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

[LegionMammal978]

The default representation does not guarantee the absence of initial padding, which is necessary for the transmute to be sound.

[seanmonstar]

Thanks!

[seanmonstar]

Sorry, I had to revert this in #3193, since I accidentally merged it before checking CI was happy. At the least, it's changed how the hyper.h file is generated. I can't yet tell if the change is important.

[LegionMammal978]

Whoops! I hadn't noticed that this project uses cbindgen. Indeed, it doesn't seem to do what we want, referring to a nonexistent Context type:

@@ -158,11 +158,6 @@
 typedef struct hyper_clientconn_options hyper_clientconn_options;
 
 /*
- An async context for a task that contains the related waker.
- */
-typedef struct hyper_context hyper_context;
-
-/*
  A more detailed error object returned by some hyper functions.
  */
 typedef struct hyper_error hyper_error;
@@ -206,15 +201,20 @@
 
 typedef int (*hyper_body_foreach_callback)(void*, const struct hyper_buf*);
 
-typedef int (*hyper_body_data_callback)(void*, struct hyper_context*, struct hyper_buf**);
+/*
+ An async context for a task that contains the related waker.
+ */
+typedef Context hyper_context;
+
+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" {
@@ -743,7 +743,7 @@
 /*
  Copies a waker out of 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 that hasn't been woken.

And if we try replacing it with #[repr(C)], then it writes out the whole struct definition, again including the Context type. Looking at cbindgen, the only official way to get it to generate an opaque item is not to place any repr() attribute on the structure. Curiously, there's also an unofficial way, which is to do anything that would make cbindgen throw an error on the struct, e.g., prefix it with /// cbindgen:==. But this is a sticky issue all around, and it's hard for me to tell which solution would be best.