Dealing with sandbox-unfriendly build scripts

In general, most build scripts used by popular Rust projects are pretty good at only attempting to write to cargo's output directory. But every once in a while it is possible to find a build script somewhere deep in the dependency tree which assumes it can happily write to any directory it wants to (i.e. wherever its own sources happen to be present). For build scripts like these the best long term approach is almost always to fix them upstream; cargo's own documentation also warns against this:

In general, build scripts should not modify any files outside of OUT_DIR. 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. cargo won't allow such scripts when packaging.

As a dire last resort it is possible to copy all vendored sources out of the (read-only) Nix store and into a writable directory. Keep in mind that doing so requires recursively copying all sources of all crates the project depends on during every single build; it comes with a performance and energy cost, and as such it is not recommended.

# You have been warned
buildPackage {
  # other attributes omitted
  postPatch = ''
    mkdir -p "$TMPDIR/nix-vendor"
    cp -Lr "$cargoVendorDir" -T "$TMPDIR/nix-vendor"
    sed -i "s|$cargoVendorDir|$TMPDIR/nix-vendor/|g" "$TMPDIR/nix-vendor/config.toml"
    chmod -R +w "$TMPDIR/nix-vendor"