Function encode_with_padding 

pub fn encode_with_padding<V: Source>(
    ast: &Ast,
    request: &ProbeRequest,
    values: &V,
) -> Result<(Vec<u8>, Vec<bool>), ProbeError>

As encode, and the offsets of the octets that carry no value.

The second half of the answer is computed by the layout, not by comparing the result with anything: Writer::align_to is the one place the walk emits a byte that XTypes 1.3 §7.4.1’s alignment rules put there instead of a value, and the writer notes each span it emits. The result is a mask over the returned byte string, one entry per octet, true where the plan says padding.

The interop comparison uses it to check its own octet rule: a peer’s differing octet that lands outside this mask is a difference in a value, whatever it looks like. See tools/interop-results/src/rust_column.rs.

Errors

As encode. Emit::KeyHash has no padding mask — the holder is a separate stream — and yields an all-false one.