Generic AST interface for convenience.

[eddyb]

Instead of the current setup where everything assumes Handle, we could have structs/enums generic over something that will let them extract their field types, e.g.:

enum Expr<L: ApplyT<Expr<L>> + ApplyT<Type<L>>> {
    Cast {
        expr: <L as ApplyT<Expr<L>>>::Out,
        ty: <L as ApplyT<Type<L>>>::Out,
    },
    Call {
        callee: <L as ApplyT<Expr<L>>>::Out,
        callee: <L as ApplyT<[Expr<L>]>>::Out,
    },
}

This would turn Handle<'.., Expr<'..>> into Handle<'.., Expr<HandleL<'..>>>, but could also be used for an AST (only replacing certain types, and using e.g. Box<Expr<MyAst>> for others), and allow for automating lowering the parse forest to that AST, including disambiguation (on the AST types).

Note that disambiguation performed on a bottom-up constructed "eager AST" would sometimes be more expensive than shallow SPPF disambiguation, but it's easier to implement.