Surprising behavior based on inclusion of `llvm_version`

[rafikk]

I have a custom toolchain configured and working in MODULE.bazel:

bazel_dep(name = "toolchains_llvm", version = "1.5.0")

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
llvm.toolchain(
    name = "llvm_toolchain",
    sha256 = {
        "darwin-aarch64": "02fb0842f75ec34f3df2162c27d01773bd122cf27e01e9e051bb56f9edec66f1",
        "darwin-x86_64": "4b382146c4483cf68b730df3329a975505205dc73dc86cfe9f3bacad06e22d49",
        "linux-aarch64": "46af59ce8c53338d90b72c401b5560d44bbb02df118f0f0daf08bf897a05f1a9",
        "linux-x86_64": "acf8c45aaa3f012d1f140893b5760daafd5965635941a4999c68d2d04a8aae4f",
    },
    urls = {
        "darwin-aarch64": ["https://github.com/MaterializeInc/toolchains/releases/download/clang-20.1.6-1/darwin_aarch64.tar.zst"],
        "darwin-x86_64": ["https://github.com/MaterializeInc/toolchains/releases/download/clang-20.1.6-1/darwin_x86_64.tar.zst"],
        "linux-aarch64": ["https://github.com/MaterializeInc/toolchains/releases/download/clang-20.1.6-1/linux_aarch64.tar.zst"],
        "linux-x86_64": ["https://github.com/MaterializeInc/toolchains/releases/download/clang-20.1.6-1/linux_x86_64.tar.zst"],
    },
)
use_repo(llvm, "llvm_toolchain")

register_toolchains("@llvm_toolchain//:all")

Note that I'm not setting llvm_version. If I update the llvm.toolchain arguments to include llvm_version = "20.1.6" ...

 llvm.toolchain(
     name = "llvm_toolchain",
+    llvm_version = "20.1.6",
     sha256 = {
         "darwin-aarch64": "02fb0842f75ec34f3df2162c27d01773bd122cf27e01e9e051bb56f9edec66f1",
         "darwin-x86_64": "4b382146c4483cf68b730df3329a975505205dc73dc86cfe9f3bacad06e22d49",

... I receive the following errors as reported in #481.

ERROR: /private/var/tmp/_bazel_rafiksalama/1d5a93ad130d5a2bb2604b5c76cd5ad7/external/rules_go+/BUILD.bazel:42:7: GoStdlib external/rules_go+/stdlib_/pkg failed: missing input file '@@toolchains_llvm++llvm+llvm_toolchain//:bin/clang-format'
ERROR: /private/var/tmp/_bazel_rafiksalama/1d5a93ad130d5a2bb2604b5c76cd5ad7/external/rules_go+/BUILD.bazel:42:7: GoStdlib external/rules_go+/stdlib_/pkg failed: missing input file '@@toolchains_llvm++llvm+llvm_toolchain//:bin/clang-tidy'
ERROR: /private/var/tmp/_bazel_rafiksalama/1d5a93ad130d5a2bb2604b5c76cd5ad7/external/rules_go+/BUILD.bazel:42:7: GoStdlib external/rules_go+/stdlib_/pkg failed: missing input file '@@toolchains_llvm++llvm+llvm_toolchain//:bin/clangd'
ERROR: /private/var/tmp/_bazel_rafiksalama/1d5a93ad130d5a2bb2604b5c76cd5ad7/external/rules_go+/BUILD.bazel:42:7: GoStdlib external/rules_go+/stdlib_/pkg failed: 3 input file(s) do not exist
ERROR: /private/var/tmp/_bazel_rafiksalama/1d5a93ad130d5a2bb2604b5c76cd5ad7/external/rules_go+/BUILD.bazel:42:7 GoStdlib external/rules_go+/stdlib_/pkg failed: 3 input file(s) do not exist

And indeed the LLVM distribution I'm using does not include the required binaries.

But why am I able to configure and register the toolchain at all without llvm_version? I assumed the toolchain was configured and working properly, but now I'm wondering if that's actually the case. If it is registered properly, why are these errors only surfaced when llvm_version is set? If llvm_version is required, shouldn't the toolchain tag raise an error?

[rafikk]

I just ran a build with --toolchain_resolution_debug=@bazel_tools//tools/cpp:toolchain_type and confirmed that the toolchain is not registered when llvm_version is unset. So it seems like the issue is just that the llvm_version attribute is required but toolchains_llvm fails silently when it's not set. Can someone confirm?

[fmeum]

Cc @helly25 as you probably understand this best

[dzbarsky]

This check looks buggy

toolchains_llvm/toolchain/rules.bzl

Lines 41 to 42 in 151ed31

	if kwargs.get("llvm_version") == kwargs.get("llvm_versions"):
	fail("Exactly one of llvm_version or llvm_versions must be set")

[helly25]

Yeah. That looks wrong indeed. The condition needs to check that only one of the two settings had a value. Thanks for spotting and reporting.

…

On Fri, Oct 24, 2025, 16:43 David Zbarsky ***@***.***> wrote: *dzbarsky* left a comment (bazel-contrib/toolchains_llvm#575) <#575 (comment)> This check looks buggy https://github.com/bazel-contrib/toolchains_llvm/blob/151ed311a9cd35802f35995b808771a7ec7f5d82/toolchain/rules.bzl#L41-43 — Reply to this email directly, view it on GitHub <#575 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABQ7NSJK3LHPQYGSL3C2DRT3ZI3IRAVCNFSM6AAAAACKDOFNW2VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTINBTGUZTEMJYGM> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[helly25]

@rafikk Could you verify whether #576 performs correctly for you?

[rafikk]

@helly25 That fixes it. Thanks!

[helly25]

Merged. The issue can be closed now.

…

On Fri, Oct 24, 2025, 20:07 Rafik Salama ***@***.***> wrote: *rafikk* left a comment (bazel-contrib/toolchains_llvm#575) <#575 (comment)> @helly25 <https://github.com/helly25> That fixes it. Thanks! — Reply to this email directly, view it on GitHub <#575 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABQ7NSOX25KV6FZYCDCZ7YL3ZJTEJAVCNFSM6AAAAACKDOFNW2VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTINBUGMYDIOJUGM> . You are receiving this because you were mentioned.Message ID: ***@***.***>