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Update global_allocator to use the attribute template #1919

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39 changes: 37 additions & 2 deletions src/runtime.md
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Expand Up @@ -6,8 +6,43 @@ This section documents features that define some aspects of the Rust runtime.
r[runtime.global_allocator]
## The `global_allocator` attribute

The *`global_allocator` attribute* is used on a [static item] implementing the
[`GlobalAlloc`] trait to set the global allocator.
r[runtime.global_allocator.intro]
The *`global_allocator` [attribute][attributes]* is used to define a [memory allocator][std::alloc].

> [!EXAMPLE]
> ```rust
> use std::alloc::{GlobalAlloc, System, Layout};
>
> struct MyAllocator;
>
> unsafe impl GlobalAlloc for MyAllocator {
> unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
> unsafe { System.alloc(layout) }
> }
>
> unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
> unsafe { System.dealloc(ptr, layout) }
> }
> }
>
> #[global_allocator]
> static GLOBAL: MyAllocator = MyAllocator;
> ```

r[runtime.global_allocator.syntax]
The `global_allocator` attribute uses the [MetaWord] syntax and thus does not take any inputs.

r[runtime.global_allocator.allowed-positions]
The `global_allocator` attribute may only be applied to a [static item] that implements the [`GlobalAlloc`] trait.

r[runtime.global_allocator.duplicates]
The `global_allocator` attribute may only be specified once on an item.

r[runtime.global_allocator.single]
At most one `global_allocator` may be specified in the crate graph.

r[runtime.global_allocator.stdlib]
The `global_allocator` attribute is exported in the [standard library prelude][core::prelude::v1].

r[runtime.windows_subsystem]
## The `windows_subsystem` attribute
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