needless_range_loop produces code with different behaviour · rust-lang/rust-clippy#6930

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A-documentationL-suggestiongood first issue

Description

needless_range_loop suggests changing:

pub fn foo_a(out: &mut Vec<usize>, inx: &[usize], base: usize) {
    for i in 1 .. base {
        out[i] = *inx.get(i).unwrap_or(&0);
    }
}

pub fn foo_b(out: &mut Vec<usize>, inx: &[usize], base: usize) {
    for (i, item) in out.iter_mut().enumerate().take(base).skip(1) {
        *item = *inx.get(i).unwrap_or(&0);
    }
}

However, these are only equivalent is out has at least base elements. If not, then this changes behaviour (the first version panics with out of bounds, the second silently truncates the loop).

See this example at https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6f00aec168a98758df574f0734a2233e (edit: this link was previously broken, thanks)

(I noticed this will discussing needless_range_loop in #6075 )

Contributor guide

Tech stack: rust
Domain: toolingdeveloper experience
Issue type: bug
Difficulty: 3
Estimated time: 1-2 days
Activity status: blocked
Clarity: clear
Prerequisites: Rust programmingclippy internalstesting lints
Newbie friendliness: 45
Research direction: Investigate the needless range loop lint implementation in clippy lints/src/loops.rs. The issue shows that the lint incorrectly suggests a transform that changes behavior when the slice length is less than `base`. To fix, ensure the transform is only suggested when the length is sufficient, or adjust the suggestion to preserve semantics. Review comments from this issue and #6075 for prior discussion. Add test cases in tests/ui/needless range loop.rs to cover edge cases where the vector length is shorter than the loop range.