The way a Mercurial plugin interacts with Mercurial has changed over time. The technique used in trees has been deprecated for a while and in 3.8, it no longer works.

I have a patch that adopts the new method using decorators, introduced in Mercurial 1.9. It also contains a fallback workaround that keeps compatibility with older versions.

For trees, there are further complications for versions between 1.9 and 3.1 which the patch also handles. I have run the automatic tests for all versions from 1.1 to latest.