fix(toolchain): do not force users to depend on optional toolchains

With this change we set the `target_settings` to something that is not
going to be ever matched if the `precompile` flag is not enabled. Users
wanting to use the `precompile` feature will need to set the
config_setting value to `enabled` instead of `auto`.

Fixes #1967

Author: aignas

CHANGELOG.md

@@ -28,7 +28,9 @@ A brief description of the categories of changes:
 * `protobuf`/`com_google_protobuf` dependency bumped to `v24.4`
 
 ### Fixed
-* Nothing yet
+* The targets would depend on the `exec_tools_toolchain_type` toolchain if it
+  was registered even though the pre-compilation was not enabled. This has been
+  fixed to not be the case anymore. Fixes [#1967](https://github.com/bazelbuild/rules_python/issues/1967).
 
 ### Removed
 * Nothing yet

python/config_settings/BUILD.bazel

@@ -1,3 +1,4 @@
+load("@bazel_skylib//lib:selects.bzl", "selects")
 load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
 load(
     "//python/private:flags.bzl",
@@ -128,3 +129,45 @@ string_flag(
     )
     for flag in INTERNAL_FLAGS
 ]
+
+config_setting(
+    name = "_precompile_enabled",
+    flag_values = {
+        ":precompile": PrecompileFlag.ENABLED,
+    },
+    # Only public because it is used in toolchain definition
+    visibility = ["//visibility:public"],
+)
+
+config_setting(
+    name = "_precompile_enabled_generated_source",
+    flag_values = {
+        ":precompile": PrecompileFlag.IF_GENERATED_SOURCE,
+    },
+    # Only public because it is used in toolchain definition
+    visibility = ["//visibility:public"],
+)
+
+config_setting(
+    name = "_precompile_enabled_force",
+    flag_values = {
+        ":precompile": PrecompileFlag.FORCE_ENABLED,
+    },
+    # Only public because it is used in toolchain definition
+    visibility = ["//visibility:public"],
+)
+
+# FIXME @aignas 2024-06-14: this currently does not handle the `auto` value of
+# the flag correctly. Right now the behaviour is to disable the pre-compilation
+# via the _precompile_flag_get_effective_value function default to disabled if
+# it the flag is "AUTO", however we may need to revisit this.
+selects.config_setting_group(
+    name = "is_precompile_enabled",
+    match_any = [
+        ":_precompile_enabled",
+        ":_precompile_enabled_force",
+        ":_precompile_enabled_generated_source",
+    ],
+    # Only public because it is used in toolchain definition
+    visibility = ["//visibility:public"],
+)

python/private/BUILD.bazel

@@ -59,6 +59,7 @@ bzl_library(
     name = "autodetecting_toolchain_bzl",
     srcs = ["autodetecting_toolchain.bzl"],
     deps = [
+        ":toolchain_types_bzl",
         "//python:py_runtime_bzl",
         "//python:py_runtime_pair_bzl",
     ],

python/private/autodetecting_toolchain.bzl

@@ -16,6 +16,7 @@
 
 load("//python:py_runtime.bzl", "py_runtime")
 load("//python:py_runtime_pair.bzl", "py_runtime_pair")
+load("//python/private:toolchain_types.bzl", "TARGET_TOOLCHAIN_TYPE")
 
 def define_autodetecting_toolchain(name):
     """Defines the autodetecting Python toolchain.
@@ -65,6 +66,6 @@ def define_autodetecting_toolchain(name):
     native.toolchain(
         name = name,
         toolchain = ":_autodetecting_py_runtime_pair",
-        toolchain_type = ":toolchain_type",
+        toolchain_type = TARGET_TOOLCHAIN_TYPE,
         visibility = ["//visibility:public"],
     )

python/private/flags.bzl

@@ -34,7 +34,7 @@ BootstrapImplFlag = enum(
 def _precompile_flag_get_effective_value(ctx):
     value = ctx.attr._precompile_flag[BuildSettingInfo].value
     if value == PrecompileFlag.AUTO:
-        value = PrecompileFlag.DISABLED
+        value = PrecompileFlag.IF_GENERATED_SOURCE
     return value
 
 # Determines if Python source files should be compiled at build time.

python/private/py_toolchain_suite.bzl

@@ -107,7 +107,10 @@ def py_toolchain_suite(*, prefix, user_repository_name, python_version, set_pyth
         ),
         toolchain_type = EXEC_TOOLS_TOOLCHAIN_TYPE,
         # The target settings capture the Python version
-        target_settings = target_settings,
+        target_settings = select({
+            Label("//python/config_settings:is_precompile_enabled"): target_settings,
+            "//conditions:default": [Label("//python/config_settings:is_precompile_enabled")],
+        }),
         exec_compatible_with = kwargs.get("target_compatible_with"),
     )