Document memory allocation APIs
Add some docs where they were missing, attempt to fix them where they were out of date.
This commit is contained in:
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@ -8,6 +8,8 @@
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Memory allocation APIs
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#![unstable(feature = "allocator_api",
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reason = "the precise API and guarantees it provides may be tweaked \
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slightly, especially to possibly take into account the \
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@ -37,27 +39,80 @@ extern "Rust" {
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fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
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}
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/// The global memory allocator.
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///
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/// This type implements the [`Alloc`] trait by forwarding calls
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/// to the allocator registered with the `#[global_allocator]` attribute
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/// if there is one, or the `std` crate’s default.
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#[derive(Copy, Clone, Default, Debug)]
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pub struct Global;
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/// Allocate memory with the global allocator.
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///
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/// This function forwards calls to the [`GlobalAlloc::alloc`] method
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/// of the allocator registered with the `#[global_allocator]` attribute
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/// if there is one, or the `std` crate’s default.
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///
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/// This function is expected to be deprecated in favor of the `alloc` method
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/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
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///
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/// # Safety
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///
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/// See [`GlobalAlloc::alloc`].
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#[unstable(feature = "allocator_api", issue = "32838")]
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#[inline]
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pub unsafe fn alloc(layout: Layout) -> *mut u8 {
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__rust_alloc(layout.size(), layout.align())
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}
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/// Deallocate memory with the global allocator.
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///
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/// This function forwards calls to the [`GlobalAlloc::dealloc`] method
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/// of the allocator registered with the `#[global_allocator]` attribute
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/// if there is one, or the `std` crate’s default.
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///
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/// This function is expected to be deprecated in favor of the `dealloc` method
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/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
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///
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/// # Safety
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///
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/// See [`GlobalAlloc::dealloc`].
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#[unstable(feature = "allocator_api", issue = "32838")]
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#[inline]
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pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
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__rust_dealloc(ptr, layout.size(), layout.align())
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}
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/// Reallocate memory with the global allocator.
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///
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/// This function forwards calls to the [`GlobalAlloc::realloc`] method
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/// of the allocator registered with the `#[global_allocator]` attribute
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/// if there is one, or the `std` crate’s default.
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///
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/// This function is expected to be deprecated in favor of the `realloc` method
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/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
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///
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/// # Safety
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///
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/// See [`GlobalAlloc::realloc`].
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#[unstable(feature = "allocator_api", issue = "32838")]
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#[inline]
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pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
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__rust_realloc(ptr, layout.size(), layout.align(), new_size)
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}
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/// Allocate zero-initialized memory with the global allocator.
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///
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/// This function forwards calls to the [`GlobalAlloc::alloc_zeroed`] method
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/// of the allocator registered with the `#[global_allocator]` attribute
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/// if there is one, or the `std` crate’s default.
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///
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/// This function is expected to be deprecated in favor of the `alloc_zeroed` method
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/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
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///
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/// # Safety
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///
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/// See [`GlobalAlloc::alloc_zeroed`].
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#[unstable(feature = "allocator_api", issue = "32838")]
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#[inline]
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pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
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@ -123,6 +178,16 @@ pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
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}
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}
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/// Abort on memory allocation error or failure.
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///
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/// Callers of memory allocation APIs wishing to abort computation
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/// in response to an allocation error are encouraged to call this function,
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/// rather than directly invoking `panic!` or similar.
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///
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/// The default behavior of this function is to print a message to standard error
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/// and abort the process.
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/// It can be replaced with [`std::alloc::set_oom_hook`]
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/// and [`std::alloc::take_oom_hook`].
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#[rustc_allocator_nounwind]
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pub fn oom(layout: Layout) -> ! {
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#[allow(improper_ctypes)]
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@ -44,6 +44,7 @@ const MIN_ALIGN: usize = 16;
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use core::alloc::{Alloc, GlobalAlloc, AllocErr, Layout};
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use core::ptr::NonNull;
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/// The default memory allocator provided by the operating system.
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#[unstable(feature = "allocator_api", issue = "32838")]
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pub struct System;
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@ -8,6 +8,8 @@
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Memory allocation APIs
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#![unstable(feature = "allocator_api",
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reason = "the precise API and guarantees it provides may be tweaked \
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slightly, especially to possibly take into account the \
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@ -315,8 +317,9 @@ impl Layout {
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}
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}
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/// The parameters given to `Layout::from_size_align` do not satisfy
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/// its documented constraints.
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/// The parameters given to `Layout::from_size_align`
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/// or some other `Layout` constructor
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/// do not satisfy its documented constraints.
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#[derive(Clone, PartialEq, Eq, Debug)]
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pub struct LayoutErr {
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private: ()
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@ -329,8 +332,8 @@ impl fmt::Display for LayoutErr {
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}
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}
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/// The `AllocErr` error specifies whether an allocation failure is
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/// specifically due to resource exhaustion or if it is due to
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/// The `AllocErr` error indicates an allocation failure
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/// that may be due to resource exhaustion or to
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/// something wrong when combining the given input arguments with this
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/// allocator.
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#[derive(Clone, PartialEq, Eq, Debug)]
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@ -346,6 +349,7 @@ impl fmt::Display for AllocErr {
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/// The `CannotReallocInPlace` error is used when `grow_in_place` or
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/// `shrink_in_place` were unable to reuse the given memory block for
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/// a requested layout.
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// FIXME: should this be in libcore or liballoc?
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#[derive(Clone, PartialEq, Eq, Debug)]
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pub struct CannotReallocInPlace;
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}
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/// Augments `AllocErr` with a CapacityOverflow variant.
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// FIXME: should this be in libcore or liballoc?
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#[derive(Clone, PartialEq, Eq, Debug)]
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#[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
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pub enum CollectionAllocErr {
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}
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}
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/// A memory allocator that can be registered to be the one backing `std::alloc::Global`
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/// A memory allocator that can be registered as the standard library’s default
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/// though the `#[global_allocator]` attributes.
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///
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/// Some of the methods require that a memory block be *currently
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/// allocated* via an allocator. This means that:
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///
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/// * the starting address for that memory block was previously
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/// returned by a previous call to an allocation method
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/// such as `alloc`, and
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///
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/// * the memory block has not been subsequently deallocated, where
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/// blocks are deallocated either by being passed to a deallocation
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/// method such as `dealloc` or by being
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/// passed to a reallocation method that returns a non-null pointer.
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///
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///
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/// # Example
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///
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/// ```no_run
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/// use std::alloc::{GlobalAlloc, Layout, alloc};
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/// use std::ptr::null_mut;
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///
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/// struct MyAllocator;
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///
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/// unsafe impl GlobalAlloc for MyAllocator {
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/// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
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/// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
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/// }
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///
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/// #[global_allocator]
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/// static A: MyAllocator = MyAllocator;
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///
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/// fn main() {
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/// unsafe {
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/// assert!(alloc(Layout::new::<u32>()).is_null())
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/// }
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/// }
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/// ```
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///
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/// # Unsafety
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///
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/// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
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/// implementors must ensure that they adhere to these contracts:
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///
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/// * Pointers returned from allocation functions must point to valid memory and
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/// retain their validity until at least the instance of `GlobalAlloc` is dropped
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/// itself.
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///
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/// * It's undefined behavior if global allocators unwind. This restriction may
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/// be lifted in the future, but currently a panic from any of these
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/// functions may lead to memory unsafety.
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///
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/// * `Layout` queries and calculations in general must be correct. Callers of
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/// this trait are allowed to rely on the contracts defined on each method,
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/// and implementors must ensure such contracts remain true.
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pub unsafe trait GlobalAlloc {
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/// Allocate memory as described by the given `layout`.
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///
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/// Returns a pointer to newly-allocated memory,
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/// or NULL to indicate allocation failure.
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/// or null to indicate allocation failure.
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///
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/// # Safety
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///
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/// **FIXME:** what are the exact requirements?
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/// This function is unsafe because undefined behavior can result
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/// if the caller does not ensure that `layout` has non-zero size.
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///
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/// (Extension subtraits might provide more specific bounds on
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/// behavior, e.g. guarantee a sentinel address or a null pointer
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/// in response to a zero-size allocation request.)
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///
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/// The allocated block of memory may or may not be initialized.
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///
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/// # Errors
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///
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/// Returning a null pointer indicates that either memory is exhausted
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/// or `layout` does not meet allocator's size or alignment constraints.
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///
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/// Implementations are encouraged to return null on memory
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/// exhaustion rather than aborting, but this is not
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/// a strict requirement. (Specifically: it is *legal* to
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/// implement this trait atop an underlying native allocation
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/// library that aborts on memory exhaustion.)
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///
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/// Clients wishing to abort computation in response to an
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/// allocation error are encouraged to call the [`oom`] function,
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/// rather than directly invoking `panic!` or similar.
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unsafe fn alloc(&self, layout: Layout) -> *mut u8;
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/// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
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///
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/// # Safety
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///
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/// **FIXME:** what are the exact requirements?
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/// In particular around layout *fit*. (See docs for the `Alloc` trait.)
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/// This function is unsafe because undefined behavior can result
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/// if the caller does not ensure all of the following:
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///
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/// * `ptr` must denote a block of memory currently allocated via
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/// this allocator,
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///
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/// * `layout` must be the same layout that was used
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/// to allocated that block of memory,
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unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
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/// Behaves like `alloc`, but also ensures that the contents
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/// are set to zero before being returned.
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///
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/// # Safety
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///
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/// This function is unsafe for the same reasons that `alloc` is.
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/// However the allocated block of memory is guaranteed to be initialized.
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///
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/// # Errors
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///
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/// Returning a null pointer indicates that either memory is exhausted
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/// or `layout` does not meet allocator's size or alignment constraints,
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/// just as in `alloc`.
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///
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/// Clients wishing to abort computation in response to an
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/// allocation error are encouraged to call the [`oom`] function,
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/// rather than directly invoking `panic!` or similar.
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unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
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let size = layout.size();
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let ptr = self.alloc(layout);
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/// Shink or grow a block of memory to the given `new_size`.
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/// The block is described by the given `ptr` pointer and `layout`.
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///
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/// Return a new pointer (which may or may not be the same as `ptr`),
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/// or NULL to indicate reallocation failure.
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/// If this returns a non-null pointer, then ownership of the memory block
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/// referenced by `ptr` has been transferred to this alloctor.
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/// The memory may or may not have been deallocated,
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/// and should be considered unusable (unless of course it was
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/// transferred back to the caller again via the return value of
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/// this method).
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///
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/// If reallocation is successful, the old `ptr` pointer is considered
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/// to have been deallocated.
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/// If this method returns null, then ownership of the memory
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/// block has not been transferred to this allocator, and the
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/// contents of the memory block are unaltered.
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///
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/// # Safety
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///
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/// `new_size`, when rounded up to the nearest multiple of `old_layout.align()`,
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/// This function is unsafe because undefined behavior can result
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/// if the caller does not ensure all of the following:
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///
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/// * `ptr` must be currently allocated via this allocator,
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///
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/// * `layout` must be the same layout that was used
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/// to allocated that block of memory,
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///
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/// * `new_size` must be greater than zero.
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///
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/// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
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/// must not overflow (i.e. the rounded value must be less than `usize::MAX`).
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///
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/// **FIXME:** what are the exact requirements?
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/// In particular around layout *fit*. (See docs for the `Alloc` trait.)
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/// (Extension subtraits might provide more specific bounds on
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/// behavior, e.g. guarantee a sentinel address or a null pointer
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/// in response to a zero-size allocation request.)
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///
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/// # Errors
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///
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/// Returns null if the new layout does not meet the size
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/// and alignment constraints of the allocator, or if reallocation
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/// otherwise fails.
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///
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/// Implementations are encouraged to return null on memory
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/// exhaustion rather than panicking or aborting, but this is not
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/// a strict requirement. (Specifically: it is *legal* to
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/// implement this trait atop an underlying native allocation
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/// library that aborts on memory exhaustion.)
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///
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/// Clients wishing to abort computation in response to a
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/// reallocation error are encouraged to call the [`oom`] function,
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/// rather than directly invoking `panic!` or similar.
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unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
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let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
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let new_ptr = self.alloc(new_layout);
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/// retain their validity until at least the instance of `Alloc` is dropped
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/// itself.
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///
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/// * It's undefined behavior if global allocators unwind. This restriction may
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/// be lifted in the future, but currently a panic from any of these
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/// functions may lead to memory unsafety. Note that as of the time of this
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/// writing allocators *not* intending to be global allocators can still panic
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/// in their implementation without violating memory safety.
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///
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/// * `Layout` queries and calculations in general must be correct. Callers of
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/// this trait are allowed to rely on the contracts defined on each method,
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/// and implementors must ensure such contracts remain true.
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///
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/// Note that this list may get tweaked over time as clarifications are made in
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/// the future. Additionally global allocators may gain unique requirements for
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/// how to safely implement one in the future as well.
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/// the future.
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pub unsafe trait Alloc {
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// (Note: existing allocators have unspecified but well-defined
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// (Note: some existing allocators have unspecified but well-defined
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// behavior in response to a zero size allocation request ;
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// e.g. in C, `malloc` of 0 will either return a null pointer or a
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// unique pointer, but will not have arbitrary undefined
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// behavior. Rust should consider revising the alloc::heap crate
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// to reflect this reality.)
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// behavior.
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// However in jemalloc for example,
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// `mallocx(0)` is documented as undefined behavior.)
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/// Returns a pointer meeting the size and alignment guarantees of
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/// `layout`.
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@ -579,8 +708,8 @@ pub unsafe trait Alloc {
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/// library that aborts on memory exhaustion.)
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///
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/// Clients wishing to abort computation in response to an
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/// allocation error are encouraged to call the allocator's `oom`
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/// method, rather than directly invoking `panic!` or similar.
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/// allocation error are encouraged to call the `oom` function,
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/// rather than directly invoking `panic!` or similar.
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unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
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/// Deallocate the memory referenced by `ptr`.
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@ -686,9 +815,9 @@ pub unsafe trait Alloc {
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/// implement this trait atop an underlying native allocation
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/// library that aborts on memory exhaustion.)
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///
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/// Clients wishing to abort computation in response to an
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/// reallocation error are encouraged to call the allocator's `oom`
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/// method, rather than directly invoking `panic!` or similar.
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/// Clients wishing to abort computation in response to a
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/// reallocation error are encouraged to call the [`oom`] function,
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/// rather than directly invoking `panic!` or similar.
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unsafe fn realloc(&mut self,
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ptr: NonNull<u8>,
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layout: Layout,
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@ -731,8 +860,8 @@ pub unsafe trait Alloc {
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/// constraints, just as in `alloc`.
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///
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/// Clients wishing to abort computation in response to an
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/// allocation error are encouraged to call the allocator's `oom`
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/// method, rather than directly invoking `panic!` or similar.
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/// allocation error are encouraged to call the [`oom`] function,
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/// rather than directly invoking `panic!` or similar.
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unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
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let size = layout.size();
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let p = self.alloc(layout);
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|
@ -757,8 +886,8 @@ pub unsafe trait Alloc {
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/// constraints, just as in `alloc`.
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///
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/// Clients wishing to abort computation in response to an
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/// allocation error are encouraged to call the allocator's `oom`
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/// method, rather than directly invoking `panic!` or similar.
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/// allocation error are encouraged to call the [`oom`] function,
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/// rather than directly invoking `panic!` or similar.
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unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
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let usable_size = self.usable_size(&layout);
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self.alloc(layout).map(|p| Excess(p, usable_size.1))
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@ -778,9 +907,9 @@ pub unsafe trait Alloc {
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/// `layout` does not meet allocator's size or alignment
|
||||
/// constraints, just as in `realloc`.
|
||||
///
|
||||
/// Clients wishing to abort computation in response to an
|
||||
/// reallocation error are encouraged to call the allocator's `oom`
|
||||
/// method, rather than directly invoking `panic!` or similar.
|
||||
/// Clients wishing to abort computation in response to a
|
||||
/// reallocation error are encouraged to call the [`oom`] function,
|
||||
/// rather than directly invoking `panic!` or similar.
|
||||
unsafe fn realloc_excess(&mut self,
|
||||
ptr: NonNull<u8>,
|
||||
layout: Layout,
|
||||
|
@ -823,7 +952,7 @@ pub unsafe trait Alloc {
|
|||
/// could fit `layout`.
|
||||
///
|
||||
/// Note that one cannot pass `CannotReallocInPlace` to the `oom`
|
||||
/// method; clients are expected either to be able to recover from
|
||||
/// function; clients are expected either to be able to recover from
|
||||
/// `grow_in_place` failures without aborting, or to fall back on
|
||||
/// another reallocation method before resorting to an abort.
|
||||
unsafe fn grow_in_place(&mut self,
|
||||
|
@ -878,7 +1007,7 @@ pub unsafe trait Alloc {
|
|||
/// could fit `layout`.
|
||||
///
|
||||
/// Note that one cannot pass `CannotReallocInPlace` to the `oom`
|
||||
/// method; clients are expected either to be able to recover from
|
||||
/// function; clients are expected either to be able to recover from
|
||||
/// `shrink_in_place` failures without aborting, or to fall back
|
||||
/// on another reallocation method before resorting to an abort.
|
||||
unsafe fn shrink_in_place(&mut self,
|
||||
|
@ -926,8 +1055,8 @@ pub unsafe trait Alloc {
|
|||
/// will *not* yield undefined behavior.
|
||||
///
|
||||
/// Clients wishing to abort computation in response to an
|
||||
/// allocation error are encouraged to call the allocator's `oom`
|
||||
/// method, rather than directly invoking `panic!` or similar.
|
||||
/// allocation error are encouraged to call the [`oom`] function,
|
||||
/// rather than directly invoking `panic!` or similar.
|
||||
fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
|
||||
where Self: Sized
|
||||
{
|
||||
|
@ -993,8 +1122,8 @@ pub unsafe trait Alloc {
|
|||
/// Always returns `Err` on arithmetic overflow.
|
||||
///
|
||||
/// Clients wishing to abort computation in response to an
|
||||
/// allocation error are encouraged to call the allocator's `oom`
|
||||
/// method, rather than directly invoking `panic!` or similar.
|
||||
/// allocation error are encouraged to call the [`oom`] function,
|
||||
/// rather than directly invoking `panic!` or similar.
|
||||
fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
|
||||
where Self: Sized
|
||||
{
|
||||
|
@ -1037,9 +1166,9 @@ pub unsafe trait Alloc {
|
|||
///
|
||||
/// Always returns `Err` on arithmetic overflow.
|
||||
///
|
||||
/// Clients wishing to abort computation in response to an
|
||||
/// reallocation error are encouraged to call the allocator's `oom`
|
||||
/// method, rather than directly invoking `panic!` or similar.
|
||||
/// Clients wishing to abort computation in response to a
|
||||
/// reallocation error are encouraged to call the [`oom`] function,
|
||||
/// rather than directly invoking `panic!` or similar.
|
||||
unsafe fn realloc_array<T>(&mut self,
|
||||
ptr: NonNull<T>,
|
||||
n_old: usize,
|
||||
|
|
|
@ -8,7 +8,7 @@
|
|||
// option. This file may not be copied, modified, or distributed
|
||||
// except according to those terms.
|
||||
|
||||
//! dox
|
||||
//! Memory allocation APIs
|
||||
|
||||
#![unstable(issue = "32838", feature = "allocator_api")]
|
||||
|
||||
|
|
Loading…
Reference in New Issue