Custom compile time warnings

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

I.e.

[%%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!

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