The code duplication of the private module, that is basically just a copy of the context_sensitive module

I've made it generic for my use case.

/// Application class
type Application<T, const TAG: u16> = CustomClass<T, TAG, 0b01>;

/// Context-specific class
type ContextSpecific<T, const TAG: u16> = CustomClass<T, TAG, 0b10>;

/// Private class
type Private<T, const TAG: u16> = CustomClass<T, TAG, 0b11>;


/// Application, Context-specific or Private class field which wraps an owned inner value.
///
/// This type decodes/encodes a field which is specific to a particular context
/// and is identified by a [`TagNumber`].
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
pub struct CustomClass<T, const TAG: u16, const CLASS: u8> {
    /// Tag mode: `EXPLICIT` VS `IMPLICIT`.
    pub tag_mode: TagMode,

    /// Value of the field.
    pub value: T,
}

Shouldn't tag_mode be generic too?

This crate is still unusable for the European Tachograph regulation.

There are many 2-byte tags.

https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A02016R0799-20200226

@dishmaker we're still open for breaking changes in this release cycle. There is a legitimate use-case, PRs are welcome!

Sorry for the slow reaction. In a week I am back and try to get in some more progress here.

@kamulos I've finished making the other changes I consider blockers for this one, though it's created quite a few conflicts.

If you can get this PR green though, I can make it a priority for the next to merge. I don't expect there will be many other major changes to der itself.

@tarcieri Thanks for your work ❤️ And sorry for being very slow, the workload at the job was high and I also had vacations.

Currently we are actually releasing this code into production and things will calm down from here. So I will get back to this next week and hopefully get some version ready that is good quality.

@kamulos no worries, we'd like to do a release "soon", but that's probably in a month or two so you have some time

@tarcieri I rebased and polished the implementation. The unit tests for the Private type are still broken, because I did not adjust them and I should definitely write some more unit Tests for the encoding and decoding of the Tag type.

But other than that I think it is ready for review. 👍

There are some noteworthy points:

1. The decode_with function in Private and ContextSpecific requires peeking for a Tag but maybe getting None as return. Therefore I added another function Tag::peek_optional that is only crate internal. This feels a little bit wring, but I did not want to modify the signature of public functions or traits.
2. Private is basically a copy of ContextSpecific with one difference: In decode_with I removed the skipping of lower tag numbers. This did not work for my use-case and I did not understand it.

Private is basically a copy of ContextSpecific

Well, I also need Application 🥺

I think the best solution is using generics:

pub type ApplicationExplicit<const TAG: u16, T> = CustomClassExplicit<T, TAG, CLASS_APPLICATION>;
pub type ContextSpecificExplicit<const TAG: u16, T> = CustomClassExplicit<T, TAG, CLASS_CONTEXT_SPECIFIC>;
pub type PrivateExplicit<const TAG: u16, T> = CustomClassExplicit<T, TAG, CLASS_PRIVATE>;

pub type ApplicationImplicit<const TAG: u16, T> = CustomClassImplicit<T, TAG, CLASS_APPLICATION>;
pub type ContextSpecificImplicit<const TAG: u16, T> = CustomClassImplicit<T, TAG, CLASS_CONTEXT_SPECIFIC>;
pub type PrivateImplicit<const TAG: u16, T> = CustomClassImplicit<T, TAG, CLASS_PRIVATE>;

/// EXPLICIT
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
pub struct CustomClassExplicit<T: for<'a> Decode<'a>, const TAG: u16, const CLASS: u8> {
    pub value: T,
}

/// IMPLICIT
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
pub struct CustomClassImplicit<T: for<'a> DecodeValue<'a>, const TAG: u16, const CLASS: u8> {
    pub value: T,
}

pub const CLASS_APPLICATION: u8 = 0b01000000;
pub const CLASS_CONTEXT_SPECIFIC: u8 = 0b10000000;
pub const CLASS_PRIVATE: u8 = 0b11000000;

I've fixed all dependencies and added these 3 custom tags as generic ones :)