MaxEncodedLen macro is not accurate for compact fields, and skipped fields and variants

[thiolliere]

AFAICT MaxEncodedLen doesn't read the attributes #[codec(skip)] and #[codec(encode_as..)] and #[codec(compact)].

Thus it can result in an inaccurate len.

For instance the type:

#[derive(MaxEncodedLen, Encode)]
struct Foo(#[codec(compact)] u8);

will have max encoded len of 1 but can actually encode in 2 bytes.

[ggwpez]

Still an issue. This fails to compile:

#[derive(MaxEncodedLen, TypeInfo, Encode, Decode)]
enum Error {
    Wrong(#[codec(skip)] &'static str),
}

no function or associated item named "max_encoded_len" found for reference "&'static str" in the current scope.
Same when using a named inner.

[bkchr]

At least codec(skip) is already supported, but just not released. This changes it: #401

[Dmitry-Borodin]

This issue is part of paritytech/polkadot-sdk#323 in a list. And looks like it's fixed.