Turn GC allocated string into a scoped heap allocation

[Per Nordlöw via Digitalmars-d-learn]

How do I turn the GC-allocation in toLower() to a scoped heap 
allocation together with toLowerInPlace() in

void f(const scope const(char)[] expr)
{
    import std.uni : toLower;
    loweredExpr = toLower(expr); // temporary
    // use loweredExpr as key in hash table
}
when `loweredExpr` is used as a temporary inside `f`?