Expose source-files and project structure

[njlr]

In order to build Bazel rules for Fable, we need access to the F# source-files and target dependency graph. This information can be used to generate fsproj files that the Fable CLI tool can understand.

This PR exposes these things, but in a generic way, so that it can be used for multiple purposes.

I was able to build a proof-of-concept set of rules for Fable on top of this. However, before going further I would like feedback on these changes, since I hope to eventually have them merged into rules_dotnet.

Thanks!

Related: #315

[purkhusid]

I'm not sure about the exact implementation you have for your Fable rules but I wonder why you need to couple the Fable rules with the rules_dotnet ones? AFAIK the Fable compiler only works with the source files so there is not need to compile a dll and then pass the results of that compilation to Fable.

If you can share the fable rules your are working on then I would maybe be able to see the usecase better?

[njlr]

I'm not sure about the exact implementation you have for your Fable rules but I wonder why you need to couple the Fable rules with the rules_dotnet ones? AFAIK the Fable compiler only works with the source files so there is not need to compile a dll and then pass the results of that compilation to Fable.

If you can share the fable rules your are working on then I would maybe be able to see the usecase better?

The idea is to be able to use rules_dotnet targets as inputs into rules_fable:

load("@aspect_rules_js//js:defs.bzl", "js_binary")
load("@npm//:defs.bzl", "npm_link_all_packages")
load("@npm//:webpack-cli/package_json.bzl", "bin")
load("@rules_dotnet//dotnet:defs.bzl", "fsharp_binary")
load("@rules_fable//fable:defs.bzl", "fable_bundle")

npm_link_all_packages(
  name = "node_modules",
)

fsharp_binary(
  name = "hello_world",
  srcs = [
    "Hello.fs",
  ],
  target_frameworks = [ "net6.0" ],
  deps = [
    "@paket.main//fsharp.core",
  ],
)

fable_bundle(
  name = "fable",
  target = ":hello_world",
)

bin.webpack_cli(
  name = "bundle",
  srcs = [
    ":fable",
  ],
  outs = [
    "dist/main.js",
  ],
  args = [
    "--mode",
    "production",
    "--entry",
    "./fable/Hello.fs.js",
  ],
  log_level = "debug",
)

js_binary(
  name = "app",
  entry_point = ":bundle",
)

This means that a user does not need to define the project structure twice.

[purkhusid]

I'm not sure I would want to add this as a public interface yet since I don't consider the providers public interfaces yet. This can be solved on the Fable rules side or on the end user side very easily by using a macro.

Sorry if that is a bummer but I'm trying to make the public API surface as small as possible to ease maintenance.