evaluate inherit (from) exprs only once per directive

desugaring inherit-from to syntactic duplication of the source expr also
duplicates side effects of the source expr (such as trace calls) and
expensive computations (such as derivationStrict).

(cherry picked from commit cefd0302b55b3360dbca59cfcb4bf6a750d6cdcf)
Change-Id: Iff519f991adef2e51683ba2c552d37a3df7a179e

3 files changed, 12 insertions, 3 deletions

diff --git a/tests/functional/lang/eval-okay-inherit-from.err.exp b/tests/functional/lang/eval-okay-inherit-from.err.exp
index 51881205b..3227501f2 100644
--- a/tests/functional/lang/eval-okay-inherit-from.err.exp
+++ b/tests/functional/lang/eval-okay-inherit-from.err.exp
@@ -1,2 +1 @@
 trace: used
-trace: used
diff --git a/tests/functional/lang/eval-okay-inherit-from.exp b/tests/functional/lang/eval-okay-inherit-from.exp
index 43bd0e899..024daff6b 100644
--- a/tests/functional/lang/eval-okay-inherit-from.exp
+++ b/tests/functional/lang/eval-okay-inherit-from.exp
@@ -1 +1 @@
-[ 1 2 { __overrides = { y = { d = [ ]; }; }; c = [ ]; d = 4; x = { c = [ ]; }; y = «repeated»; } ]
+[ 1 2 { __overrides = { y = { d = [ ]; }; }; c = [ ]; d = 4; x = { c = [ ]; }; y = «repeated»; } { inner = { c = 3; d = 4; }; } ]
diff --git a/tests/functional/lang/eval-okay-inherit-from.nix b/tests/functional/lang/eval-okay-inherit-from.nix
index d1fad7d69..b72a1c639 100644
--- a/tests/functional/lang/eval-okay-inherit-from.nix
+++ b/tests/functional/lang/eval-okay-inherit-from.nix
@@ -2,5 +2,15 @@ let
   inherit (builtins.trace "used" { a = 1; b = 2; }) a b;
   x.c = 3;
   y.d = 4;
+
+  merged = {
+    inner = {
+      inherit (y) d;
+    };
+
+    inner = {
+      inherit (x) c;
+    };
+  };
 in
-  [ a b rec { x.c = []; inherit (x) c; inherit (y) d; __overrides.y.d = []; } ]
+  [ a b rec { x.c = []; inherit (x) c; inherit (y) d; __overrides.y.d = []; } merged ]