safe transmute: use `Assume` struct to provide analysis options
This task was left as a TODO in #92268; resolving it brings [`BikeshedIntrinsicFrom`](https://doc.rust-lang.org/nightly/core/mem/trait.BikeshedIntrinsicFrom.html) more in line with the API defined in [MCP411](https://github.com/rust-lang/compiler-team/issues/411).
**Before:**
```rust
pub unsafe trait BikeshedIntrinsicFrom<
Src,
Context,
const ASSUME_ALIGNMENT: bool,
const ASSUME_LIFETIMES: bool,
const ASSUME_VALIDITY: bool,
const ASSUME_VISIBILITY: bool,
> where
Src: ?Sized,
{}
```
**After:**
```rust
pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
where
Src: ?Sized,
{}
```
`Assume::visibility` has also been renamed to `Assume::safety`, as library safety invariants are what's actually being assumed; visibility is just the mechanism by which it is currently checked (and that may change).
r? `@oli-obk`
---
Related:
- https://github.com/rust-lang/compiler-team/issues/411
- https://github.com/rust-lang/rust/issues/99571
make slice::{split_at,split_at_unchecked} const functions
Now that `slice::from_raw_parts` is const in stable 1.64, it makes sense to have `split_at` const as well, otherwise unsafe code is required to achieve a const equivalent.
mem::uninitialized: mitigate many incorrect uses of this function
Alternative to https://github.com/rust-lang/rust/pull/98966: fill memory with `0x01` rather than leaving it uninit. This is definitely bitewise valid for all `bool` and nonnull types, and also those `Option<&T>` that we started putting `noundef` on. However it is still invalid for `char` and some enums, and on references the `dereferenceable` attribute is still violated, so the generated LLVM IR still has UB -- but in fewer cases, and `dereferenceable` is hopefully less likely to cause problems than clearly incorrect range annotations.
This can make using `mem::uninitialized` a lot slower, but that function has been deprecated for years and we keep telling everyone to move to `MaybeUninit` because it is basically impossible to use `mem::uninitialized` correctly. For the cases where that hasn't helped (and all the old code out there that nobody will ever update), we can at least mitigate the effect of using this API. Note that this is *not* in any way a stable guarantee -- it is still UB to call `mem::uninitialized::<bool>()`, and Miri will call it out as such.
This is somewhat similar to https://github.com/rust-lang/rust/pull/87032, which proposed to make `uninitialized` return a buffer filled with 0x00. However
- That PR also proposed to reduce the situations in which we panic, which I don't think we should do at this time.
- The 0x01 bit pattern means that nonnull requirements are satisfied, which (due to references) is the most common validity invariant.
`@5225225` I hope I am using `cfg(sanitize)` the right way; I was not sure for which ones to test here.
Cc https://github.com/rust-lang/rust/issues/66151
Fixes https://github.com/rust-lang/rust/issues/87675
Stabilize `derive_default_enum`
This stabilizes `#![feature(derive_default_enum)]`, as proposed in [RFC 3107](https://github.com/rust-lang/rfcs/pull/3107) and tracked in #87517. In short, it permits you to `#[derive(Default)]` on `enum`s, indicating what the default should be by placing a `#[default]` attribute on the desired variant (which must be a unit variant in the interest of forward compatibility).
```````@rustbot``````` label +S-waiting-on-review +T-lang
Add SyncUnsafeCell.
This adds `SyncUnsafeCell`, which is just `UnsafeCell` except it implements `Sync`.
This was first proposed under the name `RacyUnsafeCell` here: https://github.com/rust-lang/rust/issues/53639#issuecomment-415515748 and here: https://github.com/rust-lang/rust/issues/53639#issuecomment-432741659 and here: https://github.com/rust-lang/rust/issues/53639#issuecomment-888435728
It allows you to create an UnsafeCell that is Sync without having to wrap it in a struct first (and then implement Sync for that struct).
E.g. `static X: SyncUnsafeCell<i32>`. Using a regular `UnsafeCell` as `static` is not possible, because it isn't `Sync`. We have a language workaround for it called `static mut`, but it's nice to be able to use the proper type for such unsafety instead.
It also makes implementing synchronization primitives based on unsafe cells slightly less verbose, because by using `SyncUnsafeCell` for `UnsafeCell`s that are shared between threads, you don't need a separate `impl<..> Sync for ..`. Using this type also clearly documents that the cell is expected to be accessed from multiple threads.
This is a continuation of #60109, which noted that while the ADX
intrinsics were stabilized, the corresponding target feature never was.
This PR follows the same general structure and stabilizes the ADX target
feature.
Constify `Index{,Mut}` for `[T]`, `str`, and `[T; N]`
Several panic functions were rewired (via `const_eval_select`) to simpler implementations that do not require formatting for compile-time usage.
r? ```@oli-obk```
Merge `#[deprecated]` and `#[rustc_deprecated]`
The first commit makes "reason" an alias for "note" in `#[rustc_deprecated]`, while still prohibiting it in `#[deprecated]`.
The second commit changes "suggestion" to not just be a feature of `#[rustc_deprecated]`. This is placed behind the new `deprecated_suggestion` feature. This needs a tracking issue; let me know if this PR will be approved and I can create one.
The third commit is what permits `#[deprecated]` to be used when `#![feature(staged_api)]` is enabled. This isn't yet used in stdlib (only tests), as it would require duplicating all deprecation attributes until a bootstrap occurs. I intend to submit a follow-up PR that replaces all uses and removes the remaining `#[rustc_deprecated]` code after the next bootstrap.
`@rustbot` label +T-libs-api +C-feature-request +A-attributes +S-waiting-on-review
