Move emitting cfgs to pyo3-build-config

build.rs

@@ -1,7 +1,7 @@
-use std::{env, process::Command};
+use std::env;
 
 use pyo3_build_config::pyo3_build_script_impl::{cargo_env_var, errors::Result};
-use pyo3_build_config::{bail, InterpreterConfig};
+use pyo3_build_config::{bail, print_feature_cfgs, InterpreterConfig};
 
 fn ensure_auto_initialize_ok(interpreter_config: &InterpreterConfig) -> Result<()> {
     if cargo_env_var("CARGO_FEATURE_AUTO_INITIALIZE").is_some() {
@@ -32,17 +32,6 @@ fn ensure_auto_initialize_ok(interpreter_config: &InterpreterConfig) -> Result<(
     Ok(())
 }
 
-fn rustc_minor_version() -> Option<u32> {
-    let rustc = env::var_os("RUSTC")?;
-    let output = Command::new(rustc).arg("--version").output().ok()?;
-    let version = core::str::from_utf8(&output.stdout).ok()?;
-    let mut pieces = version.split('.');
-    if pieces.next() != Some("rustc 1") {
-        return None;
-    }
-    pieces.next()?.parse().ok()
-}
-
 /// Prepares the PyO3 crate for compilation.
 ///
 /// This loads the config from pyo3-build-config and then makes some additional checks to improve UX
@@ -55,18 +44,10 @@ fn configure_pyo3() -> Result<()> {
 
     interpreter_config.emit_pyo3_cfgs();
 
-    let rustc_minor_version = rustc_minor_version().unwrap_or(0);
     ensure_auto_initialize_ok(interpreter_config)?;
 
-    // Enable use of const generics on Rust 1.51 and greater
+    // Emit cfgs like `addr_of` and `min_const_generics`
-    if rustc_minor_version >= 51 {
+    print_feature_cfgs();
-        println!("cargo:rustc-cfg=min_const_generics");
-    }
-
-    // Enable use of std::ptr::addr_of! on Rust 1.51 and greater
-    if rustc_minor_version >= 51 {
-        println!("cargo:rustc-cfg=addr_of");
-    }
 
     Ok(())
 }

pyo3-build-config/src/lib.rs

@@ -10,6 +10,7 @@ mod impl_;
 
 #[cfg(feature = "resolve-config")]
 use std::io::Cursor;
+use std::{env, process::Command};
 
 #[cfg(feature = "resolve-config")]
 use once_cell::sync::OnceCell;
@@ -115,6 +116,36 @@ fn abi3_config() -> InterpreterConfig {
     interpreter_config
 }
 
+/// Use certain features if we detect the compiler being used supports them.
+///
+/// Features may be removed or added as MSRV gets bumped or new features become available,
+/// so this function is unstable.
+#[doc(hidden)]
+pub fn print_feature_cfgs() {
+    fn rustc_minor_version() -> Option<u32> {
+        let rustc = env::var_os("RUSTC")?;
+        let output = Command::new(rustc).arg("--version").output().ok()?;
+        let version = core::str::from_utf8(&output.stdout).ok()?;
+        let mut pieces = version.split('.');
+        if pieces.next() != Some("rustc 1") {
+            return None;
+        }
+        pieces.next()?.parse().ok()
+    }
+
+    let rustc_minor_version = rustc_minor_version().unwrap_or(0);
+
+    // Enable use of const generics on Rust 1.51 and greater
+    if rustc_minor_version >= 51 {
+        println!("cargo:rustc-cfg=min_const_generics");
+    }
+
+    // Enable use of std::ptr::addr_of! on Rust 1.51 and greater
+    if rustc_minor_version >= 51 {
+        println!("cargo:rustc-cfg=addr_of");
+    }
+}
+
 /// Private exports used in PyO3's build.rs
 ///
 /// Please don't use these - they could change at any time.

pyo3-ffi/build.rs

@@ -1,5 +1,5 @@
 use pyo3_build_config::{
-    bail, ensure,
+    bail, ensure, print_feature_cfgs,
     pyo3_build_script_impl::{
         cargo_env_var, env_var, errors::Result, resolve_interpreter_config, InterpreterConfig,
         PythonVersion,
@@ -100,29 +100,12 @@ fn configure_pyo3() -> Result<()> {
         println!("{}", line);
     }
 
-    let rustc_minor_version = rustc_minor_version().unwrap_or(0);
+    // Emit cfgs like `addr_of` and `min_const_generics`
-
+    print_feature_cfgs();
-    // Enable use of std::ptr::addr_of! on Rust 1.51 and greater
-    if rustc_minor_version >= 51 {
-        println!("cargo:rustc-cfg=addr_of");
-    }
 
     Ok(())
 }
 
-fn rustc_minor_version() -> Option<u32> {
-    use std::{env, process::Command};
-
-    let rustc = env::var_os("RUSTC")?;
-    let output = Command::new(rustc).arg("--version").output().ok()?;
-    let version = core::str::from_utf8(&output.stdout).ok()?;
-    let mut pieces = version.split('.');
-    if pieces.next() != Some("rustc 1") {
-        return None;
-    }
-    pieces.next()?.parse().ok()
-}
-
 fn print_config_and_exit(config: &InterpreterConfig) {
     println!("\n-- PYO3_PRINT_CONFIG=1 is set, printing configuration and halting compile --");
     config