char[][] join ==> string

[bearophile]

Given an array of strings std.string.join() returns a single string:

import std.string;
void main() {
    string[] a1 = ["hello", "red"];
    string j1 = join(a1, " "); // OK
}


But in a program I need an array of mutable arrays of chars. If I join the arrays I get a mutable array of chars. But I need a string:

import std.string;
void main() {
    char[][] a2 = ["hello".dup, "red".dup];
    string j2 = join(a2, " "); // error
}

Error: cannot implicitly convert expression (join(a," ")) of type char[] to string

.idup avoids the error:

string j3 = join(a2, " ").idup; // OK

Given the low efficiency of the D GC it's better to reduce memory allocations as much as possible.
Here join() creates a brand new array, so idup performs a useless copy. To avoid this extra copy do I have to write another joinString() function?

Bye,
bearophile