feat: Better support for cross-compilation

README.md

@@ -181,6 +181,14 @@ cmake.source-dir = "."
 # DEPRECATED in 0.10; use build.targets instead.
 cmake.targets = ""
 
+# The CMAKE_TOOLCHAIN_FILE used for cross-compilation.
+cmake.toolchain-file = ""
+
+# Do not pass the current environment's python hints such as
+# ``Python_EXECUTABLE``. Primarily used for cross-compilation where the
+# CMAKE_TOOLCHAIN_FILE should handle it instead.
+cmake.no-python-hints = false
+
 # The versions of Ninja to allow. If Ninja is not present on the system or does
 # not pass this specifier, it will be downloaded via PyPI if possible. An empty
 # string will disable this check.
@@ -257,6 +265,12 @@ wheel.exclude = []
 # The build tag to use for the wheel. If empty, no build tag is used.
 wheel.build-tag = ""
 
+# Manually specify the wheel tags to use, ignoring other inputs such as
+# ``wheel.py-api``. Each tag must be of the format
+# {interpreter}-{abi}-{platform}. If not specified, these tags are automatically
+# calculated.
+wheel.tags = []
+
 # If CMake is less than this value, backport a copy of FindPython. Set to 0
 # disable this, or the empty string.
 backport.find-python = "3.26.1"

docs/guide/crosscompile.md

@@ -1,5 +1,9 @@
 # Cross-compiling
 
+Generally scikit-build-core will try to account for environment variables that
+specify to CMake directly how to cross-compile. Alternatively, you can define
+manually how to cross-compile as detailed in [manual cross compilation] section.
+
 ## macOS
 
 Unlike the other platforms, macOS has the ability to target older operating
@@ -50,10 +54,7 @@ correct suffix. These values are set by cibuildwheel when cross-compiling.
 
 ## Linux
 
-It should be possible to cross-compile to Linux, but due to the challenges of
-getting the manylinux RHEL devtoolkit compilers, this is currently a TODO. See
-`py-build-cmake <https://tttapa.github.io/py-build-cmake/Cross-compilation.html>`\_
-for an alternative package's usage of toolchain files.
+See [manual cross compilation] section for the general approach.
 
 ### Intel to Emscripten (Pyodide)
 
@@ -64,3 +65,71 @@ by setting `_PYTHON_SYSCONFIGDATA_NAME`. This causes values like `SOABI` and
 This is unfortunately incorrectly stripped from the cmake wrapper pyodide uses,
 so FindPython will report the wrong values, but pyodide-build will rename the
 .so's afterwards.
+
+## Manual cross compilation
+
+The manual cross compilation assumes you have [toolchain file] prepared defining
+the cross-compilers and where to search for the target development files,
+including the python library. A simple setup of this is to use the clang
+compiler and point `CMAKE_SYSROOT` to a mounted copy of the target system's root
+
+```cmake
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_SYSTEM_PROCESSOR aarch64)
+
+set(triple aarch64-linux-gnu)
+
+set(CMAKE_C_COMPILER clang)
+set(CMAKE_CXX_COMPILER clang++)
+set(CMAKE_C_COMPILER_TARGET ${triple})
+set(CMAKE_CXX_COMPILER_TARGET ${triple})
+
+set(CMAKE_SYSROOT "/path/to/aarch64/mount/")
+```
+
+For more complex environments such as embedded devices, Android or iOS see
+CMake's guide on how to write the [toolchain file].
+
+You can pass the toolchain file using the environment variable
+`CMAKE_TOOLCHAIN_FILE`, or the `cmake.toolchain-file` pyproject option. You may
+also need to use `wheel.tags` to manually specify the wheel tags to use for the
+file and `cmake.no-python-hints` if the target python should be detected using
+the toolchain file instead.
+
+:::{note}
+
+Because most of the logic in [`FindPython`] is gated by the
+`CMAKE_CROSSCOMPILING`, you generally should _not_ include the `Interpreter`
+component in the `find_package` command or use the `Python_ARTIFACTS_PREFIX`
+feature to distinguish the system and target components.
+
+:::
+
+:::{versionadded} 0.11
+
+:::
+
+### Crossenv
+
+[Crossenv] cross compilation is supported in scikit-build-core. This tool
+creates a fake virtual environment where configuration hints such as
+`EXT_SUFFIX` are overwritten with the target's values. This should work without
+specifying `wheel.tags` overwrites manually.
+
+:::{note}
+
+Because the target Python executable is being faked, the usage of
+`CMAKE_CROSSCOMPILING_EMULATOR` for the `Interpreter` would not be correct in
+this case.
+
+:::
+
+:::{versionadded} 0.11
+
+:::
+
+[manual cross compilation]: #manual-cross-compilation
+[toolchain file]:
+  https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling
+[crossenv]: https://crossenv.readthedocs.io/en/latest/
+[`FindPython`]: https://cmake.org/cmake/help/git-master/module/FindPython.html

src/scikit_build_core/build/wheel.py

@@ -10,6 +10,7 @@
 from typing import TYPE_CHECKING, Any, Literal
 
 from packaging.requirements import Requirement
+from packaging.tags import Tag
 from packaging.utils import canonicalize_name
 
 from .._compat import tomllib
@@ -260,6 +261,10 @@
         f"{{red}}({state})",
     )
 
+    override_wheel_tags = None
+    if settings.wheel.tags:
+        override_wheel_tags = {Tag(*tag.split("-")) for tag in settings.wheel.tags}
+
     with tempfile.TemporaryDirectory() as tmpdir:
         build_tmp_folder = Path(tmpdir)
         wheel_dir = build_tmp_folder / "wheel"
@@ -364,7 +369,7 @@
             wheel = WheelWriter(
                 metadata,
                 Path(metadata_directory),
-                tags.as_tags_set(),
+                override_wheel_tags or tags.as_tags_set(),
                 WheelMetadata(
                     root_is_purelib=targetlib == "purelib",
                     build_tag=settings.wheel.build_tag,
@@ -480,7 +485,7 @@
         with WheelWriter(
             metadata,
             Path(wheel_directory),
-            tags.as_tags_set(),
+            override_wheel_tags or tags.as_tags_set(),
             WheelMetadata(
                 root_is_purelib=targetlib == "purelib",
                 build_tag=settings.wheel.build_tag,

src/scikit_build_core/builder/builder.py

@@ -231,25 +231,29 @@
         python_include_dir = get_python_include_dir()
         numpy_include_dir = get_numpy_include_dir()
 
-        # Classic Find Python
-        cache_config["PYTHON_EXECUTABLE"] = sys.executable
-        cache_config["PYTHON_INCLUDE_DIR"] = python_include_dir
-        if python_library:
-            cache_config["PYTHON_LIBRARY"] = python_library
-
-        # Modern Find Python
-        for prefix in ("Python", "Python3"):
-            cache_config[f"{prefix}_EXECUTABLE"] = sys.executable
-            cache_config[f"{prefix}_ROOT_DIR"] = sys.prefix
-            cache_config[f"{prefix}_INCLUDE_DIR"] = python_include_dir
-            cache_config[f"{prefix}_FIND_REGISTRY"] = "NEVER"
-            # FindPython may break if this is set - only useful on Windows
-            if python_library and sysconfig.get_platform().startswith("win"):
-                cache_config[f"{prefix}_LIBRARY"] = python_library
-            if python_sabi_library and sysconfig.get_platform().startswith("win"):
-                cache_config[f"{prefix}_SABI_LIBRARY"] = python_sabi_library
-            if numpy_include_dir:
-                cache_config[f"{prefix}_NumPy_INCLUDE_DIR"] = numpy_include_dir
+        if self.settings.cmake.toolchain_file:
+            cache_config["CMAKE_TOOLCHAIN_FILE"] = self.settings.cmake.toolchain_file
+
+        if not self.settings.cmake.no_python_hints:
+            # Classic Find Python
+            cache_config["PYTHON_EXECUTABLE"] = sys.executable
+            cache_config["PYTHON_INCLUDE_DIR"] = python_include_dir
+            if python_library:
+                cache_config["PYTHON_LIBRARY"] = python_library
+
+            # Modern Find Python
+            for prefix in ("Python", "Python3"):
+                cache_config[f"{prefix}_EXECUTABLE"] = sys.executable
+                cache_config[f"{prefix}_ROOT_DIR"] = sys.base_exec_prefix
+                cache_config[f"{prefix}_INCLUDE_DIR"] = python_include_dir
+                cache_config[f"{prefix}_FIND_REGISTRY"] = "NEVER"
+                # FindPython may break if this is set - only useful on Windows
+                if python_library and sysconfig.get_platform().startswith("win"):
+                    cache_config[f"{prefix}_LIBRARY"] = python_library
+                if python_sabi_library and sysconfig.get_platform().startswith("win"):
+                    cache_config[f"{prefix}_SABI_LIBRARY"] = python_sabi_library
+                if numpy_include_dir:
+                    cache_config[f"{prefix}_NumPy_INCLUDE_DIR"] = numpy_include_dir
 
         cache_config["SKBUILD_SOABI"] = get_soabi(self.config.env, abi3=limited_api)
 

src/scikit_build_core/resources/scikit-build.schema.json

@@ -102,6 +102,15 @@
           },
           "description": "DEPRECATED in 0.10; use build.targets instead.",
           "deprecated": true
+        },
+        "toolchain-file": {
+          "type": "string",
+          "description": "The CMAKE_TOOLCHAIN_FILE used for cross-compilation."
+        },
+        "no-python-hints": {
+          "type": "boolean",
+          "default": false,
+          "description": "Do not pass the current environment's python hints such as ``Python_EXECUTABLE``. Primarily used for cross-compilation where the CMAKE_TOOLCHAIN_FILE should handle it instead."
         }
       }
     },
@@ -239,6 +248,13 @@
           "type": "string",
           "default": "",
           "description": "The build tag to use for the wheel. If empty, no build tag is used."
+        },
+        "tags": {
+          "type": "array",
+          "items": {
+            "type": "string"
+          },
+          "description": "Manually specify the wheel tags to use, ignoring other inputs such as ``wheel.py-api``. Each tag must be of the format {interpreter}-{abi}-{platform}. If not specified, these tags are automatically calculated."
         }
       }
     },
@@ -566,6 +582,9 @@
                   },
                   "exclude": {
                     "$ref": "#/$defs/inherit"
+                  },
+                  "tags": {
+                    "$ref": "#/$defs/inherit"
                   }
                 }
               },

src/scikit_build_core/settings/skbuild_model.py

@@ -103,6 +103,18 @@ class CMakeSettings:
     DEPRECATED in 0.10; use build.targets instead.
     """
 
+    toolchain_file: Optional[Path] = None
+    """
+    The CMAKE_TOOLCHAIN_FILE used for cross-compilation.
+    """
+
+    no_python_hints: bool = False
+    """
+    Do not pass the current environment's python hints such as ``Python_EXECUTABLE``.
+    Primarily used for cross-compilation where the CMAKE_TOOLCHAIN_FILE should handle it
+    instead.
+    """
+
 
 @dataclasses.dataclass
 class SearchSettings:
@@ -239,6 +251,13 @@ class WheelSettings:
     The build tag to use for the wheel. If empty, no build tag is used.
     """
 
+    tags: List[str] = dataclasses.field(default_factory=list)
+    """
+    Manually specify the wheel tags to use, ignoring other inputs such as
+    ``wheel.py-api``. Each tag must be of the format {interpreter}-{abi}-{platform}.
+    If not specified, these tags are automatically calculated.
+    """
+
 
 @dataclasses.dataclass
 class BackportSettings: