Add support for iOS cross-platform virtual environments

Changelog.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+* Allow iOS cross-platform virtual environments, such as those used by cibuildwheel, to imply an iOS target.
 * Fix iOS wheel naming to be compliant with PEP 730.
 * Fix generated WHEEL Tag metadata to be spec compliant.
 * Remove `add_directory()` from ModuleWriter and make it an implementation detail for the specific impl

src/build_options.rs

@@ -6,7 +6,9 @@ use crate::cross_compile::{find_sysconfigdata, parse_sysconfigdata};
 use crate::project_layout::ProjectResolver;
 use crate::pyproject_toml::ToolMaturin;
 use crate::python_interpreter::{InterpreterConfig, InterpreterKind};
-use crate::target::{detect_arch_from_python, is_arch_supported_by_pypi};
+use crate::target::{
+    detect_arch_from_python, detect_target_from_cross_python, is_arch_supported_by_pypi,
+};
 use crate::{BridgeModel, BuildContext, PyO3, PythonInterpreter, Target};
 use anyhow::{bail, format_err, Context, Result};
 use cargo_metadata::{CrateType, PackageId, TargetKind};
@@ -665,7 +667,22 @@ impl BuildContextBuilder {
         let mut target = Target::from_target_triple(target_triple.as_ref())?;
         if !target.user_specified && !universal2 {
             if let Some(interpreter) = build_options.interpreter.first() {
-                if let Some(detected_target) = detect_arch_from_python(interpreter, &target) {
+                // If there's an explicitly provided interpreter, check to see
+                // if it's a cross-compiling interpreter; otherwise, check to
+                // see if an target change is required.
+                if let Some(detected_target) = detect_target_from_cross_python(interpreter) {
+                    target = Target::from_target_triple(Some(&detected_target))?;
+                } else if let Some(detected_target) = detect_arch_from_python(interpreter, &target)
+                {
+                    target = Target::from_target_triple(Some(&detected_target))?;
+                }
+            } else {
+                // If there's no explicit user-provided target or interpreter,
+                // check the interpreter; if the interpreter identifies as a
+                // cross compiler, set the target based on the platform reported
+                // by the interpreter.
+                if let Some(detected_target) = detect_target_from_cross_python(&target.get_python())
+                {
                     target = Target::from_target_triple(Some(&detected_target))?;
                 }
             }

src/cross_compile.rs

@@ -1,3 +1,4 @@
+use crate::target::Os;
 use crate::{PythonInterpreter, Target};
 use anyhow::{bail, Result};
 use fs_err::{self as fs, DirEntry};
@@ -32,6 +33,13 @@ pub fn is_cross_compiling(target: &Target) -> Result<bool> {
         return Ok(false);
     }
 
+    if target.target_os() == Os::Ios {
+        // Not cross-compiling to compile for iOS. There's no on-device compilation,
+        // so compilation will always be in a "fake" cross-platform venv with a
+        // working python/sysconfig that can interrogated.
+        return Ok(false);
+    }
+
     Ok(true)
 }
 

src/target/mod.rs

@@ -764,3 +764,24 @@ pub(crate) fn detect_arch_from_python(python: &PathBuf, target: &Target) -> Opti
     }
     None
 }
+
+pub(crate) fn detect_target_from_cross_python(python: &PathBuf) -> Option<TargetTriple> {
+    match Command::new(python)
+        .arg("-c")
+        .arg("import sys, sysconfig; print(sysconfig.get_platform(), end='') if getattr(sys, 'cross_compiling', False) else ''")
+        .output()
+    {
+        Ok(output) if output.status.success() => {
+            let platform = String::from_utf8_lossy(&output.stdout);
+            if platform.ends_with("-arm64-iphoneos") {
+                return Some(TargetTriple::Regular("aarch64-apple-ios".to_string()));
+            } else if platform.ends_with("-arm64-iphonesimulator") {
+                return Some(TargetTriple::Regular("aarch64-apple-ios-sim".to_string()));
+            } else if platform.ends_with("-x86_64-iphonesimulator") {
+                return Some(TargetTriple::Regular("x86_64-apple-ios".to_string()));
+            }
+        }
+        _ => eprintln!("⚠️  Warning: Failed to determine python platform"),
+    }
+    None
+}