Is it feasible to support "prototypes" for "newtype enums"?

language design
1

Newtype enums

An enum is considered a newtype if all of its variants is in the form of Variant( Type ), e.g.

enum Value {
    Bin( Vec<u8> ),
    Text( String ),
}

is a newtype enum.

Prototypes of newtype enums

By erasing the name and all its variants' names of an enum, we get its prototype. For instance, the prototype of Value is

enum __2 {
    _0( Vec<u8> ),
    _1( String ),
}

The prototype trait

pub trait Proto {
    type Type;
    fn from_proto( src: Self::Type ) -> Self;
    fn into_proto( self ) -> Self::Type;
}

Can rustc auto-implement this trait for any user-defined enum type?

impl Proto for Value {
    type Type = std::enum_proto::__2;

    fn from_proto( src:: Self::Type ) -> Self {
        match src {
            std::enum_proto::__2::_0(v) => Value::Bin(v),
            std::enum_proto::__2::_1(v) => Value::Text(v),
        }
    }

    fn into_proto( self ) -> Self::Type {
        match self {
            Value::Bin(v) => std::enum_proto::__2::_0(v),
            Value::Text(v) => std::enum_proto::__2::_1(v),
        }
    }
}

Purpose of enum prototype

It is helpful for simulating language features ad-hoc enums, anonymous enums, structural enums, ... you name it.

2

derive-more can do the From portion of this.

It can't do the Into portion, but in general nothing can (how would you infallibly turn Value::Bin(_) into a String?); TryInto would be the trait to target but derive-more doesn't do that yet.

3

No, I don't think Into is an issue( I have edited the main post, see ), but this is off-topic.

It is not possible for the "derive-more" crate or any other crate to setup the equivalence of a series of enum types __0, __1, .. (as needed) in std::enum_proto. The best we can get is a predefined range of varaints up to some fixed value( e.g. 16 ). Then comes an enum of 42 variants and the game is over.

4

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