Adding select statements for build script environment and data attributes in bzlmod

[finn-ball]

An implementation to bring some select statement functionality which are available in repository rules but not bzlmod.

Since a dict of dicts is not supported, I've opted for a dict of json string which gets decoded on the other end.

[illicitonion]

Looks reasonable to me - ideally we'd have more expressive annotations, or a helper function to generate this stuff, but this opens up capabilities we currently lack :) Thanks!

[finn-ball]

or a helper function to generate this stuff

What would a helper function look like in this scenario? I'm unsure how to add functions into MODULE files for this and we're quite limited in what we can pass into a tag class.

I had thought about adding a new tag class such as:

crate.annotation_env_select(
    crate = "foo",
    platform_triplet = "linux-x86_64-unknown",
    variables = {"FOO": "bar"},
)
crate.annotation_env_select(
    crate = "foo",
    platform_triplet = "linux-arm64-unknown",
    variables = {"FOO": "baz"},
)

But felt that was a little messy too as it isn't part of the original annotation.

Have you any suggestions?

[illicitonion]

or a helper function to generate this stuff

What would a helper function look like in this scenario? I'm unsure how to add functions into MODULE files for this and we're quite limited in what we can pass into a tag class.

These aren't currently supported by Bazel, but there's perpetually discussion about them :) sorry, I was saying "ideally" as in "if Bazel were more capable" rather than "if Finn put in more work" :D

I had thought about adding a new tag class such as:

crate.annotation_env_select(
    crate = "foo",
    platform_triplet = "linux-x86_64-unknown",
    variables = {"FOO": "bar"},
)
crate.annotation_env_select(
    crate = "foo",
    platform_triplet = "linux-arm64-unknown",
    variables = {"FOO": "baz"},
)

But felt that was a little messy too as it isn't part of the original annotation.

This kind of new tag class is probably the nicest way to express this at the moment tbh, I wouldn't complain about this API at all. But also happy to merge this as-is if you don't fancy putting it together :)