Add TryFrom to convert from Varargs to tuples

godot-rust#886

Author: B-head

gdnative-core/src/export/method.rs

@@ -415,6 +415,60 @@ impl<'a> Iterator for Varargs<'a> {
     }
 }
 
+// Return a second token.
+macro_rules! replace_expr {
+    ($_t:tt $sub:expr) => {
+        $sub
+    };
+}
+
+// Count parameters.
+macro_rules! count_tts {
+    ($($tts:tt)*) => {
+        0usize $(+ replace_expr!($tts 1usize))*
+    };
+}
+
+// Convert from Varargs to tuples, implement traits.
+macro_rules! varargs_into_tuple {
+    ($($params:ident),*) => {
+        impl<'a, $($params: FromVariant),*> std::convert::TryFrom<Varargs<'a>> for ($($params,)*) {
+            type Error = VarargsError;
+
+            #[inline]
+            fn try_from(args: Varargs<'a>) -> Result<Self, Self::Error> {
+                const EXPECTED: usize = count_tts!($($params)*);
+                args.check_length(EXPECTED)?;
+                let mut i: usize = 0;
+                #[allow(unused_variables, unused_mut)]
+                let mut inc = || {
+                    let ret = i;
+                    i += 1;
+                    ret
+                };
+                Ok((
+                    $(args.get::<$params>(inc())?,)*
+                ))
+            }
+        }
+    };
+}
+
+// Define up to the length supported by standard library.
+varargs_into_tuple!();
+varargs_into_tuple!(A);
+varargs_into_tuple!(A, B);
+varargs_into_tuple!(A, B, C);
+varargs_into_tuple!(A, B, C, D);
+varargs_into_tuple!(A, B, C, D, E);
+varargs_into_tuple!(A, B, C, D, E, F);
+varargs_into_tuple!(A, B, C, D, E, F, G);
+varargs_into_tuple!(A, B, C, D, E, F, G, H);
+varargs_into_tuple!(A, B, C, D, E, F, G, H, I);
+varargs_into_tuple!(A, B, C, D, E, F, G, H, I, J);
+varargs_into_tuple!(A, B, C, D, E, F, G, H, I, J, K);
+varargs_into_tuple!(A, B, C, D, E, F, G, H, I, J, K, L);
+
 /// All possible error types for convert from Varargs.
 #[derive(Debug)]
 pub enum VarargsError {