string
is defined as alias string = immutable(char)[];
: so need to use dup
to make a mutable char array, before it can be reversed:
import std.stdio;
import std.string;
int main() {
string x = "Hello world!";
char[] x_rev = x.dup.reverse;
writeln(x_rev); // !dlrow olleH
return 0;
}