- Rewords existing Considerations section on `fetch_update` and friends
to make clear that the limitations are inherent to an implementation based on any
CAS operation, rather than the weak version of `compare_exchange` in particular
- Add Considerations to `compare_exchange` and `compare_exchange_weak`
which details similar considerations and when they may be relevant.
atomic_load intrinsic: use const generic parameter for ordering
We have a gazillion intrinsics for the atomics because we encode the ordering into the intrinsic name rather than making it a parameter. This is particularly bad for those operations that take two orderings. Let's fix that!
This PR only converts `load`, to see if there's any feedback that would fundamentally change the strategy we pursue for the const generic intrinsics.
The first two commits are preparation and could be a separate PR if you prefer.
`@BoxyUwU` -- I hope this is a use of const generics that is unlikely to explode? All we need is a const generic of enum type. We could funnel it through an integer if we had to but an enum is obviously nicer...
`@bjorn3` it seems like the cranelift backend entirely ignores the ordering?
This updates some doctests that fail to run on wasm. We will soon be
supporting cross-compiled doctests, and the test-various job fails to
run these tests. These tests fail because wasm32-wasip1 does not support
threads.
Remove `#[cfg(not(test))]` gates in `core`
These gates are unnecessary now that unit tests for `core` are in a separate package, `coretests`, instead of in the same files as the source code. They previously prevented the two `core` versions from conflicting with each other.
atomic: clarify that failing conditional RMW operations are not 'writes'
Fixes https://github.com/rust-lang/rust/issues/136669
r? ``@Amanieu``
Cc ``@rust-lang/opsem`` ``@chorman0773`` ``@gnzlbg`` ``@briansmith``
Use `std::mem::{size_of, size_of_val, align_of, align_of_val}` from the
prelude instead of importing or qualifying them.
These functions were added to all preludes in Rust 1.80.
These gates are unnecessary now that unit tests for `core` are in a
separate package, `coretests`, instead of in the same files as the
source code. They previously prevented the two `core` versions from
conflicting with each other.
Fundamentally, we have *three* disjoint categories of functions:
1. const-stable functions
2. private/unstable functions that are meant to be callable from const-stable functions
3. functions that can make use of unstable const features
This PR implements the following system:
- `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions.
- `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category.
- `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls.
Also, several holes in recursive const stability checking are being closed.
There's still one potential hole that is hard to avoid, which is when MIR
building automatically inserts calls to a particular function in stable
functions -- which happens in the panic machinery. Those need to *not* be
`rustc_const_unstable` (or manually get a `rustc_const_stable_indirect`) to be
sure they follow recursive const stability. But that's a fairly rare and special
case so IMO it's fine.
The net effect of this is that a `#[unstable]` or unmarked function can be
constified simply by marking it as `const fn`, and it will then be
const-callable from stable `const fn` and subject to recursive const stability
requirements. If it is publicly reachable (which implies it cannot be unmarked),
it will be const-unstable under the same feature gate. Only if the function ever
becomes `#[stable]` does it need a `#[rustc_const_unstable]` or
`#[rustc_const_stable]` marker to decide if this should also imply
const-stability.
Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to
use unstable const lang features (including intrinsics), or (b) `#[stable]`
functions that are not yet intended to be const-stable. Adding
`#[rustc_const_stable]` is only needed for functions that are actually meant to
be directly callable from stable const code. `#[rustc_const_stable_indirect]` is
used to mark intrinsics as const-callable and for `#[rustc_const_unstable]`
functions that are actually called from other, exposed-on-stable `const fn`. No
other attributes are required.
stabilize const_cell_into_inner
This const-stabilizes
- `UnsafeCell::into_inner`
- `Cell::into_inner`
- `RefCell::into_inner`
- `OnceCell::into_inner`
`@rust-lang/wg-const-eval` this uses `rustc_allow_const_fn_unstable(const_precise_live_drops)`, so we'd be comitting to always finding *some* way to accept this code. IMO that's fine -- what these functions do is to move out the only field of a struct, and that struct has no destructor itself. The field's destructor does not get run as it gets returned to the caller.
`@rust-lang/libs-api` this was FCP'd already [years ago](https://github.com/rust-lang/rust/issues/78729#issuecomment-811409860), except that `OnceCell::into_inner` was added to the same feature gate since then (Cc `@tgross35).` Does that mean we have to re-run the FCP? If yes, I'd honestly prefer to move `OnceCell` into its own feature gate to not risk missing the next release. (That's why it's not great to add new functions to an already FCP'd feature gate.) OTOH if this needs an FCP either way since the previous FCP was so long ago, then we might as well do it all at once.
Update docs for AtomicBool/U8/I8 with regard to alignment
Fixes#126084.
Since `AtomicBool`/`AtomicU8`/`AtomicI8` are guaranteed to have size == 1, and Rust guarantees that `size % align == 0`, they also must have alignment equal to 1, so some current docs are contradictory/confusing when describing their alignment requirements.
Specifically:
* Fix `AtomicBool::from_ptr` claiming that `align_of::<AtomicBool>() > align_of::<bool>()` on some platforms. (same for `AtomicU8::from_ptr`/`AtomicI8::from_ptr`)
* Explicitly state that `AtomicU8`/`AtomicI8` have the same alignment as `u8`/`i8` (in addition to size and bit validity)
* (internal) Change the `if_not_8_bit` macro to be `if_8_bit` and to allow an "if-else"-like structure, instead of just "if"-like.
---
I opted to leave the "`ptr` must be aligned" wording in `from_ptr`'s docs and just clarify that it is always satsified, instead of just removing the wording entirely. If that is instead preferred I can do that.
Clarify that they always have the same alignment as u8/i8, (unlike other atomic types).
Clarify in from_ptr that alignment is never an issue because of this.
