Fix cargo test with extension-module feature.

Author: davidhewitt

CHANGELOG.md

@@ -51,6 +51,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Remove `PYO3_CROSS_INCLUDE_DIR` environment variable and the associated C header parsing functionality.
 
 ### 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

@@ -587,8 +587,15 @@ fn run_python_script(interpreter: &Path, script: &str) -> Result<String> {
     }
 }
 
-fn get_rustc_link_lib(config: &InterpreterConfig) -> String {
-    let link_name = if cargo_env_var("CARGO_CFG_TARGET_OS").unwrap() == "windows" {
+fn get_library_link_name_unix(config: &InterpreterConfig) -> String {
+    match config.implementation {
+        PythonInterpreterKind::CPython => format!("python{}", config.ld_version),
+        PythonInterpreterKind::PyPy => format!("pypy{}-c", config.version.major),
+    }
+}
+
+fn get_library_link_name(config: &InterpreterConfig) -> String {
+    if cargo_env_var("CARGO_CFG_TARGET_OS").unwrap() == "windows" {
         if is_abi3() {
             // Link against python3.lib for the stable ABI on Windows.
             // See https://www.python.org/dev/peps/pep-0384/#linkage
@@ -608,17 +615,8 @@ fn get_rustc_link_lib(config: &InterpreterConfig) -> String {
             )
         }
     } else {
-        match config.implementation {
-            PythonInterpreterKind::CPython => format!("python{}", config.ld_version),
-            PythonInterpreterKind::PyPy => format!("pypy{}-c", config.version.major),
-        }
-    };
-
-    format!(
-        "cargo:rustc-link-lib={link_model}{link_name}",
-        link_model = if config.shared { "" } else { "static=" },
-        link_name = link_name
-    )
+        get_library_link_name_unix(config)
+    }
 }
 
 fn get_venv_path() -> Option<PathBuf> {
@@ -764,26 +762,35 @@ fn configure(interpreter_config: &InterpreterConfig) -> Result<()> {
     let target_os = cargo_env_var("CARGO_CFG_TARGET_OS").unwrap();
     let is_extension_module = cargo_env_var("CARGO_FEATURE_EXTENSION_MODULE").is_some();
     match (is_extension_module, target_os.as_str()) {
-        (_, "windows") => {
-            // always link on windows, even with extension module
-            println!("{}", get_rustc_link_lib(&interpreter_config));
+        (_, "windows") | (_, "android") | (false, _) => {
+            // windows or android - always link to libpython
+            // other systems - only link libs if not extension module
+
+            if let Some(libdir) = &interpreter_config.libdir {
+                println!("cargo:rustc-link-search=native={}", libdir);
+            }
+            if target_os == "windows" {
+                println!("cargo:rustc-link-search=native={}\\libs", interpreter_config.base_prefix);
+            }
+
             println!(
-                "cargo:rustc-link-search=native={}\\libs",
-                interpreter_config.base_prefix
+                "cargo:rustc-link-lib={link_model}{link_name}",
+                link_model = if interpreter_config.shared {
+                    ""
+                } else {
+                    "static="
+                },
+                link_name = get_library_link_name(&interpreter_config)
             );
         }
-        (true, "macos") => {
-            // with extension module on macos some extra linker arguments are needed
-            println!("cargo:rustc-cdylib-link-arg=-undefined");
-            println!("cargo:rustc-cdylib-link-arg=dynamic_lookup");
-        }
-        (false, _) | (_, "android") => {
-            // other systems, only link libs if not extension module
-            // android always link.
-            println!("{}", get_rustc_link_lib(&interpreter_config));
-            if let Some(libdir) = &interpreter_config.libdir {
-                println!("cargo:rustc-link-search=native={}", libdir);
+        (true, _) => {
+            // Extension module on unix system - only link non-lib targets
+            if target_os == "macos" {
+                // with extension module on macos some extra linker arguments are needed
+                println!("cargo:rustc-cdylib-link-arg=-undefined");
+                println!("cargo:rustc-cdylib-link-arg=dynamic_lookup");
             }
+
             if interpreter_config.implementation == PythonInterpreterKind::PyPy {
                 // PyPy 3.7 changed LIBDIR to point to base_prefix/lib, so need to hard-code /bin
                 // search path too: https://foss.heptapod.net/pypy/pypy/-/issues/3442
@@ -792,8 +799,13 @@ fn configure(interpreter_config: &InterpreterConfig) -> Result<()> {
                     interpreter_config.base_prefix
                 );
             }
+
+            let lib_name = get_library_link_name_unix(&interpreter_config);
+            println!("cargo:rustc-link-arg-bins=-l{}", lib_name);
+            println!("cargo:rustc-link-arg-tests=-l{}", lib_name);
+            println!("cargo:rustc-link-arg-benches=-l{}", lib_name);
+            println!("cargo:rustc-link-arg-examples=-l{}", lib_name);
         }
-        _ => {}
     }
 
     if env::var_os("CARGO_FEATURE_AUTO_INITIALIZE").is_some() {

guide/src/faq.md

@@ -15,17 +15,18 @@ 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, [#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_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!