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der: add IsConstructed trait #1741

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#1744
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der: add IsConstructed trait#1741
#1744
@dishmaker

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@dishmaker
dishmaker
opened on Mar 25, 2025

IMPLICIT decoder requires Tagged trait.

formats/der/src/asn1/context_specific.rs

Lines 58 to 63 in 937e611

pub fn decode_implicit<'a, R: Reader<'a>>(
reader: &mut R,
tag_number: TagNumber,
) -> Result<Option<Self>, T::Error>
where
T: DecodeValue<'a> + Tagged,

IMPLICIT and EXPLICIT decoders check for constructed bit.
But IMPLICIT depends on value.tag() in runtime:

formats/der/src/asn1/context_specific.rs

Lines 69 to 70 in 937e611

if header.tag.is_constructed() != value.tag().is_constructed() {
return Err(header.tag.non_canonical_error().into());

formats/der/src/asn1/context_specific.rs

Lines 161 to 164 in 937e611

fn tag(&self) -> Tag {
let constructed = match self.tag_mode {
TagMode::Explicit => true,
TagMode::Implicit => self.value.tag().is_constructed(),

I think it should check constructed bit statically, like FixedTag.

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