For TestNG, [docs] state that `@library foo` will compile `foo`, hence, no need for an explicit `@build some.class.in.foo`:

> If a library directory is used by a directory of TestNG tests,
> all the classes in a library directory will be automatically
> compiled by means of implicit `@build` tags.

CODETOOLS-7903264, integrating JUnit support, also brought this feature, but did not adapt the above statement.

[docs]: https://openjdk.org/jtreg/tag-spec.html