pyo3-build-config: inline config when not cross compiling

build.rs

@@ -1,9 +1,9 @@
-use std::{env, ffi::OsString, path::Path, process::Command};
+use std::{env, io::Cursor, path::Path, process::Command};
 
 use pyo3_build_config::{
     bail, cargo_env_var, ensure, env_var,
     errors::{Context, Result},
-    InterpreterConfig, PythonVersion,
+    make_cross_compile_config, InterpreterConfig, PythonVersion,
 };
 
 /// Minimum Python version PyO3 supports.
@@ -114,41 +114,16 @@ fn emit_cargo_configuration(interpreter_config: &InterpreterConfig) -> Result<()
 /// The result is written to pyo3_build_config::PATH, which downstream scripts can read from
 /// (including `pyo3-macros-backend` during macro expansion).
 fn configure_pyo3() -> Result<()> {
-    let write_config_file = env_var("PYO3_WRITE_CONFIG_FILE").map_or(false, |os_str| os_str == "1");
-    let custom_config_file_path = env_var("PYO3_CONFIG_FILE");
-    if let Some(path) = &custom_config_file_path {
-        ensure!(
-            Path::new(path).is_absolute(),
-            "PYO3_CONFIG_FILE must be absolute"
-        );
-    }
-    let (interpreter_config, path_to_write) = match (write_config_file, custom_config_file_path) {
-        (true, Some(path)) => {
-            // Create new interpreter config and write it to config file
-            (pyo3_build_config::make_interpreter_config()?, Some(path))
-        }
-        (true, None) => bail!("PYO3_CONFIG_FILE must be set when PYO3_WRITE_CONFIG_FILE is set"),
-        (false, Some(path)) => {
-            // Read custom config file
-            let path = Path::new(&path);
-            println!("cargo:rerun-if-changed={}", path.display());
-            let config_file = std::fs::File::open(path)
-                .with_context(|| format!("failed to read config file at {}", path.display()))?;
-            let reader = std::io::BufReader::new(config_file);
-            (
-                pyo3_build_config::InterpreterConfig::from_reader(reader)?,
-                None,
-            )
-        }
-        (false, None) => (
-            // Create new interpreter config and write it to the default location
-            pyo3_build_config::make_interpreter_config()?,
-            Some(OsString::from(pyo3_build_config::DEFAULT_CONFIG_PATH)),
-        ),
-    };
-
-    if let Some(path) = path_to_write {
+    let interpreter_config = if let Some(path) = env_var("PYO3_CONFIG_FILE") {
         let path = Path::new(&path);
+        // This is necessary because the compilations that access PYO3_CONFIG_FILE (build scripts,
+        // proc macros) have many different working directories, so a relative path is no good.
+        ensure!(path.is_absolute(), "PYO3_CONFIG_FILE must be an absolute path");
+        println!("cargo:rerun-if-changed={}", path.display());
+        InterpreterConfig::from_path(path)?
+    } else if let Some(interpreter_config) = make_cross_compile_config()? {
+        // This is a cross compile, need to write the config file.
+        let path = Path::new(&pyo3_build_config::DEFAULT_CROSS_COMPILE_CONFIG_PATH);
         let parent_dir = path.parent().ok_or_else(|| {
             format!(
                 "failed to resolve parent directory of config file {}",
@@ -165,7 +140,11 @@ fn configure_pyo3() -> Result<()> {
             .to_writer(&mut std::fs::File::create(&path).with_context(|| {
                 format!("failed to create config file at {}", path.display())
             })?)?;
-    }
+        interpreter_config
+    } else {
+        InterpreterConfig::from_reader(Cursor::new(pyo3_build_config::HOST_CONFIG))?
+    };
+
     if env_var("PYO3_PRINT_CONFIG").map_or(false, |os_str| os_str == "1") {
         print_config_and_exit(&interpreter_config);
     }
@@ -201,20 +180,8 @@ fn print_config_and_exit(config: &InterpreterConfig) {
 }
 
 fn main() {
-    // Print out error messages using display, to get nicer formatting.
     if let Err(e) = configure_pyo3() {
-        use std::error::Error;
-        eprintln!("error: {}", e);
-        let mut source = e.source();
-        if source.is_some() {
-            eprintln!("caused by:");
-            let mut index = 0;
-            while let Some(some_source) = source {
-                eprintln!("  - {}: {}", index, some_source);
-                source = some_source.source();
-                index += 1;
-            }
-        }
+        eprintln!("error: {}", e.report());
         std::process::exit(1)
     }
 }

pyo3-build-config/build.rs

@@ -1,3 +1,32 @@
+// Import some modules from this crate inline to generate the build config.
+// Allow dead code because not all code in the modules is used in this build script.
+
+#[path = "src/impl_.rs"]
+#[allow(dead_code)]
+mod impl_;
+
+#[path = "src/errors.rs"]
+#[allow(dead_code)]
+mod errors;
+
+use std::{env, path::Path};
+
+use errors::{Result, Context};
+
+fn generate_build_config() -> Result<()> {
+    // Create new interpreter config and write it to the default location
+    let interpreter_config = impl_::make_interpreter_config()?;
+
+    let path = Path::new(&env::var_os("OUT_DIR").unwrap()).join("pyo3-build-config.txt");
+    interpreter_config
+        .to_writer(&mut std::fs::File::create(&path).with_context(|| {
+            format!("failed to create config file at {}", path.display())
+        })?)
+}
+
 fn main() {
-    // Empty build script to force cargo to produce the "OUT_DIR" environment variable.
+    if let Err(e) = generate_build_config() {
+        eprintln!("error: {}", e.report());
+        std::process::exit(1)
+    }
 }

pyo3-build-config/src/errors.rs

@@ -26,6 +26,16 @@ pub struct Error {
     source: Option<Box<dyn std::error::Error>>,
 }
 
+/// Error report inspired by
+/// https://blog.rust-lang.org/inside-rust/2021/07/01/What-the-error-handling-project-group-is-working-towards.html#2-error-reporter
+pub struct ErrorReport<'a>(&'a Error);
+
+impl Error {
+    pub fn report(&self) -> ErrorReport<'_> {
+        ErrorReport(self)
+    }
+}
+
 impl std::fmt::Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{}", self.value)
@@ -38,6 +48,24 @@ impl std::error::Error for Error {
     }
 }
 
+impl std::fmt::Display for ErrorReport<'_> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        use std::error::Error;
+        self.0.fmt(f)?;
+        let mut source = self.0.source();
+        if source.is_some() {
+            writeln!(f, "\ncaused by:")?;
+            let mut index = 0;
+            while let Some(some_source) = source {
+                writeln!(f, "  - {}: {}", index, some_source)?;
+                source = some_source.source();
+                index += 1;
+            }
+        }
+        Ok(())
+    }
+}
+
 impl From<String> for Error {
     fn from(value: String) -> Self {
         Self {

pyo3-build-config/src/impl_.rs

@@ -85,6 +85,20 @@ impl InterpreterConfig {
         self.implementation == PythonImplementation::PyPy
     }
 
+    #[doc(hidden)]
+    pub fn from_path(path: impl AsRef<Path>) -> Result<Self> {
+        let path = path.as_ref();
+        let config_file =
+            std::fs::File::open(path).with_context(|| {
+                format!(
+                    "failed to open PyO3 config file at {}",
+                    path.display()
+                )
+            })?;
+        let reader = std::io::BufReader::new(config_file);
+        InterpreterConfig::from_reader(reader)
+    }
+
     #[doc(hidden)]
     pub fn from_reader(reader: impl Read) -> Result<Self> {
         let reader = BufReader::new(reader);
@@ -303,6 +317,12 @@ struct CrossCompileConfig {
     arch: String,
 }
 
+pub fn any_cross_compiling_env_vars_set() -> bool {
+    env::var_os("PYO3_CROSS").is_some()
+        || env::var_os("PYO3_CROSS_LIB_DIR").is_some()
+        || env::var_os("PYO3_CROSS_PYTHON_VERSION").is_some()
+}
+
 fn cross_compiling() -> Result<Option<CrossCompileConfig>> {
     let cross = env_var("PYO3_CROSS");
     let cross_lib_dir = env_var("PYO3_CROSS_LIB_DIR");
@@ -1029,6 +1049,30 @@ fn get_abi3_minor_version() -> Option<u8> {
         .find(|i| cargo_env_var(&format!("CARGO_FEATURE_ABI3_PY3{}", i)).is_some())
 }
 
+pub fn make_cross_compile_config() -> Result<Option<InterpreterConfig>> {
+    let abi3_version = get_abi3_minor_version();
+
+    let mut interpreter_config = if let Some(paths) = cross_compiling()? {
+        load_cross_compile_info(paths)?
+    } else {
+        return Ok(None);
+    };
+
+    // Fixup minor version if abi3-pyXX feature set
+    if let Some(abi3_minor_version) = abi3_version {
+        ensure!(
+            abi3_minor_version <= interpreter_config.version.minor,
+            "You cannot set a mininimum Python version 3.{} higher than the interpreter version 3.{}",
+            abi3_minor_version,
+            interpreter_config.version.minor
+        );
+
+        interpreter_config.version.minor = abi3_minor_version;
+    }
+
+    Ok(Some(interpreter_config))
+}
+
 pub fn make_interpreter_config() -> Result<InterpreterConfig> {
     let abi3_version = get_abi3_minor_version();
 
@@ -1059,11 +1103,7 @@ pub fn make_interpreter_config() -> Result<InterpreterConfig> {
         }
     }
 
-    let mut interpreter_config = if let Some(paths) = cross_compiling()? {
-        load_cross_compile_info(paths)?
-    } else {
-        get_config_from_interpreter(&find_interpreter()?)?
-    };
+    let mut interpreter_config = get_config_from_interpreter(&find_interpreter()?)?;
 
     // Fixup minor version if abi3-pyXX feature set
     if let Some(abi3_minor_version) = abi3_version {

pyo3-build-config/src/lib.rs

@@ -7,14 +7,14 @@
 pub mod errors;
 mod impl_;
 
-use std::{ffi::OsString, path::Path};
+use std::io::Cursor;
 
 use once_cell::sync::OnceCell;
 
 // Used in PyO3's build.rs
 #[doc(hidden)]
 pub use impl_::{
-    cargo_env_var, env_var, find_interpreter, get_config_from_interpreter, make_interpreter_config,
+    cargo_env_var, env_var, find_interpreter, get_config_from_interpreter, make_interpreter_config, make_cross_compile_config,
     InterpreterConfig, PythonImplementation, PythonVersion,
 };
 
@@ -25,20 +25,24 @@ pub use impl_::{
 pub fn get() -> &'static InterpreterConfig {
     static CONFIG: OnceCell<InterpreterConfig> = OnceCell::new();
     CONFIG.get_or_init(|| {
-        let config_path = std::env::var_os("PYO3_CONFIG_FILE")
-            .unwrap_or_else(|| OsString::from(DEFAULT_CONFIG_PATH));
-        let config_file = std::fs::File::open(DEFAULT_CONFIG_PATH).expect(&format!(
-            "failed to open PyO3 config file at {}",
-            Path::new(&config_path).display()
-        ));
-        let reader = std::io::BufReader::new(config_file);
-        InterpreterConfig::from_reader(reader).expect("failed to parse config file")
+        if let Some(path) = std::env::var_os("PYO3_CONFIG_FILE") {
+            // Config file set - use that
+            InterpreterConfig::from_path(path)
+        } else if impl_::any_cross_compiling_env_vars_set() {
+            InterpreterConfig::from_path(DEFAULT_CROSS_COMPILE_CONFIG_PATH)
+        } else {
+            InterpreterConfig::from_reader(Cursor::new(HOST_CONFIG))
+        }.expect("failed to parse PyO3 config file")
     })
 }
 
 /// Path where PyO3's build.rs will write configuration by default.
 #[doc(hidden)]
-pub const DEFAULT_CONFIG_PATH: &str = concat!(env!("OUT_DIR"), "/pyo3-build-config.txt");
+pub const DEFAULT_CROSS_COMPILE_CONFIG_PATH: &str = concat!(env!("OUT_DIR"), "/pyo3-cross-compile-config.txt");
+
+/// Build configuration discovered by `pyo3-build-config` build script. Not aware of
+/// cross-compilation settings.
+pub const HOST_CONFIG: &str = include_str!(concat!(env!("OUT_DIR"), "/pyo3-build-config.txt"));
 
 /// Adds all the [`#[cfg]` flags](index.html) to the current compilation.
 ///