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

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
+