Rollup merge of #101131 - RalfJung:ctfe-no-needs-rfc, r=oli-obk

CTFE: exposing pointers and calling extern fn is just impossible

The remaining "needs RFC" errors are just needlessly confusing, I think -- time to get rid of that error variant. They are anyway only reachable with miri-unleashed (if at all).

r? `@oli-obk`

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;
 };
 
@@ -19,4 +19,7 @@ static PTR_INT_TRANSMUTE: () = unsafe {
     //~| unable to turn pointer into raw bytes
 };
 
+// I'd love to test pointer comparison, but that is not possible since
+// their `PartialEq` impl is non-`const`.
+
 fn main() {}

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