Add a few tests around raw pointers and interior mutability

src/test/ui/consts/std/cell.rs

+use std::cell::*;
+
+// not ok, because this would create a silent constant with interior mutability.
+// the rules could be relaxed in the future
+static FOO: Wrap<*mut u32> = Wrap(Cell::new(42).as_ptr());
+//~^ ERROR cannot borrow a constant which may contain interior mutability
+
+static FOO3: Wrap<Cell<u32>> = Wrap(Cell::new(42));
+// ok
+static FOO4: Wrap<*mut u32> = Wrap(FOO3.0.as_ptr());
+
+// not ok, because the `as_ptr` call takes a reference to a type with interior mutability
+// which is not allowed in constants
+const FOO2: *mut u32 = Cell::new(42).as_ptr();
+//~^ ERROR cannot borrow a constant which may contain interior mutability
+
+struct IMSafeTrustMe(UnsafeCell<u32>);
+unsafe impl Send for IMSafeTrustMe {}
+unsafe impl Sync for IMSafeTrustMe {}
+
+static BAR: IMSafeTrustMe = IMSafeTrustMe(UnsafeCell::new(5));
+
+
+struct Wrap<T>(T);
+unsafe impl<T> Send for Wrap<T> {}
+unsafe impl<T> Sync for Wrap<T> {}
+
+static BAR_PTR: Wrap<*mut u32> = Wrap(BAR.0.get());
+
+fn main() {}
\ No newline at end of file

src/test/ui/consts/std/cell.stderr

+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/cell.rs:5:35
+   |
+LL | static FOO: Wrap<*mut u32> = Wrap(Cell::new(42).as_ptr());
+   |                                   ^^^^^^^^^^^^^
+
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/cell.rs:14:24
+   |
+LL | const FOO2: *mut u32 = Cell::new(42).as_ptr();
+   |                        ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0492`.

src/test/ui/consts/std/char.rs

+// run-pass
+
+static X: bool = 'a'.is_ascii();
+static Y: bool = 'ä'.is_ascii();
+
+fn main() {
+    assert!(X);
+    assert!(!Y);
+}
\ No newline at end of file

src/test/ui/consts/std/iter.rs

+// run-pass
+
+const I: std::iter::Empty<u32> = std::iter::empty();
+
+fn main() {
+    for i in I {
+        panic!("magical value creation: {}", i);
+    }
+}
\ No newline at end of file

src/test/ui/consts/std/slice.rs

+// compile-pass
+
+struct Wrap<T>(T);
+unsafe impl<T> Send for Wrap<T> {}
+unsafe impl<T> Sync for Wrap<T> {}
+
+static FOO: Wrap<*const u32> = Wrap([42, 44, 46].as_ptr());
+static BAR: Wrap<*const u8> = Wrap("hello".as_ptr());
+
+fn main() {}
\ No newline at end of file