Supported ASN.1 and limitations

What vest_asn1 accepts today. Anything listed as not supported is rejected by the compiler with an error.

Primitive types

Constructed types

Tagging

Components and constraints

Module-level

Additional notes

SET OF ordering. DER requires the values to be sorted by their complete DER TLV encoding. The generated prepare rejects an unsorted vector without allocating, so sorting is the caller’s job (the provided comparison abstraction is non-allocating so sorting should be efficient as well). Duplicate encodings are allowed. BER SET OF imposes no canonical order and preserves schema order on output.

Heterogeneous SET. BER lets a SET carry its components in any order, so a BER parser would have to accept every permutation of the fields. DER instead fixes them in ascending tag order. vest_asn1 currently emits SET only under DER, and only when the schema already lists the fields in canonical order. The vest_lib backend supports a group of Permute combinators that can be used to implement BER SET, but the generator does not yet emit them.

Borrowing and alloc. DER strings are contiguous, so their value types borrow from the input. BER strings may arrive fragmented across constructed encodings and are flattened into owned values, which is why BER modules need the alloc feature where the equivalent DER module may not. BMPString and UniversalString are always owned, since their wire form is not UTF-8.

High tag numbers and disjointness. CHOICE, OPTIONAL, and DEFAULT need alternatives/adjacent fields to be provably disjoint. The generated proof covers the 256 possible leading identifier octets: tags 0 through 30 are exact, but all high-tag-number forms sharing a class and constructed bit collapse onto one bit. Two such tags cannot be proven disjoint from their later tag-number octets alone, so a schema that distinguishes alternatives only by high tag numbers is conservatively rejected.