Since SubstitutionGoal::finished() in build.cc computes the hash anyway, we can prevent the inefficiency of computing the hash twice by letting the substituter tell Nix about the expected hash, which can then verify it.
Since SubstitutionGoal::finished() in build.cc computes the hash anyway, we can prevent the inefficiency of computing the hash twice by letting the substituter tell Nix about the expected hash, which can then verify it.