[WIP] Trait and Macro for exporting C API for identifiers

[twitu]

Pull Request

Macro to implement C API for identifiers

Type of change

• [ x ] Refactor

How has this change been tested?

This is what the macro generates. Requesting feedback before applying this across other identifiers.

    impl InterconvertPyString for Venue {
        #[export_name = "Venue_from_pystr"]
        unsafe extern "C" fn from_pystr(ptr: *mut ffi::PyObject) -> Self {
            Venue {
                value: Box::new(pystr_to_string(ptr)),
            }
        }
        #[export_name = "Venue_to_pystr"]
        unsafe extern "C" fn to_pystr(&self) -> *mut ffi::PyObject {
            string_to_pystr(self.value.as_str())
        }
    }

Not tested currently

[codecov]

Codecov Report

Merging #647 (b7841b7) into develop (b332e67) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head b7841b7 differs from pull request most recent head e6d6912. Consider uploading reports for the commit e6d6912 to get more accurate results

@@           Coverage Diff            @@
##           develop     #647   +/-   ##
========================================
  Coverage    90.99%   90.99%           
========================================
  Files          298      298           
  Lines        25190    25198    +8     
========================================
+ Hits         22922    22930    +8     
  Misses        2268     2268           

Impacted Files	Coverage Δ
nautilus_trader/model/data/tick.pyx	98.31% <0.00%> (+0.12%)	⬆️

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[twitu]

I've also added a DeRef trait which allows Venue to be referenced as if if were it's inner string content itself. It's a useful idiom when operating on the Rust side of things.

[twitu]

A couple of factors combine to make this impossible.

1. cbindgen does not generate bindings for functions defined inside traits.
2. this means that we need unique names for each ffi functions for e.g. uuid4_to_pystr
3. Currently it is not possible to generate identifier names from passed tokens inside a macro

This pretty much kills any chance of generating extern functions generically from macros.

SO answer
rust-lang/rust#12249
rust-lang/rust#29599