Tracking Issue for LocalWaker

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Feature gate: #![feature(local_waker)]

This is a tracking issue for support for local wakers on Context. This allows libraries to hold non thread safe data on their wakers, guaranteeing at compile time that the wakers will not be sent across threads. It includes a ContextBuilder type for building contexts.

Public API

impl Context {
    fn local_waker(&self) -> &LocalWaker;
}

impl<'a> ContextBuilder<'a> {
   fn from_waker(waker: &'a Waker) -> ContextBuilder<'a>;
   fn waker(self, waker: &'a Waker) -> ContextBuilder<'a>;
   fn local_waker(self, local_waker: &'a LocalWaker,) -> ContextBuilder<'a>
   fn build(self) -> Context;
}

impl From<&mut Context> for ContextBuilder;

pub trait LocalWake {
    fn wake(self: Rc<Self>);
}

Steps / History

• Implementation:
  • https://github.com/rust-lang/rust/pull/118960
• Have the @rust-lang/wg-async approve the API
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Should runtimes be allowed to not define a waker?

Relevant links

• https://github.com/rust-lang/libs-team/issues/191
• https://github.com/rust-lang/rust/issues/66481

Start with the local_waker feature gate and the public API listed in this tracking issue, then read implementation PR #118960 and the linked libs-team discussions. The remaining work is API approval, resolving whether runtimes may omit a waker, completing FCP, and opening a stabilization PR; no implementation file or test is named here.