Introduce base builder type-alias

[l4l]

It's quite handy to have an alias rather than manually recreating the internal type.

fn make_builder() -> FooBuilder<((), (), (), ())> { .. }

// vs

fn make_builder() -> FooBuilder { .. }

[idanarye]

1. Why would you ever need to specifically address the base builder type? I can imagine wanting to write a function that returns the builder with some fields filled in, or accept a builder with some fields filled in, or a struct that stores the builder with some fields filled in - but these all are not the base builder. The base builder itself is a fancy unit type.
2. If you still want to do it - wouldn't it be simpler to use Default Generic Type Parameters?.

[l4l]

1. I'd like to expose the type in my API, in particular builder for http client. Apparently the other, more generic option would be adding a trait, but that perhaps quite a big change.
2. That sounds better, try to look at this.

[idanarye]

Why is this required? b_generics is only used when declaring the builder type (and you want the default there) and when splitting it for the impl block (where defaults should not be looked at anyway). So why clone it instead of modifying the original?

[l4l]

That's redundant, indeed.

[idanarye]

Wouldn't this match all type parameters? The one you want to add a default to is always the first one, so you can just modify that.

Or even better - just add the default when creating b_generics.

[l4l]

Yes, that would match all parameters. I guess that barely make sense.

Though unfortunately rust doesn't allow to specify defaults for non-trailing generics and the TypedBuilderFields is the first one. I'm not sure whether it's fine to set it as the last one, perhaps it might break some code.

[idanarye]

I'm half minded to just go with your original design as a temporary solution. Once const generics support strings I plan on doing a great refactor that'll break anything (other than regular by-the-book usage) anyway, and once that come I may remove it in favor of something else.