take custom system path in actix_rt::main macro (#363)

actix-macros/tests/trybuild/main-04-system-path.rs

@@ -0,0 +1,8 @@
+mod system {
+    pub use actix_rt::System as MySystem;
+}
+
+#[actix_rt::main(system = "system::MySystem")]
+async fn main() {
+    futures_util::future::ready(()).await
+}

actix-macros/tests/trybuild/main-05-system-expect-path.rs

@@ -0,0 +1,4 @@
+#[actix_rt::main(system = "!@#*&")]
+async fn main2() {}
+
+fn main() {}

actix-macros/tests/trybuild/main-05-system-expect-path.stderr

@@ -0,0 +1,5 @@
+error: Expected path
+ --> $DIR/main-05-system-expect-path.rs:1:27
+  |
+1 | #[actix_rt::main(system = "!@#*&")]
+  |                           ^^^^^^^

actix-macros/tests/trybuild/main-06-unknown-attr.rs

@@ -0,0 +1,7 @@
+#[actix_rt::main(foo = "bar")]
+async fn async_main() {}
+
+#[actix_rt::main(bar::baz)]
+async fn async_main2() {}
+
+fn main() {}

actix-macros/tests/trybuild/main-06-unknown-attr.stderr

@@ -0,0 +1,11 @@
+error: Unknown attribute specified
+ --> $DIR/main-06-unknown-attr.rs:1:18
+  |
+1 | #[actix_rt::main(foo = "bar")]
+  |                  ^^^^^^^^^^^
+
+error: Unknown attribute specified
+ --> $DIR/main-06-unknown-attr.rs:4:18
+  |
+4 | #[actix_rt::main(bar::baz)]
+  |                  ^^^^^^^^