diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index f9edb073a2df7..f35cc08f4fbfe 100644
--- a/compiler/rustc_serialize/src/opaque.rs
+++ b/compiler/rustc_serialize/src/opaque.rs
@@ -193,6 +193,8 @@ impl FileEncoder {
         // shaves an instruction off those code paths (on x86 at least).
         assert!(capacity <= usize::MAX - max_leb128_len());
 
+        // Create the file for reading and writing, because some encoders do both
+        // (e.g. the metadata encoder when -Zmeta-stats is enabled)
         let file = File::options().read(true).write(true).create(true).truncate(true).open(path)?;
 
         Ok(FileEncoder {