A minimal Python module in Rust (move lib.rs to src/lib.rs); rename target/{profile}/rustpy.dll to rustpy.pyd.

Cargo.toml

[package]
name = "rustpy"
version = "0.1.0"
authors = ["Derrick W. Turk <dwt@terminusdatascience.com>"]
edition = "2018"

[lib]
crate-type = ["cdylib"]

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
python3-sys = "0.5.1"

lib.rs

use python3_sys::{
    PyArg_ParseTuple,
    PyMethodDef,
    PyModule_Create,
    PyModuleDef,
    PyModuleDef_HEAD_INIT,
    PyNumber_Add,
    PyObject,

    METH_VARARGS,
};

use std::{
    ptr,
    os::raw::c_char,
};

macro_rules! cstr {
  ($s:expr) => (
    concat!($s, "\0") as *const str as *const [c_char] as *const c_char
  );
}

unsafe extern fn add(_module: *mut PyObject, args: *mut PyObject
  ) -> *mut PyObject {
    let mut lhs: *mut PyObject = ptr::null_mut();
    let mut rhs: *mut PyObject = ptr::null_mut();
    if PyArg_ParseTuple(args, cstr!("OO"),
      &mut lhs as *mut _, &mut rhs as *mut _) == 0 {
        ptr::null_mut()
    } else {
        PyNumber_Add(lhs, rhs)
    }
}

static mut RUSTPY_METHODS: [PyMethodDef; 2] = [
    PyMethodDef {
        ml_name: cstr!("add"),
        ml_meth: Some(add),
        ml_flags: METH_VARARGS,
        ml_doc: cstr!("adds some stuff"),
    },

    PyMethodDef {
        ml_name: ptr::null(),
        ml_meth: None,
        ml_flags: 0,
        ml_doc: ptr::null(),
    },
];

static mut RUSTPY: PyModuleDef = PyModuleDef {
    m_base: PyModuleDef_HEAD_INIT,
    m_name: cstr!("rustpy"),
    m_doc: cstr!("it's Rust, but Py"),
    m_size: -1,
    m_methods: unsafe { &mut RUSTPY_METHODS } as *const _ as *mut _,
    m_slots: ptr::null_mut(),
    m_traverse: None,
    m_clear: None,
    m_free: None,
};

#[no_mangle]
unsafe extern fn PyInit_rustpy() -> *mut PyObject {
    PyModule_Create(&mut RUSTPY as *mut _)
}