error: Cannot build '/nix/store/n5bnwbw7wh2l4bk6y7arl70fghgk2ri7-python3.13-fb-re2-1.0.7.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/2sc10vh6wjylr4nk8rd9kylglg0i6nsd-python3.13-fb-re2-1.0.7
         /nix/store/gphgdcysf9yv6sliimd8hqb6p20vr812-python3.13-fb-re2-1.0.7-dist
       Last 25 log lines:
       > running bdist_wheel
       > running build
       > running build_py
       > creating build/lib.linux-aarch64-cpython-313
       > copying re2.py -> build/lib.linux-aarch64-cpython-313
       > running build_ext
       > building '_re2' extension
       > creating build/temp.linux-aarch64-cpython-313
       > g++ -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall -fPIC -I/nix/store/b187n72n9xglqyypj0f9nyhm9xhi3abz-python3-3.13.7/include/python3.13 -c _re2.cc -o build/temp.linux-aarch64-cpython-313/_re2.o -std=c++17
       > _re2.cc:252:1: error: cannot convert ‘std::nullptr_t’ to ‘Py_ssize_t’ {aka ‘long int’} in initialization
       >   252 | };
       >       | ^
       > _re2.cc:296:1: error: cannot convert ‘std::nullptr_t’ to ‘Py_ssize_t’ {aka ‘long int’} in initialization
       >   296 | };
       >       | ^
       > _re2.cc:340:1: error: cannot convert ‘std::nullptr_t’ to ‘Py_ssize_t’ {aka ‘long int’} in initialization
       >   340 | };
       >       | ^

I'll look at that. As for the prototype, I've tried a bunch of things, but here's where I left off yesterday.

{ sourceRoot, lib, pkgsCross, makeRustPlatform }:

pkgsCross.mingw32.callPackage
({ rustPlatform, buildPackages, pkg-config, cmake, wine }:
  buildPackages.rustPlatform.buildRustPackage (finalAttrs:
    let
      inherit (finalAttrs) src pname version meta buildInputs;
      manifest = lib.importTOML "${src}/ewext/Cargo.toml";
    in {
      pname = "ewext";
      inherit (manifest.package) version;

      src = sourceRoot;
      sourceRoot = "source/ewext";

      cargoLock.lockFile = "${src}/ewext/Cargo.lock";

      # cargoBuildTarget = "i686-pc-windows-gnu";

      strictDeps = true;
      nativeBuildInputs = [ pkg-config cmake ];

      nativeCheckInputs = [ wine ];
      doCheck = false;
      # buildInputs = with pkgsCross.mingw32; [ openssl ];

      env = { CARGO_TARGET_I686_PC_WINDOWS_GNU_RUNNER = "wine"; };

      # env = {
      #   # CARGO_TARGET_I686_PC_WINDOWS_GNU_LINKER = "${mingw32.stdenv.cc.targetPrefix}gcc";
      #   OPENSSL_DIR = "${lib.getDev pkgsCross.mingw32.openssl}";
      #   OPENSSL_LIB_DIR = "${lib.getLib pkgsCross.mingw32.openssl}/lib";
      #   OPENSSL_NO_VENDOR = 1;
      # };

      meta = { };
    })) { }