Miri does not figure out its sysroot at build time

[RalfJung]

The currently shipped Miri is broken, it fails to detect its sysroot because RUST_SYSROOT was not set at build time:

thread 'main' panicked at 'could not find sysroot. Either set `MIRI_SYSROOT` at run-time, or at build-time specify `RUST_SYSROOT` env var or use rustup or multirust', src/libcore/option.rs:1034:5

@oli-obk I thought bootstrap takes care of providing the env vars for that?

[RalfJung]

Hm, no, I cannot find any trace of a RUST_SYSROOT env var in current or past bootstrap.

Looks like the miri driver itself as shipped never had a proper sysroot set, it always relied on cargo miri providing one.

[RalfJung]

And anyway we cannot hard-code a sysroot at compile-time into a binary that we ship. ;)

And actually, just not doing anything should work here! miri and rustc are distributed in the same directory, so the existing sysroot computation should work. We just have to change miri to "trust" the default sysroot.

[RalfJung]

rust-lang/miri#772 should hopefully fix this.