Make `exhaust!` macro to define inherent impl

[sanbox-irl]

It would be nice if exhaust provided a macro_rules macro, which would appear as follows:

#[derive(exhaust::Exhaust)]
enum Foo {
  Bar,
  Baz,
}
exhaust::exhaust!(Foo);

which would expand to an inherent impl:

impl Foo {
  /// Appropriate doc comments here
  pub fn exhaust() -> impl Iterator<Item = Self> {
    <Self as exhaust::Exhaust>::exhaust()
  }
}

I freetyped the above, so it might not be correct.

This pattern has some precendence -- notably, this is what downcast-rs does. It is awkward and effectively a bodge and it can be fully implemented by users, but:

• There are many times a user would want to implement this manually, so making that easier is a good idea
• Users can choose to not use it and implement it themselves, particularly if they want to rename the main method.

[kpreid]

I'm skeptical that this is a good idea. I do agree that there is value in offering inherent methods in addition to trait methods, but this case feels like adding a single special case macro to one library for something that really should be a general language feature. You say that

Users can choose to not use it and implement it themselves, particularly if they want to rename the main method.

and I am currently of the opinion that this is how users of exhaust should do this, especially as it allows them to name the method, write documenation, define the exact RPIT bounds (or explicitly name the type, if they wish), and so on. I feel that if a crate offers a public inherent method, then for the sake of good documentation and semver compatibility, that crate should be controlling exactly how the method appears and not delegating it to a third party (exhaust).

I’m open to being convinced otherwise, though, if you have more arguments to present.