ffi: use recommended stable way to represent an opaque C struct

After `extern { type ... }` has stabilized for a while, this can
be replaced.  For now, I used a macro since it is much easier to
spot the locations to touch at that time.

fixes PyO3#1311

src/ffi/code.rs

@@ -3,7 +3,7 @@ use crate::ffi::pyport::Py_ssize_t;
 use std::os::raw::{c_char, c_int, c_uchar, c_void};
 
 #[cfg(Py_3_8)]
-pub enum _PyOpcache {}
+opaque_struct!(_PyOpcache);
 
 #[repr(C)]
 #[derive(Copy, Clone)]

src/ffi/mod.rs

@@ -2,6 +2,16 @@
 #![cfg_attr(Py_LIMITED_API, allow(unused_imports))]
 #![cfg_attr(feature = "cargo-clippy", allow(clippy::inline_always))]
 
+// Until `extern type` is stabilized, use the recommended approach to
+// model opaque types:
+// https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs
+macro_rules! opaque_struct {
+    ($name:ident) => {
+        #[repr(C)]
+        pub struct $name([u8; 0]);
+    };
+}
+
 pub use self::bltinmodule::*;
 pub use self::boolobject::*;
 pub use self::bytearrayobject::*;
@@ -165,7 +175,7 @@ pub mod structmember; // TODO supports PEP-384 only; needs adjustment for Python
 pub mod frameobject;
 #[cfg(Py_LIMITED_API)]
 pub mod frameobject {
-    pub enum PyFrameObject {}
+    opaque_struct!(PyFrameObject);
 }
 
 pub(crate) mod datetime;

src/ffi/object.rs

@@ -262,7 +262,7 @@ pub type vectorcallfunc = unsafe extern "C" fn(
 
 #[cfg(Py_LIMITED_API)]
 mod typeobject {
-    pub enum PyTypeObject {}
+    opaque_struct!(PyTypeObject);
 }
 
 #[cfg(not(Py_LIMITED_API))]

src/ffi/pyarena.rs

@@ -1 +1 @@
-pub enum PyArena {}
+opaque_struct!(PyArena);

src/ffi/pythonrun.rs

@@ -15,7 +15,7 @@ pub struct PyCompilerFlags {
 }
 
 #[cfg(not(Py_LIMITED_API))]
-pub enum _mod {}
+opaque_struct!(_mod);
 
 #[cfg(not(Py_LIMITED_API))]
 extern "C" {
@@ -90,8 +90,8 @@ extern "C" {
     ) -> *mut _mod;
 }
 
-pub enum symtable {}
-pub enum _node {}
+opaque_struct!(symtable);
+opaque_struct!(_node);
 
 #[inline]
 pub unsafe fn PyParser_SimpleParseString(s: *const c_char, b: c_int) -> *mut _node {

src/ffi/weakrefobject.rs

@@ -1,7 +1,7 @@
 use crate::ffi::object::*;
 use std::os::raw::c_int;
 
-pub enum PyWeakReference {}
+opaque_struct!(PyWeakReference);
 
 extern "C" {
     static mut _PyWeakref_RefType: PyTypeObject;