make rust-analyzer use a dedicated directory

[tshepang]

inspired by #132794

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[jieyouxu]

I feel like this was discussed #general > ✔ Setup multiple editors for work on standard lib? @ 💬 but I don't remember why we used a top-level build-rust-analyzer/ as opposed to nesting inside of build/... @Zalathar do you happen to remember why? (It's been a while)

[jieyouxu]

Oh I think it just happens to be an option that works.

[Zalathar]

It has unintuitive interactions with x clean and caching, for example. IIRC x clean won’t delete your download cache, but it will delete your nested RA download cache, because it’s just indiscriminately deleting a subdirectory it doesn’t know about.

If people choose to inflict those risks on themselves, fine, but I don’t think a nested RA build directory is a responsible default.

[jieyouxu]

Oh right. I think that's the caveat I was vaguely recalling...

[tshepang]

would it be good to get x clean not delete what it does not recognize

[mati865]

Or even just avoid deleting build/rust-analyzer.

[Zalathar]

would it be good to get x clean not delete what it does not recognize

Bootstrap “owns” the build directory, so I think it's reasonable for x clean to delete anything it doesn't know about, on the assumption that it's junk left over from an earlier version of bootstrap.

Or even just avoid deleting build/rust-analyzer.

IIRC, one of the motivations people commonly cite for using build/rust-analyzer is that they want x clean to also clean their RA build directory. (And it does accomplish that, as a consequence of deleting build/rust-analyzer entirely.)

[Zalathar]

Anyhow, I strongly agree that our default IDE configs really should be using a separate build dir (e.g. build-rust-analyzer).