Integer and float variable kinds

[AzureMarker]

Closes #327

This adds support for integer and float variable kinds. Example usage:

test! {
    program {
        trait Foo {}
        struct Bar {}

        impl Foo for usize {}
        impl Foo for Bar {}
    }

    goal {
        exists<int N> {
            N: Foo
        }
    } yields {
        "Unique; substitution [?0 := Uint(Usize)]"
    }
}

[nikomatsakis]

The main problem I see is that int/float are not 'distinct' from ordinary types. I think we need some tests like

exists<T, int N> {
    T = N, N = usize
}

this should pass, and something like

exists<T, int N> {
    T = N, T = char
}

this should have no solution.

[nikomatsakis]

I guess I didn't expect to add an enum to Ty but rather to have 5 variants:

• Ty
• IntTy
• FloatTy
• Lifetime
• Const(Ty)

but I'm not sure which setup is nicer. This one may be more elegant.

[AzureMarker]

I guess I didn't expect to add an enum to Ty but rather to have 5 variants

Adding the enum to VariableKind::Ty is effectively the same thing (now you have Ty(General), Ty(Integer), and Ty(Float)), but it's easier to group the "Ty-like" variants together (Ty(_)).

[nikomatsakis]

Nit: Maybe this would be nicer if we combined the three cases and moved the if into the arm?

I'm imagining

(
                &TyData::InferenceVar(var1, kind1),
                &TyData::InferenceVar(var2, kind2),
            ) => {
    if kind1 == kind2 {
        ...
    } else if kind1.is_general() || kind2.is_general() {
        ...
    } else {
        Err(...)
    }
}

[nikomatsakis]

let's add

exists<int N, float F> { N = F }

and check it gives an error