Allow explicit attrs in aspects that are not strings

[kgreenek]

Description of the problem / feature request:

I have implemented starlark rules for flatbuffers.

They work very similarly to the protobuf rules, e.g.

load("@rules_flatbuffers//flatbuffers:flatbuffers_library.bzl", "flatbuffers_library")
load("@rules_flatbuffers//flatbuffers:cc_flatbuffers_library.bzl", "cc_flatbuffers_library")

flatbuffers_library(
    name = "foo_fbs",
    srcs = ["foo.fbs"],
)

cc_flatbuffers_library(
    name = "foo_cc_fbs",
    deps = [":foo_fbs"],
)

The cc_flatbuffers_library rule uses aspects to generate the C++ sources from the fbs schema definitions. It's the perfect use-case for aspects.

I would like to enable users to pass a custom toolchain for the language-specific rules. The flatc compiler offers several useful customizations that users in different situations will want to enable by passing flags to the flatc compiler.

Concretely, I want to enable this type of usage:

load("@rules_flatbuffers//flatbuffers:flatbuffers_library.bzl", "flatbuffers_library")
load("@rules_flatbuffers//flatbuffers:cc_flatbuffers_library.bzl", "cc_flatbuffers_library")
load("@rules_flatbuffers//flatbuffers/toolchains:cc_flatbuffers_toolchain.bzl", "cc_flatbuffers_toolchain")

cc_flatbuffers_toolchain(
    name = "my_custom_cc_toolchain",
    flatc_args = [ ... ],
)   

flatbuffers_library(
    name = "foo_fbs",
    srcs = ["foo.fbs"],
)

cc_flatbuffers_library(
    name = "foo_cc_fbs",
    toolchain = ":my_custom_cc_toolchain",
    deps = [":foo_fbs"],
)

I have already implemented it, but in order to allow a custom toolchain arg to be passed, I need to pass the toolchain to the aspect. Currently this does not appear to be possible.

The toolchain is implemented as a simple rule which returns a FlatbuffersToolchainInfo provider. I need to be able to pass a label with this provider to the cc code generation aspect.

I was almost able to get what I need with a private attr called _toolchain. But when it is defined this way, I don't think the user can pass their own custom toolchain.

The current implementation of the rule, is implemented here for reference:
https://github.com/kgreenek/rules_flatbuffers/blob/7ec99994d4b56a2eb0735871470298e26875c780/flatbuffers/cc_flatbuffers_library.bzl#L101

Feature requests: what underlying problem are you trying to solve with this feature?

Allow non-string attrs for aspects. Specifically, allow labels to be passed as attrs to an aspect.

What operating system are you running Bazel on?

Ubuntu 18.04 & 20.04

What's the output of bazel info release?

release 4.2.1

Have you found anything relevant by searching the web?

Somewhat-related issue: #11246

[benjaminp]

#8494

[kgreenek]

That is super helpful, thank you! It seems to me like maybe my mental model for how aspects are supposed to be used was a bit wrong.

One idea for my specific use-case: instead of enabling users to pass a toolchain to the cc_flatbuffers_library rule, I could instead require them to set up the toolchain in the WORKSPACE file. That would generate a BUILD file with a toolchain target defined that everything in the workspace would use.

[kgreenek]

I made that change, and now I think it is much cleaner:
https://github.com/kgreenek/rules_flatbuffers/tree/c81acb253637eb28cb4fc4b1dd5f1d9cc6a0ca29

I think it's also more in-line with other starlark rules.

I do think it would still be useful for aspects attrs to be a bit less restrictive, but I will close this issue since the other issue is already tracking it.