A quick guided tour of what I imagine this would look like:

• Add a unit test in tests/python/pants_test/engine/test_fs.py
• Teach the engine that it knows how to take an input of type digest and return a product of type snapshot, by adding an entry here:

  pants/src/rust/engine/src/tasks.rs

  Lines 71 to 96 in 39f8080

  	let intrinsics = vec![
  	Intrinsic {
  	product: types.snapshot,
  	input: types.path_globs,
  	},
  	Intrinsic {
  	product: types.snapshot,
  	input: types.url_to_fetch,
  	},
  	Intrinsic {
  	product: types.files_content,
  	input: types.directory_digest,
  	},
  	Intrinsic {
  	product: types.directory_digest,
  	input: types.merged_directories,
  	},
  	Intrinsic {
  	product: types.directory_digest,
  	input: types.directory_with_prefix_to_strip,
  	},
  	Intrinsic {
  	product: types.process_result,
  	input: types.process_request,
  	},
  	];

• Teach the engine how to fulfil that edge, but adding a new match branch to this match:

  pants/src/rust/engine/src/nodes.rs

  Line 154 in 39f8080

  match &self.entry {

  which looks like:

&rule_graph::Rule::Intrinsic(Intrinsic { product, input }) if product == context.core.types.snapshot && input == context.core.types.directory_digest => {...}

where the ... will actually look like: Copy and paste the code from one of the other branches which uses a directory_digest (e.g. the one that produces files_content), and then call Snapshot::from_digest on the result. Probably with a map_err to a throw copied and pasted from one of the other branches too. Roughly:

self
  .select_product(&context, context.core.types.directory_digest, "intrinsic")
  .and_then(|directory_digest_val| lift_digest(&directory_digest_val).map_err(|str| throw(&str)))
  .and_then(|digest| Snapshot::from_digest(context.core.store(), digest).map_err(|str| throw(&str)))
  .map(move |snapshot| Snapshot::store_snapshot(&core, &snapshot))
  .to_boxed()

probably with some extra clones and map_errs around.