Forbid casting to/from a pointer of unknown kind

src/test/ui/issue-45730.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::fmt;
+fn main() {
+    let x: *const _ = 0 as _;
+
+    let x: *const _ = 0 as *const _;
+    let y: Option<*const fmt::Debug> = Some(x) as _;
+
+    let x = 0 as *const i32 as *const _ as *mut _;
+}

src/test/ui/issue-45730.stderr

@@ -0,0 +1,32 @@
+error[E0641]: cannot cast to a pointer of an unknown kind
+  --> $DIR/issue-45730.rs:13:23
+   |
+13 |     let x: *const _ = 0 as _;
+   |                       ^^^^^-
+   |                            |
+   |                            help: consider giving more type information
+   |
+   = note: The type information given here is insufficient to check whether the pointer cast is valid
+
+error[E0641]: cannot cast to a pointer of an unknown kind
+  --> $DIR/issue-45730.rs:15:23
+   |
+15 |     let x: *const _ = 0 as *const _;
+   |                       ^^^^^--------
+   |                            |
+   |                            help: consider giving more type information
+   |
+   = note: The type information given here is insufficient to check whether the pointer cast is valid
+
+error[E0641]: cannot cast to a pointer of an unknown kind
+  --> $DIR/issue-45730.rs:18:13
+   |
+18 |     let x = 0 as *const i32 as *const _ as *mut _;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^------
+   |                                            |
+   |                                            help: consider giving more type information
+   |
+   = note: The type information given here is insufficient to check whether the pointer cast is valid
+
+error: aborting due to 3 previous errors
+