c++ code-generating rule in cc_library deps? - bazel

Question

I have automatically-generated .cc sources and a Starlark rule running the .cc generator:
BUILD file:
generate_cc(
name = "foo_generated"
)  # runs an executable that generates foo.h, foo.cc
I'd like the above foo_generated to act also as a cc_library, so that it can be a valid dependency of a subsequent cc_library:
cc_library(
  name = "bar",
  deps = [":foo_generated"]  # foo_generated used like a cc_library()
)
Can generate_cc be implemented in a single rule, without macros, so that a target of type generate_cc would work as other cc_library's deps?
(I realize that generate_cc could be a macro that calls the actual rule and then calls a cc_library rule, thereby creating two separate targets / labels - this is what I'd like to avoid).
If a rule implementation could call another rule, then generate_cc's implementation could
wrap the sources it generates in a cc_library
return the CcInfo provider returned by cc_library
as in (hypothetical .bzl file):
def generate_cc_impl(ctx):
  # generate .h, .cc files
  # ...
cc_info = native.cc_library(...)  # wrap .h, .cc files
  return cc_info
But I suppose calling one rule from another is not possible?

Answer 1

Rules cannot call other rules. However, support was added fairly recently for rules to reuse most of the native C++ functionality, which supports this use case. There's a section of documentation about implementing Starlark rules that depend on C++ rules and/or that C++ rules can depend on.
The my_c_archive example shows a lot of the boilerplate to use this functionality (finding the cc_toolchain and feature_configuration in particular). cc_common.compile is the function to create actions to compile your source files. cc_common.create_linking_context_from_compilation_outputs will convert the CcCompilationOutputs from compile into a CcLinkingContext for creating the CcInfo to return.
You can choose to pull some/all of the files out of the CcCompilationOutputs and CcLinkingOutputs to return as your rule's DefaultInfo, depending on your use case.
create_linking_context_from_compilation_outputs returns (CcLinkingContext, CcLinkingOutputs) for reference. I created bazel#10253 just now to add that to the docs.