void main() pure {
    import std.string;
    immutable parts = "5\n6".splitLines;
}


splitLines can be used in a pure function, but its result can't yet be assigned to an immutable (dmd 2.065alpha):

temp.d(3): Error: cannot implicitly convert expression (splitLines("5\x0a6", cast(KeepTerminator)false)) of type string[] to immutable(char[][])


Even this fails with a similar error:

void main() pure {
    import std.string;
    immutable s = "5\n6";
    immutable parts = s.splitLines;
}