Overloading

A POC crate that utilizes Fn* traits to implement partial overloading. Caveat: only parameters can be overloaded, not return types.

TLDR

```rust

![feature(unboxed_closures)]

![feature(fn_traits)]

use overloading::overloading;

[overloading]

fn overloaded(abc: String) -> i32 { abc.parse().unwrap() }

[overloading(overloaded)]

fn overloaded() -> i32 { 114514 }

[test]

fn test() { let res = overloaded("123".toowned()); asserteq!(res, 123);

let res = overloaded();
assert_eq!(res, 114514);

} ```

Expanded code:

```rust

[allow(noncamelcase_types)]

struct overloaded; impl std::ops::FnOnce<(String,)> for overloaded { type Output = i32; extern "rust-call" fn callonce(self, (abc,): (String,)) -> Self::Output { abc.parse().unwrap() } } impl std::ops::FnMut<(String,)> for overloaded { extern "rust-call" fn callmut(&mut self, (abc,): (String,)) -> Self::Output { abc.parse().unwrap() } } impl std::ops::Fn<(String,)> for overloaded { extern "rust-call" fn call(&self, (abc,): (String,)) -> Self::Output { abc.parse().unwrap() } } impl std::ops::FnOnce<()> for overloaded { type Output = i32; extern "rust-call" fn callonce(self, _: ()) -> Self::Output { 114514 } } impl std::ops::FnMut<()> for overloaded { extern "rust-call" fn callmut(&mut self, _: ()) -> Self::Output { 114514 } } impl std::ops::Fn<()> for overloaded { extern "rust-call" fn call(&self, _: ()) -> Self::Output { 114514 } } ```