Migrating existing APIs

If a library with an existing API could benefit from using context objects in its public API, is there a soft migration path that doesn't require both the library and its users to change at the same time?

Injectable arguments

An earlier proposal for #[inject]able functions hints at an answer: make contexts regular function arguments, and allow callers to leave off arguments that are already given.

#![allow(unused)]
fn main() {
fn do_stuff(
    num: u32,
    #[inject(given log::logger)]
    logger: Logger,
) {
    info!(logger, "do_stuff({})", num);
}
}

This is similar to how implicit arguments work in Scala. There are some challenges, however.

• What happens if some of your #[inject] arguments are provided and some aren't?
• Should the function signature effectively be rewritten to remove the provided arguments in a given scope? Spooky!
• It's tempting to only allow #[inject] arguments at the end to reduce confusion. However, it would severely limit its usefulness for migrating existing APIs.