[phobos] substring for std.metastrings

[Igor Lesik]

I believe, I have a function that is good candidate to be in std.metastrings.
Feedback is appreciated.

/**
 * Returns:
 *         .found = true if string t found in s,
 *         .found = false if string t not found in s
 */
template substring(string s, string t)
{
    static if (s == null || t == null || t.length == 0 || s.length == 0) {
        const match = false;
        const found = false;
    }
    else static if (s[0] == t[0])
    {
        const match = true;
        static if (t.length == 1)
            const found = true;
        else static if (s.length == 1)
            const found = false;
        else static if (substring!(s[1..$],t[1..$]).match)
            const found = substring!(s[1..$],t[1..$]).found;
        else
            const found = substring!(s[1..$],t[0..$]).found;
    }
    else
    {
        const match = false;
        const found = substring!(s[1..$],t[0..$]).found;
    }
}

unittest
{
    assert(!substring!(null,"fun").found);
    assert(!substring!("","fun").found);
    assert(!substring!("fun",null).found);
    assert(!substring!("fun","").found);
    assert(!substring!("","").found);
    assert(substring!("fun with gun","fun").found);
    assert(substring!("fun with gun","gun").found);
    assert(!substring!("fun with gun","peace").found);
}