(this issue originally reported by @alandonovan)

Observe the output of the program below:

$ cat t/BUILD
load(":inc.bzl", "r")

r(
    name = "myrule",
    src = "a",
)

print(type(native.existing_rule("myrule")["src"]))  # "string"

$ cat t/inc.bzl

def _implicit_outputs(src):
  return {"": "%s" % type(src)} # "Label"

def _implementation():
  pass

r = rule(
    outputs = _implicit_outputs,
    implementation = _implementation,
    attrs = {
        "src": attr.label(),
    },
)

$ blaze query t:*
WARNING: /google/src/cloud/adonovan/gbuild/google3/t/BUILD:8:1: string.
//t:Label
//t:myrule
//t:a
//t:BUILD

The existing_rule function converts a rule's attribute dictionary back into Skylark form. Similarly, the rule.outputs function converts some rule attributes into Skylark form. However, they use different conversion logic. For example, existing_rule converts labels to strings whereas rule.outputs preserves labels as labels.

This is confusing to users, since it requires twice as much documentation to specify (or would do, if either conversion were actually documented), and confusing to implementors, since it requires twice as much logic to implement.

They should use the same logic.