Merge pull request #157480 from MatthewCroughan/in-pure-eval-mode

lib: add inPureEvalMode

lib/default.nix

@@ -71,7 +71,7 @@ let
       info showWarnings nixpkgsVersion version isInOldestRelease
       mod compare splitByAndCompare
       functionArgs setFunctionArgs isFunction toFunction
-      toHexString toBaseDigits;
+      toHexString toBaseDigits inPureEvalMode;
     inherit (self.fixedPoints) fix fix' converge extends composeExtensions
       composeManyExtensions makeExtensible makeExtensibleWithCustomName;
     inherit (self.attrsets) attrByPath hasAttrByPath setAttrByPath

lib/trivial.nix

@@ -229,6 +229,13 @@ rec {
   */
   inNixShell = builtins.getEnv "IN_NIX_SHELL" != "";
 
+  /* Determine whether the function is being called from inside pure-eval mode
+     by seeing whether `builtins` contains `currentSystem`. If not, we must be in
+     pure-eval mode.
+
+     Type: inPureEvalMode :: bool
+  */
+  inPureEvalMode = ! builtins ? currentSystem;
 
   ## Integer operations