Rename missing! to extern_ty!

Really what we are indicating with this macro invocation is that we need
a type that can be specified behind indirection but never used directly,
which is an extern type. Using an uninhabited enum gets us most of the
way there and is about the best we can do for now.

(backport <#4759>)
(cherry picked from commit fe5ecb8)

Author: tgross35

src/macros.rs

@@ -203,15 +203,20 @@ macro_rules! s_no_extra_traits {
     );
 }
 
-/// Specify that an enum should have no traits that aren't specified in the macro
-/// invocation, i.e. no `Clone` or `Copy`.
-macro_rules! missing {
+/// Create an uninhabited type that can't be constructed. It implements `Debug` but no
+/// other traits.
+///
+/// Really what we want here is something that also can't be named without indirection (in
+/// ADTs or function signatures), but this doesn't exist.
+macro_rules! extern_ty {
     ($(
         $(#[$attr:meta])*
         pub enum $i:ident {}
     )*) => ($(
         $(#[$attr])*
         #[allow(missing_copy_implementations)]
+        // FIXME(1.0): the type is uninhabited so this trait is unreachable.
+        #[::core::prelude::v1::derive(::core::fmt::Debug)]
         pub enum $i { }
     )*);
 }

src/unix/aix/powerpc64.rs

@@ -2,8 +2,7 @@ use crate::off_t;
 use crate::prelude::*;
 
 // Define lock_data_instrumented as an empty enum
-missing! {
-    #[derive(Debug)]
+extern_ty! {
     pub enum lock_data_instrumented {}
 }
 

src/unix/linux_like/linux/mod.rs

@@ -67,8 +67,7 @@ pub type eventfd_t = u64;
 
 cfg_if! {
     if #[cfg(not(target_env = "gnu"))] {
-        missing! {
-            #[derive(Debug)]
+        extern_ty! {
             pub enum fpos64_t {} // FIXME(linux): fill this out with a struct
         }
     }

src/unix/linux_like/mod.rs

@@ -8,8 +8,7 @@ pub type timer_t = *mut c_void;
 pub type key_t = c_int;
 pub type id_t = c_uint;
 
-missing! {
-    #[derive(Debug)]
+extern_ty! {
     pub enum timezone {}
 }
 

src/unix/mod.rs

@@ -37,8 +37,7 @@ cfg_if! {
     }
 }
 
-missing! {
-    #[derive(Debug)]
+extern_ty! {
     pub enum DIR {}
 }
 pub type locale_t = *mut c_void;
@@ -567,15 +566,13 @@ cfg_if! {
 
 cfg_if! {
     if #[cfg(not(all(target_os = "linux", target_env = "gnu")))] {
-        missing! {
-            #[derive(Debug)]
+        extern_ty! {
             pub enum fpos_t {} // FIXME(unix): fill this out with a struct
         }
     }
 }
 
-missing! {
-    #[derive(Debug)]
+extern_ty! {
     pub enum FILE {}
 }