Allow disabling the FFI integration with a feature

[bjorn3]

cargo check -Zunstable-options --compile-time-deps is taking almost 2 minutes because it is building capstone and libffi. The entire rest of the rust workspace takes only 1m34s to build with these options. When rust-lang/rust#143785 lands, this means that half of the time it takes rust-analyzer before it can do any analysis with a fresh build dir would be taken up by miri.

[RalfJung]

Yeah I noticed capstone taking forever as well...
Cc @nia-e

[nia-e]

Doesn't CI build with --all-features? Then if we add a feature to disable it, we'll never be able to test it in CI...

[RalfJung]

Features can't disable things, they must be positive. Cargo doesn't work well otherwise.

Given that libffi is also not super fast to build, we could indeed have a feature for the entire native-lib support, and put all these dependencies behind it. It's fine for work in the Miri repo, one can use --feature and our CI already sets --all-features.

But in the rustc repo -- we'd want the dist builder and CI to still enable some but not all features. Also when I work on Miri in the rustc repo I need a way to test native-lib mode. I don't think that is currently possible, which means I don't think we currently can move this behind a feature gate. Cc @Kobzol

Of course, this also always risks people breaking the build and then noticing that only when CI runs -- and then they'll not know how to reproduce the failure locally. I don't think anything else in the rustc repo has feature that are "off by default when building locally but turned on on CI"?

half of the time it takes rust-analyzer before it can do any analysis with a fresh build dir would be taken up by miri.

Clippy AFAIK still takes longer to build than Miri, so the other question here is -- why do you even build Miri in your RA setup?

[RalfJung]

Fundamentally the real problem is that we don't even need capstone and libffi to build their C code for a check build, but cargo has no support for skipping C dependency builds in check builds (rust-lang/cargo#4001). So all we can do is hack around this in various ways...

[bjorn3]

The tool_extended!() macro has an add_features field with which you can enable arbitrary features when a tool is built. Also conditionally on doing a check mode using builder.kind == Kind::Check.