Make it easy for users to define hegel-style builders for generators

[DRMacIver]

For example, if I define a generator as

#[hegel::composite]
fn my_gen(tc: hegel::TestCase, some_param: usize) -> MyType {
...
}

Then it would be nice if the following worked:

let x = my_gen(42);
let y = x.some_param(11);

It's probably not super critical and we might want to think about surrounding ergonomics (e.g. this is less useful without being able to default the parameters), but in general it would be nice if it were easy for people to write generators in the style we write them in Hegel.

More generally: Because Generator is a private trait, there's no other way to do this at the moment! I think that's something we probably want to figure out how to fix.

[Liam-DeVoe]

I'm not sure I follow. It might be nice to have some sort of builder ux for #[composite], but I'd only want it as a pure alternative to the standard my_gen(42) pattern. If I'm already specifying all my parameters up-front, what's the benefit of combining it with a builder pattern later?

[DRMacIver]

I'm not sure I follow. It might be nice to have some sort of builder ux for #[composite], but I'd only want it as a pure alternative to the standard my_gen(42) pattern. If I'm already specifying all my parameters up-front, what's the benefit of combining it with a builder pattern later?

Yeah, I think as is this isn't a very useful feature, though not completely useless. I've renamed and updated the description a bit.

My main objection is that we've committed ourselves to a style of generator definition that we've also committed to excluding users from, and that sucks.

[Liam-DeVoe]

Right, so the discussion here is that we don't provide a way for users to write builder-style generators, despite using the builder pattern heavily in our public generators. I agree we should think about ways to fix this.