digest: newtype support

[baloo]

This is an attempt at fixing #1069

example use:

digest::newtype!(
    "SHA-256 hasher",
    Sha256,
    CoreWrapper<CtVariableCoreWrapper<Sha256VarCore, U32, OidSha256>>
);

RustCrypto/hashes#659

[baloo]

I only implemented enough to make tests for sha2 pass. But I'd like your thoughts before going further.

Example doc output:

[newpavlov]

Sorry, but I still don't quite understand the relevance of this wrapper. Why can't the macro implement the struct as a direct thin wrapper around CoreCtWrapper and other wrapper types?

[baloo]

An isolated example is probably easier to showcase the purpose of the wrapper
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=2c559bf521cd586849fb5ffc8ba5850f

But this allows me to implement trait for the newtype only if the inner carries an implementation for it.
This is only meant as a hack until https://doc.rust-lang.org/beta/unstable-book/language-features/trivial-bounds.html lands.

The wrapper was a workaround found in this comment: taiki-e/pin-project#102 (comment)

[newpavlov]

Why do you need to deal with lifetimes here?

This snippet works just fine: https://play.rust-lang.org/?gist=9f86f1ec8c12cbb176cb8e1cce852e56

[baloo]

Yes, but this doesn't work anymore: https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=0c16e92508e7903da2d6099d956f57dd

This is what the lifetime gives me: a bound on the trait implementation of the wrapper.

[newpavlov]

I do not understand. Why do you expect for wrapper!(String => Email); to work in you snippet?

[baloo]

Because the inner type may or may not implement Foo, and I want the wrapper to implement Foo only if the inner implements it.

This is what the lifetime hack gives me.

[newpavlov]

Ah, I see. It's certainly a neat trick, but it's a bit too much magic for my taste. The macro itself already obscures the implementation a bit, so I would prefer to have something more straightforward.

We can either have macros for common scenarious (e.g. impl_fixed_digest!) or explicitly list traits which we want to implement by delegation to the wrapped type. I am not sure which solution would be better, so I will need to experiment and think about it a bit.

[baloo]

alright well I'll pause a bit then :)

[newpavlov]

Alternative WIP PR: #1799