Add support for enum variants scoped under their type (`enum mod`)

[Kimundi]

Current situation

Right now, enum types and their variants are scoped at the same level, which means this:

mod foo {
    enum Speed {
        Fast,
        Slow
    }
}

leads to these paths:

foo::Speed;
foo::Fast;
foo::Slow;

We will likely not change this, because it often makes working with enums easier, and because many languages with similar enum types work that way.

The problem

However, the restriction to a flat hierarchy currently leads to a lot of variants getting their type name embedded in the variant name to avoid collisions and/or ambiguity. For example, a random enum picked out of libsyntax reads like this:

pub enum Sigil {
    BorrowedSigil,
    OwnedSigil,
    ManagedSigil
}

The possible solution

To improve this situation, it would be convenient to have a enum declaration syntax for scoping the variants under their type instead, like this:

mod foo {
    enum mod Speed {
        Fast,
        Slow
    }
}

foo::Speed;
foo::Speed::Fast;
foo::Speed::Slow;

The above example could then get rewritten to

pub enum mod Sigil {
    Borrowed,
    Owned,
    Managed
}

and get used as Sigil::Borrowed, Sigil::Owned etc, or directly imported in the local scope if there is no collision.

Emulating it

You can currently kinda emulate it with

mod foo {
    pub type Speed = self::Speed::Speed;
    pub mod Speed {
        pub enum Speed {
            Fast,
            Slow
        }
    }
}

but this is very verbose, complicated, and creates more definitions and moving parts than necessary.

Working example

This uses the workaround above to show how using such a scoped enum would look like in practice.

#[feature(globs)];

use foo::Speed;

mod foo {
    pub type Speed = self::Speed::Speed;
    pub mod Speed {
        pub enum Speed {
            Fast,
            Slow
        }
    }
}

fn main() {
    let a: Speed = Speed::Fast;
    let b: Speed = Speed::Slow;

    match a {
        Speed::Fast => {}
        Speed::Slow => {}
    }

    {
        use foo::Speed::*;
        match b {
            Fast => {}
            Slow => {}
        }
    }
}

[sfackler]

I'm a big fan of this. It'd be nice if the enum type name wasn't required in a match statement over a variable of the type, i.e.

pub enum mod Speed {
    Fast,
    Slow
}

let x = Speed::Fast;

match x {
    Fast => {}
    Slow => {}
}

As opposed to

match x {
    Speed::Fast => {}
    Speed::Slow => {}
}

[Kimundi]

@sfackler: I updated the Issue text with an example showing how that would be doable with the existing scoping primitives.

[auroranockert]

I really like this idea.

[lilyball]

I quite like this. I assume that the final code example in "The possible solution" was meant to say pub enum mod Sigil instead of pub enum Sigil?

[Kimundi]

@kballard Indeed, I forgot the mod there. 😃

[SimonSapin]

Yes please. Servo's style system has tons of deeply nested modules that would become unnecessary with enum mod

[brendanzab]

+1 here. Both styles - ie. enum and enum mod have their place. It would be good if the variants of an enum mod could be imported separately though. ie. use foo::Speed::Fast;

[sfackler]

If we allow use foo::Speed::Fast, which seems pretty reasonable to me, what is the use case for normal enums? It might be too extreme of a change, but we could just change the semantics of enum to those of enum mod. Heavily used variants like Some would still be imported in std::prelude. I've personally always thought the way enum worked in C, C++, Rust etc was kind of weird.

[nikomatsakis]

So I was thinking "can't this just be a macro?" and the answer is "not quite" -- in particular, the type of the enum would have to be Speed::t or something like that, since Speed is taken as the name of the module and modules/types inhabit the same namespace.

[SimonSapin]

@nikomatsakis said:

in particular, the type of the enum would have to be Speed::t or something like that, since Speed is taken as the name of the module and modules/types inhabit the same namespace.

Do they really? The working example in the initial message seems to use Speed both as a module and a type.

[sfackler]

@SimonSapin Something like this will fail:

pub use Thing::Thing;

pub mod Thing {
    pub enum Thing {}
}

pub fn do_a_thing(thing: Thing) {}

src/enum-mod/lib.rs:28:25: 28:30 error: found module name used as a type: mod Thing (id=15)
src/enum-mod/lib.rs:28 pub fn do_a_thing(thing: Thing) {}

@nikomatsakis My biggest problem with a macro-based solution is that it makes generated documentation enormously confusing. If I see a foo::Thing being used as an enum somewhere, and look at the documentation for foo, I would expect to see Thing in the "Enums" section of the documentation, but instead it's actually two layers of indirection away, hidden in the middle of a bunch of reexports and modules.

For example, here's rustdoc that uses a normal enum: http://sfackler.com/doc/enum/ and here's rustdoc for the same setup, but using the current workaround enum mod solution: http://sfackler.com/doc/enum-mod/

[sfackler]

Oops, I was using a reexport above instead of a type alias. The http://sfackler.com/doc/enum-mod/ example is updated.

[alexcrichton]

The consensus on this at today's meeting is that this feature should not be added to the language now. We may want to revisit this later (post-1.0), but for now we have decided to leave this out of 1.0