fix: auto-generate .def file for zig + windows-gnu to export PyInit symbol

When using zig as the linker for windows-gnu targets, the `PyInit_<module>`
symbol is not exported, causing Python to fail to import the module.

This generates a `.def` file in `target/maturin/` that explicitly exports the
symbol, passed to the linker via `-C link-arg`.

Also refactors `target/maturin` directory creation into a shared
`ensure_target_maturin_dir()` helper used by compile and repair.

Fixes #922

Author: messense

src/build_context/repair.rs

@@ -321,8 +321,7 @@ impl BuildContext {
     /// reflink-or-copy directly; the concurrent-modification window is
     /// unlikely in cross-device setups.
     pub(super) fn stage_artifact(&self, artifact: &mut BuildArtifact) -> Result<()> {
-        let maturin_build = self.target_dir.join(env!("CARGO_PKG_NAME"));
-        fs::create_dir_all(&maturin_build)?;
+        let maturin_build = crate::compile::ensure_target_maturin_dir(&self.target_dir);
         let artifact_path = &artifact.path;
         let new_artifact_path = maturin_build.join(artifact_path.file_name().unwrap());
         // Remove any stale file at the destination so that `fs::rename`

src/compile.rs

@@ -265,6 +265,10 @@ fn cargo_build_command(
         bridge_model,
         &context.module_name,
         python_interpreter,
+        #[cfg(feature = "zig")]
+        context.zig,
+        #[cfg(feature = "zig")]
+        &context.target_dir,
     );
 
     if context.strip {
@@ -325,14 +329,16 @@ fn configure_bin_lib_flags(
     }
 }
 
-/// Configure platform-specific linker arguments (macOS PyO3, Emscripten).
+/// Configure platform-specific linker arguments (macOS PyO3, Emscripten, Windows GNU + zig).
 fn configure_platform_linker_args(
     cargo_rustc: &mut cargo_options::Rustc,
     rustflags: &mut cargo_config2::Flags,
     target: &Target,
     bridge_model: &BridgeModel,
     module_name: &str,
     python_interpreter: Option<&PythonInterpreter>,
+    #[cfg(feature = "zig")] zig: bool,
+    #[cfg(feature = "zig")] target_dir: &Path,
 ) {
     if target.is_macos() {
         if let BridgeModel::PyO3 { .. } = bridge_model {
@@ -346,6 +352,35 @@ fn configure_platform_linker_args(
     } else if target.is_emscripten() {
         configure_emscripten_args(cargo_rustc, rustflags, target);
     }
+
+    // When using zig as the linker for windows-gnu targets, the PyInit symbol
+    // is not exported. Work around this by generating a .def file that explicitly
+    // exports the symbol.
+    // See https://github.com/PyO3/maturin/issues/922
+    #[cfg(feature = "zig")]
+    if zig
+        && target.is_windows()
+        && !target.is_msvc()
+        && matches!(
+            bridge_model,
+            BridgeModel::PyO3 { .. } | BridgeModel::Cffi | BridgeModel::UniFfi
+        )
+    {
+        let py_init = format!("PyInit_{module_name}");
+        let maturin_dir = ensure_target_maturin_dir(target_dir);
+        let def_path = maturin_dir.join(format!("{module_name}.def"));
+        let def_contents = format!("LIBRARY {module_name}\nEXPORTS\n    {py_init}\n");
+        if let Err(e) = fs::write(&def_path, def_contents) {
+            eprintln!("⚠️  Warning: Failed to write .def file for zig windows-gnu workaround: {e}");
+        } else {
+            debug!(
+                "Generated .def file at {} for zig windows-gnu export workaround",
+                def_path.display()
+            );
+            rustflags.push("-C");
+            rustflags.push(&format!("link-arg={}", def_path.display()));
+        }
+    }
 }
 
 /// Configure macOS-specific linker arguments for PyO3 builds.
@@ -632,12 +667,11 @@ fn configure_pyo3_env(
             build_command.env("PYTHON_SYS_EXECUTABLE", &interpreter.executable);
         } else if bridge_model.is_pyo3() && env::var_os("PYO3_CONFIG_FILE").is_none() {
             let pyo3_config = interpreter.pyo3_config_file();
-            let maturin_target_dir = context.target_dir.join(env!("CARGO_PKG_NAME"));
+            let maturin_target_dir = ensure_target_maturin_dir(&context.target_dir);
             let config_file = maturin_target_dir.join(format!(
                 "pyo3-config-{}-{}.{}.txt",
                 target_triple, interpreter.major, interpreter.minor
             ));
-            fs::create_dir_all(&maturin_target_dir)?;
             // We don't want to rewrite the file every time as that will make cargo
             // trigger a rebuild of the project every time
             let existing_pyo3_config = fs::read_to_string(&config_file).unwrap_or_default();
@@ -930,6 +964,14 @@ pub fn warn_missing_py_init(artifact: &Path, module_name: &str) -> Result<()> {
     Ok(())
 }
 
+/// Ensures the `maturin` subdirectory inside the target directory exists
+/// and returns its path. This directory is used for maturin-generated artifacts.
+pub(crate) fn ensure_target_maturin_dir(target_dir: &Path) -> PathBuf {
+    let dir = target_dir.join(env!("CARGO_PKG_NAME"));
+    let _ = fs::create_dir_all(&dir);
+    dir
+}
+
 fn pyo3_version(cargo_metadata: &cargo_metadata::Metadata) -> Option<(u64, u64, u64)> {
     let packages: HashMap<&str, &cargo_metadata::Package> = cargo_metadata
         .packages