In some use cases like debugging over UART, if DEBUG_TX/DEBUG_RX are defined, the static buffer is provided by serial_early_init with the call of common_hal_busio_uart_construct.

Mircopython originally recommended to use list of initialisers instead of calling ringbuf_alloc in case the buffer cannot be allocated from heap internally and static buffer must be provided by user code.

// Static initialization:
// byte buf_array[N];
// ringbuf_t buf = {buf_array, sizeof(buf_array)};

In the meantime the ring buffer interface has been modified in micropython project.

This approach can be error prone because the user code relies on certain order of structure members.
Initialising the structure with designators addresses this issue but then the names of structure members are exposed.

Using dedicated function is cleaner and more portable way of initialisation.
The overhead is single bool structure member.
Appending this member to the structure preserves the backward compatibility with initialiser list approach.