Fix the soundness bug in the representation of extern types

Since the very first import dafaca9 ("Initial import of liblibc"),
`libc` has used uninhabited enums to represent C's incomplete/opaque
types. While this is, as far as I know, techincally okay when working
behind raw pointers, it means that using reference types like `&FILE`
can lead to easy UB.

Resolve this by changing the representation to a `!Sync + !Send +
!Unpin` ZST, as recommended by the nomicon [1]. The loss of auto traits
technically makes this user-visible, but it is unlikely that anybody who
is doing sound things was relying on these. 

I also used this as an opportunity to add a forward-compatibility note
about intended use that should allow us to switch to real extern types
once those are available.

[1]: https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs

Author: tgross35

src/fuchsia/mod.rs

@@ -79,9 +79,9 @@ pub type fsfilcnt_t = c_ulonglong;
 pub type rlim_t = c_ulonglong;
 
 extern_ty! {
-    pub enum timezone {}
-    pub enum DIR {}
-    pub enum fpos64_t {} // FIXME(fuchsia): fill this out with a struct
+    pub type timezone;
+    pub type DIR;
+    pub type fpos64_t; // FIXME(fuchsia): fill this out with a struct
 }
 
 // PUB_STRUCT
@@ -3192,8 +3192,8 @@ fn __MHDR_END(mhdr: *const msghdr) -> *mut c_uchar {
 extern "C" {}
 
 extern_ty! {
-    pub enum FILE {}
-    pub enum fpos_t {} // FIXME(fuchsia): fill this out with a struct
+    pub type FILE;
+    pub type fpos_t; // FIXME(fuchsia): fill this out with a struct
 }
 
 extern "C" {

src/macros.rs

@@ -246,17 +246,37 @@ macro_rules! s_no_extra_traits {
 macro_rules! extern_ty {
     ($(
         $(#[$attr:meta])*
-        pub enum $i:ident {}
+        pub type $i:ident;
     )*) => ($(
         $(#[$attr])*
+        /// This is an extern type ("opaque" or "incomplete" type in C).
+        ///
+        /// <div class="warning">
+        /// This type's current representation allows inspecting some properties, such as via
+        /// <code>size_of</code>, and it is technically possible to construct the type within
+        /// <code>MaybeUninit</code>, However, this <strong>MUST NOT</strong> be relied upon
+        /// because a future version of <code>libc</code> may switch to a proper
+        /// <a href="https://rust-lang.github.io/rfcs/1861-extern-types.html">extern type</a>
+        /// representation when available.
+        /// </div>
+        // ^ unfortunately warning blocks currently don't render markdown so we need to
+        // use raw HTML.
+        //
+        // Representation based on the Nomicon:
+        // <https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs>.
+        //
         // FIXME(1.0): the type is uninhabited so these traits are unreachable and could be
         // removed.
         #[::core::prelude::v1::derive(
             ::core::clone::Clone,
             ::core::marker::Copy,
             ::core::fmt::Debug,
         )]
-        pub enum $i { }
+        #[repr(C)]
+        pub struct $i {
+            _data: (),
+            _marker: ::core::marker::PhantomData<(*mut u8, ::core::marker::PhantomPinned)>,
+        }
     )*);
 }
 

src/new/qurt/mod.rs

@@ -60,7 +60,7 @@ pub type fd_set = c_ulong;
 
 // Standard C library types
 extern_ty! {
-    pub enum FILE {}
+    pub type FILE;
 }
 pub type fpos_t = c_long;
 pub type clock_t = c_long;

src/solid/mod.rs

@@ -397,8 +397,8 @@ pub const SIGUSR2: c_int = 31;
 pub const SIGPWR: c_int = 32;
 
 extern_ty! {
-    pub enum FILE {}
-    pub enum fpos_t {}
+    pub type FILE;
+    pub type fpos_t;
 }
 
 extern "C" {

src/unix/aix/powerpc64.rs

@@ -3,7 +3,7 @@ use crate::prelude::*;
 
 // Define lock_data_instrumented as an empty enum
 extern_ty! {
-    pub enum lock_data_instrumented {}
+    pub type lock_data_instrumented;
 }
 
 s! {

src/unix/bsd/apple/mod.rs

@@ -176,7 +176,7 @@ pub type attrgroup_t = u32;
 pub type vol_capabilities_set_t = [u32; 4];
 
 extern_ty! {
-    pub enum timezone {}
+    pub type timezone;
 }
 
 c_enum! {

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -49,7 +49,7 @@ pub type vm_map_entry_t = *mut vm_map_entry;
 pub type pmap = __c_anonymous_pmap;
 
 extern_ty! {
-    pub enum sem {}
+    pub type sem;
 }
 
 c_enum! {

src/unix/bsd/freebsdlike/mod.rs

@@ -60,7 +60,7 @@ cfg_if! {
 // link.h
 
 extern_ty! {
-    pub enum timezone {}
+    pub type timezone;
 }
 
 impl siginfo_t {

src/unix/bsd/netbsdlike/mod.rs

@@ -17,8 +17,8 @@ pub type sem_t = *mut sem;
 pub type key_t = c_long;
 
 extern_ty! {
-    pub enum timezone {}
-    pub enum sem {}
+    pub type timezone;
+    pub type sem;
 }
 
 s! {

src/unix/bsd/netbsdlike/netbsd/mod.rs

@@ -45,7 +45,7 @@ c_enum! {
 }
 
 extern_ty! {
-    pub enum _cpuset {}
+    pub type _cpuset;
 }
 
 cfg_if! {