typeck: Make sure casts from fat to thin pointers are illegal

Fixes an ICE on casting *const [T] to *const T and similar cases.

Fixes #21397
Fixes #22955

Author: Ulrik Sverdrup

src/librustc_typeck/check/cast.rs

@@ -170,9 +170,9 @@ pub fn check_cast<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>, cast: &CastCheck<'tcx>) {
                 demand::coerce(fcx, e.span, t_1, &e);
             }
         }
-    } else if fcx.type_is_fat_ptr(t_e, span) && !fcx.type_is_fat_ptr(t_1, span) {
+    } else if fcx.type_is_fat_ptr(t_e, span) != fcx.type_is_fat_ptr(t_1, span) {
         fcx.type_error_message(span, |actual| {
-            format!("illegal cast; cast from fat pointer: `{}` as `{}`",
+            format!("illegal cast; cast between thin and fat pointer: `{}` as `{}`",
                     actual, fcx.infcx().ty_to_string(t_1))
         }, t_e, None);
     } else if !(t_e_is_scalar && t_1_is_trivial) {

src/test/compile-fail/fat-ptr-cast.rs

@@ -8,12 +8,24 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// Make sure casts between thin pointer <-> fat pointer are illegal.
+
+pub trait Trait {}
+
 fn main() {
     let a: &[i32] = &[1, 2, 3];
     let b: Box<[i32]> = Box::new([1, 2, 3]);
     let p = a as *const [i32];
+    let q = a.as_ptr();
+
+    a as usize; //~ ERROR illegal cast
+    b as usize; //~ ERROR illegal cast
+    p as usize; //~ ERROR illegal cast
+
+    // #22955
+    q as *const [i32]; //~ ERROR illegal cast
 
-    a as usize; //~ ERROR cast from fat pointer
-    b as usize; //~ ERROR cast from fat pointer
-    p as usize; //~ ERROR cast from fat pointer
+    // #21397
+    let t: *mut (Trait + 'static) = 0 as *mut _; //~ ERROR illegal cast
+    let mut fail: *const str = 0 as *const str; //~ ERROR illegal cast
 }