lib: make extendDerivation lighter on eval

the fix to extendDerivation in #140051 unwittingly worsened eval performance by
quite a bit. set elements alone needed over 1GB extra after the change, which
seems disproportionate to how small it was. if we flip the logic used to
determine which outputs to install around and keep a "this one exactly" flag in
the specific outputs instead of a "all of them" in the root we can avoid most
of that cost.

2 files changed, 3 insertions, 4 deletions

diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index 31fddc59e20..812521ce6d1 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -487,7 +487,7 @@ rec {
        => "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"
   */
   getOutput = output: pkg:
-    if pkg.outputUnspecified or false
+    if ! pkg ? outputSpecified || ! pkg.outputSpecified
       then pkg.${output} or pkg.out or pkg
       else pkg;
 
diff --git a/lib/customisation.nix b/lib/customisation.nix
index a794b673d70..234a528527d 100644
--- a/lib/customisation.nix
+++ b/lib/customisation.nix
@@ -145,14 +145,14 @@ rec {
     let
       outputs = drv.outputs or [ "out" ];
 
-      commonAttrs = (removeAttrs drv [ "outputUnspecified" ]) //
-        (builtins.listToAttrs outputsList) //
+      commonAttrs = drv // (builtins.listToAttrs outputsList) //
         ({ all = map (x: x.value) outputsList; }) // passthru;
 
       outputToAttrListElement = outputName:
         { name = outputName;
           value = commonAttrs // {
             inherit (drv.${outputName}) type outputName;
+            outputSpecified = true;
             drvPath = assert condition; drv.${outputName}.drvPath;
             outPath = assert condition; drv.${outputName}.outPath;
           };
@@ -160,7 +160,6 @@ rec {
 
       outputsList = map outputToAttrListElement outputs;
     in commonAttrs // {
-      outputUnspecified = true;
       drvPath = assert condition; drv.drvPath;
       outPath = assert condition; drv.outPath;
     };