How do I carry around dynamic dependencies on e.g. headers?

Thanks for the great work. I have a design question for a problem I've been trying to solve. I'm new to buck2 and I'm not a previous Bazel or buck user, so it's been a little rough getting started. If I make any conceptual mistakes, please point them out :)

For context, this is without the prelude -- I would like to keep much simpler rules on hand that don't need to be as flexible and get better at implementing rules.

I have a project made up a bunch of logical groupings of C files, created by a rule `c_inputs` that returns the provider `CInputsInfo`:

```starlark
CInputsInfo = provider(
doc = "Info on a set of C inputs.",
fields = [
"srcs",
"hdrs", # dict mapping { str (header path): source }, globbed from all qualified_inc_paths
"qualified_inc_paths",
]
)
```

Downstream rules consuming `CInputsInfo` might compile the sources into a binary, run a linter on them, and so on.

I want to have a dynamic dependency on the headers consumed by these C files, generated by the compiler (`-M` family of options in gcc/clang). I would like to do the actual dependency generation only once and then get to use the sources many times. I see a couple of general approaches to doing this:

1. `dynamic_outputs`: I implemented this for one use case (`c_objects`), and it works well, but I'm having trouble seeing how I would do this for `c_inputs` because we wouldn't yet have an artifact to bind.

```starlark
def _c_objects_impl(ctx: "context"):
...

for src in ctx.attrs.inputs[CInputsInfo].srcs:
...

def compile_with_header_deps(ctx, artifacts, outputs, dep_file_json=dep_file_json, object=object, src=src):
cmd = cmd_args([compiler, src, flags, inc_flags, "-c", "-o", outputs[object].as_output()])

# for every header in the list, add a hidden dependency on it to the cmd
dep_header_list = artifacts[dep_file_json].read_json()
for header in dep_header_list:
cmd.hidden(ctx.attrs.inputs[CInputsInfo].hdrs[header])

ctx.actions.run(cmd, category = "compile", identifier = str(outputs[object]))

ctx.actions.dynamic_output(dynamic=[dep_file_json], inputs=[src], outputs=[object], f=compile_with_header_deps)
```
2. `dep_file`: Unless I'm mistaken, these can't be used across actions.

Do you have any ideas on how to approach this kind of thing in buck2?