Introduce top-level structuredAttrs field in JSON derivation format

Makes the behavoral change of #13263 without the underlying refactor.
Hopefully this clearly safe from a perf and GC perspective, and will
make it easier to benchmark #13263.

doc/manual/source/language/advanced-attributes.md

@@ -53,23 +53,13 @@ Derivations can declare some infrequently used optional attributes.
 
   - [`__structuredAttrs`]{#adv-attr-structuredAttrs}\
     If the special attribute `__structuredAttrs` is set to `true`, the other derivation
-    attributes are serialised into a file in JSON format. The environment variable
-    `NIX_ATTRS_JSON_FILE` points to the exact location of that file both in a build
-    and a [`nix-shell`](../command-ref/nix-shell.md). This obviates the need for
-    [`passAsFile`](#adv-attr-passAsFile) since JSON files have no size restrictions,
-    unlike process environments.
+    attributes are serialised into a file in JSON format.
 
-    It also makes it possible to tweak derivation settings in a structured way; see
-    [`outputChecks`](#adv-attr-outputChecks) for example.
+    This obviates the need for [`passAsFile`](#adv-attr-passAsFile) since JSON files have no size restrictions, unlike process environments.
+    It also makes it possible to tweak derivation settings in a structured way;
+    see [`outputChecks`](#adv-attr-outputChecks) for example.
 
-    As a convenience to Bash builders,
-    Nix writes a script that initialises shell variables
-    corresponding to all attributes that are representable in Bash. The
-    environment variable `NIX_ATTRS_SH_FILE` points to the exact
-    location of the script, both in a build and a
-    [`nix-shell`](../command-ref/nix-shell.md). This includes non-nested
-    (associative) arrays. For example, the attribute `hardening.format = true`
-    ends up as the Bash associative array element `${hardening[format]}`.
+    See the [corresponding section in the derivation page](@docroot@/store/derivation/index.md#structured-attrs) for further details.
 
     > **Warning**
     >

doc/manual/source/protocols/json/derivation.md

@@ -91,3 +91,7 @@ is a JSON object with the following fields:
 
 * `env`:
   The environment passed to the `builder`.
+
+* `structuredAttrs`:
+  [Strucutured Attributes](@docroot@/store/derivation/index.md#structured-attrs), only defined if the derivation contains them.
+  Structured attributes are JSON, and thus embedded as-is.

doc/manual/source/store/derivation/index.md

@@ -138,6 +138,17 @@ See [Wikipedia](https://en.wikipedia.org/wiki/Argv) for details.
 
 Environment variables which will be passed to the [builder](#builder) executable.
 
+#### Structured Attributes {#structured-attrs}
+
+Nix also has special support for embedding JSON in the derivations.
+
+The environment variable `NIX_ATTRS_JSON_FILE` points to the exact location of that file both in a build and a [`nix-shell`](@docroot@/command-ref/nix-shell.md).
+
+As a convenience to Bash builders, Nix writes a script that initialises shell variables corresponding to all attributes that are representable in Bash.
+The environment variable `NIX_ATTRS_SH_FILE` points to the exact location of the script, both in a build and a [`nix-shell`](@docroot@/command-ref/nix-shell.md).
+This includes non-nested (associative) arrays.
+For example, the attribute `hardening.format = true` ends up as the Bash associative array element `${hardening[format]}`.
+
 ### Placeholders
 
 Placeholders are opaque values used within the [process creation fields] to [store objects] for which we don't yet know [store path]s.