feat: multi-toolchain support

python/config_settings/transition.bzl

@@ -15,15 +15,20 @@ _transition_python_version = transition(
 )
 
 def _transition_py_impl(ctx):
-    target = ctx.attr.target[0]
-    executable = ctx.actions.declare_file(ctx.attr.name + (".exe" if ctx.attr.is_windows else ""))
+    target = ctx.attr.target
+    windows_constraint = ctx.attr._windows_constraint[platform_common.ConstraintValueInfo]
+    target_is_windows = ctx.target_platform_has_constraint(windows_constraint)
+    executable = ctx.actions.declare_file(ctx.attr.name + (".exe" if target_is_windows else ""))
     ctx.actions.symlink(
         is_executable = True,
         output = executable,
         target_file = target[DefaultInfo].files_to_run.executable,
     )
     zipfile_symlink = None
-    if ctx.attr.is_windows:
+    if target_is_windows:
+        # Under Windows, the expected "<name>.zip" does not exist, so we have to
+        # create the symlink ourselves to achieve the same behaviour as in macOS
+        # and Linux.
         zipfile = None
         expected_target_path = target[DefaultInfo].files_to_run.executable.short_path[:-4] + ".zip"
         for file in target[DefaultInfo].default_runfiles.files.to_list():
@@ -38,6 +43,7 @@ def _transition_py_impl(ctx):
     env = {}
     for k, v in ctx.attr.env.items():
         env[k] = ctx.expand_location(v)
+
     providers = [
         DefaultInfo(
             executable = executable,
@@ -46,12 +52,16 @@ def _transition_py_impl(ctx):
         ),
         target[PyInfo],
         target[PyRuntimeInfo],
-        target[InstrumentedFilesInfo],
+        # Ensure that the binary we're wrapping is included in code coverage.
+        coverage_common.instrumented_files_info(
+            ctx,
+            dependency_attributes = ["target"],
+        ),
         target[OutputGroupInfo],
-        # TODO(f0rmiga): testing.TestEnvironment is deprecated in favour of RunEnvironmentInfo but
+        # testing.TestEnvironment is deprecated in favour of RunEnvironmentInfo but
         # RunEnvironmentInfo is not exposed in Bazel < 5.3.
         # https://github.com/bazelbuild/bazel/commit/dbdfa07e92f99497be9c14265611ad2920161483
-        testing.TestEnvironment(environment = env),
+        (RunEnvironmentInfo if hasattr(native, "RunEnvironmentInfo") else testing.TestEnvironment)(environment = env),
     ]
     return providers
 
@@ -62,9 +72,6 @@ _COMMON_ATTRS = {
     "env": attr.string_dict(
         mandatory = False,
     ),
-    "is_windows": attr.bool(
-        mandatory = True,
-    ),
     "python_version": attr.string(
         mandatory = True,
     ),
@@ -74,7 +81,7 @@ _COMMON_ATTRS = {
     ),
     "target": attr.label(
         executable = True,
-        cfg = _transition_python_version,
+        cfg = "exec",
         mandatory = True,
         providers = [PyInfo],
     ),
@@ -96,17 +103,22 @@ _COMMON_ATTRS = {
     "_allowlist_function_transition": attr.label(
         default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
     ),
+    "_windows_constraint": attr.label(
+        default = "@platforms//os:windows",
+    ),
 }
 
 _transition_py_binary = rule(
     _transition_py_impl,
     attrs = _COMMON_ATTRS,
+    cfg = _transition_python_version,
     executable = True,
 )
 
 _transition_py_test = rule(
     _transition_py_impl,
     attrs = _COMMON_ATTRS,
+    cfg = _transition_python_version,
     test = True,
 )
 
@@ -180,10 +192,6 @@ def _py_rule(rule_impl, transition_rule, name, python_version, **kwargs):
         args = args,
         deps = deps,
         env = env,
-        is_windows = select({
-            "@platforms//os:windows": True,
-            "//conditions:default": False,
-        }),
         python_version = python_version,
         srcs = srcs,
         tags = tags,