Serialize to disk if descriptor path is set

prost-build/src/lib.rs

@@ -886,7 +886,15 @@ impl Config {
 
         #[cfg(feature = "vendored")]
         if protoc() == Path::new("linked") {
-            return Self::serialize_file_descriptor_set(protos, includes);
+            let file_descriptor_set = Self::serialize_file_descriptor_set(protos, includes)?;
+
+            // If the user has set the file descriptor path manually then also save it
+            // to disk in case they are counting on that behavior
+            if let Some(path) = &self.file_descriptor_set_path {
+                std::fs::write(path, &file_descriptor_set)?;
+            }
+
+            return Ok(file_descriptor_set);
         }
 
         let tmp;