Improve cast_ptr_alignment lint

* print alignment in bytes in the lint message
* ignore ZST left-hand types

clippy_lints/src/types.rs

@@ -1215,12 +1215,14 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Casts {
                 if from_align < to_align;
                 // with c_void, we inherently need to trust the user
                 if !is_c_void(cx, from_ptr_ty.ty);
+                // when casting from a ZST, we don't know enough to properly lint
+                if !(Some(true) == cx.layout_of(from_ptr_ty.ty).ok().map(|a| a.is_zst()));
                 then {
                     span_lint(
                         cx,
                         CAST_PTR_ALIGNMENT,
                         expr.span,
-                        &format!("casting from `{}` to a more-strictly-aligned pointer (`{}`)", cast_from, cast_to)
+                        &format!("casting from `{}` to a more-strictly-aligned pointer (`{}`) ({} < {})", cast_from, cast_to, from_align.bytes(), to_align.bytes())
                     );
                 }
             }

tests/ui/cast_alignment.rs

@@ -22,4 +22,6 @@ fn main() {
     // For c_void, we should trust the user. See #2677
     (&1u32 as *const u32 as *const std::os::raw::c_void) as *const u32;
     (&1u32 as *const u32 as *const libc::c_void) as *const u32;
+    // For ZST, we should trust the user. See #4256
+    (&1u32 as *const u32 as *const ()) as *const u32;
 }

tests/ui/cast_alignment.stderr

@@ -1,12 +1,12 @@
-error: casting from `*const u8` to a more-strictly-aligned pointer (`*const u16`)
+error: casting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2)
   --> $DIR/cast_alignment.rs:12:5
    |
 LL |     (&1u8 as *const u8) as *const u16;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::cast-ptr-alignment` implied by `-D warnings`
 
-error: casting from `*mut u8` to a more-strictly-aligned pointer (`*mut u16`)
+error: casting from `*mut u8` to a more-strictly-aligned pointer (`*mut u16`) (1 < 2)
   --> $DIR/cast_alignment.rs:13:5
    |
 LL |     (&mut 1u8 as *mut u8) as *mut u16;