diff --git a/src/tools/compiletest/src/header.rs b/src/tools/compiletest/src/header.rs
index e70e01e8757..7c65e37119d 100644
--- a/src/tools/compiletest/src/header.rs
+++ b/src/tools/compiletest/src/header.rs
@@ -650,7 +650,7 @@ fn iter_header_extra(
 
     let comment = if testfile.extension().is_some_and(|e| e == "rs") { "//" } else { "#" };
 
-    let mut rdr = BufReader::new(rdr);
+    let mut rdr = BufReader::with_capacity(1024, rdr);
     let mut ln = String::new();
     let mut line_number = 0;