BUILD

load('//:dark-underbelly.bzl', 'du')
du(name = "whatever")

load('//:very-dark-underbelly.bzl', 'vdu')
vdu(
    name = "dark-whatever", 
    one_result = "foo"
)

dark-underbelly.bzl

def my_output():
  return {'txt' : '%{name}.txt'}

def _impl(ctx):
  ctx.file_action(
      output = ctx.outputs.txt,
      content = "I'm bespoke.",
  )

du = rule(
  implementation = _impl, 
  outputs = my_output
)

very-dark-underbelly.bzl

def my_output(one_result):
  # One result is a label.
  first_char = one_result.name[0:1]
  return {'txt' : '%s.txt' % first_char}

def _impl(ctx):
  ctx.file_action(
      output = ctx.outputs.one_result,
      content = "I'm bespoke.",
  )
  ctx.file_action(
      output = ctx.outputs.txt,
      content = "I'm also bespoke.",
  )

vdu = rule(
  implementation = _impl, 
  attrs = {'one_result' : attr.output()},
  outputs = my_output,
)

You do realize that I will immediately start using that now, right.

@kchodorow does something similar (attribute functions) work for attrs?