Process-specific rule syntax

[stuhood]

As discussed with @illicitonion (and previously on #4289), we will likely want to make some process-specific syntax for rules, since they are guaranteed to need to wrap inputs and outputs (which would otherwise just be Snapshots and Snapshots, or ExecuteProcessRequests and ExecuteProcessResults) in order to disambiguate them from other process executions.

An expanded version of the syntax that we came up with was:

@process(ClasspathEntry, [Select(Javac), SelectProjection(Snapshot, SourcesField, 'source_field', Target)])
def javac(binary, sources_snapshot):
  out_dir = "doesnt_matter"
  process_request = SnapshottedProcessRequest(args=(binary.name,
                                                    '-d', out_dir)+
                                                    tuple(f.path for f in sources_snapshot.files),
                                              snapshots=(binary.snapshot, sources_snapshot),
                                              directories_to_create=(out_dir.path,))
  process_result = yield process_request
  if process_result.exit_code:
    raise ...
  return ClasspathEntry(process_result.snapshot.re_root(outdir))

I expect that either as-part-of, or immediately-after @ity's current PR, we will want to do something like this. In the meantime though, continuing to have input_conversion and output_conversion functions would likely be ok:

pants/src/python/pants/engine/isolated_process.py

Lines 150 to 167 in 4d64273

	@staticmethod
	def create(product_type, binary_type, input_selectors, input_conversion, output_conversion):
	"""TODO: Not clear that `binary_type` needs to be separate from the input selectors."""
	# Select the concatenation of the snapshot directory, binary, and input selectors.
	inputs = [Select(_Snapshots), Select(binary_type)] + list(input_selectors)
	# Apply the input/output conversions to a top-level process-execution function which
	# will receive all inputs, convert in, execute, and convert out.
	func = functools.partial(_snapshotted_process,
	input_conversion,
	output_conversion)
	func.__name__ = '{}_and_then_snapshotted_process_and_then_{}'.format(
	input_conversion.__name__, output_conversion.__name__
	)
	# Return a task triple that executes the function to produce the product type.
	return TaskRule(product_type, inputs, func)

[stuhood]

This is a higher priority now, but is blocked on @ity's rust-isolated-process ticket. cc @illicitonion

[stuhood]

Resolved via #5580.