Document order of operations for attributes

[jam1garner]

Implementation notes:

• Add a section at the bottom of the corresponding attributes page
• Link to it from a sentence at the bottom of the attribute overview tables

[stevecheckoway]

I spent a while reading through the code and this is the order in which the attributes for binread are acted on. Any corrections appreciated.

I didn't look at how byte order is determined in any detail, but I assume you can just treat each of the "Byte order determination" below to mean big, little, is_big, and is_little.

This took longer than I expected and I didn't have time to do binwrite.

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Reading unit structs

1. import, import_tuple
2. Byte order determination
3. magic
4. pre_assert

Reading structs

1. import, import_tuple
2. Byte order determination
3. magic
4. pre_assert
5. Read each field
6. Calls BinRead::after_parse()
7. assert

Reading a field

1. If temp and either default or ignore, then no other attributes are consulted and the field is not read
2. offset
3. import, import_tuple
4. Byte order determination
5. magic
6. restore_position's position is computed here
7. if (if the condition is not satisfied skip to the assert step)
8. seek_before
9. pad_before
10. align_before
11. pad_size_to's position is computed here
12. default, ignore (which acts like default), or calc actions are taken; otherwise the field is read via parse_with or normally, passing args, args_raw, or count
13. If none of default, ignore, and calc, then try or err_context
14. map or try_map
15. postprocess_now or deref_now cause BinRead::after_parse() to be called
16. pad_size_to
17. pad_after
18. align_after
19. assert
20. restore_position

Reading unit enums

1. import, import_tuple
2. Byte order determination
3. magic
4. pre_assert
5. repr or fields' magic and pre_assert

Reading data enums

1. import, import_tuple
2. Byte order determination
3. magic
4. pre_assert
5. return_all_errors, return_unexpected_error
6. If a unit variant, read according to Reading unit structs, otherwise read according to Reading structs

[octylFractal]

Maybe a separate issue, but IMO for a more intuitive order, magic and loading restore_position should be swapped for fields, so that the position is before the magic is read.