Add a constant ‘nixpkgsVersion’

This makes it a bit easier to find out the current Nixpkgs version,
e.g.

  $ nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion
  "14.02.35657.66f51a9"

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/trivial.nix b/lib/trivial.nix
index 760a74ce6664..190cda6abd67 100644
--- a/lib/trivial.nix
+++ b/lib/trivial.nix
@@ -41,4 +41,10 @@ rec {
     pathExists readFile isBool isFunction
     isInt add sub lessThan;
 
+  # Return the Nixpkgs version number.
+  nixpkgsVersion =
+    let suffixFile = ../.version-suffix; in
+    readFile ../.version
+    + (if pathExists suffixFile then readFile suffixFile else "pre-git");
+
 }