Is (de)serializing `usize` with Serde platform-dependent?

Question

For example, if I serialize usize::MAX on a 64-bit system and then deserialize it on a 32-bit system, what would happen? An overflow error, be truncated to a u32 and cause misaligned read for the remaining data, or it depends on how some certain data format is implemented? Would casting usize to u64 be overly defensive?

Answer 1

Serde unconditionally casts an usize/isize to u64/i64 before serializing:

primitive_impl!(isize, serialize_i64 as i64);
// …
primitive_impl!(usize, serialize_u64 as u64);

and when deserializing depending on the type visited (defaulting to u64 for non self describing formats) it uses an as-cast or TryFrom:

impl_deserialize_num! {
    usize, NonZeroUsize, deserialize_u64
    num_as_self!(u8:visit_u8 u16:visit_u16);
    int_to_uint!(i8:visit_i8 i16:visit_i16 i32:visit_i32 i64:visit_i64);
    uint_to_self!(u32:visit_u32 u64:visit_u64);
}

Where num_as_self uses an as-cast, int_to_uint checks for >= 0 and uses TryFrom and uint_to_self uses TryFrom

That means a bare usize causes an Err to be returned when it's deserialized from a value that's bigger than usize::MAX.

To achieve saturating semantics you can wrap your usize in Saturating.

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Would casting usize to u64 be overly defensive?

For serializing yes, it would be. serde already does that.

For deserializing, it would allow any value up to u64::MAX on all platforms instead of just usize::MAX.