Fix cargo test with extension-module feature.

CHANGELOG.md

@@ -57,6 +57,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Fix `cargo test` with `extension-module` feature. (Requires `cargo +nightly` for now.) #[2135](https://github.com/PyO3/pyo3/pull/2135)
 - Fix undefined symbol for `PyObject_HasAttr` on PyPy. [#2025](https://github.com/PyO3/pyo3/pull/2025)
 - Fix memory leak in `PyErr::into_value`. [#2026](https://github.com/PyO3/pyo3/pull/2026)
 - Fix clippy warning `needless-option-as-deref` in code generated by `#[pyfunction]` and `#[pymethods]`. [#2040](https://github.com/PyO3/pyo3/pull/2040)

guide/src/faq.md

@@ -15,17 +15,17 @@ PyO3 provides a struct [`GILOnceCell`] which works equivalently to `OnceCell` bu
 
 [`GILOnceCell`]: {{#PYO3_DOCS_URL}}/pyo3/once_cell/struct.GILOnceCell.html
 
-## I can't run `cargo test`: I'm having linker issues like "Symbol not found" or "Undefined reference to _PyExc_SystemError"!
+## I can't run `cargo test` or `cargo run`: I'm having linker issues like "Symbol not found" or "Undefined reference to _PyExc_SystemError"!
 
-Currently, [#340](https://github.com/PyO3/pyo3/issues/340) causes `cargo test` to fail with linking errors when the `extension-module` feature is activated. For now you can work around this by making the `extension-module` feature optional and running the tests with `cargo test --no-default-features`:
+On unix operating systems the `extension-module` feature is required to disable linking against libpython to meet criteria of how Python extension modules should be built.
 
-```toml
-[dependencies.pyo3]
-{{#PYO3_CRATE_VERSION}}
+PyO3 is able to re-enable linking for binaries and tests in the project, but this requires a `cargo` version capable of understanding extra linker arguments. This is only supported on nightly cargoes from February of 2022 or newer.
 
-[features]
-extension-module = ["pyo3/extension-module"]
-default = ["extension-module"]
+```text
+# For cargo test
+cargo +nightly test
+# For cargo run
+cargo +nightly run
 ```
 
 ## I can't run `cargo test`: my crate cannot be found for tests in `tests/` directory!

pyo3-build-config/src/lib.rs

@@ -37,24 +37,36 @@ pub fn use_pyo3_cfgs() {
     get().emit_pyo3_cfgs();
 }
 
-/// Adds linker arguments (for macOS) suitable for PyO3's `extension-module` feature.
+/// Adds linker arguments suitable for PyO3's `extension-module` feature.
 ///
 /// This should be called from a build script.
-///
-/// This is currently a no-op on non-macOS platforms, however may emit additional linker arguments
-/// in future if deemed necessarys.
+#[cfg(feature = "resolve-config")]
 pub fn add_extension_module_link_args() {
+    let interpreter_config = get();
+
     _add_extension_module_link_args(
+        interpreter_config,
         &impl_::cargo_env_var("CARGO_CFG_TARGET_OS").unwrap(),
         std::io::stdout(),
     )
 }
 
-fn _add_extension_module_link_args(target_os: &str, mut writer: impl std::io::Write) {
+fn _add_extension_module_link_args(
+    interpreter_config: &InterpreterConfig,
+    target_os: &str,
+    mut writer: impl std::io::Write,
+) {
     if target_os == "macos" {
         writeln!(writer, "cargo:rustc-cdylib-link-arg=-undefined").unwrap();
         writeln!(writer, "cargo:rustc-cdylib-link-arg=dynamic_lookup").unwrap();
     }
+    if target_os != "windows" && target_os != "android" {
+        let lib_name = interpreter_config.lib_name.as_ref().unwrap();
+        writeln!(writer, "cargo:rustc-link-arg-bins=-l{}", lib_name).unwrap();
+        writeln!(writer, "cargo:rustc-link-arg-tests=-l{}", lib_name).unwrap();
+        writeln!(writer, "cargo:rustc-link-arg-benches=-l{}", lib_name).unwrap();
+        writeln!(writer, "cargo:rustc-link-arg-examples=-l{}", lib_name).unwrap();
+    }
 }
 
 /// Loads the configuration determined from the build environment.
@@ -179,15 +191,32 @@ mod tests {
     fn extension_module_link_args() {
         let mut buf = Vec::new();
 
+        let interpreter_cfg = InterpreterConfig {
+            version: PythonVersion { major: 3, minor: 7 },
+            implementation: PythonImplementation::CPython,
+            shared: true,
+            abi3: false,
+            lib_name: Some("test".to_owned()),
+            lib_dir: None,
+            executable: None,
+            pointer_width: None,
+            build_flags: BuildFlags::default(),
+            suppress_build_script_link_lines: false,
+            extra_build_script_lines: vec![],
+        };
         // Does nothing on non-mac
-        _add_extension_module_link_args("windows", &mut buf);
+        _add_extension_module_link_args(&interpreter_cfg, "windows", &mut buf);
         assert_eq!(buf, Vec::new());
 
-        _add_extension_module_link_args("macos", &mut buf);
+        _add_extension_module_link_args(&interpreter_cfg, "macos", &mut buf);
         assert_eq!(
             std::str::from_utf8(&buf).unwrap(),
             "cargo:rustc-cdylib-link-arg=-undefined\n\
-             cargo:rustc-cdylib-link-arg=dynamic_lookup\n"
+             cargo:rustc-cdylib-link-arg=dynamic_lookup\n\
+             cargo:rustc-link-arg-bins=-ltest\n\
+             cargo:rustc-link-arg-tests=-ltest\n\
+             cargo:rustc-link-arg-benches=-ltest\n\
+             cargo:rustc-link-arg-examples=-ltest\n"
         );
     }
 }