fix(ffi): add repr(C) to hyper_context

See mozilla/cbindgen#579 (comment)

capi/cbindgen.toml

@@ -13,3 +13,9 @@ sys_includes = ["stdint.h", "stddef.h", "stdbool.h"]
 cpp_compat = true
 documentation_style = "c"
 documentation_length = "short"
+
+[export]
+include = ["hyper_context_ffi"]
+
+[export.rename]
+hyper_context_ffi = "hyper_context"

capi/include/hyper.h

@@ -194,15 +194,15 @@ typedef struct hyper_waker hyper_waker;
 
 typedef int (*hyper_body_foreach_callback)(void*, const struct hyper_buf*);
 
-typedef int (*hyper_body_data_callback)(void*, struct hyper_context*, struct hyper_buf**);
+typedef int (*hyper_body_data_callback)(void*, hyper_context*, struct hyper_buf**);
 
 typedef void (*hyper_request_on_informational_callback)(void*, struct hyper_response*);
 
 typedef int (*hyper_headers_foreach_callback)(void*, const uint8_t*, size_t, const uint8_t*, size_t);
 
-typedef size_t (*hyper_io_read_callback)(void*, struct hyper_context*, uint8_t*, size_t);
+typedef size_t (*hyper_io_read_callback)(void*, hyper_context*, uint8_t*, size_t);
 
-typedef size_t (*hyper_io_write_callback)(void*, struct hyper_context*, const uint8_t*, size_t);
+typedef size_t (*hyper_io_write_callback)(void*, hyper_context*, const uint8_t*, size_t);
 
 #ifdef __cplusplus
 extern "C" {
@@ -525,7 +525,7 @@ void *hyper_task_userdata(struct hyper_task *task);
 /*
  Creates a waker associated with the task context.
  */
-struct hyper_waker *hyper_context_waker(struct hyper_context *cx);
+struct hyper_waker *hyper_context_waker(hyper_context *cx);
 
 /*
  Free a waker.

src/ffi/task.rs

@@ -127,8 +127,20 @@ 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>
+///
+/// We ignore this type in cbindgen because we want to expose an opaque type. See hyper_context_ffi.
+/// cbindgen:ignore
+#[repr(C)] // we transmute references
 pub struct hyper_context<'a>(Context<'a>);
 
+/// An async context for a task that contains the related waker.
+///
+/// This creates an opaque type for use in cbindgen. Use
+/// `hyper_context` in Rust code.
+#[doc(hidden)]
+#[allow(dead_code)]
+pub struct hyper_context_ffi<'a>(Context<'a>);
+
 /// A waker that is saved and used to waken a pending task.
 ///
 /// This is provided to `hyper_io`'s read and write callbacks via `hyper_context`
@@ -500,7 +512,7 @@ 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) }
     }
 }

src/proto/h2/mod.rs

@@ -384,6 +384,7 @@ fn h2_to_io_error(e: h2::Error) -> std::io::Error {
     }
 }
 
+#[repr(transparent)]
 struct UpgradedSendStream<B>(SendStream<SendBuf<Neutered<B>>>);
 
 impl<B> UpgradedSendStream<B>