rust-lang/rust-clippy

`needless_range_loop` suggestion removes panics

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#16003 opened on Nov 1, 2025

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C-bugI-suggestion-causes-buggood first issue

Description

Description

needless_range_loop currently suggests using take(n) when the iteration is not bounded by the vec's length. This is dangerous since it will silently truncate the iterated range which can very easily lead to data corruption. It may be fine to do so, but we don't have the necessary information to make that decision from clippy.

The following:

fn f() {
    let n = 15;
    let v = vec![1, 2];
    
    for i in 0..n {
        let _ = n;
        let _ = v[i];
    }
}

The current suggestion is effectively:

fn f() {
    let n = 15;
    let v = vec![1, 2];
    
    for (i, x) in v.iter().enumerate().take(n) {
        let _ = n;
        let _ = *x;
    }
}

It should be something that preserves the panic. e.g.:

fn f() {
    let n = 15;
    let v = vec![1, 2];
    
    for (i, x) in v[..n].iter().enumerate() {
        let _ = n;
        let _ = *x;
    }
}

Note that this will move the panic to occur before iteration rather than on the iteration that goes out of bounds. This behaviour is likely to be fine, but it should be called out by the lint when making this suggestion.

Version

clippy 0.1.91 (f8297e351a 2025-10-28)

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Contributor guide

Tech stack
rust
Domain
developer experiencetooling
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.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
RustClippy internalslint creation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The issue identifies that the `needless range loop` lint suggests using `.take(n)` which can silently truncate the iteration, removing potential panics. The fix should replace `.take(n)` with `v[..n].iter().enumerate()` to preserve the panic at the start of iteration. Look at the lint's implementation in files like `clippy lints/src/needless range loop.rs` or similar, and update the suggestion logic accordingly. Ensure that the new suggestion also calls out the change in panic behavior as noted. Review comments for any additional constraints or edge cases.