Support const evaluation

[mirsella]

the Builder method generated isn't const, and so we can't create use bon when declaring const without using a lazycell.

my use case is im writing a game with a lot of cards, and im declaring the stats of the card in a struct and building with bon.
ideally my Trait Card would have a Card::STATS with my Stats struct.

thanks !

A note for the community from the maintainers

Please vote on this issue by adding a 👍 reaction to help the maintainers with prioritizing it. You may add a comment describing your real use case related to this issue for us to better understand the problem domain.

[Veetaha]

Hi, thank you for creating the issue!

I tried to test if this is at all possible. I expanded the code for the following macro usage via Rust Analyzer, edited it manually and managed to make it const-compatible:

#[derive(bon::Builder)]
struct Point {
    x: i32,
    y: i32,
}

What did I have to change?

• Replace the usage of Default::default() that constructs a tuple of Option<T> with manual (None, None)
• Replace the usage of Option::unwrap_unchecked() with manual match ... { Some(x) => x, None => std::hint::unreachable_unchecked() }

What other limitations are anticipated

Other than that I expect the following to never work for const builders unless Rust language ships rust-lang/rust#67792:

• #[builder(default)] can not work because Default::default() is not const. You'd have to write #[builder(default = ...)] instead to construct the default value manually
• #[builder(into)] can not work because Into::into() is not const

How would the API look like?

I imagine having a #[builder(const)] attribute for #[derive(Builder)] that would enable const on all builder methods. For function syntax: marking them const using the usual rust syntax (i.e. const fn) should also make the builder const-compatible.

I can immediately tell that parsing the attribute #[builder(const)] will be a PITA because syn doesn't support parsing keywords like that. It expects a Path as a key (dtolnay/syn#1458). Easier solutions would be #[builder(const_)] or #[builder(konst)] or #[builder(sig(const))], but that sucks, and I guess this feature would require a big rewrite of bon's attribute parsing code that currently depends on darling and nested syn::Meta syntax.

I fear the problem of parsing #[builder(const)] will require doing something like what David colin-kiegel/rust-derive-builder#310 attempted to do in derive_builder. I imagine this would unlock vis = pub (without quotes) syntax and potentially some other things in the future where the right-hand-side of key = value is not an Expr.

[mirsella]

Thanks for the detailed answer ! if this is not doable now in rust, or would require too much weird behavior within bon, this can be closed (or kept open as a tracking issue for the future).

for my case, ill just alloc the struct instead of having a &'static.

Have a good day !

[Veetaha]

This is definitely doable, but with some compromises that I think would be obvious to the user.
To me personally, the biggest problem is parsing #[builder(const)], which seems like a trivial thing. It could be localized though and not require rewriting parsing of all attributes with a workaround custom FromMeta impl like there already is one for #[builder(on())].

Indeed, let's keep this issue open and track 👍 reactions to see if there is more demand for this.