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:
  • Add LocalWaker and ContextBuilder types to core, and LocalWake trait to alloc. #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

• Add LocalWaker support libs-team#191
• Context and Waker might be accidentally Sync #66481