Respect _PYTHON_HOST_PLATFORM env variable

docs/guide/crosscompile.md

@@ -64,3 +64,6 @@ 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.
+
+pyodide-build will also set `_PYTHON_HOST_PLATFORM` to the target Pyodide
+platform, so scikit-build-core can use that to compute the correct wheel name.

src/scikit_build_core/builder/wheel_tag.py

@@ -2,6 +2,7 @@
 
 import dataclasses
 import itertools
+import os
 import sys
 import sysconfig
 from typing import TYPE_CHECKING
@@ -60,7 +61,12 @@ def compute_best(
         interp, abi, *plats = (best_tag.interpreter, best_tag.abi, best_tag.platform)
         pyvers = [interp]
 
-        if sys.platform.startswith("win") and archs:
+        # Check for _PYTHON_HOST_PLATFORM environment variable to override platform
+        host_platform = os.environ.get("_PYTHON_HOST_PLATFORM")
+        if host_platform:
+            # Convert sysconfig platform format to wheel platform tag format
+            plats = [host_platform.replace("-", "_").replace(".", "_")]
+        elif sys.platform.startswith("win") and archs:
             plats = [x.replace("-", "_") for x in archs]
         elif sys.platform.startswith("darwin"):
             pairs: Iterable[tuple[str | None, bool]]

tests/test_builder.py

@@ -264,3 +264,22 @@ def test_wheel_tag_with_abi_darwin(monkeypatch):
 
     tags = WheelTag.compute_best(["x86_64"], py_api="py2.py3")
     assert str(tags) == "py2.py3-none-macosx_10_10_x86_64"
+
+
+def test_wheel_tag_host_platform_override(monkeypatch):
+    """Test that _PYTHON_HOST_PLATFORM environment variable overrides platform detection."""
+    get_config_var = sysconfig.get_config_var
+    monkeypatch.setattr(
+        sysconfig,
+        "get_config_var",
+        lambda x: None if x == "Py_GIL_DISABLED" else get_config_var(x),
+    )
+    monkeypatch.setattr(sys, "platform", "linux")
+
+    monkeypatch.setenv("_PYTHON_HOST_PLATFORM", "macosx-11.0-arm64")
+    tags = WheelTag.compute_best(["x86_64"], py_api="py3")
+    assert str(tags) == "py3-none-macosx_11_0_arm64"
+
+    monkeypatch.setenv("_PYTHON_HOST_PLATFORM", "emscripten-4.0.9-wasm32")
+    tags = WheelTag.compute_best(["x86_64"], py_api="py3")
+    assert str(tags) == "py3-none-emscripten_4_0_9_wasm32"