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clarfonthey opened this issue Jun 6, 2025 · 3 comments · Fixed by #142208
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Function only used in const assert is treated as dead code #142104

clarfonthey opened this issue Jun 6, 2025 · 3 comments · Fixed by #142208
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A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. C-bug Category: This is a bug. L-dead_code Lint: dead_code T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@clarfonthey
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clarfonthey commented Jun 6, 2025 •
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I tried this code: playground link

const fn is_nonzero(x: u8) -> bool {
    x != 0
}
const _: () = {
    assert!(is_nonzero(2));
};

I expected to compile without any warnings.

Instead, it indicates that is_nonzero is dead code:

warning: function `is_nonzero` is never used
 --> src/lib.rs:1:10
  |
1 | const fn is_nonzero(x: u8) -> bool {
  |          ^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

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rustc --version --verbose:

rustc 1.89.0-nightly (5d707b07e 2025-06-02)
binary: rustc
commit-hash: 5d707b07e42766c080c5012869c9988a18dcbb83
commit-date: 2025-06-02
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5
@clarfonthey clarfonthey added the C-bug Category: This is a bug. label Jun 6, 2025
@rustbot rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Jun 6, 2025
@Noratrieb Noratrieb added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. L-dead_code Lint: dead_code A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. and removed needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. labels Jun 6, 2025
@Noratrieb
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Noratrieb commented Jun 6, 2025 •
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Constants with the name _ should be seen as live for the dead code analysis.

@hanna-kruppe
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Some previous discussion in #142075

@DoubleHyphen
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I swear, myself and @clarfonthey are completely different people.

@Urgau Urgau mentioned this issue Jun 8, 2025
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@bors bors closed this as completed in facc5da Jun 9, 2025
rust-timer added a commit that referenced this issue Jun 9, 2025
@rust-timer
Unrolled build for #142208
ca7b3ba 
Rollup merge of #142208 - Urgau:dead_code-const_, r=petrochenkov

Always consider `const _` items as live for dead code analysis

This PR alters dead code analysis to always consider `const _: () = { ... };` to be live.

This doesn't address the `_name` pattern from #142075.

Fixes #142104
tgross35 added a commit to tgross35/compiler-builtins that referenced this issue Jun 9, 2025
@tgross35
Rollup merge of #142208 - Urgau:dead_code-const_, r=petrochenkov
4bad7eb 
Always consider `const _` items as live for dead code analysis

This PR alters dead code analysis to always consider `const _: () = { ... };` to be live.

This doesn't address the `_name` pattern from rust-lang/rust#142075.

Fixes rust-lang/rust#142104
github-actions bot pushed a commit to rust-lang/miri that referenced this issue Jun 10, 2025
@tgross35
Rollup merge of #142208 - Urgau:dead_code-const_, r=petrochenkov
752f764 
Always consider `const _` items as live for dead code analysis

This PR alters dead code analysis to always consider `const _: () = { ... };` to be live.

This doesn't address the `_name` pattern from rust-lang/rust#142075.

Fixes rust-lang/rust#142104
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A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. C-bug Category: This is a bug. L-dead_code Lint: dead_code T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Always consider const _ items as live for dead code analysis Urgau/rust
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@hanna-kruppe @clarfonthey @DoubleHyphen @rustbot @Noratrieb