Bazel: Feature request: allow attrs accept selects.

[ahyangyi]

I tried to put a select of lists of strings to a rule's attr expecting a string_list or a label_list, but that didn't work. This seems an important feature missing, since without selects we have absolutely no flexibility to write BUILDs that work under different configurations.

Thanks.

[ahyangyi]

When I think about it, shouldn't rules and aspects also have access to user configuration inputs, like the values of --cpu, --compiler and -c?

The built-in rules indeed behave differently when these configuration inputs change, so I can't see this being a technical impossibility. Supporting this would make rules closer to their built-in counterparts.

[ahyangyi]

FYI:

Currently one can work around the problem where attrs cannot accept selects by adding a level of indirection: attrs can use labels and labels can point to filegroups or genrules that uses selects.

Similarly, rules and aspects can access various config_settings if you create some dummy files and a filegroup that uses select to conditionally depend on different dummy files. You can use an implicit dependency to depend on such a dummy filegroup and parse its filename to figure out whatever you need in config_settings.

These are tedious, but before the official Bazel team fixes this issue, you can use these tricks to work around the limits.

[ulfjack]

Do you have an example that doesn't work and Bazel output?

[ahyangyi]

@ulfjack Thanks. That's a good question. I revisited this feature request and tested and can confirm that said rules can indeed accept selects.

My original intention, however, was to set the default value of an attr to a select. For example:

def _cat_impl(ctx):
    srcs = ctx.files.srcs
    output = ctx.outputs.cat

    ctx.action(
        command = "cat %s > %s" % (" ".join([x.path for x in srcs]), output.path),
        inputs = srcs,
        outputs = [output],
        )

cat = rule(
    implementation = _cat_impl,
    attrs = {
        "srcs": attr.label_list(allow_files=True, default=select({"//conditions:default": []})),
        },
    outputs = {
        "cat": "%{name}.cat",
        },
)

This fails:

ERROR: /home/<redacted-but-thats-me>/test/cat/cat.bzl:14:17: Traceback (most recent call last):
        File "/home/<redacted-but-thats-me>/test/cat/cat.bzl", line 11
                rule(implementation = _cat_impl, attrs ...": []}))}, ..."})
        File "/home/<redacted-but-thats-me>/test/cat/cat.bzl", line 14, in rule
                attr.label_list(allow_files = True, default = sele...": []}))
expected sequence of Labels or sequence of Labels-returning function for 'default' while calling label_list but got select of list instead: selector({"//conditions:default": []}).
ERROR: error loading package 'cat': Extension file 'cat/cat.bzl' has errors.

But I wish that could be accepted.

[ulfjack]

@laurentlb

[laurentlb]

Thanks for the feature request, that makes sense.

The workaround for now is to wrap the rule with a macro, and use the select in the macro.

[excitoon]

This is the same issue as #287.

[gregestren]

Duplicate of #287