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/build.rs

@@ -0,0 +1,39 @@
+fn main() {
+    use std::process::Command;
+    
+    let project_root = std::env::current_dir()
+        .unwrap()
+        .parent()  // flatbuffers/tests/rust_usage test
+        .unwrap()
+        .parent()  // flatbuffers/tests
+        .unwrap()
+        .parent()  // flatbuffers/
+        .unwrap()
+        .to_path_buf();
+    
+    let sample_schema = {
+        let mut s = project_root.to_path_buf();
+        s.push("samples");
+        s.push("monster.fbs");
+        s
+    };
+
+    let flatc = {
+        let mut f = project_root.to_path_buf();
+        f.push("flatc");
+        f
+    };
+
+    let out_dir = std::path::Path::new(&std::env::var("OUT_DIR").unwrap()).to_path_buf();
+
+    Command::new(&flatc)
+        .arg("--rust")
+        .arg(&sample_schema)
+        .arg("--filename-suffix")
+        .arg("_gen")
+        .output()
+        .expect("Failed to generate file");
+
+    let genfile = "monster_gen.rs";
+    std::fs::rename(&genfile, out_dir.join("monster_generated.rs")).unwrap();
+}