So Q is a "tuple-struct". In rust you can define a struct like

struct Q<T> { value: Option<T> }

with a named field or you can define it like

struct<T> Q(Some<T>)

which is the same struct layout but without an explicit name for the value. In that case, the field is implicitly names `0`

So q.0 = Some(foo) is equivalent to q.value = Some(foo).

The Some(..) is just one variant of the Option enum:

enum Option<T> { None, Some(T) }

Option represents a value that may or may not be initialized. For uninitialized values you use the None variant and then you can initialize it to Some(value).

The C rough equivalent would be

struct Q { void* value; }

struct Q q = q { NULL }; q->value = ...