add a test for dereferencing a pointer to a `!`

src/error.rs

@@ -52,6 +52,7 @@ pub enum EvalError<'tcx> {
     ReallocatedFrozenMemory,
     DeallocatedFrozenMemory,
     Layout(layout::LayoutError<'tcx>),
+    Unreachable,
 }
 
 pub type EvalResult<'tcx, T> = Result<T, EvalError<'tcx>>;
@@ -122,6 +123,8 @@ fn description(&self) -> &str {
                 "rustc layout computation failed",
             EvalError::UnterminatedCString(_) =>
                 "attempted to get length of a null terminated string, but no null found before end of allocation",
+            EvalError::Unreachable =>
+                "entered unreachable code",
         }
     }
 

src/terminator/mod.rs

@@ -155,7 +155,7 @@ pub(super) fn eval_terminator(
 
             DropAndReplace { .. } => unimplemented!(),
             Resume => unimplemented!(),
-            Unreachable => unimplemented!(),
+            Unreachable => return Err(EvalError::Unreachable),
         }
 
         Ok(())

tests/compile-fail/never_say_never.rs

+#![feature(never_type)]
+#![allow(unreachable_code)]
+
+fn main() {
+    let y = &5;
+    let x: ! = unsafe {
+        *(y as *const _ as *const !) //~ ERROR entered unreachable code
+    };
+    f(x)
+}
+
+fn f(x: !) -> ! { x }