Fix cargo test with extension-module feature.

Author: davidhewitt

CHANGELOG.md

@@ -33,6 +33,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Remove unused `python3` feature. [#1235](https://github.com/PyO3/pyo3/pull/1235)
 
 ### 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)
 - Fix missing field in `PyCodeObject` struct (`co_posonlyargcount`) - caused invalid access to other fields in Python >3.7. [#1260](https://github.com/PyO3/pyo3/pull/1260)
 - Fix building for `x86_64-unknown-linux-musl` target from `x86_65-unknown-linux-gnu` host. [#1267](https://github.com/PyO3/pyo3/pull/1267)
 

build.rs

@@ -594,8 +594,8 @@ fn get_library_link_name(version: &PythonVersion, ld_version: &str) -> String {
     }
 }
 
-fn get_rustc_link_lib(config: &InterpreterConfig) -> Result<String> {
-    match env::var("CARGO_CFG_TARGET_OS").unwrap().as_str() {
+fn get_rustc_link_lib(config: &InterpreterConfig, target_os: &str) -> Result<String> {
+    match target_os {
         "windows" => get_rustc_link_lib_windows(config),
         "macos" => get_rustc_link_lib_macos(config),
         _ => get_rustc_link_lib_unix(config),
@@ -762,19 +762,26 @@ fn configure(interpreter_config: &InterpreterConfig) -> Result<String> {
     }
 
     check_target_architecture(interpreter_config)?;
-    let target_os = env::var_os("CARGO_CFG_TARGET_OS").unwrap();
+    let target_os = env::var("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" {
-        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 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(&interpreter_config.version, &interpreter_config.ld_version);
+        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, &target_os)?);
     }
 
     let mut flags = String::new();

guide/src/faq.md

@@ -15,15 +15,16 @@ 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
 ```