Structural exhaustive matching

[weaselcat via Digitalmars-d-learn]

On Wednesday, 22 April 2015 at 04:54:39 UTC, Martin Nowak wrote:
> On Tuesday, 21 April 2015 at 15:36:28 UTC, Jadbox wrote:
>> What's the best equivalent to Rust's structural enum/pattern 
>> (match)ing? Is it also possible to enforce exhaustive matches? 
>> Basically, I'm curious on what the best way to do ADTs in D.
>
> If it needs to be really fast, use final switch on the tag of a 
> discriminated union.
>
> enum Tag { A, B, C }
> struct Val
> {
>   Tag tag;
>   union
>   {
>     A a;
>     B b;
>     C c;
>   }
> }
>
> void too(Val val)
> {
>   final switch (val.tag)
>   {
>   case Tag.A: writeln(val.a); break;
>   case Tag.B: writeln(val.b); break;
>   case Tag.C: writeln(val.c); break;
>   }
> }

there's no reason this should be faster than Algebraic(restricted 
variant) from std.variant, is there? implementation issue?