Impl core::error::Error instead of std::error::Error

[DeppLearning]

Hey there,

thanks for the great library! I noticed that the Error trait is not implemented for BuilderErrors when the std feature is inactive. Rust 1.81 stabilized core::error::Error, it would be nice if this crate could make use of it and lift the former restriction. Since, as implemented, this would be a breaking change, I'm wondering what the preferred way forward is. I can think of

• add a feature flag, like core_error that opts into using core::error::Error instead of the std version
• use something like https://crates.io/crates/rustversion/ to gate the core version behind a #[rustversion::since(1.81)]
• others?

[TedDriggs]

add a feature flag, like core_error that opts into using core::error::Error instead of the std version

I think I'd rather bump the MSRV than use a cargo flag for it. If the only difference between 0.20.x and 0.21.0 is an MSRV bump for this purpose, upgrading will be very easy for those who can upgrade, and those who can't upgrade won't be missing out on anything.

use something like https://crates.io/crates/rustversion/ to gate the core version behind a #[rustversion::since(1.81)]

I wouldn't consider a version that included rustversion in the generated code, but would be willing to have the derive_builder_core crate take that dependency. Would the code in the macro be something like...

if rustversion::cfg!(since(1.81)) {
    // generate core::error::Error impl
}

[TedDriggs]

In ≥1.81, is there some blanket impl on std::error::Error that means all core::error::Error impls are satisfactory? If not, do we need to emit both impls to avoid breaking compatibility?