example transitions only change platforms, leave toolchains for WORKSPACE

WORKSPACE

@@ -50,17 +50,19 @@ load(
 zig_toolchains()
 
 register_toolchains(
-    # if no `--platform` is selected, these toolchains will be used.
+    # if no `--platform` is specified, these toolchains will be used for
+    # (linux,darwin)x(amd64,arm64)
     "@zig_sdk//toolchain:linux_amd64_gnu.2.19",
     "@zig_sdk//toolchain:linux_arm64_gnu.2.28",
     "@zig_sdk//toolchain:darwin_amd64",
     "@zig_sdk//toolchain:darwin_arm64",
 
-    # when a libc-aware platform is selected, these will be used. arm64:
+    # amd64 toolchains for libc-aware platforms:
-    "@zig_sdk//libc_aware/toolchain:linux_arm64_gnu.2.28",
-    "@zig_sdk//libc_aware/toolchain:linux_arm64_musl",
-    # ditto, amd64:
     "@zig_sdk//libc_aware/toolchain:linux_amd64_gnu.2.19",
     "@zig_sdk//libc_aware/toolchain:linux_amd64_gnu.2.28",
+    "@zig_sdk//libc_aware/toolchain:linux_amd64_gnu.2.31",
     "@zig_sdk//libc_aware/toolchain:linux_amd64_musl",
+    # arm64 toolchains for libc-aware platforms:
+    "@zig_sdk//libc_aware/toolchain:linux_arm64_gnu.2.28",
+    "@zig_sdk//libc_aware/toolchain:linux_arm64_musl",
 )

rules/platform.bzl

@@ -2,7 +2,6 @@ def _platform_transition_impl(settings, attr):
     _ignore = settings
     return {
         "//command_line_option:platforms": "@zig_sdk{}".format(attr.platform),
-        "//command_line_option:extra_toolchains": ["@zig_sdk{}".format(tc) for tc in attr.extra_toolchains],
     }
 
 _platform_transition = transition(
@@ -10,7 +9,6 @@ _platform_transition = transition(
     inputs = [],
     outputs = [
         "//command_line_option:platforms",
-        "//command_line_option:extra_toolchains",
     ],
 )
 
@@ -40,16 +38,12 @@ _attrs = {
     "platform": attr.string(
         doc = "The platform to build the target for.",
     ),
-    "extra_toolchains": attr.string_list(
-        doc = "The toolchains to provide as extra_toolchains.",
-    ),
     "_allowlist_function_transition": attr.label(
         default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
     ),
 }
 
-# wrap a single exectable and build it for the specified platform passing in
+# wrap a single exectable and build it for the specified platform.
-# the extra_toolchains.
 platform_binary = rule(
     implementation = _platform_binary_impl,
     cfg = _platform_transition,
@@ -57,8 +51,7 @@ platform_binary = rule(
     executable = True,
 )
 
-# wrap a single test target and build it for the specified platform passing in
+# wrap a single test target and build it for the specified platform.
-# the extra_toolchains.
 platform_test = rule(
     implementation = _platform_binary_impl,
     cfg = _platform_transition,

test/c/BUILD

@@ -13,16 +13,6 @@ cc_binary(
         platform_binary(
             name = "which_libc_{}".format(name),
             src = "which_libc",
-            extra_toolchains = [
-                # toolchains for specific libc versions
-                "//libc_aware/toolchain:linux_amd64_gnu.2.19",
-                "//libc_aware/toolchain:linux_amd64_gnu.2.28",
-                "//libc_aware/toolchain:linux_amd64_gnu.2.31",
-                "//libc_aware/toolchain:linux_amd64_musl",
-                # fallback toolchains
-                "//toolchain:linux_amd64_gnu.2.19",
-                "//toolchain:linux_arm64_gnu.2.28",
-            ],
             platform = platform,
         ),
         sh_test(

test/go/BUILD

@@ -24,31 +24,29 @@ go_test(
 
 [
     platform_binary(
-        name = "go_{}".format(toolchain),
+        name = "go_{}".format(name),
         src = "go",
-        extra_toolchains = ["//toolchain:{}".format(toolchain)],
+        platform = platform,
-        platform = "//platform:{}".format(platform),
     )
-    for platform, toolchain in [
+    for name, platform in [
-        ("linux_amd64", "linux_amd64_musl"),
+        ("linux_amd64_musl", "//libc_aware/platform:linux_amd64_musl"),
-        ("linux_amd64", "linux_amd64_gnu.2.19"),
+        ("linux_amd64_gnu.2.19", "//libc_aware/platform:linux_amd64_gnu.2.19"),
-        ("linux_arm64", "linux_arm64_musl"),
+        ("linux_arm64_musl", "//libc_aware/platform:linux_arm64_musl"),
-        ("linux_arm64", "linux_arm64_gnu.2.28"),
+        ("linux_arm64_gnu.2.28", "//libc_aware/platform:linux_arm64_gnu.2.28"),
-        ("darwin_amd64", "darwin_amd64"),
+        ("darwin_amd64", "//platform:darwin_amd64"),
     ]
 ]
 
 [
     platform_test(
-        name = "go_test_{}".format(toolchain),
+        name = "go_test_{}".format(name),
         src = "go_test",
-        extra_toolchains = ["//toolchain:{}".format(toolchain)],
+        platform = platform,
-        platform = "//platform:{}".format(platform),
     )
-    for platform, toolchain in [
+    for name, platform in [
-        ("linux_amd64", "linux_amd64_musl"),
+        ("linux_amd64_musl", "//libc_aware/platform:linux_amd64_musl"),
-        ("linux_amd64", "linux_amd64_gnu.2.19"),
+        ("linux_amd64_gnu.2.19", "//libc_aware/platform:linux_amd64_gnu.2.19"),
-        ("linux_arm64", "linux_arm64_musl"),
+        ("linux_arm64_musl", "//libc_aware/platform:linux_arm64_musl"),
-        ("linux_arm64", "linux_arm64_gnu.2.28"),
+        ("linux_arm64_gnu.2.28", "//libc_aware/platform:linux_arm64_gnu.2.28"),
     ]
 ]