chore(semantics): Remove determine_main

Determining the main using suffix matching was always a goofy idea. The
underlying rules have long required that main be a label. Rip out
determine_main and provide minimal default semantics for main as part of
the existing wrapper macros.

Those too should go away, but for now we have them.

Replaces #641 with thanks to Keith.

Author: arrdem

py/BUILD.bazel

@@ -28,7 +28,6 @@ bzl_library(
     visibility = ["//visibility:public"],
     deps = [
         "//py/private:py_binary",
-        "//py/private:py_executable",
         "//py/private:py_image_layer",
         "//py/private:py_library",
         "//py/private:py_pex_binary",

py/defs.bzl

@@ -37,7 +37,6 @@ python.toolchain(python_version = "3.9", is_default = True)
 
 load("@aspect_bazel_lib//lib:utils.bzl", "propagate_common_rule_attributes")
 load("//py/private:py_binary.bzl", _py_binary = "py_binary", _py_test = "py_test")
-load("//py/private:py_executable.bzl", "determine_main")
 load("//py/private:py_image_layer.bzl", _py_image_layer = "py_image_layer")
 load("//py/private:py_library.bzl", _py_library = "py_library")
 load("//py/private:py_pex_binary.bzl", _py_pex_binary = "py_pex_binary")
@@ -66,23 +65,12 @@ def _py_binary_or_test(name, rule, srcs, main, data = [], deps = [], resolutions
     exec_properties = kwargs.pop("exec_properties", {})
     non_test_exec_properties = {k: v for k, v in exec_properties.items() if not k.startswith("test.")}
 
-    # Compatibility with rules_python, see docs in py_executable.bzl
-    main_target = "{}.find_main".format(name)
-    determine_main(
-        name = main_target,
-        target_name = name,
-        main = main,
-        srcs = srcs,
-        exec_properties = non_test_exec_properties,
-        **propagate_common_rule_attributes(kwargs)
-    )
-
     package_collisions = kwargs.pop("package_collisions", None)
 
     rule(
         name = name,
         srcs = srcs,
-        main = main_target,
+        main = main,
         data = data,
         deps = deps,
         resolutions = resolutions,
@@ -113,10 +101,8 @@ def py_binary(name, srcs = [], main = None, **kwargs):
     Args:
         name: Name of the rule.
         srcs: Python source files.
-        main: Entry point.
-            Like rules_python, this is treated as a suffix of a file that should appear among the srcs.
-            If absent, then `[name].py` is tried. As a final fallback, if the srcs has a single file,
-            that is used as the main.
+        main: The label of an entrypoint file which will be used as the binary's entrypoint.
+            If srcs is length 1, main is implied to be the only src.
         **kwargs: additional named parameters to `py_binary_rule`.
     """
 
@@ -126,6 +112,11 @@ def py_binary(name, srcs = [], main = None, **kwargs):
     if resolutions:
         resolutions = resolutions.to_label_keyed_dict()
 
+    if not main and len(srcs) == 1:
+        main = srcs[0]
+    elif main and main not in srcs:
+        srcs = srcs + [main]
+
     _py_binary_or_test(
         name = name,
         rule = _py_binary,
@@ -141,10 +132,8 @@ def py_test(name, srcs = [], main = None, pytest_main = False, **kwargs):
     Args:
         name: Name of the rule.
         srcs: Python source files.
-        main: Entry point.
-            Like rules_python, this is treated as a suffix of a file that should appear among the srcs.
-            If absent, then `[name].py` is tried. As a final fallback, if the srcs has a single file,
-            that is used as the main.
+        main: The label of an entrypoint file which will be used as the test runner.
+            Must exist as a listed src.
         pytest_main: If set, generate a [py_pytest_main](#py_pytest_main) script and use it as the main.
             The deps should include the pytest package (as well as the coverage package if desired).
         **kwargs: additional named parameters to `py_binary_rule`.
@@ -160,7 +149,7 @@ def py_test(name, srcs = [], main = None, pytest_main = False, **kwargs):
         resolutions = resolutions.to_label_keyed_dict()
 
     deps = kwargs.pop("deps", [])
-    if pytest_main:
+    if pytest_main or not main:
         if main:
             fail("When pytest_main is set, the main attribute should not be set.")
         pytest_main_target = name + ".pytest_main"

py/private/BUILD.bazel

@@ -92,11 +92,6 @@ bzl_library(
     srcs = ["providers.bzl"],
 )
 
-bzl_library(
-    name = "py_executable",
-    srcs = ["py_executable.bzl"],
-)
-
 bzl_library(
     name = "transitions",
     srcs = ["transitions.bzl"],