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Auto merge of #126877 - GrigorenkoPV:clone_to_uninit, r=dtolnay 

CloneToUninit impls

As per #126799.

Also implements it for `Wtf8` and both versions of `os_str::Slice`.

Maybe it is worth to slap `#[inline]` on some of those impls.

r? `@dtolnay`
This commit is contained in:
bors 2024-08-17 11:39:08 +00:00
parent 426a60abc2 110c273f4f
commit c6f81a452e
11 changed files with 290 additions and 105 deletions
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127
library/core/src/clone.rs
View File

@ -36,8 +36,7 @@
#![stable(feature = "rust1", since = "1.0.0")]
use crate::mem::{self, MaybeUninit};
use crate::ptr;
mod uninit;
/// A common trait for the ability to explicitly duplicate an object.
///
@ -248,7 +247,7 @@ pub unsafe trait CloneToUninit {
/// * `dst` must be properly aligned.
/// * `dst` must have the same [pointer metadata] (slice length or `dyn` vtable) as `self`.
///
/// [valid]: ptr#safety
/// [valid]: crate::ptr#safety
/// [pointer metadata]: crate::ptr::metadata()
///
/// # Panics
@ -272,124 +271,42 @@ pub unsafe trait CloneToUninit {
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Clone> CloneToUninit for T {
default unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::write().
unsafe {
// We hope the optimizer will figure out to create the cloned value in-place,
// skipping ever storing it on the stack and the copy to the destination.
ptr::write(dst, self.clone());
}
}
}
// Specialized implementation for types that are [`Copy`], not just [`Clone`],
// and can therefore be copied bitwise.
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Copy> CloneToUninit for T {
#[inline]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::copy_nonoverlapping().
unsafe {
ptr::copy_nonoverlapping(self, dst, 1);
}
// SAFETY: we're calling a specialization with the same contract
unsafe { <T as self::uninit::CopySpec>::clone_one(self, dst) }
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Clone> CloneToUninit for [T] {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
default unsafe fn clone_to_uninit(&self, dst: *mut Self) {
let len = self.len();
// This is the most likely mistake to make, so check it as a debug assertion.
debug_assert_eq!(
len,
dst.len(),
"clone_to_uninit() source and destination must have equal lengths",
);
// SAFETY: The produced `&mut` is valid because:
// * The caller is obligated to provide a pointer which is valid for writes.
// * All bytes pointed to are in MaybeUninit, so we don't care about the memory's
// initialization status.
let uninit_ref = unsafe { &mut *(dst as *mut [MaybeUninit<T>]) };
// Copy the elements
let mut initializing = InitializingSlice::from_fully_uninit(uninit_ref);
for element_ref in self.iter() {
// If the clone() panics, `initializing` will take care of the cleanup.
initializing.push(element_ref.clone());
}
// If we reach here, then the entire slice is initialized, and we've satisfied our
// responsibilities to the caller. Disarm the cleanup guard by forgetting it.
mem::forget(initializing);
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: we're calling a specialization with the same contract
unsafe { <T as self::uninit::CopySpec>::clone_slice(self, dst) }
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Copy> CloneToUninit for [T] {
unsafe impl CloneToUninit for str {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
let len = self.len();
// This is the most likely mistake to make, so check it as a debug assertion.
debug_assert_eq!(
len,
dst.len(),
"clone_to_uninit() source and destination must have equal lengths",
);
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::copy_nonoverlapping().
unsafe {
ptr::copy_nonoverlapping(self.as_ptr(), dst.as_mut_ptr(), len);
}
// SAFETY: str is just a [u8] with UTF-8 invariant
unsafe { self.as_bytes().clone_to_uninit(dst as *mut [u8]) }
}
}
/// Ownership of a collection of values stored in a non-owned `[MaybeUninit<T>]`, some of which
/// are not yet initialized. This is sort of like a `Vec` that doesn't own its allocation.
/// Its responsibility is to provide cleanup on unwind by dropping the values that *are*
/// initialized, unless disarmed by forgetting.
///
/// This is a helper for `impl<T: Clone> CloneToUninit for [T]`.
struct InitializingSlice<'a, T> {
data: &'a mut [MaybeUninit<T>],
/// Number of elements of `*self.data` that are initialized.
initialized_len: usize,
}
impl<'a, T> InitializingSlice<'a, T> {
#[inline]
fn from_fully_uninit(data: &'a mut [MaybeUninit<T>]) -> Self {
Self { data, initialized_len: 0 }
}
/// Push a value onto the end of the initialized part of the slice.
///
/// # Panics
///
/// Panics if the slice is already fully initialized.
#[inline]
fn push(&mut self, value: T) {
MaybeUninit::write(&mut self.data[self.initialized_len], value);
self.initialized_len += 1;
}
}
impl<'a, T> Drop for InitializingSlice<'a, T> {
#[cold] // will only be invoked on unwind
fn drop(&mut self) {
let initialized_slice = ptr::slice_from_raw_parts_mut(
MaybeUninit::slice_as_mut_ptr(self.data),
self.initialized_len,
);
// SAFETY:
// * the pointer is valid because it was made from a mutable reference
// * `initialized_len` counts the initialized elements as an invariant of this type,
// so each of the pointed-to elements is initialized and may be dropped.
unsafe {
ptr::drop_in_place::<[T]>(initialized_slice);
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl CloneToUninit for crate::ffi::CStr {
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: For now, CStr is just a #[repr(trasnsparent)] [c_char] with some invariants.
// And we can cast [c_char] to [u8] on all supported platforms (see: to_bytes_with_nul).
// The pointer metadata properly preserves the length (NUL included).
// See: `cstr_metadata_is_length_with_nul` in tests.
unsafe { self.to_bytes_with_nul().clone_to_uninit(dst as *mut [u8]) }
}
}

128
library/core/src/clone/uninit.rs Normal file
View File

@ -0,0 +1,128 @@
use crate::mem::{self, MaybeUninit};
use crate::ptr;
/// Private specialization trait used by CloneToUninit, as per
/// [the dev guide](https://std-dev-guide.rust-lang.org/policy/specialization.html).
pub(super) unsafe trait CopySpec: Clone {
unsafe fn clone_one(src: &Self, dst: *mut Self);
unsafe fn clone_slice(src: &[Self], dst: *mut [Self]);
}
unsafe impl<T: Clone> CopySpec for T {
#[inline]
default unsafe fn clone_one(src: &Self, dst: *mut Self) {
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::write().
unsafe {
// We hope the optimizer will figure out to create the cloned value in-place,
// skipping ever storing it on the stack and the copy to the destination.
ptr::write(dst, src.clone());
}
}
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
default unsafe fn clone_slice(src: &[Self], dst: *mut [Self]) {
let len = src.len();
// This is the most likely mistake to make, so check it as a debug assertion.
debug_assert_eq!(
len,
dst.len(),
"clone_to_uninit() source and destination must have equal lengths",
);
// SAFETY: The produced `&mut` is valid because:
// * The caller is obligated to provide a pointer which is valid for writes.
// * All bytes pointed to are in MaybeUninit, so we don't care about the memory's
// initialization status.
let uninit_ref = unsafe { &mut *(dst as *mut [MaybeUninit<T>]) };
// Copy the elements
let mut initializing = InitializingSlice::from_fully_uninit(uninit_ref);
for element_ref in src {
// If the clone() panics, `initializing` will take care of the cleanup.
initializing.push(element_ref.clone());
}
// If we reach here, then the entire slice is initialized, and we've satisfied our
// responsibilities to the caller. Disarm the cleanup guard by forgetting it.
mem::forget(initializing);
}
}
// Specialized implementation for types that are [`Copy`], not just [`Clone`],
// and can therefore be copied bitwise.
unsafe impl<T: Copy> CopySpec for T {
#[inline]
unsafe fn clone_one(src: &Self, dst: *mut Self) {
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::copy_nonoverlapping().
unsafe {
ptr::copy_nonoverlapping(src, dst, 1);
}
}
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_slice(src: &[Self], dst: *mut [Self]) {
let len = src.len();
// This is the most likely mistake to make, so check it as a debug assertion.
debug_assert_eq!(
len,
dst.len(),
"clone_to_uninit() source and destination must have equal lengths",
);
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::copy_nonoverlapping().
unsafe {
ptr::copy_nonoverlapping(src.as_ptr(), dst.as_mut_ptr(), len);
}
}
}
/// Ownership of a collection of values stored in a non-owned `[MaybeUninit<T>]`, some of which
/// are not yet initialized. This is sort of like a `Vec` that doesn't own its allocation.
/// Its responsibility is to provide cleanup on unwind by dropping the values that *are*
/// initialized, unless disarmed by forgetting.
///
/// This is a helper for `impl<T: Clone> CloneToUninit for [T]`.
struct InitializingSlice<'a, T> {
data: &'a mut [MaybeUninit<T>],
/// Number of elements of `*self.data` that are initialized.
initialized_len: usize,
}
impl<'a, T> InitializingSlice<'a, T> {
#[inline]
fn from_fully_uninit(data: &'a mut [MaybeUninit<T>]) -> Self {
Self { data, initialized_len: 0 }
}
/// Push a value onto the end of the initialized part of the slice.
///
/// # Panics
///
/// Panics if the slice is already fully initialized.
#[inline]
fn push(&mut self, value: T) {
MaybeUninit::write(&mut self.data[self.initialized_len], value);
self.initialized_len += 1;
}
}
impl<'a, T> Drop for InitializingSlice<'a, T> {
#[cold] // will only be invoked on unwind
fn drop(&mut self) {
let initialized_slice = ptr::slice_from_raw_parts_mut(
MaybeUninit::slice_as_mut_ptr(self.data),
self.initialized_len,
);
// SAFETY:
// * the pointer is valid because it was made from a mutable reference
// * `initialized_len` counts the initialized elements as an invariant of this type,
// so each of the pointed-to elements is initialized and may be dropped.
unsafe {
ptr::drop_in_place::<[T]>(initialized_slice);
}
}
}

40
library/core/tests/clone.rs
View File

@ -1,5 +1,7 @@
use core::clone::CloneToUninit;
use core::ffi::CStr;
use core::mem::MaybeUninit;
use core::ptr;
#[test]
#[allow(suspicious_double_ref_op)]
@ -81,3 +83,41 @@ fn test_clone_to_uninit_slice_drops_on_panic() {
drop(a);
assert_eq!(COUNTER.load(Relaxed), 0);
}
#[test]
fn test_clone_to_uninit_str() {
let a = "hello";
let mut storage: MaybeUninit<[u8; 5]> = MaybeUninit::uninit();
unsafe { a.clone_to_uninit(storage.as_mut_ptr() as *mut [u8] as *mut str) };
assert_eq!(a.as_bytes(), unsafe { storage.assume_init() }.as_slice());
let mut b: Box<str> = "world".into();
assert_eq!(a.len(), b.len());
assert_ne!(a, &*b);
unsafe { a.clone_to_uninit(ptr::from_mut::<str>(&mut b)) };
assert_eq!(a, &*b);
}
#[test]
fn test_clone_to_uninit_cstr() {
let a = c"hello";
let mut storage: MaybeUninit<[u8; 6]> = MaybeUninit::uninit();
unsafe { a.clone_to_uninit(storage.as_mut_ptr() as *mut [u8] as *mut CStr) };
assert_eq!(a.to_bytes_with_nul(), unsafe { storage.assume_init() }.as_slice());
let mut b: Box<CStr> = c"world".into();
assert_eq!(a.count_bytes(), b.count_bytes());
assert_ne!(a, &*b);
unsafe { a.clone_to_uninit(ptr::from_mut::<CStr>(&mut b)) };
assert_eq!(a, &*b);
}
#[test]
fn cstr_metadata_is_length_with_nul() {
let s: &CStr = c"abcdef";
let p: *const CStr = ptr::from_ref(s);
let bytes: *const [u8] = p as *const [u8];
assert_eq!(s.to_bytes_with_nul().len(), bytes.len());
}

13
library/std/src/ffi/os_str.rs
View File

@ -3,10 +3,13 @@
#[cfg(test)]
mod tests;
use core::clone::CloneToUninit;
use crate::borrow::{Borrow, Cow};
use crate::collections::TryReserveError;
use crate::hash::{Hash, Hasher};
use crate::ops::{self, Range};
use crate::ptr::addr_of_mut;
use crate::rc::Rc;
use crate::str::FromStr;
use crate::sync::Arc;
@ -1261,6 +1264,16 @@ impl Clone for Box<OsStr> {
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl CloneToUninit for OsStr {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: we're just a wrapper around a platform-specific Slice
unsafe { self.inner.clone_to_uninit(addr_of_mut!((*dst).inner)) }
}
}
#[stable(feature = "shared_from_slice2", since = "1.24.0")]
impl From<OsString> for Arc<OsStr> {
/// Converts an [`OsString`] into an <code>[Arc]<[OsStr]></code> by moving the [`OsString`]

17
library/std/src/ffi/os_str/tests.rs
View File

@ -1,4 +1,6 @@
use super::*;
use crate::mem::MaybeUninit;
use crate::ptr;
#[test]
fn test_os_string_with_capacity() {
@ -286,3 +288,18 @@ fn slice_surrogate_edge() {
assert_eq!(post_crab.slice_encoded_bytes(..4), "🦀");
assert_eq!(post_crab.slice_encoded_bytes(4..), surrogate);
}
#[test]
fn clone_to_uninit() {
let a = OsStr::new("hello.txt");
let mut storage = vec![MaybeUninit::<u8>::uninit(); size_of_val::<OsStr>(a)];
unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()) as *mut OsStr) };
assert_eq!(a.as_encoded_bytes(), unsafe { MaybeUninit::slice_assume_init_ref(&storage) });
let mut b: Box<OsStr> = OsStr::new("world.exe").into();
assert_eq!(size_of_val::<OsStr>(a), size_of_val::<OsStr>(&b));
assert_ne!(a, &*b);
unsafe { a.clone_to_uninit(ptr::from_mut::<OsStr>(&mut b)) };
assert_eq!(a, &*b);
}

1
library/std/src/lib.rs
View File

@ -319,6 +319,7 @@
// tidy-alphabetical-start
#![feature(c_str_module)]
#![feature(char_internals)]
#![feature(clone_to_uninit)]
#![feature(core_intrinsics)]
#![feature(core_io_borrowed_buf)]
#![feature(duration_constants)]

12
library/std/src/path.rs
View File

@ -70,6 +70,8 @@
#[cfg(test)]
mod tests;
use core::clone::CloneToUninit;
use crate::borrow::{Borrow, Cow};
use crate::collections::TryReserveError;
use crate::error::Error;
@ -3109,6 +3111,16 @@ impl Path {
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl CloneToUninit for Path {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: Path is just a wrapper around OsStr
unsafe { self.inner.clone_to_uninit(core::ptr::addr_of_mut!((*dst).inner)) }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl AsRef<OsStr> for Path {
#[inline]

19
library/std/src/path/tests.rs
View File

@ -3,6 +3,8 @@ use core::hint::black_box;
use super::*;
use crate::collections::{BTreeSet, HashSet};
use crate::hash::DefaultHasher;
use crate::mem::MaybeUninit;
use crate::ptr;
#[allow(unknown_lints, unused_macro_rules)]
macro_rules! t (
@ -2054,3 +2056,20 @@ fn bench_hash_path_long(b: &mut test::Bencher) {
black_box(hasher.finish());
}
#[test]
fn clone_to_uninit() {
let a = Path::new("hello.txt");
let mut storage = vec![MaybeUninit::<u8>::uninit(); size_of_val::<Path>(a)];
unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()) as *mut Path) };
assert_eq!(a.as_os_str().as_encoded_bytes(), unsafe {
MaybeUninit::slice_assume_init_ref(&storage)
});
let mut b: Box<Path> = Path::new("world.exe").into();
assert_eq!(size_of_val::<Path>(a), size_of_val::<Path>(&b));
assert_ne!(a, &*b);
unsafe { a.clone_to_uninit(ptr::from_mut::<Path>(&mut b)) };
assert_eq!(a, &*b);
}

13
library/std/src/sys/os_str/bytes.rs
View File

@ -1,6 +1,9 @@
//! The underlying OsString/OsStr implementation on Unix and many other
//! systems: just a `Vec<u8>`/`[u8]`.
use core::clone::CloneToUninit;
use core::ptr::addr_of_mut;
use crate::borrow::Cow;
use crate::collections::TryReserveError;
use crate::fmt::Write;
@ -345,3 +348,13 @@ impl Slice {
self.inner.eq_ignore_ascii_case(&other.inner)
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl CloneToUninit for Slice {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: we're just a wrapper around [u8]
unsafe { self.inner.clone_to_uninit(addr_of_mut!((*dst).inner)) }
}
}

13
library/std/src/sys/os_str/wtf8.rs
View File

@ -1,5 +1,8 @@
//! The underlying OsString/OsStr implementation on Windows is a
//! wrapper around the "WTF-8" encoding; see the `wtf8` module for more.
use core::clone::CloneToUninit;
use core::ptr::addr_of_mut;
use crate::borrow::Cow;
use crate::collections::TryReserveError;
use crate::rc::Rc;
@ -268,3 +271,13 @@ impl Slice {
self.inner.eq_ignore_ascii_case(&other.inner)
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl CloneToUninit for Slice {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: we're just a wrapper around Wtf8
unsafe { self.inner.clone_to_uninit(addr_of_mut!((*dst).inner)) }
}
}

12
library/std/src/sys_common/wtf8.rs
View File

@ -19,12 +19,14 @@
mod tests;
use core::char::{encode_utf16_raw, encode_utf8_raw};
use core::clone::CloneToUninit;
use core::str::next_code_point;
use crate::borrow::Cow;
use crate::collections::TryReserveError;
use crate::hash::{Hash, Hasher};
use crate::iter::FusedIterator;
use crate::ptr::addr_of_mut;
use crate::rc::Rc;
use crate::sync::Arc;
use crate::sys_common::AsInner;
@ -1046,3 +1048,13 @@ impl Hash for Wtf8 {
0xfeu8.hash(state)
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl CloneToUninit for Wtf8 {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: we're just a wrapper around [u8]
unsafe { self.bytes.clone_to_uninit(addr_of_mut!((*dst).bytes)) }
}
}