Try transitioning and avoiding default target config

py/private/toolchain/venv/BUILD.bazel

@@ -2,7 +2,7 @@ load("//py/private/toolchain:tools.bzl", "source_toolchain")
 
 source_toolchain(
     name = "venv",
-    bin = "//py/tools/venv_bin",
+    bin = "//py/tools/venv_bin:venv_bin_for_platform",
     # When running from source, we assume the target is the exec platform.
     # TODO: when we have cross-compile Mac -> Linux, pre-release users should be able to
     # build a py_image on their Mac and have the linux venv rust_binary end up in the container.

py/tools/venv_bin/BUILD.bazel

@@ -6,9 +6,6 @@ rust_binary(
     srcs = [
         "src/main.rs",
     ],
-    visibility = [
-        "//visibility:public",
-    ],
     deps = [
         "//py/tools/py",
         "@crate_index//:clap",
@@ -23,6 +20,18 @@ alias(
     tags = ["manual"],
 )
 
+alias(
+    name = "venv_bin_for_platform",
+    actual = select({
+        "//tools/platforms:macos_aarch64": ":venv_macos_aarch64_build",
+        "//tools/platforms:macos_x86_64": ":venv_macos_x86_64_build",
+        "//tools/platforms:linux_aarch64": ":venv_linux_aarch64_build",
+        "//tools/platforms:linux_x86_64": ":venv_linux_x86_64_build",
+    }),
+    tags = ["manual"],
+    visibility = ["//visibility:public"],
+)
+
 multi_arch_rust_binary_release(
     name = "macos",
     src = ":venv",

tools/platforms/BUILD.bazel

@@ -0,0 +1,35 @@
+config_setting(
+    name = "linux_x86_64",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:linux",
+    ],
+    visibility = ["//visibility:public"],
+)
+
+config_setting(
+    name = "linux_aarch64",
+    constraint_values = [
+        "@platforms//cpu:arm64",
+        "@platforms//os:linux",
+    ],
+    visibility = ["//visibility:public"],
+)
+
+config_setting(
+    name = "macos_x86_64",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:macos",
+    ],
+    visibility = ["//visibility:public"],
+)
+
+config_setting(
+    name = "macos_aarch64",
+    constraint_values = [
+        "@platforms//cpu:arm64",
+        "@platforms//os:macos",
+    ],
+    visibility = ["//visibility:public"],
+)

tools/release/defs.bzl

@@ -1,7 +1,7 @@
 "Make releases for platforms supported by rules_py"
 
 load("@aspect_bazel_lib//lib:copy_file.bzl", "copy_file")
-load("@aspect_bazel_lib//lib:transitions.bzl", "platform_transition_filegroup")
+load("@aspect_bazel_lib//lib:transitions.bzl", "platform_transition_binary")
 load("@aspect_bazel_lib//tools/release:hashes.bzl", "hashes")
 
 def _map_os_to_triple(os):
@@ -15,11 +15,12 @@ def _map_os_to_triple(os):
 # buildozer: disable=function-docstring
 def multi_arch_rust_binary_release(name, src, os, archs = ["aarch64", "x86_64"], **kwargs):
     outs = []
+    actuals = {}
     for arch in archs:
         bin = Label(src).name
-        platform_transition_filegroup(
+        platform_transition_binary(
             name = "{}_{}_{}_build".format(bin, os, arch),
-            srcs = [src],
+            binary = src,
             target_platform = "//tools/release:{}_{}".format(os, arch),
             target_compatible_with = ["@platforms//os:{}".format(os)],
         )