per-toolchain cache of std

[matthiaskrgr]

would be nice if miri was able to have separate caches for its std build for cargo +master miri... and cargo +nightly miri... for example, to avoid frequent rebuilds when experimenting with different toolchains at the same time.

[RalfJung]

That should be fairly easy, the main problem being that we'd never clean up when a toolchain gets uninstalled.

The folder logic is here:

miri/cargo-miri/src/setup.rs

Lines 65 to 66 in e9e11de

	let user_dirs = directories::ProjectDirs::from("org", "rust-lang", "miri").unwrap();
	user_dirs.cache_dir().to_owned()

AFAIK rustup sets an env var with the active toolchain, that we could simply take into account here.

[oli-obk]

the main problem being that we'd never clean up when a toolchain gets uninstalled.

Could we pick a miri folder inside the toolchain? Won't work for distro based miri, but once that's a thing we'll figure sth out.

[bjorn3]

Or maybe ship the miri sysroot as part of the miri component?

[oli-obk]

Or maybe ship the miri sysroot as part of the miri component?

That's not possible as we'd have to ship a miri sysroot per target

[RalfJung]

Not sure if messing with rustup's toolchain folder is a good idea. Also that won't work on nix (where the toolchain folder is read-only).