cast(string) for mutable array

[Vindex9]

> You frequently can, but remember that when you mark something 
> as @trusted, you're telling the compiler that you've verified 
> that the code is memory-safe (whereas with @safe, the compiler 
> has verified that). So, if you mark code with @trusted when 
> it's not actually memory-safe, you make it so that @safe code 
> can call code which isn't memory-safe, and the @safe code then 
> isn't actually memory-safe.
>
> So, ideally, you'd do whatever @system stuff you need to do 
> within a function and encapsulate the @system stuff in a way 
> that you can mark the function as @trusted and have an API 
> which is memory-safe, but whether you can do that or not 
> depends on what your code is doing.
>
> - Jonathan M Davis

This is a really good explanation, thank you.