build: revert usage of cargo:rustc-cdylib-link-arg

CHANGELOG.md

@@ -11,10 +11,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Added
 
 - Add `indexmap` feature to add `ToPyObject`, `IntoPy` and `FromPyObject` implementations for `indexmap::IndexMap`. [#1728](https://github.com/PyO3/pyo3/pull/1728)
+- Add `pyo3_build_config::add_extension_module_link_args()` to use in build scripts to set linker arguments (for macOS). [#1755](https://github.com/PyO3/pyo3/pull/1755)
 
 ### Changed
 
 - `PyErr::new` no longer acquires the Python GIL internally. [#1724](https://github.com/PyO3/pyo3/pull/1724)
+- Reverted PyO3 0.14.0's use of `cargo:rustc-cdylib-link-arg` in its build script, as Cargo unintentionally allowed crates to pass linker args to downstream crates in this way. Projects supporting macOS may need to restore `.cargo/config.toml` files. [#1755](https://github.com/PyO3/pyo3/pull/1755)
 
 ### Fixed
 

build.rs

@@ -121,11 +121,6 @@ fn emit_cargo_configuration(interpreter_config: &InterpreterConfig) -> Result<()
                 println!("cargo:rustc-link-search=native={}\\libs", base_prefix);
             }
         }
-        (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.

guide/src/building_and_distribution.md

@@ -82,6 +82,36 @@ You can then open a Python shell in the output directory and you'll be able to r
 
 See, as an example, Bazel rules to build PyO3 on Linux at https://github.com/TheButlah/rules_pyo3.
 
+#### macOS
+
+On macOS, because the `extension-module` feature disables linking to `libpython` ([see the next section](#the-extension-module-feature)), some additional linker arguments need to be set. `maturin` and `setuptools-rust` both pass these arguments for PyO3 automatically, but projects using manual builds will need to set these directly in order to support macOS.
+
+The easiest way to set the correct linker arguments is to add a [`build.rs`](https://doc.rust-lang.org/cargo/reference/build-scripts.html) with the following content:
+
+```rust,ignore
+fn main() {
+  pyo3_build_config::add_extension_module_link_args();
+}
+```
+
+Remember to also add `pyo3-build-config` to the `build-dependencies` section in `Cargo.toml`.
+
+An alternative to using `pyo3-build-config` is add the following to a cargo configuration file (e.g. `.cargo/config.toml`):
+
+```toml
+[target.x86_64-apple-darwin]
+rustflags = [
+  "-C", "link-arg=-undefined",
+  "-C", "link-arg=dynamic_lookup",
+]
+
+[target.aarch64-apple-darwin]
+rustflags = [
+  "-C", "link-arg=-undefined",
+  "-C", "link-arg=dynamic_lookup",
+]
+```
+
 ### The `extension-module` feature
 
 PyO3's `extension-module` feature is used to disable [linking](https://en.wikipedia.org/wiki/Linker_(computing)) to `libpython` on unix targets.

pyo3-build-config/src/lib.rs

@@ -1,33 +1,20 @@
 //! Configuration used by PyO3 for conditional support of varying Python versions.
 //!
-//! The only public API currently exposed is [`use_pyo3_cfgs`], which is intended to be used in
-//! build scripts to add a standard set of `#[cfg]` attributes for handling multiple Python
-//! versions.
-//!
-//! The full list of attributes added are the following:
-//!
-//! | Flag | Description |
-//! | ---- | ----------- |
-//! | `#[cfg(Py_3_6)]`, `#[cfg(Py_3_7)]`, `#[cfg(Py_3_8)]`, `#[cfg(Py_3_9)]`, `#[cfg(Py_3_10)]` | These attributes mark code only for a given Python version and up. For example, `#[cfg(Py_3_6)]` marks code which can run on Python 3.6 **and newer**. |
-//! | `#[cfg(Py_LIMITED_API)]` | This marks code which is run when compiling with PyO3's `abi3` feature enabled. |
-//! | `#[cfg(PyPy)]` | This marks code which is run when compiling for PyPy. |
-//!
-//! For examples of how to use these attributes, [see PyO3's guide](https://pyo3.rs/latest/building_and_distribution/multiple_python_versions.html).
+//! The public APIs exposed, [`use_pyo3_cfgs`] and [`add_extension_module_link_args`] are intended
+//! to be called from build scripts to simplify building crates which depend on PyO3.
 
 #[doc(hidden)]
 pub mod errors;
 mod impl_;
 
 use once_cell::sync::OnceCell;
 
-pub use impl_::{
-    find_interpreter, get_config_from_interpreter, InterpreterConfig, PythonImplementation,
-    PythonVersion,
-};
-
 // Used in PyO3's build.rs
 #[doc(hidden)]
-pub use impl_::{cargo_env_var, env_var, make_interpreter_config};
+pub use impl_::{
+    cargo_env_var, env_var, find_interpreter, get_config_from_interpreter, make_interpreter_config,
+    InterpreterConfig, PythonImplementation, PythonVersion,
+};
 
 /// Reads the configuration written by PyO3's build.rs
 ///
@@ -46,6 +33,32 @@ pub fn get() -> &'static InterpreterConfig {
 #[doc(hidden)]
 pub const PATH: &str = concat!(env!("OUT_DIR"), "/pyo3-build-config.txt");
 
+/// Adds all the [`#[cfg]` flags](index.html) to the current compilation.
+///
+/// This should be called from a build script.
+///
+/// The full list of attributes added are the following:
+///
+/// | Flag | Description |
+/// | ---- | ----------- |
+/// | `#[cfg(Py_3_6)]`, `#[cfg(Py_3_7)]`, `#[cfg(Py_3_8)]`, `#[cfg(Py_3_9)]`, `#[cfg(Py_3_10)]` | These attributes mark code only for a given Python version and up. For example, `#[cfg(Py_3_6)]` marks code which can run on Python 3.6 **and newer**. |
+/// | `#[cfg(Py_LIMITED_API)]` | This marks code which is run when compiling with PyO3's `abi3` feature enabled. |
+/// | `#[cfg(PyPy)]` | This marks code which is run when compiling for PyPy. |
+///
+/// For examples of how to use these attributes, [see PyO3's guide](https://pyo3.rs/latest/building_and_distribution/multiple_python_versions.html).
 pub fn use_pyo3_cfgs() {
     get().emit_pyo3_cfgs();
 }
+
+/// Adds linker arguments (for macOS) 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.
+pub fn add_extension_module_link_args() {
+    if cargo_env_var("CARGO_CFG_TARGET_OS").unwrap() == "macos" {
+        println!("cargo:rustc-cdylib-link-arg=-undefined");
+        println!("cargo:rustc-cdylib-link-arg=dynamic_lookup");
+    }
+}