--- a/xpcom/rust/gtest/xpcom/test.rs
+++ b/xpcom/rust/gtest/xpcom/test.rs
@@ -4,16 +4,17 @@
 
 #![allow(non_snake_case)]
 
 #[macro_use]
 extern crate xpcom;
 
 extern crate nserror;
 
+use std::os::raw::c_char;
 use std::ptr;
 use std::ffi::{CStr, CString};
 use xpcom::interfaces;
 use nserror::{NsresultExt, nsresult, NS_OK};
 
 #[no_mangle]
 pub unsafe extern fn Rust_ObserveFromRust() -> *const interfaces::nsIObserverService {
     let obssvc = xpcom::services::get_ObserverService().unwrap();
@@ -24,17 +25,17 @@ pub unsafe extern fn Rust_ObserveFromRus
     #[refcnt = "nonatomic"]
     struct InitObserver {
         run: *mut bool
     }
     impl Observer {
         unsafe fn Observe(
             &self,
             _subject: *const interfaces::nsISupports,
-            topic: *const i8,
+            topic: *const c_char,
             _data: *const i16
         ) -> nsresult {
             *self.run = true;
             assert!(CStr::from_ptr(topic).to_str() == Ok("test-rust-observe"));
             NS_OK
         }
     }