Cargo should provide --remap-path-prefix or not hash it from RUSTFLAGS.

[eddyb]

After #6503, reproducible build setups that were setting RUSTFLAGS="--remap-path-prefix=..." in order to normalize paths in rustc, stopped being reproducible, as Cargo is now hashing the whole RUSTFLAGS value, resulting in different -C metadata values.

This was originally reported in rust-lang/rust#59542, and one of the solutions suggested there was that Cargo should provide its own --remap-path-prefix, which would not be hashed, and that becomes the supported way set this flag.

Another possibility could be Cargo parsing --remap-path-prefix out of RUSTFLAGS, before hashing the rest, but that seems perhaps a bit brittle.

cc @michaelwoerister @Eh2406

[jhfrontz]

Can I petition to have this changed from "feature request" to "use-blocking bug"? I mean, it was working fine until it got broken.

[alexcrichton]

One workaround for this which I've forgotten to mention earlier is something like this:

$ RUSTC=./my-rustc-wrapper.sh cargo build

and the contents of my-rustc-wrapper.sh are:

#!/bin/sh
exec rustc "$@" --remap-path-prefix ...

and that will subvert Cargo's hashing.