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(

src/test/ui/const-ptr/ptr_to_usize_cast.rs

@@ -0,0 +1,13 @@
+#![feature(const_raw_ptr_to_usize_cast)]
+
+fn main() {
+    const OK: usize = unsafe { 0 as *const i32 as usize };
+
+    const _ERROR: usize = unsafe { &0 as *const i32 as usize };
+    //~^ ERROR [const_err]
+    //~| NOTE cannot cast pointer to integer because it was not created by cast from integer
+    //~| NOTE
+    //~| NOTE `#[deny(const_err)]` on by default
+    //~| WARN this was previously accepted by the compiler but is being phased out
+    //~| NOTE see issue #71800
+}

src/test/ui/const-ptr/ptr_to_usize_cast.stderr

@@ -0,0 +1,14 @@
+error: any use of this value will cause an error
+  --> $DIR/ptr_to_usize_cast.rs:6:36
+   |
+LL |     const _ERROR: usize = unsafe { &0 as *const i32 as usize };
+   |     -------------------------------^^^^^^^^^^^^^^^^^^^^^^^^^---
+   |                                    |
+   |                                    cannot cast pointer to integer because it was not created by cast from integer
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to previous error
+