Why we cannot use string in mixins?

[mahdi via Digitalmars-d-learn]

I read this criticism about D on Reddit and it claims that you 
cannot use strings in mixins. Can you please elaborate about this 
and the reason behind it?

QUOTE:
Look at strings: they are defined as immutable(char []). 
"immutable" as in "you could put it in ROM", ... CTFE can't see 
strings correctly (can't be determined at compile time). So 
rather than fix the compiler so that strings can be used in 
mixins in the way that people expect it to work even if that 
means making them something other than immutable(char []), they 
decided to start using enums where strings actually go. 
Seriously: take the same code and replace 'string "foo"' with 
'enum "foo"' and it starts to work!
END QUOTE