How to extend the string class to return this inside the square bracket?

[Marcone]

On Friday, 13 August 2021 at 21:47:22 UTC, Steven Schveighoffer 
wrote:
> On 8/13/21 5:05 PM, Marcone wrote:
>> How to extend the string class to return this inside the 
>> square bracket the same way opDollar $ returns the length of 
>> the string? Thank you.
>> 
>>      import std;
>> 
>>      void main(){
>>          writeln("Hello World!"[0..this.indexOf("o")]);
>>      }
>
> There is no string class to extend.
>
> `$` is a special token the compiler changes to `arr.length` for 
> arrays.
>
> -Steve

Isn't there some unario operator template that I can use with 
lambda to handle a string literal?