diff --git a/src/compiletest/procsrv.rs b/src/compiletest/procsrv.rs
index aab08ffffe78..312c48a24065 100644
--- a/src/compiletest/procsrv.rs
+++ b/src/compiletest/procsrv.rs
@@ -106,7 +106,7 @@ fn readclose(fd: c_int) -> ~str {
     let mut buf = ~"";
     while !reader.eof() {
         let bytes = reader.read_bytes(4096u);
-        buf += str::from_bytes(bytes);
+        str::push_str(buf, str::from_bytes(bytes));
     }
     os::fclose(file);
     return buf;