PR #756 -imacros redacted_dates.h breaks rules_rust cargo_build_script with --strategy=CargoBuildScriptRun=local

[jayakasadev]

Summary

PR #756 injects -imacros external/toolchains_llvm++llvm+llvm_toolchain/redacted_dates.h into unfiltered_compile_flags. The path is sandbox-relative, which is fine for normal cc_library/cc_binary actions but breaks consumers that re-serialize toolchain flags into out-of-sandbox compiler invocations.

The most common surface: rules_rust cargo_build_script actions invoking cc-rs when --strategy=CargoBuildScriptRun=local is set. cc-rs calls cc_wrapper.sh with the toolchain's flags but resolves CWD outside the sandbox where external/... paths don't exist.

Reproducer

Any Rust target whose dep graph pulls a *-sys crate with a C build script (ring, libtcmalloc-sys, snmalloc-sys, etc.) plus:

# .bazelrc
build:linux --strategy=CargoBuildScriptRun=local

(--strategy=CargoBuildScriptRun=local is the recommended workaround for the libtcmalloc-sys sandbox-symlink issue, per rules_rust discussion.)

Failure

cargo:warning=<built-in>:1:19: fatal error: 'external/toolchains_llvm++llvm+llvm_toolchain/redacted_dates.h' file not found
cargo:warning=    1 | #__include_macros "external/toolchains_llvm++llvm+llvm_toolchain/redacted_dates.h"
cargo:warning=      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cargo:warning=1 error generated.

Reproduces on darwin-aarch64 + linux-x86_64. v1.8.0 (just released) carries PR #756, so every consumer that bumps past v1.7.0 is affected.

Cause

toolchain/internal/configure.bzl:841:

redacted_dates_path = "external/{}/redacted_dates.h".format(rctx.name),

The sandbox-relative path is correct for in-sandbox actions but unusable for any consumer that doesn't have cxx_builtin_include_files in its runfiles. rules_rust cargo_build_script_runner doesn't propagate cxx_builtin_include_files to local-strategy invocations.

PR #756's docstring foresaw this: "tools that re-serialize the toolchain's compile flags into their own build scripts (rules_foreign_cc make/cmake builds, embedded compiler-flag strings) drop the quotes" — the new -imacros form doesn't have the quote-loss bug but instead has an analogous file-path-loss bug for the same class of consumers.

Proposed fixes (pick one)

1. Feature flag: Add a `disable_date_redaction` toolchain feature (default off). Consumers can enable it to skip the -imacros redacted_dates_path block. Smallest API surface; opt-in; ships immediately.
2. Bazel-resolved absolute path: redacted_dates_path = str(rctx.path("redacted_dates.h")). Bakes machine-specific path into config — breaks remote execution cache.
3. Restore -D__DATE__="redacted": revert PR Redact date macros via -imacros header instead of -D on command line #756. Trades cargo_build_script bug for rules_foreign_cc quote-loss bug.
4. Bazel runfile path propagation: cooperate with rules_rust to propagate cxx_builtin_include_files to local-strategy cargo_build_script. Out of scope here.

Happy to send a PR implementing option 1 (disable_date_redaction feature) if maintainers want.

Environment

• bazel 7.5.0
• toolchains_llvm `332d6856` (current HEAD, v1.8.0 + 4 commits)
• rules_rust 0.70.0
• LLVM 21.1.8 / 20.1.8 (darwin-aarch64)
• macOS 26.4 + Ubuntu 24.04 Linux

[dzbarsky]

I personally don't think disabling sandboxing for Rust build scripts is a good solution. They are already giving arbitrary people arbitrary code execution in your environment. While Bazel's sandboxing isn't a real security boundary, it's definitely better than nothing. Given the recent slew of supply chain attacks I would take every defensive measure possible...

It sounds like the bug is in rules_rust and should be fixed there instead of adding knobs in other rulesets to further disable hermeticity/correctness.

[tamird]

but breaks consumers that re-serialize toolchain flags into out-of-sandbox compiler invocations.

Buried in the AI slop of this issue text is this key sentence fragment. I don't know what the guarantees are, but it seems that the consumer is doing something bad that it should stop doing.

EDIT: was wrong of me to assume AI was behind the slop.