refactor: add rule to do analysis time evaluation of environment markers

python/private/pypi/dependency_specifier_flag.bzl

@@ -0,0 +1,97 @@
+load("//python/private:toolchain_types.bzl", "TARGET_TOOLCHAIN_TYPE")
+load(":pep508_evaluate.bzl", "evaluate")
+
+def _impl(ctx):
+    env = {}
+
+    runtime = ctx.toolchains[TARGET_TOOLCHAIN_TYPE].py3_runtime
+    if runtime.interpreter_version_info:
+        version_info = runtime.interpreter_version_info
+        env["python_version"] = "{v.major}.{v.minor}".format(v = version_info)
+        full_version = format_full_version(version_info)
+        env["python_full_version"] = full_version
+        env["implementation_version"] = full_version
+    else:
+        env["python_version"] = get_flag(ctx.attr._python_version)
+        full_version = get_flag(ctx.attr._python_full_version)
+        env["python_full_version"] = full_version
+        env["implementation_version"] = full_version
+
+    # We assume cpython if the toolchain doesn't specify because it's most
+    # likely to be true.
+    env["implementation_name"] = runtime.implementation_name or "cpython"
+
+    # todo: map from os constraint
+    env["os_name"] = struct()
+
+    # todo: map from os constraint
+    env["sys_platform"] = struct()
+
+    # todo: map from cpu flag (x86_64, etc)
+    env["platform_machine"] = struct()
+
+    # todo: add PyRuntimeInfo.platform_python_implementation
+    # The values are slightly different to implementation_name
+    env["platform_python_implementation"] = runtime.implementation_name
+
+    # todo: add flag to carry this
+    env["platform_release"] = struct()
+
+    # todo: map from os constraint
+    env["platform_system"] = struct()
+
+    # todo: add flag to carry this
+    env["platform_version"] = struct()
+
+    if evalute(ctx.attr.expression, env):
+        value = "yes"
+    else:
+        value = "no"
+    return [config_common.FeatureFlagInfo(value = value)]
+
+# Adapted from spec code at:
+# https://packaging.python.org/en/latest/specifications/dependency-specifiers/#environment-markers
+def format_full_info(info):
+    kind = info.releaselevel
+    if kind == "final":
+        kind = ""
+        serial = ""
+    else:
+        kind = kind[0] if kind else ""
+        serial = str(info.serial) if info.serial else ""
+
+    return "{v.major}.{v.minor}.{v.micro}{kind}{serial}".format(
+        v = version_info,
+        kind = kind,
+        serial = serial,
+    )
+    return version
+
+pypa_dependency_specification = rule(
+    implementation = _impl,
+    attrs = {
+        "expression": attt.string(),
+        "_os_name": attr.label(),
+        "_sys_platform_flag": attr.label(),
+        "_platform_release_flag": attr.label(),
+        "_platform_system_flag": attr.label(),
+        "_platform_version_flag": attr.label(),
+        "_python_version_flag": attr.label(
+            default = "//python/config_settings:_python_version_major_minor",
+        ),
+        "_python_full_version_flag": attr.label(
+            default = "//python/config_settings:python_version",
+        ),
+        "_extra_flag": attr.label(),
+    },
+    toolchains = [
+        TARGET_TOOLCHAIN_TYPE,
+    ],
+)
+
+def _get_flag(t):
+    if config_common.FeatureFlagInfo in t:
+        return t[config_common.FeatureFlagInfo].value
+    if BuildSettingInfo in t:
+        return t[BuildSettingInfo].value
+    fail("Should not occur: {} does not have necessary providers")