Use hint-mostly-unused to speedup compilation

[NobodyXu]

hint-mostly-unused enables lazy codegen for crates, can be enabled for libraries by adding:

[hints]
mostly-unused = true

and enable -Zprofile-hint-mostly-unused using cargo nightly.

Given that gix has lots of function and for many simple use cases, they are just left unused, probably makes sense to turn this on for the crate gix at least.

It might even speedup the tests

https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/

[Byron]

Thanks for bringing it up.

I was also intrigued when reading about it, and thought that what's really missing now to set it up is usage statistics to know which crates would be eligible.

Tests usually test the entire surface of their crates, but third-party crates are probably mostly unused.
Definitely interesting to explore that more though, and PRs would definitely be welcome.

[NobodyXu]

For cargo-binstall, we use gix through simple-git which uses a tiny, tiny bit of the API gix provided, might be interested to test it

[Byron]

That would be interesting to see. I also don't quite know how it works, but would think that then it only will defer building gix (if only that has the hint), so it would still build all dependencies (i.e. gix-* plumbing) which probably quite a lot of work on its own.

Maybe there is a wild-card way of specifying hints, but probably, even if so, this is only possible for direct dependencies and not indirect ones, i.e. gix-* crates.

Despite that, I also think it's an obvious win for huge generated crates that take insane amounts of time to compile on their own while being mostly unused, I am thinking of you, aws-ec2 :D.