Allow async-specific generic parameters and return type

[lyeb]

Changes

Allow async_signature to have:

• Return type: async_signature(...) -> ReturnType
• Generic parameters: async_signature<T: TraitA, U>(...) where U: TraitB

If omitted, the synchronous implementation’s generic parameters and return type are used.

Motivation

Sometimes when working with both sync and async implementations, there is a situation where the generic parameters of a function and/or the return value are not the same.

E.g. if we have traits:

trait SyncTrait {
    type SyncReturn;
    fn foo() -> Self::SyncReturn;
}

trait AsyncTrait {
    type AsyncReturn;
    fn foo() -> Future<Output = Self::AsyncReturn>;
}

it is currently not possible to write a function like:

#[async_generic(...)]
fn generic_foo<T: *SyncOrAsyncTrait*>() -> T::*SyncOrAsyncReturn* { ... }

Solution

This PR adds return types and generic parameters to the async_signature argument.

The syntax is just like writing a function named async_signature:

// With return type
#[async_generic(async_signature(...) -> AsyncReturnType)]
fn foo(...) -> SyncReturnType { ... }

// With generics
#[async_generic(async_signature<T: TraitA>(...))]
fn foo<T: TraitB>(...) { ... }

// Or as complex as we want
#[async_generic(async_signature<'a, T: TraitA, U: TraitB, ...>(...) -> AsyncReturnType where C<T, U>: 'a + TraitC + ...)]
fn foo<'a, T: TraitD, ...>(...) -> SyncReturnType where D<T>: 'a + ... { ... }

If the generics or return type are omitted, those of the sync implementation are used. This means there is no breaking change in this PR, the behavior is the same as before, unless some generic parameters or a return type is specified.

Notes

• I think this PR is in spirit quite similar to feat!: Implement support for generic params #17 but much simpler
• This would also close Generic dependent on sync/async #16
• Simplified the argument parsing by using syn::Parse instead of manually parsing the TokenTree
• Added some tests but not that many (I can add some more if you'd prefer)