derive(IntoBytes) erroneously accepts repr(C, align(X)) DSTs with dynamic trailing padding; e.g.

#[derive(IntoBytes)]
#[repr(C, align(4))]
struct Unsound([u8]);

The derive accepts this because it merely confirms that [u8] is Unaligned, whereas it should be failing on the align directive. The sized analysis codepath correctly takes this into account; e.g.:

#[derive(IntoBytes)] // Compile error!
#[repr(C, align(4))]
struct Sized([u8; 3]);

Bug confirmed on v0.8.39 (latest release at time of reporting), but likely present on earlier releases. Looks like this slipped in with #2679, and was probably missed because the testing infra borrowed for the sized case also doesn't reason about outer align directives.