feat(sterni/nix/fun): add hasEllipsis

As a complementation to builtins.functionArgs this function checks if
the function has a set pattern that contains an ellipsis
(i. e. `{ [arg, [ arg1, [ … ]]] ... }:`). The implementation of this is
pretty cursed however since there is no clean way to do this in vanilla
nix: We need to match on the output of builtins.toXML which does try to
serialize functions by outputting their argument and information about
it (whether it is a normal argument or a attribute set pattern, in the
latter case it also serialize every component of the pattern).

Change-Id: I0f33721811a3180cec205a0c98e6d92e10e92075
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2950
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/nix/fun/default.nix

@@ -26,6 +26,19 @@ let
   lrs = x: fs:
     builtins.foldl' (v: f: f v) x fs;
 
+  # Warning: cursed function
+  #
+  # Check if a function has an attribute
+  # set pattern with an ellipsis as its argument.
+  #
+  # s/o to puck for discovering that you could use
+  # builtins.toXML to introspect functions more than
+  # you should be able to in Nix.
+  hasEllipsis = f:
+    builtins.isFunction f &&
+    builtins.match ".*<attrspat ellipsis=\"1\">.*"
+      (builtins.toXML f) != null;
+
 in
 
 {
@@ -41,5 +54,6 @@ in
     rls
     lr
     lrs
+    hasEllipsis
     ;
 }