Merge pull request RustPython#647 from RustPython/joey/no-arg-functions

Allow functions that don't take any args

Author: rmliddle

vm/src/pyobject.rs

@@ -1379,6 +1379,20 @@ where
     }
 }
 
+// For functions that accept no arguments. Implemented explicitly instead of via
+// macro below to avoid unused warnings.
+impl FromArgs for () {
+    fn from_args<I>(
+        _vm: &mut VirtualMachine,
+        _args: &mut iter::Peekable<I>,
+    ) -> Result<Self, ArgumentError>
+    where
+        I: Iterator<Item = PyArg>,
+    {
+        Ok(())
+    }
+}
+
 // A tuple of types that each implement `FromArgs` represents a sequence of
 // arguments that can be bound and passed to a built-in function.
 //
@@ -1465,25 +1479,26 @@ impl IntoPyNativeFunc<PyFuncArgs, PyResult> for PyNativeFunc {
 //
 // Note that this could be done without a macro - it is simply to avoid repetition.
 macro_rules! into_py_native_func_tuple {
-    ($(($n:tt, $T:ident)),+) => {
-        impl<F, $($T,)+ R> IntoPyNativeFunc<($($T,)+), R> for F
+    ($(($n:tt, $T:ident)),*) => {
+        impl<F, $($T,)* R> IntoPyNativeFunc<($($T,)*), R> for F
         where
-            F: Fn($($T,)+ &mut VirtualMachine) -> R + 'static,
-            $($T: FromArgs,)+
-            ($($T,)+): FromArgs,
+            F: Fn($($T,)* &mut VirtualMachine) -> R + 'static,
+            $($T: FromArgs,)*
+            ($($T,)*): FromArgs,
             R: IntoPyObject,
         {
             fn into_func(self) -> PyNativeFunc {
                 Box::new(move |vm, args| {
-                    let ($($n,)+) = args.bind::<($($T,)+)>(vm)?;
+                    let ($($n,)*) = args.bind::<($($T,)*)>(vm)?;
 
-                    (self)($($n,)+ vm).into_pyobject(&vm.ctx)
+                    (self)($($n,)* vm).into_pyobject(&vm.ctx)
                 })
             }
         }
     };
 }
 
+into_py_native_func_tuple!();
 into_py_native_func_tuple!((a, A));
 into_py_native_func_tuple!((a, A), (b, B));
 into_py_native_func_tuple!((a, A), (b, B), (c, C));