global allocators: add a few comments
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@ -21,6 +21,10 @@ use core::usize;
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pub use core::alloc::*;
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extern "Rust" {
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// These are the magic symbols to call the global allocator. rustc generates
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// them from the `#[global_allocator]` attribute if there is one, or uses the
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// default implementations in libstd (`__rdl_alloc` etc in `src/libstd/alloc.rs`)
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// otherwise.
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#[allocator]
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#[rustc_allocator_nounwind]
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fn __rust_alloc(size: usize, align: usize) -> *mut u8;
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@ -142,6 +142,7 @@ pub use alloc_crate::alloc::*;
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#[derive(Debug, Copy, Clone)]
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pub struct System;
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// The Alloc impl just forwards to the GlobalAlloc impl, which is in `std::sys::*::alloc`.
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#[unstable(feature = "allocator_api", issue = "32838")]
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unsafe impl Alloc for System {
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#[inline]
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@ -226,6 +227,10 @@ pub fn rust_oom(layout: Layout) -> ! {
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#[unstable(feature = "alloc_internals", issue = "0")]
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pub mod __default_lib_allocator {
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use super::{System, Layout, GlobalAlloc};
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// These magic symbol names are used as a fallback for implementing the
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// `__rust_alloc` etc symbols (see `src/liballoc/alloc.rs) when there is
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// no `#[global_allocator]` attribute.
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// for symbol names src/librustc/middle/allocator.rs
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// for signatures src/librustc_allocator/lib.rs
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