build: not cross-compiling when musl from gnu

CHANGELOG.md

@@ -6,8 +6,6 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-### Fixed
-- 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)
 ### Packaging
 - Drop support for Python 3.5 (as it is now end-of-life). [#1250](https://github.com/PyO3/pyo3/pull/1250)
 
@@ -27,6 +25,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Remove deprecated ffi definitions `PyUnicode_AsUnicodeCopy`, `PyUnicode_GetMax`, `_Py_CheckRecursionLimit`, `PyObject_AsCharBuffer`, `PyObject_AsReadBuffer`, `PyObject_CheckReadBuffer` and `PyObject_AsWriteBuffer`, which will be removed in Python 3.10. [#1217](https://github.com/PyO3/pyo3/pull/1217)
 - Remove unused `python3` feature. [#1235](https://github.com/PyO3/pyo3/pull/1235)
 
+### Fixed
+- 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)
+
 ## [0.12.3] - 2020-10-12
 ### Fixed
 - Fix support for Rust versions 1.39 to 1.44, broken by an incorrect internal update to paste 1.0 which was done in PyO3 0.12.2. [#1234](https://github.com/PyO3/pyo3/pull/1234)

build.rs

@@ -150,15 +150,34 @@ impl CrossCompileConfig {
 fn cross_compiling() -> Result<Option<CrossCompileConfig>> {
     let target = env::var("TARGET")?;
     let host = env::var("HOST")?;
-    if target == host || (target == "i686-pc-windows-msvc" && host == "x86_64-pc-windows-msvc") {
+    if target == host {
+        // Not cross-compiling
+        return Ok(None);
+    }
+
+    if target == "i686-pc-windows-msvc" && host == "x86_64-pc-windows-msvc" {
+        // Not cross-compiling to compile for 32-bit Python from windows 64-bit
+        return Ok(None);
+    }
+
+    if host.starts_with(&format!(
+        "{}-{}-{}",
+        env::var("CARGO_CFG_TARGET_ARCH")?,
+        env::var("CARGO_CFG_TARGET_VENDOR")?,
+        env::var("CARGO_CFG_TARGET_OS")?
+    )) {
+        // Not cross-compiling if arch-vendor-os is all the same
+        // e.g. x86_64-unknown-linux-musl on x86_64-unknown-linux-gnu host
         return Ok(None);
     }
 
     if env::var("CARGO_CFG_TARGET_FAMILY")? == "windows" {
-        Ok(Some(CrossCompileConfig::both()?))
+        // Windows cross-compile uses both header includes and sysconfig
-    } else {
+        return Ok(Some(CrossCompileConfig::both()?));
-        Ok(Some(CrossCompileConfig::lib_only()?))
     }
+
+    // Cross-compiling on any other platform
+    Ok(Some(CrossCompileConfig::lib_only()?))
 }
 
 /// A list of python interpreter compile-time preprocessor defines that