rust-lang/rust-clippy

`manual_flatten` false positives with reference slices

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#7,514 opened on Jul 29, 2021

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Description

Lint name: manual_flatten

I tried this code:

struct Test {
    a: Option<usize>,
    b: Option<usize>,
}

fn main() {
    let t = Test {
        a: Some(1),
        b: Some(2),
    };

    for x in [&t.a, &t.b] {
        if let Some(y) = x {
            println!("{}", y);     
        }
    }
}

This also applies to mutable references.

Clippy suggests this, which is both incomplete as a refactor and does not compile. Try as I might I could not get into_iter().flatten() to compile.

for x in [&t.a, &t.b].into_iter().flatten() {
    if let Some(y) = x {
        println!("{}", y);
    }
}

Meta

I saw https://github.com/rust-lang/rust-clippy/issues/6893 which appears to be the same issue, but it was claimed to be fixed there and that fix should have landed in 1.53. I'm on 1.54 now and still seeing it with both mutable and immutable reference slices.

  • cargo clippy -V: clippy 0.1.54 (a178d03 2021-07-26)
  • rustc -Vv:
rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-unknown-linux-gnu
release: 1.54.0
LLVM version: 12.0.1

Contributor guide

Tech stack
rust
Domain
toolingdeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust programmingClippy lint development
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Examine the previous fix for issue #6893 (PR #7039) and understand why reference slices still produce false positives for 'manual flatten'. Investigate the 'manual flatten' lint implementation in clippy utils/src/ty.rs and the relevant test files in tests/ui/manual flatten.rs to identify the missing case for reference slices (immutable and mutable). Consider updating the lint to skip references or adjust the pattern matching.