populate: deliver flake and flake-input sources natively in Rust

src/flake_input.rs reads what nix flake metadata and nix flake archive
print as JSON itself: it selects the flake or input, decides what the
target fetches or gets copied, and computes the store paths to root,
which are what selected.jq, selfIsRemote.jq, selfLocked.jq, and the Nix
expressions of the shell implementation do there.  On the target, fixed
snippets in sh::flake run Nix, getting values as positional parameters,
and send JSON or store paths back.  The jq test's cases are unit tests of
the Rust equivalents.

1 files changed, 5 insertions, 4 deletions

diff --git a/pkgs/populate/docs/design.md b/pkgs/populate/docs/design.md
index fd64c2b..3340ccf 100644
--- a/pkgs/populate/docs/design.md
+++ b/pkgs/populate/docs/design.md
@@ -177,10 +177,11 @@ Populate runs rsync, Git, ssh, and Nix as programs, found in `PATH`.
 ## Status
 
 The program delivers all source types, draws inputs from sources, makes
-backups, and prunes.  The code for flakes, in `src/flake.rs`, builds the
-same shell code that `shell.nix` generates, with every value quoted, so
-both implementations behave alike; it moves to Rust piece by piece, with
-the scenarios telling whether it still behaves alike.
+backups, and prunes.  Drawing inputs, in `src/flake.rs`, builds the same
+shell code that `shell.nix` generates, with every value quoted; it moves
+to Rust like delivering flake and flake-input sources did, in
+`src/flake_input.rs`, which reads what Nix prints as JSON itself, and runs
+fixed snippets on the target that send JSON or store paths back.
 
 The jq programs in `flake-jq.nix`, which the shell implementation and the
 jq test use, have equivalents in Rust, with the jq test's cases as unit