improve error message for disallowed ptr-to-int casts in const eval

compiler/rustc_mir/src/const_eval/error.rs

@@ -16,6 +16,7 @@ use crate::interpret::{
 #[derive(Clone, Debug)]
 pub enum ConstEvalErrKind {
     NeedsRfc(String),
+    PtrToIntCast,
     ConstAccessesStatic,
     ModifiedGlobal,
     AssertFailure(AssertKind<ConstInt>),
@@ -39,6 +40,12 @@ impl fmt::Display for ConstEvalErrKind {
             NeedsRfc(ref msg) => {
                 write!(f, "\"{}\" needs an rfc before being allowed inside constants", msg)
             }
+            PtrToIntCast => {
+                write!(
+                    f,
+                    "cannot cast pointer to integer because it was not created by cast from integer"
+                )
+            }
             ConstAccessesStatic => write!(f, "constant accesses static"),
             ModifiedGlobal => {
                 write!(f, "modifying a static's initial value from another static's initializer")

compiler/rustc_mir/src/const_eval/machine.rs

@@ -352,7 +352,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
     }
 
     fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
-        Err(ConstEvalErrKind::NeedsRfc("pointer-to-integer cast".to_string()).into())
+        Err(ConstEvalErrKind::PtrToIntCast.into())
     }
 
     fn binary_ptr_op(