experimental auto-currying for rust functions

this currently works only on nightly with the type_alias_impl_trait feature enabled.

#![feature(type_alias_impl_trait)]

#[cutlass::curry]
fn add(x: u32, y: u32, z: u32) -> u32 {
    return x + y + z;
}

fn main() {
    let plus_3 = add(1)(2);
    let v: Vec<u32> = (1..=3).map(plus_3).collect();
    assert_eq!(v, vec![4, 5, 6]);
}

the #[curry] proc-macro expands the above add function to something like this (roughly):

type T0 = u32;
type T1 = impl Fn(u32) -> T0;
type T2 = impl Fn(u32) -> T1;
fn add(x: u32) -> T2 {
    return (move |y| move |z| x + y + z);
}

• doesn't yet work for method functions (signatures with self)
• the function has to have a return value
• works only on nightly with type_alias_impl_trait enabled

test this crate with cargo +nightly test.

to view macro expansions, install cargo-expand and run cargo expand --test <test name>. expand tests/smoke.rs for example:

cargo expand --test smoke