Gate representation in C bindings

[alecandido]

Note

This is mostly a note to myself

There might be an issue if we're going to generate C bindings directly from the current Gate layout.

The reason is the following:

The representation of a repr(C) enum with fields is a repr(C) struct with two fields, also called a "tagged union" in C:

• a repr(C) version of the enum with all fields removed ("the tag")
• a repr(C) union of repr(C) structs for the fields of each variant that had them ("the payload")

[...]

Note: unions with non-Copy fields are unstable, see 55149.

https://doc.rust-lang.org/nightly/reference/type-layout.html#reprc-enums-with-fields

Even though the mentioned issue has been closed (still have to read that).

Thus, we should make sure that whatever is in a gate will be Copy, or choose a different scheme for the C API and implement a mapping in the bindings.

Size

Also consider that the size of a union is the size of its biggest field (rounded to the alignment). This means that, even if we limited to store arrays and whatever needed in a Copy-struct, every gate would have the size of an arbitrary Unitary (or the largest gate we have).

This might be irrelevant, but it would certainly blow up the memory consumption.