Remove usage of extern_types feature gate

Author: bjorn3

library/proc_macro/src/bridge/closure.rs

@@ -1,24 +1,25 @@
 //! Closure type (equivalent to `&mut dyn FnMut(A) -> R`) that's `repr(C)`.
 
+use std::marker::PhantomData;
+
 #[repr(C)]
 pub struct Closure<'a, A, R> {
-    call: unsafe extern "C" fn(&mut Env, A) -> R,
-    env: &'a mut Env,
+    call: unsafe extern "C" fn(*mut Env, A) -> R,
+    env: *mut Env,
+    _marker: PhantomData<&'a mut ()>,
 }
 
-extern "C" {
-    type Env;
-}
+struct Env;
 
 impl<'a, A, R> !Sync for Closure<'a, A, R> {}
 impl<'a, A, R> !Send for Closure<'a, A, R> {}
 
 impl<'a, A, R, F: FnMut(A) -> R> From<&'a mut F> for Closure<'a, A, R> {
     fn from(f: &'a mut F) -> Self {
-        unsafe extern "C" fn call<A, R, F: FnMut(A) -> R>(env: &mut Env, arg: A) -> R {
+        unsafe extern "C" fn call<A, R, F: FnMut(A) -> R>(env: *mut Env, arg: A) -> R {
             (*(env as *mut _ as *mut F))(arg)
         }
-        Closure { call: call::<A, R, F>, env: unsafe { &mut *(f as *mut _ as *mut Env) } }
+        Closure { call: call::<A, R, F>, env: f as *mut _ as *mut Env, _marker: PhantomData }
     }
 }
 

library/proc_macro/src/lib.rs

@@ -24,7 +24,6 @@
 #![cfg_attr(bootstrap, feature(const_fn_fn_ptr_basics))]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]
-#![feature(extern_types)]
 #![feature(negative_impls)]
 #![feature(restricted_std)]
 #![feature(rustc_attrs)]