Common/shared defaults across different trait macro crates.

[crabdancing]

So, I often find myself in a position where I have some struct representing a layered config system or somesuch, where I need to have multiple crates, like serde, bon, and clap, all know about a particular default. I could newtype everything, but this gets to be a bit deranged -- so I end up needing to override defaults for all crates that need to know it. Is there anything stopping the ecosystem from standardizing on some universal macro for marking field defaults in structs, such that each crate can pull that information? Or, even better, does something like that already exist? For example, if I want a specific struct field to be at Duration::from_millis(20).

[Veetaha]

Hi, thank you for creating this discussion!

There is indeed a problem of sharing the defaults between different data literals parsing crates (be it JSON, CLI syntax, or Rust builder syntax). I think, ideally, all of these crates will eventually align on a shared syntax thanks to the effort of rust-lang/rust#132162. That RFC seems to be on a healthy path to becoming stable.

Once default field values syntax arrives in syn (dtolnay/syn#1774), bon should add support for using it. I've just subscribed to that issue to make sure I don't miss any progress there.

However, the caveat here is that default field value syntax will support only const expressions, while #[builder(default)] supports the Default trait and arbitrary expressions. I'm not sure how big of a problem const limitation will be in practice though.

I, personally, work around the problem of sharing the defaults by using an Option<T> type of the field and using unwrap_or/unwrap_or_else right at the usage site of the value. However, the downside of this approach is that crates like clap or bon can no longer benefit from the knowledge of the default value, which they use to render the default value in --help or the documentation.