Use .0 instead of .as_usize

Author: tautschnig

library/core/src/ptr/alignment.rs

@@ -16,7 +16,7 @@ use crate::{cmp, fmt, hash, mem, num};
 #[unstable(feature = "ptr_alignment_type", issue = "102070")]
 #[derive(Copy, Clone, PartialEq, Eq)]
 #[repr(transparent)]
-#[invariant(self.as_usize().is_power_of_two())]
+#[invariant(self.0.is_power_of_two())] // uses .0 instead of .as_usize() to permit proving as_usize
 pub struct Alignment(AlignmentEnum);
 
 // Alignment is `repr(usize)`, but via extra steps.