diff --git a/src/ffi/task.rs b/src/ffi/task.rs
index f53a7b1f5a..e745710633 100644
--- a/src/ffi/task.rs
+++ b/src/ffi/task.rs
@@ -127,6 +127,7 @@ struct TaskFuture {
 /// its only purpose is to provide access to the waker. See `hyper_waker`.
 ///
 /// Corresponding Rust type: <https://doc.rust-lang.org/std/task/struct.Context.html>
+#[repr(C)] // we transmute references
 pub struct hyper_context<'a>(Context<'a>);
 
 /// A waker that is saved and used to waken a pending task.
@@ -500,7 +501,7 @@ where
 
 impl hyper_context<'_> {
     pub(crate) fn wrap<'a, 'b>(cx: &'a mut Context<'b>) -> &'a mut hyper_context<'b> {
-        // A struct with only one field has the same layout as that field.
+        // A repr(C) newtype has the same layout as its only field.
         unsafe { std::mem::transmute::<&mut Context<'_>, &mut hyper_context<'_>>(cx) }
     }
 }
diff --git a/src/proto/h2/mod.rs b/src/proto/h2/mod.rs
index 15afe48d23..66879aa6c8 100644
--- a/src/proto/h2/mod.rs
+++ b/src/proto/h2/mod.rs
@@ -386,6 +386,7 @@ fn h2_to_io_error(e: h2::Error) -> std::io::Error {
     }
 }
 
+#[repr(transparent)]
 struct UpgradedSendStream<B>(SendStream<SendBuf<Neutered<B>>>);
 
 impl<B> UpgradedSendStream<B>