bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests by Zalathar · Pull Request #159705 · rust-lang/rust

Zalathar · 2026-07-22T07:44:30Z

Zulip thread: Reducing conditional compilation in bootstrap?

Bootstrap contains a few code paths that need to be stubbed out when running bootstrap's unit tests, so that they don't download files or cause other problems. (That's unfortunate, but fixing it is outside the scope of this PR.)

Currently those code paths are skipped using #[cfg(test)] and #[cfg(not(test))], which does achieve the desired effect, but has a few downsides. The stubbed-out code triggers dead-code and unused-import warnings, and makes IDE navigation harder when rust-analyzer treats cfg(test) as true when analyzing the code.

If we switch to using cfg!(test) and cfg!(not(test)) to “dynamically” skip the relevant code paths, it still gets skipped, but the skipped code is still considered live for the purpose of dead-code warnings and IDE analysis.

rustbot · 2026-07-22T07:44:34Z

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR changes how GCC is built. Consider updating src/bootstrap/download-ci-gcc-stamp.

rustbot · 2026-07-22T07:44:36Z

r? @clubby789

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Kobzol · 2026-07-22T07:55:23Z

This is much better than repeating function signatures or expecting dead code on top of imports. Thank you!

r? @Kobzol

@bors r+ rollup

rust-bors · 2026-07-22T07:55:25Z

📌 Commit db20e5d has been approved by Kobzol

It is now in the queue for this repository.

Zalathar · 2026-07-22T08:15:46Z

The clippy false-positive for assert!(cfg!(..)) was supposedly fixed in rust-lang/rust-clippy#7319, but I guess it broke again at some point?

Kobzol · 2026-07-22T08:16:45Z

@bors r-

rust-bors · 2026-07-22T08:16:50Z

This pull request was unapproved.

View changes since this unapproval

Assertions have been added to some code paths that were previously not built during `cfg(test)`, to make sure they aren't accidentally executed.

Zalathar · 2026-07-22T09:14:04Z

I have pushed an update that allows clippy::assertions_on_constants crate-wide, due to the false positives.

(Allowing the individual asserts would be possible, but IMO the lint is pretty low-value so I don’t mind the blanket allow in this case.)

@rustbot ready

Kobzol · 2026-07-22T09:20:52Z

Yeah, agreed that the global allow is fine. I'll r+ once CI is green.

Kobzol · 2026-07-22T10:31:31Z

@bors r+ rollup

Thanks!

rust-bors · 2026-07-22T10:31:34Z

📌 Commit 480c37d has been approved by Kobzol

It is now in the queue for this repository.

bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests - [Zulip thread: Reducing conditional compilation in bootstrap?](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Reducing.20conditional.20compilation.20in.20bootstrap.3F/with/611882645) --- Bootstrap contains a few code paths that need to be stubbed out when running bootstrap's unit tests, so that they don't download files or cause other problems. (That's unfortunate, but fixing it is outside the scope of this PR.) Currently those code paths are skipped using `#[cfg(test)]` and `#[cfg(not(test))]`, which does achieve the desired effect, but has a few downsides. The stubbed-out code triggers dead-code and unused-import warnings, and makes IDE navigation harder when rust-analyzer treats `cfg(test)` as true when analyzing the code. If we switch to using `cfg!(test)` and `cfg!(not(test))` to “dynamically” skip the relevant code paths, it still gets skipped, but the skipped code is still considered live for the purpose of dead-code warnings and IDE analysis.

…uwer Rollup of 8 pull requests Successful merges: - #159712 (Subtree sync for rustc_codegen_cranelift) - #155697 (Stabilize c-variadic function definitions) - #159285 (Simplify `apply_effects_in_range`) - #159607 (test: update riscv32e-registers.rs for LLVM 24 MC diagnostic changes) - #159659 (Move `Limit` out of `rustc_hir`) - #159451 (Remove config cloning in compiletest) - #159646 (Increase depth for float infer var fallback hack) - #159705 (bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests)

bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests - [Zulip thread: Reducing conditional compilation in bootstrap?](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Reducing.20conditional.20compilation.20in.20bootstrap.3F/with/611882645) --- Bootstrap contains a few code paths that need to be stubbed out when running bootstrap's unit tests, so that they don't download files or cause other problems. (That's unfortunate, but fixing it is outside the scope of this PR.) Currently those code paths are skipped using `#[cfg(test)]` and `#[cfg(not(test))]`, which does achieve the desired effect, but has a few downsides. The stubbed-out code triggers dead-code and unused-import warnings, and makes IDE navigation harder when rust-analyzer treats `cfg(test)` as true when analyzing the code. If we switch to using `cfg!(test)` and `cfg!(not(test))` to “dynamically” skip the relevant code paths, it still gets skipped, but the skipped code is still considered live for the purpose of dead-code warnings and IDE analysis.

…uwer Rollup of 12 pull requests Successful merges: - #159712 (Subtree sync for rustc_codegen_cranelift) - #155697 (Stabilize c-variadic function definitions) - #159285 (Simplify `apply_effects_in_range`) - #159596 (unify the AST repr of type const and const RHS) - #159607 (test: update riscv32e-registers.rs for LLVM 24 MC diagnostic changes) - #159659 (Move `Limit` out of `rustc_hir`) - #159707 (fix error when a dangling ref in a ManuallyDrop is used in a pattern) - #158479 (Reject static item as direct const generic arg) - #158738 (next_trait_solver: Recover from GCE const exprs) - #159451 (Remove config cloning in compiletest) - #159646 (Increase depth for float infer var fallback hack) - #159705 (bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests)

…uwer Rollup of 11 pull requests Successful merges: - #159712 (Subtree sync for rustc_codegen_cranelift) - #155697 (Stabilize c-variadic function definitions) - #159285 (Simplify `apply_effects_in_range`) - #159596 (unify the AST repr of type const and const RHS) - #159607 (test: update riscv32e-registers.rs for LLVM 24 MC diagnostic changes) - #159659 (Move `Limit` out of `rustc_hir`) - #159707 (fix error when a dangling ref in a ManuallyDrop is used in a pattern) - #158738 (next_trait_solver: Recover from GCE const exprs) - #159451 (Remove config cloning in compiletest) - #159646 (Increase depth for float infer var fallback hack) - #159705 (bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests)

Rollup merge of #159705 - Zalathar:cfg-test, r=Kobzol bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests - [Zulip thread: Reducing conditional compilation in bootstrap?](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Reducing.20conditional.20compilation.20in.20bootstrap.3F/with/611882645) --- Bootstrap contains a few code paths that need to be stubbed out when running bootstrap's unit tests, so that they don't download files or cause other problems. (That's unfortunate, but fixing it is outside the scope of this PR.) Currently those code paths are skipped using `#[cfg(test)]` and `#[cfg(not(test))]`, which does achieve the desired effect, but has a few downsides. The stubbed-out code triggers dead-code and unused-import warnings, and makes IDE navigation harder when rust-analyzer treats `cfg(test)` as true when analyzing the code. If we switch to using `cfg!(test)` and `cfg!(not(test))` to “dynamically” skip the relevant code paths, it still gets skipped, but the skipped code is still considered live for the purpose of dead-code warnings and IDE analysis.

…uwer Rollup of 11 pull requests Successful merges: - rust-lang/rust#159712 (Subtree sync for rustc_codegen_cranelift) - rust-lang/rust#155697 (Stabilize c-variadic function definitions) - rust-lang/rust#159285 (Simplify `apply_effects_in_range`) - rust-lang/rust#159596 (unify the AST repr of type const and const RHS) - rust-lang/rust#159607 (test: update riscv32e-registers.rs for LLVM 24 MC diagnostic changes) - rust-lang/rust#159659 (Move `Limit` out of `rustc_hir`) - rust-lang/rust#159707 (fix error when a dangling ref in a ManuallyDrop is used in a pattern) - rust-lang/rust#158738 (next_trait_solver: Recover from GCE const exprs) - rust-lang/rust#159451 (Remove config cloning in compiletest) - rust-lang/rust#159646 (Increase depth for float infer var fallback hack) - rust-lang/rust#159705 (bootstrap: Prefer `cfg!(not(test))` when skipping code paths during unit tests)

Prefer cfg!(not(test)) when skipping code paths during unit tests

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) labels Jul 22, 2026

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Prefer cfg!(not(test)) when skipping downloads

480c37d

Assertions have been added to some code paths that were previously not built during `cfg(test)`, to make sure they aren't accidentally executed.

Zalathar force-pushed the cfg-test branch from db20e5d to 480c37d Compare July 22, 2026 08:24

Zalathar mentioned this pull request Jul 22, 2026

Const eval changes rust-lang/rust-clippy#15773

Merged

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JonathanBrouwer mentioned this pull request Jul 22, 2026

Rollup of 8 pull requests #159715

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JonathanBrouwer mentioned this pull request Jul 22, 2026

Rollup of 12 pull requests #159723

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JonathanBrouwer mentioned this pull request Jul 22, 2026

Rollup of 11 pull requests #159728

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rust-bors Bot merged commit 6d43869 into rust-lang:main Jul 22, 2026
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rustbot added this to the 1.99.0 milestone Jul 22, 2026

Zalathar deleted the cfg-test branch July 23, 2026 00:16

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