CTFE: exposing pointers and calling extern fn doesn't need an RFC, it is just impossible

Author: RalfJung

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -269,9 +269,10 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                     );
                     throw_inval!(AlreadyReported(guar));
                 } else {
+                    // `find_mir_or_eval_fn` checks that this is a const fn before even calling us,
+                    // so this should be unreachable.
                     let path = ecx.tcx.def_path_str(def.did);
-                    Err(ConstEvalErrKind::NeedsRfc(format!("calling extern function `{}`", path))
-                        .into())
+                    bug!("trying to call extern function `{path}` at compile-time");
                 }
             }
             _ => Ok(ecx.tcx.instance_mir(instance)),
@@ -469,7 +470,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _ptr: Pointer<AllocId>,
     ) -> InterpResult<'tcx> {
-        Err(ConstEvalErrKind::NeedsRfc("exposing pointers".to_string()).into())
+        // This is only reachable with -Zunleash-the-miri-inside-of-you.
+        throw_unsup_format!("exposing pointers is not possible at compile-time")
     }
 
     #[inline(always)]

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -490,6 +490,7 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     ) -> InterpResult<$tcx, Pointer<Option<AllocId>>> {
         // Allow these casts, but make the pointer not dereferenceable.
         // (I.e., they behave like transmutation.)
+        // This is correct because no pointers can ever be exposed in compile-time evaluation.
         Ok(Pointer::from_addr(addr))
     }
 

src/test/ui/consts/miri_unleashed/ptr_arith.rs

@@ -8,7 +8,7 @@
 static PTR_INT_CAST: () = {
     let x = &0 as *const _ as usize;
     //~^ ERROR could not evaluate static initializer
-    //~| "exposing pointers" needs an rfc before being allowed inside constants
+    //~| exposing pointers
     let _v = x == x;
 };
 

src/test/ui/consts/miri_unleashed/ptr_arith.stderr

@@ -2,7 +2,7 @@ error[E0080]: could not evaluate static initializer
   --> $DIR/ptr_arith.rs:9:13
    |
 LL |     let x = &0 as *const _ as usize;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^ "exposing pointers" needs an rfc before being allowed inside constants
+   |             ^^^^^^^^^^^^^^^^^^^^^^^ exposing pointers is not possible at compile-time
 
 error[E0080]: could not evaluate static initializer
   --> $DIR/ptr_arith.rs:17:14