Add a `position_of!` macro to reference earlier fields position

[MrNbaYoh]

This issue is a feature proposal for adding a position_of! macro in the generated code for reading/writing.

#[binrw]
struct Test {
 a: u32,
 b: u32,
 #[br(calc = position_of!(b) + 1000]
 c: u64,
}

Main features

• it is defined in the BinRead::read/BinWrite::write function, so it is not visible from outside
• position_of!(x) gives the stream position at which the field x has been read

Additional ideas

• to make look less like it's popping out of nowhere, add a struct-level position_of attribute such as:
  • if it is absent, the macro is not added in the generated code
  • #[br(position_of)] means we add it with it's default name
  • #[br(position_of = my_name)] means we add it and name it my_name

Concerns

• what to return when the field is not actually read/written:
  • when it's conditionally read/written (with and if attribute for example), should it be:
  • an Option<u64> which is None when the field is not read and Some when it is (preferred)
  • a regular u64 that gives the position of the stream where the field could've been read (introduce concerns with conditionally applied seek/align directives)
  • when it's calculated with calc, should it be:
  • not defined at all (preferred)
  • a regular u64 that gives the position of the stream at the time the field is calculated
• any other edge case?

Implementation proposals

The main difficulty is that we need to define a macro that allows us to reference earlier fields only:

• we can redefine the macro each time a new field position is known, so for the given example you'd have:

  macro_rules! position_of {
    (a) => {
         __binrw_generated_position_of_a
    };
  }

  once the position of a is known, and as soon as the position of b is known, redefine it this way:

  macro_rules! position_of {
    (a) => {
         __binrw_generated_position_of_a
    };
    (b) => {
         __binrw_generated_position_of_b
    };
  }

• or we can define it once and make it craft new idents with something like paste