Disallow casting directly between C-like enums and unsafe pointers

This closes issue #17444

Author: bkoropoff

src/librustc/middle/typeck/check/mod.rs

@@ -1468,7 +1468,7 @@ fn check_cast(fcx: &FnCtxt,
     // casts to scalars other than `char` and `bare fn` are trivial
     let t_1_is_trivial = t_1_is_scalar && !t_1_is_char && !t_1_is_bare_fn;
     if ty::type_is_c_like_enum(fcx.tcx(), t_e) && t_1_is_trivial {
-        if t_1_is_float {
+        if t_1_is_float || ty::type_is_unsafe_ptr(t_1) {
             fcx.type_error_message(span, |actual| {
                 format!("illegal cast; cast through an \
                          integer first: `{}` as `{}`",