Function check_decode_error_envelope 

pub fn check_decode_error_envelope<D>(dec: D, malformed: &[u8]) -> Conformance
where
    D: DecoderConfig + 'static,

A codec’s ErrorCategory and originating codec name survive dyn-dispatch type erasure — the runtime half of the Pattern-B contract.

Drives the decoder through the dyn boundary (&dyn DynDecoderConfig → dyn_job → probe) on input the codec rejects, so the typed At<CodecError> is erased to the BoxedError a generic pipeline actually holds. It then recovers the envelope from that Box<dyn Error> and asserts both error_category and the codec name come back.

A Pattern-A codec (native-enum type Error) fails here: its category may exist on the typed value, but it is unrecoverable once erased — which is the whole point of the envelope, and what this check exists to catch. assert_uses_codec_error_envelope is the compile-time companion; this proves the category genuinely propagates. Not part of check_all (see that note).

malformed must be bytes this codec rejects — any non-decodable input, a short garbage buffer is usually enough. If the codec accepts them, the check says so rather than passing silently.