Fix cargo test with extension-module feature.

Author: davidhewitt

CHANGELOG.md

@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
   - `PyObject_Check`, `PySuper_Check`, and `FreeFunc` [#1438](https://github.com/PyO3/pyo3/pull/1438)
 
 ### Fixed
+- Fix `cargo test` with `extension-module` feature. (Requires `cargo +nightly -Zextra-link-arg test` for now.) #[1123](https://github.com/PyO3/pyo3/pull/1123)
 - Remove FFI definition `PyCFunction_ClearFreeList` for Python 3.9 and later. [#1425](https://github.com/PyO3/pyo3/pull/1425)
 - `PYO3_CROSS_LIB_DIR` enviroment variable no long required when compiling for x86-64 Python from macOS arm64 and reverse. [#1428](https://github.com/PyO3/pyo3/pull/1428)
 - Fix FFI definition `_PyEval_RequestCodeExtraIndex` which took an argument of the wrong type. [#1429](https://github.com/PyO3/pyo3/pull/1429)

build.rs

@@ -605,6 +605,13 @@ fn run_python_script(interpreter: &Path, script: &str) -> Result<String> {
     }
 }
 
+fn get_library_link_name_unix(config: &InterpreterConfig) -> String {
+    match config.version.implementation {
+        PythonInterpreterKind::CPython => format!("python{}", config.ld_version),
+        PythonInterpreterKind::PyPy => format!("pypy{}-c", config.version.major),
+    }
+}
+
 fn get_rustc_link_lib(config: &InterpreterConfig) -> String {
     let link_name = if env::var("CARGO_CFG_TARGET_OS").unwrap().as_str() == "windows" {
         if env::var("CARGO_CFG_TARGET_ENV").unwrap().as_str() == "gnu" {
@@ -628,10 +635,7 @@ fn get_rustc_link_lib(config: &InterpreterConfig) -> String {
             }
         }
     } else {
-        match config.version.implementation {
-            PythonInterpreterKind::CPython => format!("python{}", config.ld_version),
-            PythonInterpreterKind::PyPy => format!("pypy{}-c", config.version.major),
-        }
+        get_library_link_name_unix(config)
     };
 
     format!(
@@ -764,17 +768,23 @@ fn configure(interpreter_config: &InterpreterConfig) -> Result<()> {
     check_target_architecture(interpreter_config)?;
     let target_os = env::var_os("CARGO_CFG_TARGET_OS").unwrap();
 
+    if let Some(libdir) = &interpreter_config.libdir {
+        println!("cargo:rustc-link-search=native={}", libdir);
+    } else if target_os == "windows" {
+        println!(
+            "cargo:rustc-link-search=native={}\\libs",
+            interpreter_config.base_prefix
+        );
+    }
+
     let is_extension_module = env::var_os("CARGO_FEATURE_EXTENSION_MODULE").is_some();
-    if !is_extension_module || target_os == "windows" || target_os == "android" {
+    if is_extension_module && !(target_os == "windows" || target_os == "android") {
+        // Extension module on unix - only link non-lib targets
+        let lib_name = get_library_link_name_unix(&interpreter_config);
+        println!("cargo:rustc-link-arg-bins=-l{}", lib_name);
+    } else {
+        // Not extension module, or is windows or android - always link to libpython.
         println!("{}", get_rustc_link_lib(&interpreter_config));
-        if let Some(libdir) = &interpreter_config.libdir {
-            println!("cargo:rustc-link-search=native={}", libdir);
-        } else if target_os == "windows" {
-            println!(
-                "cargo:rustc-link-search=native={}\\libs",
-                interpreter_config.base_prefix
-            );
-        }
     }
 
     if interpreter_config.shared {

guide/src/faq.md

@@ -15,17 +15,18 @@ PyO3 provides a struct [`GILOnceCell`] which works equivalently to `OnceCell` bu
 
 [`GILOnceCell`]: https://docs.rs/pyo3/latest/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, [#341](https://github.com/PyO3/pyo3/issues/341) 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]
-version = "*"
+PyO3 is able to re-enable linking for binaries and tests in the project, but it requires a nightly cargo feature. To use this feature, you must opt into it, e.g.:
 
-[features]
-extension-module = ["pyo3/extension-module"]
-default = ["extension-module"]
+```
+# For cargo test
+cargo +nightly -Zextra-link-arg test
+
+# For cargo run
+cargo +nightly -Zextra-link-arg run
 ```
 
 ## I can't run `cargo test`: my crate cannot be found for tests in `tests/` directory!