Add PyBytes::new_with_writer

[bschoenmaeckers]

Adds a new constructor on PyBytes that allows creating a python bytes object using a std::io::Write writer. I've exposed the writer as &mut dyn Write dyn object so we do not have to expose the internal writer implementation. We can change this to &mut PyBytesWriter at a later stage if that seems necessary.

closes #4003

[bschoenmaeckers]

How can I make pyo3-ffi-check happy? It is complaining that PyBytesWriter does not exist on pre-1.15. This is expected but I cannot let it point to the existing _PyBytesWriter because the layout is different.

  error[E0412]: cannot find type `PyBytesWriter` in module `bindings`
    --> src\main.rs:70:5
     |
  70 |     pyo3_ffi_check_macro::for_all_structs!(check_struct);
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `bindings`

[bschoenmaeckers]

A possible future extension would be to implement std::io::Seek on PyBytesWriter or std::io::Write on Cursor<PyBytesWriter>

[davidhewitt]

Very nice, thank you! It's cool to see these newer APIs improving ergonomics / solving cases which couldn't be done previously.

[davidhewitt]

Probably missing a cfg here?

Suggested change

	opaque_struct!(pub PyBytesWriter);
	#[cfg(Py_3_15)]
	opaque_struct!(pub PyBytesWriter);

[davidhewitt]

Ah I see, the compat APIs produce a PyBytesWriter too, which exists before 3.15.

Maybe we should have pyo3_ffi::compat::PyBytesWriter opaque struct defined for pre-3.15? (On 3.15 it can be a re-export.)

[bschoenmaeckers]

I could not get ffi-check to pass, with a public exposed struct that does not have a definition in the generated binding. So my only working solution so far was to force it to ignore this struct on pre-3.15.

For reference this is the error

  error[E0412]: cannot find type `PyBytesWriter` in module `bindings`
    --> src\main.rs:70:5
     |
  70 |     pyo3_ffi_check_macro::for_all_structs!(check_struct);
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `bindings`