I read the following bazel manual about make variables (or custom variables):
https://bazel.build/reference/be/make-variables#custom_variables

“Attributes marked as “Subject to ‘Make variable’ substitution” can reference the “Make” variable FOO as follows:”

my_attr = "prefix $(FOO) suffix"

My question is: how to mark attribute as “Subject to ‘Make variable’ substitution” ?

There is nothing about this in the documentation about rule attributes:
https://bazel.build/rules/lib/toplevel/attr

Please help

In my example

### BUILD.bazel

load("version.bzl", "version_rule")
load("file.bzl", "file")

version_rule(
    name = "version_provider",
    varname = "VERSION",
)

file(
    name = "hello",
    version = ":version_provider",
    output_name = "hello",
    content = "Hello world $(VERSION)n",
)

### version.bzl

def _version_rule_impl(ctx):
    print("n------------------- _version_rule_impln")
    return [
        platform_common.TemplateVariableInfo({
            'VERSION': '1-2-3-4',
        }),
    ]

version_rule = rule(
    implementation = _version_rule_impl,
    attrs = {
        "varname": attr.string(mandatory = True),
    },
)

### file.bzl

def file(**kwargs):
    _file(out = "{output_name}.txt".format(**kwargs), **kwargs)

def _impl(ctx):
    output = ctx.outputs.out
    ctx.actions.write(output = output, content = ctx.attr.content)

_file = rule(
    implementation = _impl,
    attrs = {
        "version": attr.label(providers=[platform_common.TemplateVariableInfo]),
        "output_name": attr.string(mandatory=True),
        "content": attr.string(),
        "out": attr.output()
    },
)

I expect that generated file contents will be “Hello world 1-2-3-4n” but it is “Hello world $(VERSION)n”