diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 9ed4d241bd0..3800b270794 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -59,9 +59,9 @@ const METADATA_VERSION: u8 = 7;
 
 /// Metadata header which includes `METADATA_VERSION`.
 ///
-/// This header is followed by the position of the `CrateRoot`,
-/// which is encoded as a 32-bit big-endian unsigned integer,
-/// and further followed by the rustc version string.
+/// This header is followed by the length of the compressed data, then
+/// the position of the `CrateRoot`, which is encoded as a 32-bit big-endian
+/// unsigned integer, and further followed by the rustc version string.
 pub const METADATA_HEADER: &[u8] = &[b'r', b'u', b's', b't', 0, 0, 0, METADATA_VERSION];
 
 /// A value of type T referred to by its absolute position