lib === Any native code (C/C++/Rust) that Mercurial (either core or extensions) depends on should go here. Python code, or native code that depends on Python code (e.g. `#include ` or `use cpython`) is disallowed. As we start to convert more of Mercurial into Rust, and write new paths entrirely in native code, we'll want to limit our dependency on Python, which is why this barrier exists. See also `hgext/extlib/README.md`, `mercurial/cext/README.mb`. How do I choose between `lib` and `extlib` (and `cext`)? -------------------------------------------------------- If your code is native and doesn't depend on Python (awesome!), it goes here. Otherwise, put it in `hgext/extlib` (if it's only used by extensions) or `mercurial/cext` (if it's used by extensions or core).