Enforce signatures on user get/set/index functions

codegen/ui_tests/rhai_fn_index_setter_multiple.rs

@@ -0,0 +1,28 @@
+use rhai::plugin::*;
+
+#[derive(Clone)]
+pub struct Point {
+    x: f32,
+    y: f32,
+}
+
+#[export_module]
+pub mod test_module {
+    pub use super::Point;
+    #[rhai_fn(name = "foo", index_set, index_set)]
+    pub fn test_fn(input: Point) -> bool {
+        input.x > input.y
+    }
+}
+
+fn main() {
+    let n = Point {
+        x: 0.0,
+        y: 10.0,
+    };
+    if test_module::test_fn(n) {
+        println!("yes");
+    } else {
+        println!("no");
+    }
+}