Hygienic synthetic targets

[kaos]

Stu:

This problem (from #17365) seems loosely related to the idea of "hygienic macros": i.e., that a macro should be able to define anonymous variables which can't collide with or pollute the original namespace. But I don't know how to do that and still have consumers later depend-on/find them... maybe by having the request for the synthetic target use something other than its name/address?

Originally posted by @stuhood in #17365 (review)

My idea being we could open up a discussion about synthetic target hygiene. I know @Eric-Arellano wanted the synthetic targets to live in their own namespace, or something like that.

While the synthetic targets needs to exist also in the regular target namespace (to be able to being target for dependencies and extension by BUILD file targets), perhaps it would be useful to be able to reach also shadowed synthetic targets in some way?

[benjyw]

... uh, you're welcome? :)

[kaos]

haha.. yeah. was trying to take a short-cut to create the discussion from a review comment. turned out meh. (was prepared to make some edits, but not to get a reply before having done so 😂 )

[stuhood]

But I don't know how to do that and still have consumers later depend-on/find them... maybe by having the request for the syntethic target use something other than its name/address?

This is sortof like what find_owners does: locate Target(s) that own a file. So if synthetic targets were given illegal or impossible to guess names, you could use find_owners on the relevant file to locate them, without knowing the name ahead of time.

Alternatively, if synthetic targets had names which were impossible to collide with (because they exist in a namespace that targets defined in BUILD files cannot be in), then you could use a static name for them.

Finally: assuming that you can think of an API that amounts to "the caller creating a synthetic target at request time, without having to choose a name for it", then the macro that wants to consume it wouldn't have to choose or know a name.