Use select as default value

[Yannic]

Description of the problem / feature request:

Use select as default value.

Feature requests: what underlying problem are you trying to solve with this feature?

Selecting the default value of a rule attribute based on, e.g. the host or target OS.
For example, protoc's --proto_path or --descriptor_set_in uses a different path separator based on the host OS: https://github.com/protocolbuffers/protobuf/blob/61301f01552dd84d744a05c88af95833c600a1a7/src/google/protobuf/compiler/command_line_interface.cc#L778

The following rule definition fails because the default value is not of type string. I see the same behavior with other attribute types.

def _foo_impl(ctx):
    pass

foo = rule(
    implementation = _foo_impl,
    attrs = {
        "_target_platform": attr.string(
            default = select({
                # Definition of //:os_windows omitted for brevity. Does what you'd expect it to do.
                "//:os_windows": "windows",
                "//conditions:default": "posix",
            }),
        )
    },
)

ERROR: /.../defs.bzl:??:??: Traceback (most recent call last):
	File "/.../defs.bzl", line ??
		rule(implementation = _foo_impl, attrs =..."}))})
	File "/.../defs.bzl", line ?? in rule
		attr.string(default = select({"//:os_windows..."}))
expected value of type 'string' for parameter 'default', for call to method string(default = '', doc = '', mandatory = False, values = []) of 'attr (a language module)'

As a workaround for this issue, it's possible to wrap foo in a macro and set the default value in the macro. However, this requires the attribute to be "public".

# ...

_foo = rule(
    implementation = _foo_impl,
    attrs = {
        "target_platform": attr.string()
    },
)

def foo(**kargs):
    _foo(
        target_platform = select(...),
        **kargs)

Have you found anything relevant by searching the web?

A similar issue that asked to use select as default value in a repository_rule (#2026) was closed as WAI. However, this feature request is about using it in rule.

[gregestren]

Marking duplicate of #287 (with a comment to the same workaround you identified).