Custom compile time warnings

The ppxlib manual suggests that ppx developers use Location.error_extensionf to insert an error extension node into the parsetree.


[%%ocaml.error "My custom error"]

Is there a similar extension point to this that will emit a custom warning at compile time? ocaml.warn(ing) doesn’t seem to exist. I thought about using alerts but those only really work if the attached structure item/etc. is used somewhere.

Any help is appreciated!