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yezhizhen opened this issue May 8, 2025 · 2 comments · May be fixed by #14757
Open

False positive needless_match for even simple example #14754

yezhizhen opened this issue May 8, 2025 · 2 comments · May be fixed by #14757
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@samueltardieu
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C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have I-suggestion-causes-bug Issue: The suggestion compiles but changes the code to behave in an unintended way

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@yezhizhen
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yezhizhen commented May 8, 2025 •
edited by rustbot
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Summary

needless_match is warned, while the suggestion would really change the execution logic.

Discovered in servo/servo#36908 (comment)

Lint Name

needless_match

Reproducer

I tried this code:

fn side_effect() {
    println!("Side effect!");
}

fn t() -> Result<i32, &'static str> {
    if 5 > 3 { Ok(3) } else { Err("Error") }
}

fn logic() -> Result<i32, &'static str> {
    let x = match t() {
        Ok(v) => Ok(v),
        err @ Err(_) => return err,
    };
    // Only when Ok
    side_effect();
    x
}

fn main() {
    let _ = logic();
}

I saw this happen:

warning: this match expression is unnecessary
  --> src\main.rs:10:13
   |
10 |       let x = match t() {
   |  _____________^
11 | |         Ok(v) => Ok(v),
12 | |         err @ Err(_) => return err,
13 | |     };
   | |_____^ help: replace it with: `t()`

I expected to see this happen:
No Warning. As the suggestion would change the execution logic.

Version

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-pc-windows-msvc
release: 1.85.1
LLVM version: 19.1.7

Additional Labels

No response
@yezhizhen yezhizhen added C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have labels May 8, 2025
@samueltardieu
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@rustbot claim
@rustbot label +I-suggestion-causes-error

@rustbot rustbot added the I-suggestion-causes-error Issue: The suggestions provided by this Lint cause an ICE/error when applied label May 8, 2025
@samueltardieu samueltardieu linked a pull request May 8, 2025 that will close this issue
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@samueltardieu
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@rustbot label -I-suggestion-causes-error +I-suggestion-causes-bug

@rustbot rustbot added I-suggestion-causes-bug Issue: The suggestion compiles but changes the code to behave in an unintended way and removed I-suggestion-causes-error Issue: The suggestions provided by this Lint cause an ICE/error when applied labels May 8, 2025
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@samueltardieu samueltardieu

Labels
C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have I-suggestion-causes-bug Issue: The suggestion compiles but changes the code to behave in an unintended way
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@samueltardieu @yezhizhen @rustbot