lib: add inPureEvalMode

This makes a value that is true if builtins does not contain the
currentSystem function, but false if it does.

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/trivial.nix b/lib/trivial.nix
index 18616a189c269..902c56a2d0aad 100644
--- a/lib/trivial.nix
+++ b/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