Builders require generic types to impl Default even when not needed.

[ColonelThirtyTwo]

Example code:

use derive_builder::Builder;

// Does not impl Default
struct Foo;

#[derive(Builder)]
struct Bar<T> {
	value: T
}

fn main() {
	BarBuilder::default()
		.value(Foo)
		.build()
}

This doesn't build; it complains that Foo does not implement Default. It obviously doesn't need to, but the Default implementation that the #[derive(Default)] adds the bound anyway. This makes the builder impossible to instantiate for types that take generic types that do not implement Default.

PS Is this crate still maintained? Last release was a year ago.

[rardiol]

I believe you can use the derivative crate to derive Default and then set Default to boundless: https://mcarton.github.io/rust-derivative/Default.html#custom-bound

[ColonelThirtyTwo]

@rardiol Well, I can't, because the generated builder has to have the derive and there's no way to add the derive to it.

I made a PR fixing this issue here, #179, but since has gone 17 days without any sort of response I can only conclude that this crate is not actively maintained.

[TedDriggs]

You're correct; I've been short on time and haven't had resources to make sure changes to this crate don't break anyone depending on it. The past few efforts have led to revving the minor version for most changes, but that's not great for people either.

The standard library applies the Default bound to all type parameters, even if it technically doesn't need them. I think your change is safe, but I'm concerned about deviating from the native derive behaviors since I'm not sure if it sets us up for problems down the line. For example, are there any cases we'd need other bounds on those type parameters? (I think not because we initialize everything to None)

[ColonelThirtyTwo]

Well, this crate is literally unusuable with generic types, unless they all happen to derive Default. It's frustrating that the seemingly most popular builder crate can't get this fairly basic case right.

None should be valid for every Option<T>. If for some reason you need more bounds on the builder, you can do what serde does and expose an attribute for adding them.

[TedDriggs]

This is now live in 0.10.0-alpha. Please test it out and make sure it's working as-expected.