docs(tvix/TODO): add builtins.path roundtrip for flat

Change-Id: I8291d2fdfa2ea31ec24c82dcfb447fa9eaa3a961
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11819
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/docs/src/TODO.md

@@ -115,6 +115,15 @@ Some more fetcher-related builtins need work:
  - `fetchTree` (hairy, seems there's no proper spec and the URL syntax seems
    subject to change/underdocumented)
 
+### `builtins.path` roundtrip for flat
+`builtins.path` currently uses `filtered_ingest` also for the non-recursive
+case, then reads through the blob contents again to get the sha256.
+
+We should take care of assembling the root node on our own, and pipe the data
+through sha256 too (via `InspectReader`, see `glue/fetcher` for an example).
+
+This avoids some roundtrips, and is probably faster.
+
 ### Derivation -> Build
 While we have some support for `structuredAttrs` and `fetchClosure` (at least
 enough to calculate output hashes, aka produce identical ATerm), the code