In Rust it is the best practice to have features to be strictly additive. extension-module, instead, subtracts from the crate by not linking python. This can result in weird and difficult to resolve failure modes in e.g. workspaces.

For instance consider…

💥 Reproducing

[workspace] members = [ "extension", "some_binary", ]

where both extension and some_binary use as a dependency pyo3, but extension enables the extension-module feature, while some_binary explicitly disables the features.

When cargo build is run from the workspace root, then both of the binaries will get a pyo3 with extension-module feature enabled and some_binary will fail to link.

This behaviour is entirely valid from cargo’s standpoint – it dictates that features should be additive and thus there must not be a way to write any single crate in a way that would fail when additional features are enabled in its dependencies.

To be honest, I don’t see a good way to fix it, other than spiting pyo3 into two separate crates.