`attrs.any()` doesn't accept records

[lf-]

I bet this is because the types available as attrs are special somehow that's not documented in the docs, and probably even for a good reason. However, the use case I have here is to have type safe nested structures in BUCK files (which one might rightfully question why. avoiding reinventing configerator while having type safe configs is the motive).

BUCK:

load("//dux/rules:json_document.bzl", "json_document", "Oops")

json_document(name = "foo", contents = Oops(a = 1))

//dux/rules/json_document.bzl:

"""
Allows you to write a JSON document in a Starlark file, to get comments and
schema and such.
"""

Oops = record(a = int)

def _json_document_impl(ctx: AnalysisContext):
    out = ctx.actions.write(
        "{}.json".format(ctx.label.name),
        ctx.attrs.content,
        pretty = True
    )
    return [DefaultInfo(default_outputs=[out])]

json_document = rule(
    impl = _json_document_impl,
    attrs = {
        "contents": attrs.any(doc = "What to put in the JSON document; should generally be a record."),
    }
)

$ buck build //dux/code-intel/config:foo
BUILD FAILED
Error evaluating build file: `root//dux/code-intel/config:BUCK`

Caused by:
    0: Traceback (most recent call last):
         * dux/code-intel/config/BUCK:5, in <module>
             json_document(name = "foo", contents = Oops(a = 1))
       
       error: Error coercing attribute `contents` of `root//dux/code-intel/config:foo`
        --> dux/code-intel/config/BUCK:5:1
         |
       5 | json_document(name = "foo", contents = Oops(a = 1))
         | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
    1: Error coercing attribute `contents` of type `attrs.any()`
    2: Error coercing record[Oops](a=1)
    3: Cannot coerce value of type `record` to any: `record[Oops](a=1)`

[cormacrelf]

If the fix is to document it, the description of the current behaviour is that it's "any other attr type" but not "typing.Any".

If you want to make this accept records, currently it is trying to get the value via DictRef::from_value which is looking for an actual starlark dict. I think records might have one of these somewhere inside. So you could try digging around.