A common pitfall when doing build.rs code generation in Rust is to write generated files directly to /src. It may seem fine on the first blush, but it does cause problems when you use such crate as a dependency, because there's an implicit invariant that sources in .cargo/registry should be immutable. Breaking this invariant sometimes is fine, but sometimes leads to pretty bewildering bugs:

• Error during cargo doc. abonander/mime_guess#31
• Unable to depend on larlpop from read-only vendor directory lalrpop/lalrpop#272

So, I think we, first of all, should formally declare and document that build scripts should not modify stuff outside of OUT_DIR. We can enforce this rule during pre-flight check in cargo publish, by making the directory with sources read-only before the build. That is, in this line, we should add an equivalent of chown ua-w -r dst.