Phobos' std.conv.to-conversion from enum to string doesn't scale beyond hundreds of enumerators

[Per Nordlöw via Digitalmars-d]

I've discovered the annoying fact that std.conv.to doesn't scale 
for enum to string conversion when the enum has hundreds of 
members. This because of a call to `NoDuplicates` which has (at 
least) O(n*log(n) time and space complexity.

So I've come up with

/** Faster implementation of `std.conv.to`.
 */
string toString(T)(T value) @safe pure nothrow @nogc
if (is(T == enum))
{
    final switch (value)
    {
        static foreach (member; __traits(allMembers, T))
        {
        case __traits(getMember, T, member):
            return member;
        }
    }
}

///
@safe pure nothrow @nogc unittest
{
    enum E { unknown, x, y, z, }
    assert(E.x.toString == "x");
    assert(E.y.toString == "y");
    assert(E.z.toString == "z");
}


The question know is: How do I make this support enums with 
enumerator aliases without needing to call `NoDuplicates`?

For instance, this should work

///
@safe pure nothrow @nogc unittest
{
    enum E { unknown, x, y, z, z_ = z, }
    assert(E.x.toString == "x");
    assert(E.y.toString == "y");
    assert(E.z.toString == "z");
    assert(E.z_.toString == "z");
}