Make cast generated enums to i32 non-scalar

[EPashkin]

No description provided.

[EPashkin]

Its second part of enum as i32. See gtk-rs/examples#88

[EPashkin]

AppVeyor build succeeded, @gkoz, please, merge this

[EPashkin]

Seems I missread matches, need more (_)

[EPashkin]

Or not?

[EPashkin]

I tried micro-optimisation on match.

[gkoz]

__Nonexhaustive(_) => panic!(),

This got me thinking: if we somehow get an unexpected variant from the library, should it be possible to pass it back unchanged? This would require complicated workarounds and making enums much fatter.

[EPashkin]

We can add flag for some enums, that convert it to __Nonexhaustive(i32) on from_glib and decode it in to_glib

[EPashkin]

These enums still defended against direct converting, so I don't see any problems.

[gkoz]

Problem is we can't allow putting arbitrary i32s there and that's where complicated workarounds come in.

[EPashkin]

Sorry, don't understand :(

[gkoz]

Enum members can't have private fields.
Type safety (and likely memory safety too) require that a user can't just put any random value in that variant and also can't copy it from one enum into another, so it'd be along the lines of

pub struct Discriminant<T> {
    value: i32,
    _dummy: PhantomData<*const T>,
};

impl<T> struct Discriminant<T> {
    // there shouldn't be a safe way to put arbitrary discriminants into enums
    pub unsafe fn new(value: i32) -> Self {
        Discriminant {
            value: value,
            _dummy: PhantomData::new(),
        }
    }
}

pub enum Foo {
    Member1,
    Member2,
    Member3,
    __Nonexhaustive(Discriminant<Self>),
}

This looks like it's not worth the trouble at this point.

[EPashkin]

IMHO it not much related to this PR ;)
Currently we safe from direct converting enum <-> int (not checked int -> enum but FromPrimitive trait seems discarded),
way make defence against enum1 -> int -> enum2 you show IMHO not needed until some example arises.