Rust: remove inner attributes (#6410)

* remove inner attributes

* Added test for outdir in Rust

* add bin/outdir

* Moved outdir test to its own package and only run it if flatc is available

Co-authored-by: Casper Neo <cneo@google.com>

tests/rust_usage_test/outdir/src/main.rs

@@ -0,0 +1,38 @@
+// In this example, a build.rs file generates the code and then copies it into $OUT_DIR. 
+extern crate flatbuffers;
+
+#[cfg(target_family = "unix")]
+#[allow(dead_code, unused_imports)]
+mod generated {
+    include!(concat!(env!("OUT_DIR"), "/monster_generated.rs"));
+}
+
+#[cfg(target_family = "windows")]
+#[allow(dead_code, unused_imports)]
+mod generated {
+    include!(concat!(env!("OUT_DIR"), "\\monster_generated.rs"));
+}
+
+use generated::my_game::sample::{Monster, MonsterArgs};
+
+
+fn main() {
+    let mut fbb = flatbuffers::FlatBufferBuilder::new();
+    let name = Some(fbb.create_string("bob"));
+    let m = Monster::create(&mut fbb, &MonsterArgs {
+        hp: 1,
+        mana: 2,
+        name,
+        ..Default::default()
+    });
+    fbb.finish(m, None);
+    let mon = flatbuffers::root::<Monster>(fbb.finished_data()).unwrap();
+    assert_eq!(mon.hp(), 1);
+    assert_eq!(mon.mana(), 2);
+    assert_eq!(mon.name().unwrap(), "bob");
+}
+
+#[test]
+fn test_main() {
+    main()
+}