rust-lang/rust-clippy

needless_range_loop produces code with different behaviour

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#6,930 opened on Mar 18, 2021

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A-documentationL-suggestiongood first issue

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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);
    }
}

to

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

Research direction
Analyze the needless range loop lint logic and ensure it only suggests the transformation when the container length is guaranteed to be at least as large as the range bound, to avoid behavioral differences.
Tech stack
rust
Domain
tooling
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3
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