Source complete env in `nix-shell` with `__structuredAttrs = true;`

This is needed to push the adoption of structured attrs[1] forward. It's
now checked if a `__json` exists in the environment-map of the derivation
to be openend in a `nix-shell`.

Derivations with structured attributes enabled also make use of a file
named `.attrs.json` containing every environment variable represented as
JSON which is useful for e.g. `exportReferencesGraph`[2]. To
provide an environment similar to the build sandbox, `nix-shell` now
adds a `.attrs.json` to `cwd` (which is mostly equal to the one in the
build sandbox) and removes it using an exit hook when closing the shell.

To avoid leaking internals of the build-process to the `nix-shell`, the
entire logic to generate JSON and shell code for structured attrs was
moved into the `ParsedDerivation` class.

[1] https://nixos.mayflower.consulting/blog/2020/01/20/structured-attrs/
[2] https://nixos.org/manual/nix/unstable/expressions/advanced-attributes.html#advanced-attributes

1 files changed, 19 insertions, 0 deletions

diff --git a/tests/structured-attrs-shell.nix b/tests/structured-attrs-shell.nix
new file mode 100644
index 000000000..0c01c2568
--- /dev/null
+++ b/tests/structured-attrs-shell.nix
@@ -0,0 +1,19 @@
+with import ./config.nix;
+let
+  dep = mkDerivation {
+    name = "dep";
+    buildCommand = ''
+      mkdir $out; echo bla > $out/bla
+    '';
+  };
+in
+mkDerivation {
+  name = "structured2";
+  __structuredAttrs = true;
+  outputs = [ "out" "dev" ];
+  my.list = [ "a" "b" "c" ];
+  exportReferencesGraph.refs = [ dep ];
+  buildCommand = ''
+    touch ''${outputs[out]}; touch ''${outputs[dev]}
+  '';
+}