Compile time evaluation

If I write

enum chars = digits ~ uppercase;

will the string be concatenated at compile time? I'm assuming it will. If I replace it with a string literal or a CTFE function I can't measure any significant performance differences (even calling it a hundred million times). I do get a difference if I replace enum with const. I've been told it's inefficient to write it like this. I thought it was kind of convenient and I don't see the inefficiency. (BTW, the line is in a function that's called recursively).

The full code (converting to a numeral system with a different base)

import std.string;

string toBase(long n, int b) 
in {
    assert(2 <= b && b <= 35);
} body {
    static string sign;
    if (n < 0) {
        n *= -1;
        sign = "-";
    }
    enum chars = digits ~ uppercase;
    size_t r = cast(size_t)(n % b);
    if (n == r) {
        return sign ~ chars[r];
    }
    return toBase((n - r) / b, b) ~ chars[r];
}

Edit: updated code, in response to comments, not relevant to the question

string toBase(long n, int b) 
in {
    assert(2 <= b && b <= 35);
} body {
    enum chars = digits ~ uppercase;
    long r = n % b;
    char c = chars[cast(size_t) abs(r)];
    if (n == r) {
        return (n < 0 ? "-" : "") ~ c;
    }
    return toBase((n - r) / b, b) ~ c;
}