nix run: Allow passing a command to execute

E.g.

  nix run nixpkgs.hello -c hello --greeting Hallo

Note that unlike "nix-shell --command", no quoting of arguments is
necessary.

"-c" (short for "--command") cannot be combined with "--" because they
both consume all remaining arguments. But since installables shouldn't
start with a dash, this is unlikely to cause problems.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index fd910b965..044ed209e 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -26,6 +26,8 @@ public:
 
 protected:
 
+    static const size_t ArityAny = std::numeric_limits<size_t>::max();
+
     /* Flags. */
     struct Flag
     {