plus-one dependency tracking does not handle custom rules

[jdai8]

The plus-one reduced classpath is implemented using an aspect that propagates along deps and exports. This doesn't work with custom rules that declare a custom attribute and call java_common.compile directly.

[jdai8]

We can work around this by exporting the custom dependencies. This is not ideal for all cases though.

[jdai8]

One potential solution is to expose a provider (maybe PlusOneDeps itself) so that custom rules can report their direct dependencies.

[liucijus]

@jdai8 I'm not sure I fully follow your idea. Any chance you can make a small POC PR to demonstrate what you have in mind?

[jdai8]

Consider a custom rule my_java_rule with a custom_dep attribute:

def my_java_rule_impl(ctx):
  ...
  java_info = java_common.compile(
    ctx = ctx,
    source_files = ctx.attrs.srcs,
    deps = [ctx.attr.custom_dep[JavaInfo]],
    ...
  )

  return [java_info]

my_java_rule(
  name = "foo",
  ...
  custom_dep = "//:second_order_dep",
)

scala_library(
  name = "bar",
  srcs = ...,
  deps = [
    ":foo",
  ]
)

If we use the plus-one mode, I'd expect second_order_dep to be on the classpath for bar, but it's not.