improve CSharp plugin policy logic

Author: Heliozoa

plugins/csharp/src/plugin/policy.rs

@@ -1,6 +1,5 @@
 //! Student file policy for C#
 
-use std::ffi::OsString;
 use std::path::{Path, PathBuf};
 use tmc_langs_framework::StudentFilePolicy;
 
@@ -17,16 +16,12 @@ impl CSharpStudentFilePolicy {
 
     /// Goes up directories until a bin or obj directory is found
     fn is_child_of_binary_dir(&self, path: &Path) -> bool {
-        let mut parent = path.parent();
-        while let Some(next) = parent {
-            if let Some(file_name) = next.file_name() {
-                if next.is_dir()
-                    && (file_name == OsString::from("bin") || file_name == OsString::from("obj"))
-                {
+        for ancestor in path.ancestors() {
+            if let Some(file_name) = ancestor.file_name() {
+                if ancestor.is_dir() && (file_name == "bin" || file_name == "obj") {
                     return true;
                 }
             }
-            parent = next.parent();
         }
         false
     }
@@ -35,11 +30,7 @@ impl CSharpStudentFilePolicy {
 impl StudentFilePolicy for CSharpStudentFilePolicy {
     // false for files in bin or obj directories, true for other files in src
     fn is_student_source_file(&self, path: &Path) -> bool {
-        if self.is_child_of_binary_dir(path) {
-            false
-        } else {
-            path.starts_with("src")
-        }
+        path.starts_with("src") && !self.is_child_of_binary_dir(path)
     }
 
     fn get_config_file_parent_path(&self) -> &Path {