lib: add callLocklessFlake

This is essentially a copy of the function of the same name, from
flake-compat. callLocklessFlake is useful when trying to utilise a
flake.nix without a lock file, often for when you want to create a
subflake from within a parent flake.

Co-authored-by: Tom Bereknyei <tomberek@gmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

lib/default.nix

@@ -64,7 +64,7 @@ let
       hasAttr head isAttrs isBool isInt isList isString length
       lessThan listToAttrs pathExists readFile replaceStrings seq
       stringLength sub substring tail trace;
-    inherit (self.trivial) id const pipe concat or and bitAnd bitOr bitXor
+    inherit (self.trivial) id callLocklessFlake const pipe concat or and bitAnd bitOr bitXor
       bitNot boolToString mergeAttrs flip mapNullable inNixShell isFloat min max
       importJSON importTOML warn warnIf warnIfNot throwIf throwIfNot checkListOfEnum
       info showWarnings nixpkgsVersion version isInOldestRelease

lib/trivial.nix

@@ -13,6 +13,22 @@ rec {
     # The value to return
     x: x;
 
+  /* imports a flake.nix without acknowledging its lock file, useful for
+     referencing subflakes from a parent flake. The second argument allows
+     specifying the inputs of this flake.
+
+     Example:
+       callLocklessFlake {
+         path = ./directoryContainingFlake;
+         inputs = { inherit nixpkgs; };
+       }
+  */
+  callLocklessFlake = { path, inputs ? {} }: let
+    self = {outPath = path;} //
+    ((import (path + "/flake.nix")).outputs (inputs // {self = self;}));
+  in
+    self;
+
   /* The constant function
 
      Ignores the second argument. If called with only one argument,