asn1rust: acn_enc_int_twos_complement_const_size panics on i64::MIN

[philphauler]

Hey 👋

Found a panic in the ACN two's complement encoder in the Rust runtime while writing round-trip tests for asn1rust.

How to reproduce:

let mut buf = [0u8; 16];
let mut bs = BitStream::new(&mut buf);
acn::acn_enc_int_twos_complement_const_size(&mut bs, i64::MIN, 64);
// panics: "attempt to subtract with overflow" at acn.rs:372

Root cause:

Line 372 in asn1rust/src/acn.rs:

let abs_val = (int_val.wrapping_neg() - 1) as Asn1SccUint;

i64::MIN.wrapping_neg() returns i64::MIN (the value has no positive i64 representation). The subsequent - 1 is an unchecked subtraction that overflows in Rust debug mode. The C equivalent wraps silently.

Expected behavior:

The encoder should produce the correct two's complement bit pattern for any value in the i64 range, including the minimum.

Impact:

Any ASN.1 INTEGER type whose range includes the minimum 64-bit signed value will panic during ACN encoding in the Rust backend. The C backend handles this correctly.

Happy to send a fix if this is confirmed 🙏