Command and CommandView code duplication, avoidable edge cases

[dishmaker]

Currently Command and CommandView<'a> implement the same logic that could be, respectively:

• CAPDU<heapless::Vec<u8>>,
• CAPDU<&[u8]>.

Also, current code covers edge cases that can be eliminated with rust enum of Short/Extended variants:

iso7816/src/command.rs

Lines 193 to 198 in 4eb8ac1

	| (_, false, false, ExtendedLen::Forced) => {
	unreachable!("Can't have non extended Lc and extended Le")
	}
	(_, true, true, _) => {
	unreachable!("Can't have both no data and data extended length")
	}

Maybe let's create a new crate?

• ISO7816-4: apdu der-like Encode/Decode crate RustCrypto/formats#2211

[robin-nitrokey]

I was working on a similar idea in #3:

- pub struct Command<const S: usize>
+ pub struct Command<B: AsRef<[u8]>>

Since this PR, iso7816 became more complex, especially with the Writer and DataSource traits. I would be open to revisit a change like this, but only if it does not increase the overall complexity. @sosthene-nitrokey What do you think?

[dishmaker]

So with changes in your PR, could we both:

• decode into Command<&[u8]>
• encode from Command<&[u8]> ?

Such thing is already possible in der: https://docs.rs/der/0.8.0-rc.10/der/asn1/struct.OctetStringRef.html

In the diff, looks like you implemented such case (?)

[sosthene-nitrokey]

We could unify them, ideally in a way that would also allow us to get Unsize coercion from Command to CommandView by using the View types from heapless 0.9