Error: local variable 'cc_toolchain' is referenced before assignment.

[iffyio]

Hi! I get this error from rust_library targets after updating to commit 76c44206c4d37491767446c6ccdf1a98274a0887

Traceback (most recent call last):
        File "/home/iffy/.cache/bazel/_bazel_iffy/7f16182b13b4a660f93298499007a04c/external/rules_rust/rust/toolchain.bzl", line 154, column 72, in _rust_toolchain_impl
                libstd_and_allocator_ccinfo = _make_libstd_and_allocator_ccinfo(ctx, ctx.attr.rust_lib, ctx.attr.allocator_library),
        File "/home/iffy/.cache/bazel/_bazel_iffy/7f16182b13b4a660f93298499007a04c/external/rules_rust/rust/toolchain.bzl", line 43, column 60, in _make_libstd_and_allocator_ccinfo
                cc_toolchain, feature_configuration = find_cc_toolchain(ctx)
        File "/home/iffy/.cache/bazel/_bazel_iffy/7f16182b13b4a660f93298499007a04c/external/rules_rust/rust/private/utils.bzl", line 43, column 24, in find_cc_toolchain
                cc_toolchain = cc_toolchain,
Error: local variable 'cc_toolchain' is referenced before assignment.

it seems like somehow @bazel_tools//tools/cpp:toolchain_type was supposed to be set but no longer isnt?

[UebelAndre]

Can you provide an example that demonstrates this issue?

[iffyio]

The issue occurs when using --incompatible_enable_cc_toolchain_resolution

given WORKSPACE file

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "rules_rust",
    strip_prefix = "rules_rust-76c44206c4d37491767446c6ccdf1a98274a0887",
    urls = [
        "https://github.com/bazelbuild/rules_rust/archive/76c44206c4d37491767446c6ccdf1a98274a0887.tar.gz",
    ],
)
load("@rules_rust//rust:repositories.bzl", "rust_repositories")
rust_repositories(version="1.49.0")

given the target

load("@rules_rust//rust:rust.bzl", "rust_library")
rust_library(
    name = "hello-world",
    srcs = [
        "src/main.rs",
    ],
    edition = "2018",
    visibility = ["//visibility:public"],
)

$ bazel build --incompatible_enable_cc_toolchain_resolution hello-world
INFO: SHA256 (https://github.com/bazelbuild/rules_rust/archive/76c44206c4d37491767446c6ccdf1a98274a0887.tar.gz) = 0044473997e40c5467aa410912ae8771aa328e2d44d71df073b87f147ab5cfcf
DEBUG: Rule 'rules_rust' indicated that a canonical reproducible form can be obtained by modifying arguments sha256 = "0044473997e40c5467aa410912ae8771aa328e2d44d71df073b87f147ab5cfcf"
DEBUG: Repository rules_rust instantiated at:
  /tmp/repro/WORKSPACE:3:13: in <toplevel>
Repository rule http_archive defined at:
  /home/iffy/.cache/bazel/_bazel_iffy/a81b5ac6b5d7900969957b5de3807d8b/external/bazel_tools/tools/build_defs/repo/http.bzl:336:31: in <toplevel>
ERROR: /home/iffy/.cache/bazel/_bazel_iffy/a81b5ac6b5d7900969957b5de3807d8b/external/rust_linux_x86_64/BUILD.bazel:82:15: in rust_toolchain rule @rust_linux_x86_64//:toolchain_for_x86_64-unknown-linux-gnu_impl:
Traceback (most recent call last):
        File "/home/iffy/.cache/bazel/_bazel_iffy/a81b5ac6b5d7900969957b5de3807d8b/external/rules_rust/rust/toolchain.bzl", line 154, column 72, in _rust_toolchain_impl
                libstd_and_allocator_ccinfo = _make_libstd_and_allocator_ccinfo(ctx, ctx.attr.rust_lib, ctx.attr.allocator_library),
        File "/home/iffy/.cache/bazel/_bazel_iffy/a81b5ac6b5d7900969957b5de3807d8b/external/rules_rust/rust/toolchain.bzl", line 43, column 60, in _make_libstd_and_allocator_ccinfo
                cc_toolchain, feature_configuration = find_cc_toolchain(ctx)
        File "/home/iffy/.cache/bazel/_bazel_iffy/a81b5ac6b5d7900969957b5de3807d8b/external/rules_rust/rust/private/utils.bzl", line 43, column 24, in find_cc_toolchain
                cc_toolchain = cc_toolchain,
Error: local variable 'cc_toolchain' is referenced before assignment.
ERROR: While resolving toolchains for target //hello-world:hello-world: Analysis of target '@rust_linux_x86_64//:toolchain_for_x86_64-unknown-linux-gnu_impl' failed
ERROR: Analysis of target '//hello-world:hello-world' failed; build aborted: Analysis of target '@rust_linux_x86_64//:toolchain_for_x86_64-unknown-linux-gnu_impl' failed
INFO: Elapsed time: 4.123s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (28 packages loaded, 27940 targets configured)

[illicitonion]

I reproduce, this looks like it was introduced in in #624, but looks really weird - the actual error message given:
Error: local variable 'cc_toolchain' is referenced before assignment.
seems weirdly spurious, as the variable is clearly assigned, and that file didn't even change in that pull request, which suggests at least there's a bug in Bazel's starlark interpreter somewhere...

I'll keep digging for a bit...

[iffyio]

The problematic part occurs here while trying to resolve the toolchain https://github.com/bazelbuild/bazel/blob/master/tools/cpp/toolchain_utils.bzl#L41. Even though the check succeeds, ctx.toolchains["@bazel_tools//tools/cpp:toolchain_type"] returns weirdly null which causes the referenced before assignment error when hasattr tries to use toolchain_info. Not at all clear on why this happens, it sounds potentially internal to bazel.

This works around the issue by skipping that code path entirely and always falling back

diff --git rust/private/utils.bzl rust/private/utils.bzl
index 06ed842..be8904e 100644
--- rust/private/utils.bzl
+++ rust/private/utils.bzl
@@ -14,7 +14,11 @@
 
 """Utility functions not specific to the rust toolchain."""
 
-load("@bazel_tools//tools/cpp:toolchain_utils.bzl", find_rules_cc_toolchain = "find_cpp_toolchain")
+def find_rules_cc_toolchain(ctx):
+    if hasattr(ctx.attr, "_cc_toolchain"):
+        return ctx.attr._cc_toolchain[cc_common.CcToolchainInfo]
+    fail("In order to use find_cpp_toolchain, you must define the '_cc_toolchain' attribute on your rule or aspect.")
+
 
 def find_toolchain(ctx):
     """Finds the first rust toolchain that is configured.

[illicitonion]

I probably don't have time to dig much deeper into this at the moment, but definitely feels like a bug internal to bazel to me

[hlopko]

One thing that we missed is that rust_toolchain doesn't declare

    toolchains = [
        "@bazel_tools//tools/cpp:toolchain_type",
    ],

But even with that the build still fails. I am starting to worry that toolchains cannot depend on other toolchains. @katre may help us there?

[katre]

Let me take a look and see if I can reproduce this.

[katre]

Using the setup from @iffyio (and a hello world for rust that I grabbed from somewhere), I can see the same problem. Even after adding the toolchain type (and the incompatible_use_toolchain_transition argument to rule) it's still happening.

Debugging into Bazel I can see that the toolchain context exists, and has no toolchains, which is very odd, so I'm going to debug further.

[katre]

Ah ha, found the problem. In ConfiguredTargetFunction.computeUnloadedToolchainContexts, we remove all toolchains from the context to prevent a self-loop. What is actually needed is to remove only the current configured target, which conveniently we know about and can do.

I'll work up a patch for Bazel and send it shortly. I'll also upload a PR to add the proper toolchain declaration to rust_toolchain. Technically, it's not needed: when loaded as a toolchain, all rust_toolchain targets use the parent's toolchain context, not their own, but it's better to be correct. I will take a look at a way to fix this, as well.

Filed bazelbuild/bazel#13243 to track this on Bazel's side.

[katre]

I've fixed bazelbuild/bazel#13243 with Bazel at head. My repro now works, please test yourselves and let me know if there are still problems.