes it. Before:  After:  r? ```@notriddle```
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UnsafeCell<T>#101717 (Add documentation about the memory layout ofUnsafeCell<T>)Failed merges:
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