Propagate the testonly= attribute in the pybind_extension macro.

[apronchenkov]

Motivation: Enable test-only pybind_extensions to depend on test-only targets.

Note: Tested manually, using git_override.

[junyer]

Thanks for the PR! Just before I merge it, I want to fill the gap in my Starlark knowledge: How is kwargs actually propagating through the implementation? Is it happening implicitly or magically via the rule context somehow? Do you know, @rickeylev?

[rickeylev]

how is kwargs propagating

So recall there are two phases: first loading phase and then analysis phase.

Loading phase is when macros run. Macros are the more "regular" looking functions that take arbitrary args, including **kwargs. The propagation of common args (testsonly, compatible_with, etc) to other function calls must be done manually. These common args can only be propagating during loading phase. When loading phase ends, the graph of targets has been decided/finalized/fixed.

When analysis phase runs, that's when the functions that have the ctx arg are run. The ctx object contains all the target-specific state. It doesn't magically propagate anywhere; if you have a helper function, then you have to manually pass it to that helper function.

HTH

[junyer]

Sorry, what I meant was that testonly, visibility et al. aren't explicitly named anywhere, so... how do they end up where they need to be such that this PR does what it does? :)