Document Cargo metadata hash #1761
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The collector uses
RUSTC_FORCE_RUSTC_VERSIONandRUSTC_FORCE_INCR_COMP_ARTIFACT_HEADERto try to limit noise introduced by different stable hashes and mangled method names.@bjorn3 realized that there is another source of this noise. Cargo uses
rustc -vVand hashes the output, and then passes this hash torustcusing-Cmetadata. This happens here.I experimented with the hash, and it looks like it's mostly a non-issue for rustc-perf, because all try and master builds should have the same hash, because Cargo only hashes the part of the
rustcversion after the dash, which should benightlyfor all try and master builds (it also hashes the host, but that is, at least for now, constant on the collector server). The hash will be different for beta/stable benchmarks, but there are so rare that this is probably not a problem in practice.This PR documents this behavior.