Custom, locally-sourced output filenames

Skylark lets you use templates in your output file name, e.g., this would create a file called target.timestamp:

touch = rule(
    outputs = {"date_and_time": "%{name}.timestamp"},
    implementation = _impl,
)

So if you had touch(name = "foo") in a BUILD file and built :foo, you’d get foo.timestamp.

I’d always used %{name}, but I found out the other day that you can actually use other attributes, too. For example, you could have:

greet = rule(
    attrs = {"my_name": attr.string()},
    outputs = {"greeting": "hi-there-%{my_name}"},
    implementation = _impl,
)

Then if you have greet(name = "a-greeting", my_name = "kristina") and build :a-greeting, you’ll get “hi-there-kristina” as an output file.

The entire source for this example is available as a GitHub gist (all four lines of implementation function not shown above).