Name: attribute based effects
Proposed by: @oli-obk
Original proposal (optional): (url)

Design

Use function and trait attributes to make a function/trait have effect-like behaviour instead of adding new syntax. There's still some new syntax in trait bounds, but these are removed by the attribute at attribute expansion time.

This is experimentally being built with a proc macro in https://github.com/yoshuawuyts/maybe-async-channel.

base (reference)


#![allow(unused)]
fn main() {
/// A trimmed-down version of the `std::Iterator` trait.
pub trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
    fn size_hint(&self) -> (usize, Option<usize>);
}

/// An adaptation of `Iterator::find` to a free-function
pub fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
where
    I: Iterator<Item = T> + Sized,
    P: FnMut(&T) -> bool;
}

always async


#![allow(unused)]
fn main() {
#[async]
pub trait Iterator {
    type Item;
    #[async]
    fn next(&mut self) -> Option<Self::Item>;
    fn size_hint(&self) -> (usize, Option<usize>);
}

/// An adaptation of `Iterator::find` to a free-function
#[async]
fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
where
    I: async Iterator<Item = T> + Sized,
    P: async FnMut(&T) -> bool;
}

maybe async


#![allow(unused)]
fn main() {
#[maybe_async]
pub trait Iterator {
    type Item;
    #[maybe_async]
    fn next(&mut self) -> Option<Self::Item>;
    fn size_hint(&self) -> (usize, Option<usize>);
}

/// An adaptation of `Iterator::find` to a free-function
#[maybe_async]
fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
where
    I: async Iterator<Item = T> + Sized,
    P: async FnMut(&T) -> bool;
}

generic over all modifier keywords


#![allow(unused)]
fn main() {
#[effect]
pub trait Iterator {
    type Item;
    #[effect]
    fn next(&mut self) -> Option<Self::Item>;
    fn size_hint(&self) -> (usize, Option<usize>);
}

/// An adaptation of `Iterator::find` to a free-function
#[effect]
fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
where
    I: effect Iterator<Item = T> + Sized,
    P: effect FnMut(&T) -> bool;
}

Notes