Pre-RFC: Anonymous Trait Impls

[jdahlstrom]

Some prior related discussions:

Closure syntax for non-function traits

As impl Trait is getting closer to stabilization, I have been wondering about some syntactic sugaring. Perhaps this has been discussed before. If so, please let me know where. Current state On current nightly with #[feature(conservative_impl_trait)] enabled, if I want to write a function that returns an unboxed closure, I can do this: fn add_number(a: i32) -> impl Fn(i32) -> i32 { move |b| a + b } This is nice, I can provide an implementation without going through all the trouble of (1) d…

Idea: Anonymous `impl` types that support capturing like closures

I'm using the listener patterns heavily, and I hope there are anonymous impl types that support capturing like closures. When people are defining a listener, they have to define a struct and implement the trait for it. But since the type is used only once. using anonymous impl type can reduce the code to write and make it clearer. I know that it is a bad idea to use anonymous types in library code. But anonymous types (including closures) can help the client codes or test codes easier to write. …

Closures and functional traits

TL;DR: Assume we have following trait and a function that uses it: trait SingleMethodTrait<T> { fn do_stuff<'a>(self, &'a T) -> impl Future<Output = ()> + use<'a>; } async fn do_bigger_stuff( do_stuff: impl SingleMethodTrait<u32>, ) { ... let inner_param = 42u32; do_stuff.do_stuff(&inner_param).await; ... } Then, we should be able to provide that trait's implementation by writing closure: do_bigger_stuff(async |i| { println!("{i}"); }).await; This should work if …