Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

.bazelrc

@@ -6,7 +6,4 @@ build --worker_sandboxing
 build --compilation_mode=opt
 
 build --incompatible_enable_cc_toolchain_resolution
-build --extra_toolchains @zig_sdk//toolchain:linux_amd64_gnu.2.19
+build --action_env BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
-build --extra_toolchains @zig_sdk//toolchain:linux_arm64_gnu.2.28
-build --extra_toolchains @zig_sdk//toolchain:darwin_amd64
-build --extra_toolchains @zig_sdk//toolchain:darwin_arm64

.build.yml

@@ -20,10 +20,7 @@ tasks:
       sudo apt-get install libc6:arm64 -y
   - list_toolchains_platforms: |
       cd bazel-zig-cc; . .envrc
-      echo "Available toolchains:"
+      ./ci/list_toolchains_platforms
-      bazel query @zig_sdk//toolchain:*
-      echo "Available platforms:"
-      bazel query @zig_sdk//platform:*
   - test: |
       cd bazel-zig-cc; . .envrc
       ./ci/test --color=yes --curses=yes

README.md

@@ -23,67 +23,168 @@ http_archive(
     urls = ["https://git.sr.ht/~motiejus/bazel-zig-cc/archive/{}.tar.gz".format(BAZEL_ZIG_CC_VERSION)],
 )
 
-load("@bazel-zig-cc//toolchain:defs.bzl", zig_register_toolchains = "register_toolchains")
+load("@bazel-zig-cc//toolchain:defs.bzl", zig_toolchains = "toolchains")
 
-zig_register_toolchains()
+zig_toolchains()
-
-# Or, if you are using this in production, you probably want more control:
-zig_register_toolchains(
-    version = "<...>",
-    url_formats = [
-        "https://example.internal/zig/zig-{host_platform}-{version}.tar.xz",
-    ],
-    host_platform_sha256 = { ... },
-)
 ```
 
+>   ### zig sdk download control
+>
+>   If you are using this in production, you probably want more control over
+>   where the zig sdk is downloaded from:
+>   ```
+>   zig_register_toolchains(
+>       version = "<...>",
+>       url_formats = [
+>           "https://example.internal/zig/zig-{host_platform}-{version}.tar.xz",
+>       ],
+>       host_platform_sha256 = { ... },
+>   )
+>   ```
+
 And this to `.bazelrc`:
 
 ```
 build --incompatible_enable_cc_toolchain_resolution
-build --extra_toolchains @zig_sdk//toolchain:linux_amd64_gnu.2.19
+```
-build --extra_toolchains @zig_sdk//toolchain:linux_arm64_gnu.2.28
+
-build --extra_toolchains @zig_sdk//toolchain:darwin_amd64
+The snippets above will download the zig toolchain and make the bazel
-build --extra_toolchains @zig_sdk//toolchain:darwin_arm64
+toolchains available for registration and usage.
+
+The next steps depend on your use case.
+
+## I want to manually build a single target with a specific zig cc toolchain
+
+You may explicitly request Bazel to use a specific toolchain (compatible with
+the specified platform). For example, if you wish to compile a specific binary
+(or run tests) on linux/amd64/musl, you may specify:
+
+```
+bazel build \
+    --platforms @zig_sdk//platform:linux_arm64 \
+    --extra_toolchains @zig_sdk//toolchain:linux_arm64_musl \
+    //test/go:go
+```
+
+This registers the toolchain `@zig_sdk//toolchain:linux_arm64_musl` for linux
+arm64 targets. This toolchains links code statically with musl. We also specify
+that we want to build //test/go:go for linux arm64.
+
+## I want to use zig cc as the default compiler
+
+Replace the call to `zig_register_toolchains` with
+```
+register_toolchains(
+    "@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",
+)
 ```
 
 The snippets above will download the zig toolchain and register it for the
-following platforms:
+following configurations:
 
-- `x86_64-linux-gnu.2.19` for `["@platforms//os:linux", "@platforms//cpu:x86_64"]`.
+- `toolchain:linux_amd64_gnu.2.19` for `["@platforms//os:linux", "@platforms//cpu:x86_64", "@zig_sdk//libc:unconstrained"]`.
-- `x86_64-linux-gnu.2.28` for `["@platforms//os:linux", "@platforms//cpu:aarch64"]`.
+- `toolchain:linux_arm64_gnu.2.28` for `["@platforms//os:linux", "@platforms//cpu:aarch64", "@zig_sdk//libc:unconstrained"]`.
-- `x86_64-macos-gnu` for `["@platforms//os:macos", "@platforms//cpu:x86_64"]`.
+- `toolchain:darwin_arm64` for `["@platforms//os:macos", "@platforms//cpu:x86_64"]`.
-- `aarch64-macos-gnu` for `["@platforms//os:macos", "@platforms//cpu:aarch64"]`.
+- `toolchain:darwin_arm64` for `["@platforms//os:macos", "@platforms//cpu:aarch64"]`.
 
-Note that both Go and Bazel naming schemes are accepted. For convenience with
+>   ### Naming
-Go, the following Go-style toolchain aliases are created:
+>
+>   Both Go and Bazel naming schemes are accepted. For convenience with
+>   Go, the following Go-style toolchain aliases are created:
+>
+>   |Bazel (zig) name | Go name  |
+>   |---------------- | -------- |
+>   |`x86_64`         | `amd64`  |
+>   |`aarch64`        | `arm64`  |
+>   |`macos`          | `darwin` |
+>
+>   For example, the toolchain `linux_amd64_gnu.2.28` is aliased to
+>   `x86_64-linux-gnu.2.28`. To find out which toolchains can be registered or
+>   used, run:
+>
+>   ```
+>   $ bazel query @zig_sdk//toolchain/...
+>   ```
 
-|Bazel (zig) name | Go name  |
+>   ### Disabling the default bazel cc toolchain
-|---------------- | -------- |
+>
-|`x86_64`         | `amd64`  |
+>   It may be useful to disable the default toolchain that bazel configures for
-|`aarch64`        | `arm64`  |
+>   you, so that configuration issues can be caught early on:
-|`macos`          | `darwin` |
+>
+>   .bazelrc
+>   ```
+>   build:zig_cc --action_env BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
+>   ```
+>
+>   This is not documented in bazel, so use at your own peril.
 
-For example, the toolchain `linux_amd64_gnu.2.28` is aliased to
+## I want to start using the zig cc toolchain gradually
-`x86_64-linux-gnu.2.28`. To find out which toolchains can be registered or
-used, run:
 
+You can register your zig cc toolchains under a config in your .bazelrc
 ```
-$ bazel query @zig_sdk//toolchain/...
+build:zig_cc --extra_toolchains @zig_sdk//toolchain:linux_amd64_gnu.2.19
+build:zig_cc --extra_toolchains @zig_sdk//toolchain:linux_arm64_gnu.2.28
+build:zig_cc --extra_toolchains @zig_sdk//toolchain:darwin_amd64
+build:zig_cc --extra_toolchains @zig_sdk//toolchain:darwin_arm64
 ```
 
-## Specifying non-default toolchains
+Then for your builds/tests you need to specify that the `zig_cc` config
-
+should be used:
-You may explicitly request Bazel to use a specific toolchain, even though a
-different one is registered using `--extra_toolchains <toolchain>` in
-`.bazelrc`. For example, if you wish to compile a specific binary (or run
-tests) on linux/amd64/musl, you may specify:
-
 ```
---extra_toolchains @zig_sdk//toolchain:linux_amd64_musl
+bazel build --config zig_cc //test/go:go
 ```
 
-## UBSAN and "SIGILL: Illegal Instruction"
+You can build a target for a different platform like so:
+```
+bazel build --config zig_cc \
+    --platforms @zig_sdk//platform:linux_arm64 \
+    //test/go:go
+```
+
+## I want to use zig to build targets for multiple libc variants
+
+If you have targets that need to be build with different glibc versions or with
+musl, you can register a linux toolchain declared under `libc_aware/toolchains`.
+It will only be selected when building for a specific libc version. For example
+
+- `libc_aware/toolchain:linux_amd64_gnu.2.19` for `["@platforms//os:linux", "@platforms//cpu:x86_64", "@zig_sdk//libc:gnu.2.19"]`.
+- `libc_aware/toolchain:linux_amd64_gnu.2.28` for `["@platforms//os:linux", "@platforms//cpu:x86_64", "@zig_sdk//libc:gnu.2.28"]`.
+- `libc_aware/toolchain:x86_64-linux-musl` for `["@platforms//os:linux", "@platforms//cpu:x86_64", "@zig_sdk//libc:musl"]`.
+
+With these toolchains registered, you can build a project for a specific libc
+aware platform:
+```
+$ bazel build --platforms @zig_sdk//libc_aware/platform:linux_amd64_gnu.2.19 //test/go:go
+$ bazel build --platforms @zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 //test/go:go
+$ bazel build --platforms @zig_sdk//libc_aware/platform:linux_amd64_musl //test/go:go
+```
+
+You can see the list of libc aware toolchains and platforms by running:
+```
+$ bazel query @zig_sdk//libc_aware/toolchain/...
+$ bazel query @zig_sdk//libc_aware/platform/...
+ ```
+
+This is especially useful if you are relying on [transitions][transitions], as
+transitioning `extra_platforms` will cause your host tools to be rebuilt with
+the specific libc version, which takes time, and your host may not be able to
+run them.
+
+The `@zig_sdk//libc:variant` constraint is used to select a matching toolchain.
+If you are using your own platform definitions, add a `@zig_sdk//libc:variant`
+constraint to them. See the list of available values:
+```
+$ bazel query "attr(constraint_setting, @zig_sdk//libc:variant, @zig_sdk//...)"
+```
+
+`@zig_sdk//libc:unconstrained` is a special value that indicates that no value
+for the constraint is specified. The non libc aware linux toolchains are only
+compatible with this value to prevent accidental silent fallthrough to them.
+
+# UBSAN and "SIGILL: Illegal Instruction"
 
 `zig cc` differs from "mainstream" compilers by [enabling UBSAN by
 default][ubsan1]. Which means your program may compile successfully and crash
@@ -200,3 +301,4 @@ the issues promptly.
 [sysroot]: https://github.com/ziglang/zig/issues/10299#issuecomment-989153750
 [ubsan1]: https://github.com/ziglang/zig/issues/4830#issuecomment-605491606
 [ubsan2]: https://github.com/ziglang/zig/issues/5163
+[transitions]: https://docs.bazel.build/versions/main/skylark/config.html#user-defined-transitions

WORKSPACE

@@ -44,7 +44,14 @@ gazelle_dependencies(go_repository_default_config = "@//:WORKSPACE")
 
 load(
     "//toolchain:defs.bzl",
-    zig_register_toolchains = "register_toolchains",
+    zig_toolchains = "toolchains",
 )
 
-zig_register_toolchains()
+zig_toolchains()
+
+register_toolchains(
+    "@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",
+)

ci/list_toolchains_platforms

@@ -0,0 +1,15 @@
+#!/bin/bash
+set -euo pipefail
+
+indent() { sed 's/^/    /'; }
+
+echo "Available toolchains:"
+bazel query '@zig_sdk//toolchain:*' | indent
+echo "Available platforms:"
+bazel query '@zig_sdk//platform:*' | indent
+echo "Available libc aware toolchains:"
+bazel query '@zig_sdk//libc_aware/toolchain:*' | indent
+echo "Available libc aware platforms:"
+bazel query '@zig_sdk//libc_aware/platform:*' | indent
+echo "Available libc variants:"
+bazel query "attr(constraint_setting, @zig_sdk//libc:variant, @zig_sdk//...)" | indent

rules/platform.bzl

@@ -1,8 +1,8 @@
 def _platform_transition_impl(settings, attr):
     _ignore = settings
     return {
-        "//command_line_option:platforms": "@zig_sdk//platform:{}".format(attr.platform),
+        "//command_line_option:platforms": "@zig_sdk{}".format(attr.platform),
-        "//command_line_option:extra_toolchains": ["@zig_sdk//toolchain:{}".format(tc) for tc in attr.extra_toolchains],
+        "//command_line_option:extra_toolchains": ["@zig_sdk{}".format(tc) for tc in attr.extra_toolchains],
     }
 
 _platform_transition = transition(

test/c/BUILD

@@ -0,0 +1,46 @@
+load("@bazel-zig-cc//rules:platform.bzl", "platform_binary")
+
+cc_binary(
+    name = "which_libc",
+    srcs = ["main.c"],
+    target_compatible_with = [
+        "@platforms//os:linux",
+    ],
+)
+
+[
+    (
+        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(
+            name = "test_libc_{}".format(name),
+            srcs = ["test.sh"],
+            data = ["which_libc_{}".format(name)],
+            env = {
+                "WANT": want,
+                "BINARY": "$(location which_libc_{})".format(name),
+            },
+        ),
+    )
+    for name, platform, want in [
+        ("linux_amd64_musl", "//libc_aware/platform:linux_amd64_musl", "non-glibc"),
+        ("linux_amd64_gnu.2.19", "//libc_aware/platform:linux_amd64_gnu.2.19", "glibc_2.19"),
+        ("linux_amd64_gnu.2.28", "//libc_aware/platform:linux_amd64_gnu.2.28", "glibc_2.28"),
+        ("linux_amd64_gnu.2.31", "//libc_aware/platform:linux_amd64_gnu.2.31", "glibc_2.31"),
+        ("linux_amd64", "//platform:linux_amd64", "glibc_2.19"),
+        ("linux_arm64", "//platform:linux_arm64", "glibc_2.28"),
+    ]
+]

test/c/main.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <features.h>
+
+int main() {
+    #ifdef __GLIBC__
+    printf("glibc_%d.%d", __GLIBC__, __GLIBC_MINOR__);
+    #else
+    puts("non-glibc");
+    #endif
+    return 0;
+}

test/c/test.sh

@@ -0,0 +1,16 @@
+#/bin/bash
+
+set -euo pipefail
+
+want=$WANT
+binary=$BINARY
+
+got=$($binary)
+
+if [[ "$got" != "$want" ]]; then
+    echo wanted:
+    echo \ \ "$want"
+    echo got:
+    echo \ \ "$got"
+    exit 1
+fi

test/go/BUILD

@@ -26,8 +26,8 @@ go_test(
     platform_binary(
         name = "go_{}".format(toolchain),
         src = "go",
-        extra_toolchains = [toolchain],
+        extra_toolchains = ["//toolchain:{}".format(toolchain)],
-        platform = platform,
+        platform = "//platform:{}".format(platform),
     )
     for platform, toolchain in [
         ("linux_amd64", "linux_amd64_musl"),
@@ -42,8 +42,8 @@ go_test(
     platform_test(
         name = "go_test_{}".format(toolchain),
         src = "go_test",
-        extra_toolchains = [toolchain],
+        extra_toolchains = ["//toolchain:{}".format(toolchain)],
-        platform = platform,
+        platform = "//platform:{}".format(platform),
     )
     for platform, toolchain in [
         ("linux_amd64", "linux_amd64_musl"),

toolchain/BUILD.sdk.bazel

@@ -4,6 +4,8 @@ package(
     default_visibility = ["//visibility:public"],
 )
 
+
+
 declare_files(
     zig_include_root = {zig_include_root},
 )

toolchain/defs.bzl

@@ -1,4 +1,3 @@
-load("@bazel_skylib//lib:shell.bzl", "shell")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "read_user_netrc", "use_netrc")
 load("@bazel-zig-cc//toolchain/private:defs.bzl", "DEFAULT_INCLUDE_DIRECTORIES", "ZIG_TOOL_PATH", "target_structs")
@@ -42,15 +41,15 @@ _HOST_PLATFORM_SHA256 = {
     "macos-x86_64": "78220a4460a7c0f563d7365313fcd3ea028ed38166ebac55ba22f17ab6404851",
 }
 
-def register_toolchains(
+def toolchains(
-        register = [],
         version = _VERSION,
         url_formats = [URL_FORMAT_JAKSTYS],
         host_platform_sha256 = _HOST_PLATFORM_SHA256):
     """
-        Download zig toolchain and register some.
+        Download zig toolchain and declare bazel toolchains.
-        @param register registers the given toolchains to the system using
+        The platforms are not registered automatically, that should be done by
-        native.register_toolchains(). See README for possible choices.
+        the user with register_toolchains() in the WORKSPACE file. See README
+        for possible choices.
     """
     zig_repository(
         name = "zig_sdk",
@@ -65,9 +64,6 @@ def register_toolchains(
         },
     )
 
-    toolchains = ["@zig_sdk//toolchain:%s" % t for t in register]
-    native.register_toolchains(*toolchains)
-
 ZIG_TOOL_WRAPPER = """#!/bin/bash
 set -e
 
@@ -95,6 +91,9 @@ _ZIG_TOOLS = [
     "wasm-ld",  # WebAssembly
 ]
 
+def _quote(s):
+    return "'" + s.replace("'", "'\\''") + "'"
+
 def _zig_repository_impl(repository_ctx):
     arch = repository_ctx.os.arch
     if arch == "amd64":
@@ -139,32 +138,26 @@ def _zig_repository_impl(repository_ctx):
         content = _fcntl_h,
     )
 
-    repository_ctx.symlink(
+    for dest, src in {
-        Label("//toolchain/platform:BUILD"),
+        "platform/BUILD": "//toolchain/platform:BUILD",
-        "platform/BUILD",
+        "toolchain/BUILD": "//toolchain/toolchain:BUILD",
-    )
+        "libc/BUILD": "//toolchain/libc:BUILD",
+        "libc_aware/platform/BUILD": "//toolchain/libc_aware/platform:BUILD",
+        "libc_aware/toolchain/BUILD": "//toolchain/libc_aware/toolchain:BUILD",
+    }.items():
+        repository_ctx.symlink(Label(src), dest)
 
+    for dest, src in {
+        "BUILD": "//toolchain:BUILD.sdk.bazel",
+        "private/BUILD": "//toolchain/private:BUILD.sdk.bazel",
+    }.items():
         repository_ctx.template(
-        "BUILD",
+            dest,
-        Label("//toolchain:BUILD.sdk.bazel"),
+            Label(src),
             executable = False,
             substitutions = {
-            "{zig_include_root}": shell.quote(zig_include_root),
+                "{absolute_path}": _quote(str(repository_ctx.path(""))),
-        },
+                "{zig_include_root}": _quote(zig_include_root),
-    )
-
-    repository_ctx.symlink(
-        Label("//toolchain/toolchain:BUILD"),
-        "toolchain/BUILD",
-    )
-
-    repository_ctx.template(
-        "private/BUILD",
-        Label("//toolchain/private:BUILD.sdk.bazel"),
-        executable = False,
-        substitutions = {
-            "{absolute_path}": shell.quote(str(repository_ctx.path(""))),
-            "{zig_include_root}": shell.quote(zig_include_root),
             },
         )
 

toolchain/libc/BUILD

@@ -0,0 +1,17 @@
+load("@bazel-zig-cc//toolchain/libc:defs.bzl", "declare_libcs")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+constraint_setting(
+    name = "variant",
+    default_constraint_value = "unconstrained",
+)
+
+constraint_value(
+    name = "unconstrained",
+    constraint_setting = "variant",
+)
+
+declare_libcs()

toolchain/libc/defs.bzl

@@ -0,0 +1,8 @@
+load("@bazel-zig-cc//toolchain/private:defs.bzl", "LIBCS")
+
+def declare_libcs():
+    for libc in LIBCS:
+        native.constraint_value(
+            name = libc,
+            constraint_setting = "variant",
+        )

toolchain/libc_aware/platform/BUILD

@@ -0,0 +1,7 @@
+load("@bazel-zig-cc//toolchain/platform:defs.bzl", "declare_libc_aware_platforms")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+declare_libc_aware_platforms()

toolchain/libc_aware/toolchain/BUILD

@@ -0,0 +1,7 @@
+load("@bazel-zig-cc//toolchain/toolchain:defs.bzl", "declare_libc_aware_toolchains")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+declare_libc_aware_toolchains()

toolchain/platform/BUILD

@@ -1,3 +1,7 @@
 load("@bazel-zig-cc//toolchain/platform:defs.bzl", "declare_platforms")
 
+package(
+    default_visibility = ["//visibility:public"],
+)
+
 declare_platforms()

toolchain/platform/defs.bzl

@@ -1,17 +1,40 @@
+load("@bazel-zig-cc//toolchain/private:defs.bzl", "LIBCS")
+
+_CPUS = (("x86_64", "amd64"), ("aarch64", "arm64"))
+
 def declare_platforms():
     # create @zig_sdk//{os}_{arch}_platform entries with zig and go conventions
-    for zigcpu, gocpu in (("x86_64", "amd64"), ("aarch64", "arm64")):
+    for zigcpu, gocpu in _CPUS:
         for bzlos, oss in {"linux": ["linux"], "macos": ["macos", "darwin"]}.items():
             for os in oss:
+                declare_platform(gocpu, zigcpu, bzlos, os)
+
+def declare_libc_aware_platforms():
+    # create @zig_sdk//{os}_{arch}_platform entries with zig and go conventions
+    # with libc specified
+    for zigcpu, gocpu in _CPUS:
+        for libc in LIBCS:
+            declare_platform(
+                gocpu,
+                zigcpu,
+                "linux",
+                "linux",
+                suffix = "_{}".format(libc),
+                extra_constraints = ["@zig_sdk//libc:{}".format(libc)],
+            )
+
+def declare_platform(gocpu, zigcpu, bzlos, os, suffix = "", extra_constraints = []):
     constraint_values = [
         "@platforms//os:{}".format(bzlos),
         "@platforms//cpu:{}".format(zigcpu),
-                ]
+    ] + extra_constraints
+
     native.platform(
-                    name = "{os}_{zigcpu}".format(os = os, zigcpu = zigcpu),
+        name = "{os}_{zigcpu}{suffix}".format(os = os, zigcpu = zigcpu, suffix = suffix),
         constraint_values = constraint_values,
     )
+
     native.platform(
-                    name = "{os}_{gocpu}".format(os = os, gocpu = gocpu),
+        name = "{os}_{gocpu}{suffix}".format(os = os, gocpu = gocpu, suffix = suffix),
         constraint_values = constraint_values,
     )

toolchain/private/defs.bzl

@@ -28,6 +28,8 @@ _GLIBCS = [
     "2.34",
 ]
 
+LIBCS = ["musl", "gnu"] + ["gnu.{}".format(glibc) for glibc in _GLIBCS]
+
 def target_structs():
     ret = []
     for zigcpu, gocpu in (("x86_64", "amd64"), ("aarch64", "arm64")):
@@ -100,6 +102,7 @@ def _target_linux_gnu(gocpu, zigcpu, glibc_version = ""):
             "@platforms//os:linux",
             "@platforms//cpu:{}".format(zigcpu),
         ],
+        libc_constraint = "@zig_sdk//libc:{}".format(glibc_suffix),
         tool_paths = {"ld": "ld.lld"},
     )
 
@@ -120,5 +123,6 @@ def _target_linux_musl(gocpu, zigcpu):
             "@platforms//os:linux",
             "@platforms//cpu:{}".format(zigcpu),
         ],
+        libc_constraint = "@zig_sdk//libc:musl",
         tool_paths = {"ld": "ld.lld"},
     )

toolchain/toolchain/defs.bzl

@@ -1,16 +1,37 @@
-load("@bazel-zig-cc//toolchain/private:defs.bzl", "DEFAULT_INCLUDE_DIRECTORIES", "ZIG_TOOL_PATH", "target_structs")
+load("@bazel-zig-cc//toolchain/private:defs.bzl", "target_structs")
 
 def declare_toolchains():
     for target_config in target_structs():
         gotarget = target_config.gotarget
         zigtarget = target_config.zigtarget
 
+        # if the toolchain is libc aware, create two variants for it: one that
+        # is only selected if libc is not expicitly set and another one that is
+        # only selected if the specific libc variant is selected.
+        extra_constraints = []
+        if hasattr(target_config, "libc_constraint"):
+            extra_constraints = ["@zig_sdk//libc:unconstrained"]
+
+        _declare_toolchain(gotarget, zigtarget, target_config.constraint_values + extra_constraints)
+
+def declare_libc_aware_toolchains():
+    for target_config in target_structs():
+        gotarget = target_config.gotarget
+        zigtarget = target_config.zigtarget
+
+        # if the toolchain is libc aware, create two variants for it: one that
+        # is only selected if libc is not expicitly set and another one that is
+        # only selected if the specific libc variant is selected.
+        if hasattr(target_config, "libc_constraint"):
+            _declare_toolchain(gotarget, zigtarget, target_config.constraint_values + [target_config.libc_constraint])
+
+def _declare_toolchain(gotarget, zigtarget, target_compatible_with):
     # register two kinds of toolchain targets: Go and Zig conventions.
     # Go convention: amd64/arm64, linux/darwin
     native.toolchain(
         name = gotarget,
         exec_compatible_with = None,
-            target_compatible_with = target_config.constraint_values,
+        target_compatible_with = target_compatible_with,
         toolchain = "@zig_sdk//private:%s_cc" % zigtarget,
         toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
     )
@@ -19,7 +40,7 @@ def declare_toolchains():
     native.toolchain(
         name = zigtarget,
         exec_compatible_with = None,
-            target_compatible_with = target_config.constraint_values,
+        target_compatible_with = target_compatible_with,
         toolchain = "@zig_sdk//private:%s_cc" % zigtarget,
         toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
     )