poll_fn and Unpin: fix pinning
See [IRLO](https://internals.rust-lang.org/t/surprising-soundness-trouble-around-pollfn/17484) for details: currently `poll_fn` is very subtle to use, since it does not pin the closure, so creating a `Pin::get_unchcked(&mut capture)` inside the closure is unsound. This leads to actual miscompilations with `futures::join!`.
IMO the proper fix is to pin the closure when the future is pinned, which is achieved by changing the `Unpin` implementation. This is a breaking change though. 1.64.0 was *just* released, so maybe this is still okay?
The alternative would be to add some strong comments to the docs saying that closure captures are *not pinned* and doing `Pin::get_unchecked` on them is unsound.
Clarify documentation about the memory layout of `UnsafeCell`
This PR addresses a [comment](https://github.com/rust-lang/rust/pull/101717#issuecomment-1279908390) by `@RalfJung` in PR #101717 to further clarify the documentation of `UnsafeCell<T>`. The previous PR was merged already before we had a chance to correct this, hence this second PR :)
To goal of this PR is:
1. Split the paragraph about the memory layout of `UnsafeCell<T>` and the usage of `UnsafeCell::(raw_)get()` into two paragraphs, so that it is easier to digest for the reader.
2. Slightly simplify the previously added examples in order to reduce redundancy between the new examples and the examples that already [existed](ddd119b2fe/library/core/src/cell.rs (L1858-L1908)) before these 2 PRs (which remained untouched by both PRs).
remove redundant Send impl for references
Also explain why the other instance is not redundant, move it next to the trait they are implementing, and out of the redundant module. This seems to go back all the way to 35ca50bd56, not sure why the module was added.
The instance for `&mut` is the default instance we get anyway, and we don't have anything similar for `Sync`, so IMO we should be consistent and not have the redundant instance here, either.
Try to say that memory outside the AM is always exposed
cc ``@Gankra`` ``@thomcc``
I want to confidently tell people that they can use `from_exposed_addr` to get a pointer for doing MMIO and/or other hardware interactions done with volatile reads/writes at particular addresses outside the Rust AM. Currently, the docs indicate that would be UB.
With this change, now the docs indicate that this is intended to be a valid use of `from_exposed_addr`.
r? ``@RalfJung``
Even nicer errors from assert_unsafe_precondition
For example, now running `cargo test` with this patch I get things like:
```
$ cargo +stage1 test
Finished test [unoptimized + debuginfo] target(s) in 0.01s
Running unittests src/lib.rs (target/debug/deps/malloc_buf-9d105ddf86862995)
running 5 tests
thread 'tests::test_null_buf' panicked at 'unsafe precondition violated: is_aligned_and_not_null(data) &&
crate::mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize', /home/ben/rust/library/core/src/slice/raw.rs:93:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread panicked while panicking. aborting.
error: test failed, to rerun pass `--lib`
Caused by:
process didn't exit successfully: `/tmp/malloc_buf-1.0.0/target/debug/deps/malloc_buf-9d105ddf86862995` (signal: 6, SIGABRT: process abort signal)
```
This is still not perfect, but these are better for another PR:
* `stringify!` is trying to do clever pretty-printing on the `expr` inside `assert_unsafe_precondition` and can even add a newline.
* It would be nice to print a bit more information about where the problem is. Perhaps this is `cfg_attr(debug_assertions, track_caller)`, or perhaps it the function name added to `Location`.
cc ``@RalfJung`` this is what I was thinking of for https://github.com/rust-lang/rust/pull/102732#discussion_r989068907
ptr::eq: clarify that comparing dyn Trait is fragile
Also remove the dyn trait example from `ptr::eq` since those tests are not actually guaranteed to pass due to how unstable vtable comparison is.
Cc ``@rust-lang/libs-api``
Cc discussion following https://github.com/rust-lang/rust/pull/80505
Use a faster allocation size check in slice::from_raw_parts
I've been perusing through the codegen changes that result from turning on the standard library debug assertions. The previous check in here uses saturating arithmetic, which in my experience sometimes makes LLVM just fail to optimize things around the saturating operation.
Here is a demo of the codegen difference: https://godbolt.org/z/WMEqrjajW
Before:
```asm
example::len_check_old:
mov rax, rdi
mov ecx, 3
mul rcx
setno cl
test rax, rax
setns al
and al, cl
ret
example::len_check_old:
mov rax, rdi
mov ecx, 8
mul rcx
setno cl
test rax, rax
setns al
and al, cl
ret
```
After:
```asm
example::len_check_new:
movabs rax, 3074457345618258603
cmp rdi, rax
setb al
ret
example::len_check_new:
shr rdi, 60
sete al
ret
```
Running rustc-perf locally, this looks like up to a 4.5% improvement when `debug-assertions-std = true`.
Thanks ```@LegionMammal978``` (I think that's you?) for turning my idea into a much cleaner implementation.
r? ```@thomcc```
Stabilize `duration_checked_float`
## Stabilization Report
This stabilization report is for a stabilization of `duration_checked_float`, tracking issue: https://github.com/rust-lang/rust/issues/83400.
### Implementation History
- https://github.com/rust-lang/rust/pull/82179
- https://github.com/rust-lang/rust/pull/90247
- https://github.com/rust-lang/rust/pull/96051
- Changed error type to `FromFloatSecsError` in https://github.com/rust-lang/rust/pull/90247
- https://github.com/rust-lang/rust/pull/96051 changes the rounding mode to round-to-nearest instead of truncate.
## API Summary
This stabilization report proposes the following API to be stabilized in `core`, along with their re-exports in `std`:
```rust
// core::time
impl Duration {
pub const fn try_from_secs_f32(secs: f32) -> Result<Duration, TryFromFloatSecsError>;
pub const fn try_from_secs_f64(secs: f64) -> Result<Duration, TryFromFloatSecsError>;
}
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TryFromFloatSecsError { ... }
impl core::fmt::Display for TryFromFloatSecsError { ... }
impl core::error::Error for TryFromFloatSecsError { ... }
```
These functions are made const unstable under `duration_consts_float`, tracking issue #72440.
There is an open question in the tracking issue around what the error type should be called which I was hoping to resolve in the context of an FCP.
In this stabilization PR, I have altered the name of the error type to `TryFromFloatSecsError`. In my opinion, the error type shares the name of the method (adjusted to accommodate both types of floats), which is consistent with other error types in `core`, `alloc` and `std` like `TryReserveError` and `TryFromIntError`.
## Experience Report
Code such as this is ready to be converted to a checked API to ensure it is panic free:
```rust
impl Time {
pub fn checked_add_f64(&self, seconds: f64) -> Result<Self, TimeError> {
// Fail safely during `f64` conversion to duration
if seconds.is_nan() || seconds.is_infinite() {
return Err(TzOutOfRangeError::new().into());
}
if seconds.is_sign_positive() {
self.checked_add(Duration::from_secs_f64(seconds))
} else {
self.checked_sub(Duration::from_secs_f64(-seconds))
}
}
}
```
See: https://github.com/artichoke/artichoke/issues/2194.
`@rustbot` label +T-libs-api -T-libs
cc `@mbartlett21`
Use ptr::metadata in <[T]>::len implementation
This avoids duplication of ptr::metadata code.
I believe this is acceptable as the previous approach essentially duplicated `ptr::metadata` because back then `rustc_allow_const_fn_unstable` annotation did not exist.
I would like somebody to ping `@rust-lang/wg-const-eval` as the documentation says:
> Always ping `@rust-lang/wg-const-eval` if you are adding more rustc_allow_const_fn_unstable attributes to any const fn.
`MaybeUninit`: use `assume_init_drop()` in the partially initialized array example
The `assume_init_drop()` method does the same thing as the pointer conversion, and makes the example more straightforward.
Clarify `array::from_fn` documentation
I've seen quite a few of people on social media confused of where the length of array is coming from in the newly stabilized `array::from_fn` example.
This PR tries to clarify the documentation on this.
Adjust argument type for mutable with_metadata_of (#75091)
The method takes two pointer arguments: one `self` supplying the pointer value, and a second pointer supplying the metadata.
The new parameter type more clearly reflects the actual requirements. The provenance of the metadata parameter is disregarded completely. Using a mutable pointer in the call site can be coerced to a const pointer while the reverse is not true.
In some cases, the current parameter type can thus lead to a very slightly confusing additional cast. [Example](cad93775eb).
```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;
let ptr = val as *const _ as *mut T;
let ptr = uninit.as_ptr().with_metadata_of(ptr);
```
This could then instead be simplified to:
```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;
let ptr = uninit.as_ptr().with_metadata_of(&**val);
```
Tracking issue: https://github.com/rust-lang/rust/issues/75091
``@dtolnay`` you're reviewed #95249, would you mind chiming in?
Add default trait implementations for "c-unwind" ABI function pointers
Following up on #92964, only add default trait implementations for the `c-unwind` family of function pointers. The previous attempt in #92964 added trait implementations for many more ABIs and ran into concerns regarding the increase in size of the libcore rlib.
An attempt to abstract away function pointer types behind a unified trait to reduce the duplication of trait impls is being discussed in #99531 but this change looks to be blocked on a lang MCP.
Following `@RalfJung's` suggestion in https://github.com/rust-lang/rust/pull/99531#issuecomment-1233440142, this commit is another cut at #92964 but it _only_ adds the impls for `extern "C-unwind" fn` and `unsafe extern "C-unwind" fn`.
I am interested in landing this patch to unblock the stabilization of the `c_unwind` feature.
RFC: https://github.com/rust-lang/rfcs/pull/2945
Tracking Issue: https://github.com/rust-lang/rust/issues/74990
The method takes two pointer arguments: one `self` supplying the pointer
value, and a second pointer supplying the metadata.
The new parameter type more clearly reflects the actual requirements.
The provenance of the metadata parameter is disregarded completely.
Using a mutable pointer in the call site can be coerced to a const
pointer while the reverse is not true.
An example of the current use:
```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;
let ptr = val as *const _ as *mut T;
let ptr = uninit.as_ptr().with_metadata_of(ptr);
```
This could then instead be simplified to:
```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;
let ptr = uninit.as_ptr().with_metadata_of(&**val);
```
Adjust `transmute{,_copy}` to be clearer about which of `T` and `U` is input vs output
This is essentially a documentation-only change (although it does touch code in an irrelevant way).
Following up on #92964, only add default trait implementations for the
`c-unwind` family of function pointers. The previous attempt in #92964
added trait implementations for many more ABIs and ran into concerns
regarding the increase in size of the libcore rlib.
An attempt to abstract away function pointer types behind a unified
trait to reduce the duplication of trait impls is being discussed in #99531
but this change looks to be blocked on a lang MCP.
Following @RalfJung's suggestion in
https://github.com/rust-lang/rust/pull/99531#issuecomment-1233440142,
this commit is another cut at #92964 but it _only_ adds the impls for
`extern "C-unwind" fn` and `unsafe extern "C-unwind" fn`.
I am interested in landing this patch to unblock the stabilization of
the `c_unwind` feature.
RFC: https://github.com/rust-lang/rfcs/pull/2945
Tracking Issue: https://github.com/rust-lang/rust/issues/74990
Make transpose const and inline
r? `@scottmcm`
- These should have been const from the beginning since we're never going to do more than a transmute.
- Inline these always because that's what every other method in MaybeUninit which simply casts does. :) Ok, but a stronger justification is that because we're taking in arrays by `self`, not inlining would defeat the whole purpose of using `MaybeUninit` due to the copying.
Optimize `slice_iter.copied().next_chunk()`
```
OLD:
test iter::bench_copied_array_chunks ... bench: 371 ns/iter (+/- 7)
NEW:
test iter::bench_copied_array_chunks ... bench: 31 ns/iter (+/- 0)
```
The default `next_chunk` implementation suffers from having to assemble the array byte by byte via `next()`, checking the `Option<&T>` and then dereferencing `&T`. The specialization copies the chunk directly from the slice.
More slice::partition_point examples
After seeing the discussion of `binary_search` vs `partition_point` in #101999, I thought some more example code could be helpful.
doc: rewrite doc for uint::{carrying_add,borrowing_sub}
Reword the documentation for bigint helper methods `uint::{carrying_add,borrowing_sub}` (#85532).
The examples were also rewritten to demonstrate how the methods can be used in bignum arithmetic. No loops are used in the examples, but the variable names were chosen to include indices so that it is clear how this can be used in a loop if required.
Also, previously `carrying_add` had an example to say that if the input carry is false, the method is equivalent to `overflowing_add`. While the note was kept, the example was removed and an extra note was added to make sure this equivalence is not assumed for signed integers as well.
Remove the redundant `Some(try_opt!(..))` in `checked_pow`
The final return value doesn't need to be tried at all -- we can just
return the checked option directly. The optimizer can probably figure
this out anyway, but there's no need to make it work here.
The final return value doesn't need to be tried at all -- we can just
return the checked option directly. The optimizer can probably figure
this out anyway, but there's no need to make it work here.
