diff --git a/src/librustc_data_structures/blake2b.rs b/src/librustc_data_structures/blake2b.rs
index bdef9fefd41e4..5adeef1ab3a6d 100644
--- a/src/librustc_data_structures/blake2b.rs
+++ b/src/librustc_data_structures/blake2b.rs
@@ -23,6 +23,7 @@
 use std::mem;
 use std::slice;
 
+#[repr(C)]
 pub struct Blake2bCtx {
     b: [u8; 128],
     h: [u64; 8],