nixos docs: allow displaying package references

This is an improved version of original #12357.
For the purpose of generating docs, evaluate options with each derivation
in `pkgs` (recursively) replaced by a fake with path "\${pkgs.attribute.path}".
It isn't perfect, but it seems to cover a vast majority of use cases.
Caveat: even if the package is reached by a different means,
the path above will be shown and not e.g. `${config.services.foo.package}`.

As before, defaults created by `mkDefault` aren't displayed,
but documentation shouldn't (mostly) be a reason to use that anymore.

Note: t wouldn't be enough to just use `lib.mapAttrsRecursive`,
because derivations are also (special) attribute sets.

1 files changed, 22 insertions, 6 deletions

diff --git a/nixos/modules/services/misc/nixos-manual.nix b/nixos/modules/services/misc/nixos-manual.nix
index 3e1f53e79f3e..8ef1b035ca6f 100644
--- a/nixos/modules/services/misc/nixos-manual.nix
+++ b/nixos/modules/services/misc/nixos-manual.nix
@@ -17,16 +17,32 @@ let
       nixpkgs.system = config.nixpkgs.system;
     };
 
-  eval = evalModules {
-    modules = [ versionModule ] ++ baseModules;
-    args = (config._module.args) // { modules = [ ]; };
-  };
-
+  /* For the purpose of generating docs, evaluate options with each derivation
+    in `pkgs` (recursively) replaced by a fake with path "\${pkgs.attribute.path}".
+    It isn't perfect, but it seems to cover a vast majority of use cases.
+    Caveat: even if the package is reached by a different means,
+    the path above will be shown and not e.g. `${config.services.foo.package}`. */
   manual = import ../../../doc/manual {
     inherit pkgs;
     version = config.system.nixosVersion;
     revision = config.system.nixosRevision;
-    options = eval.options;
+    options =
+      let
+        scrubbedEval = evalModules {
+          modules = [ versionModule ] ++ baseModules;
+          args = (config._module.args) // { modules = [ ]; };
+          specialArgs = { pkgs = scrubDerivations "pkgs" pkgs; };
+        };
+        scrubDerivations = namePrefix: pkgSet: mapAttrs
+          (name: value:
+            let wholeName = "${namePrefix}.${name}"; in
+            if isAttrs value then
+              scrubDerivations wholeName value
+              // (optionalAttrs (isDerivation value) { outPath = "\${${wholeName}}"; })
+            else value
+          )
+          pkgSet;
+      in scrubbedEval.options;
   };
 
   entry = "${manual.manual}/share/doc/nixos/index.html";