Revert "Add implementations of last in terms of next_back on a bunch of DoubleEndedIterators."
This changed observable behavior for several iterator types.
r? @alexcrichton
Simplify RefCell minimum_spanning_tree example
This simplifies the implementation of the `minimum_spanning_tree` example of `RefCell` in the `cell` module-level docs, avoiding an unnecessary recursive call. This also eliminates the need for a block to contain the scope of the borrow in this example. But since that use of a block served an important didactic purpose, we make up for this by instead introducing a block in the initial, simpler example of `RefCell`, where the point will hopefully be conveyed to the reader more easily.
This allows types like Option<NonZeroU8> to be used in FFI without triggering the improper_ctypes lint. This works by changing the is_repr_nullable_ptr function to consider an enum E to be FFI-safe if:
- E has no explicit #[repr(...)].
- It only has two variants.
- One of those variants is empty (meaning it has no fields).
- The other variant has only one field.
- That field is one of the following:
- &T
- &mut T
- extern "C" fn
- core::num::NonZero*
- core::ptr::NonNull<T>
- #[repr(transparent)] struct wrapper around one of the types in this list.
- The size of E and its field are both known and are both the same size (implying E is participating in the nonnull optimization).
remove confusing remarks about mixed volatile and non-volatile accesses
These comments were originally added by @ecstatic-morse in 911d35f0bf and then later edited by me. The intention, I think, was to make sure people do both their reads and writes with these methods if the affected memory really is used for communication with external devices.
However, [people read this as saying that mixed volatile/non-volatile accesses are UB](https://github.com/rust-lang/rust/issues/58599#issuecomment-493791130), which -- to my knowledge -- they are not. So better remove this.
Cc @rkruppe @rust-lang/wg-unsafe-code-guidelines
Fix intra-doc link resolution failure on re-exporting libstd
Currently, re-exporting libstd items as below will [occur a lot of failures](https://gist.github.com/taiki-e/e33e0e8631ef47f65a74a3b69f456366).
```rust
pub use std::*;
```
Until the underlying issue (#56922) fixed, we can fix that so they don't propagate to downstream crates.
Related: https://github.com/rust-lang/rust/pull/56941 (That PR fixed failures that occur when re-exporting from libcore to libstd.)
r? @QuietMisdreavus
Fix the data types indication in basic examples of the Trait std::fmt::LowerExp and std::fmt::UpperExp.
Since there aren’t any type annotation on the let statement using the number 42.0, they are of type f64 according to The Book:
https://doc.rust-lang.org/book/ch03-02-data-types.html#floating-point-types
Use iter() for iterating arrays by slice
These `into_iter()` calls will change from iterating references to
values if we ever get `IntoIterator` for arrays, which may break the
code using that iterator. Calling `iter()` is future proof.
Mark core::alloc::Layout::from_size_align_unchecked const
Makes it possible (pending stabilization of #57563 (`const_fn`)) to rewrite code like
```rust
const BUFFER_SIZE: usize = 0x2000;
const BUFFER_ALIGN: usize = 0x1000;
fn foo() {
let layout = std::alloc::Layout::from_size_align(BUFFER_SIZE, BUFFER_ALIGN)
.unwrap();
let buffer = std::alloc::alloc(layout);
}
```
to
```rust
const BUFFER_LAYOUT: std::alloc::Layout = unsafe {
std::alloc::Layout::from_size_align_unchecked(0x2000, 0x1000)
};
fn foo() {
let buffer = std::alloc::alloc(BUFFER_LAYOUT);
}
```
which (although `unsafe` is used) looks somewhat cleaner and is easier to read.
These `into_iter()` calls will change from iterating references to
values if we ever get `IntoIterator` for arrays, which may break the
code using that iterator. Calling `iter()` is future proof.
as_ptr returns a read-only pointer
Add comments to `as_ptr` methods to warn that these are read-only pointers, and writing to them is UB.
[It was pointed out](https://internals.rust-lang.org/t/as-ptr-vs-as-mut-ptr/9940) that `CStr` does not even have an `as_mut_ptr`. I originally was going to add one, but there is no method at all that would mutate a `CStr`. Was that a deliberate choice or should I add an `as_mut_ptr` (similar to [what I did for `str`](https://github.com/rust-lang/rust/pull/58200))?
Add implementations of last in terms of next_back on a bunch of DoubleEndedIterators
Provided a `DoubleEndedIterator` has finite length, `Iterator::last` is equivalent to `DoubleEndedIterator::next_back`. But searching forwards through the iterator when it's unnecessary is obviously not good for performance. I ran into this on one of the collection iterators.
I tried adding appropriate overloads for a bunch of the iterator adapters like filter, map, etc, but I ran into a lot of type inference failures after doing so.
The other interesting case is what to do with `Repeat`. Do we consider it part of the contract that `Iterator::last` will loop forever on it? The docs do say that the iterator will be evaluated until it returns None. This is also relevant for the adapters, it's trivially easy to observe whether a `Map` adapter invoked its closure a zillion times or just once for the last element.
coretest: Downgrade deny to warn
The `deny` causes a build failure in https://github.com/RalfJung/miri-test-libstd. Since we use `-D warnings` for rustc builds, `warn` should be enough to lead to compile errors here, without impeding external builds.
impl From<Infallible> for TryFromSliceError
I believe this was missed when TryFrom was stabilized. I think `TryFromSliceError` and `TryFromIntError` are the only two `TryFrom` error types that appear in `std`. I think trait implementations have to be insta-stable, but I'm not sure.
Revert "Disable big-endian simd in swap_nonoverlapping_bytes"
This reverts commit 77bd4dc654 (#43159).
Issue #42778 was formerly easy to reproduce on two big-endian targets,
`powerpc64` and `s390x`, so we disabled SIMD on this function for all
big-endian targets as a workaround.
I have re-tested this code on `powerpc64` and `s390x`, each with the
bundled LLVM 8 and with external LLVM 7 and LLVM 6, and the problems no
longer appear. So it seems safe to remove this workaround, although I'm
still a little uncomfortable that we never found a root-cause...
Closes#42778.
r? @arielb1
Add a `cast` method to raw pointers.
This is similar to `NonNull::cast`.
Compared to the `as` operator (which has a wide range of meanings depending on the input and output types), a call to this method:
* Can only go from a raw pointer to a raw pointer
* Cannot change the pointer’s `const`ness
… even when the pointed types are inferred based on context.
This is similar to `NonNull::cast`.
Compared to the `as` operator (which has a wide range of meanings
depending on the input and output types), a call to this method:
* Can only go from a raw pointer to a raw pointer
* Cannot change the pointer’s `const`ness
… even when the pointed types are inferred based on context.
This reverts commit 77bd4dc654.
Issue #42778 was formerly easy to reproduce on two big-endian targets,
`powerpc64` and `s390x`, so we disabled SIMD on this function for all
big-endian targets as a workaround.
I have re-tested this code on `powerpc64` and `s390x`, each with the
bundled LLVM 8 and with external LLVM 7 and LLVM 6, and the problems no
longer appear. So it seems safe to remove this workaround, although I'm
still a little uncomfortable that we never found a root-cause...
... on different platforms.
Official rustdoc of
[`usize::to_le_bytes`](https://doc.rust-lang.org/std/primitive.usize.html#method.to_le_bytes)
displays signature
```
pub fn to_ne_bytes(self) -> [u8; 8]
```
which might be misleading: this function returns 4 bytes on 32-bit
systems.
