Thread#thread_variable_set

static VALUE
rb_thread_variable_set(VALUE thread, VALUE key, VALUE val)
{
    VALUE locals;
    VALUE symbol;

    if (OBJ_FROZEN(thread)) {
        rb_frozen_error_raise(thread, "can't modify frozen thread locals");
    }

    symbol = rb_to_symbol(key);

    if (NIL_P(val)) {
        if (LIKELY(!THREAD_LOCAL_STORAGE_INITIALISED_P(thread))) {
            return Qnil;
        }

        locals = rb_thread_local_storage(thread);
        rb_hash_delete(locals, symbol);
        return Qnil;
    }

    locals = rb_thread_local_storage(thread);
    return rb_hash_aset(locals, symbol, val);
}