In bug 1510490 we hit a problem where we started building the typenum crate, and that caused a Btup job failure, because tup seems to assume that all the build scripts in rust crates are called "build.rs". In fact the Cargo.toml can point to some other .rs file to use as the build script. In the case of typenum it's a build/main.rs file. So to unblock WR updates I wrote a patch to loosen this assumption a bit, but I'm filing this bug to have a more general fix for this problem. The patch landed as part of bug 1510490, and can be seen here: https://hg.mozilla.org/integration/autoland/rev/d244292c2a127fafda775e1db5e0a8e4decea5ff