rust-lang/rust-clippy
Auf GitHub ansehenLint for `fold` closure that never moves the accumulator
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#6.053 geöffnet am 16. Sept. 2020
A-lintL-perfgood first issue
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Beschreibung
What it does
Lint when a fold closure always returns the accumulator from the input, having only used it by reference. (As opposed to consuming the accumulator, or returning something else.)
Categories (optional)
- Kind:
perf
The compiler currently cannot always optimize away passing along the accumulator every time (see https://github.com/rust-lang/rust/issues/76725), so it's better to not do that if it's actually the same thing every time anyway.
Drawbacks
None.
(Well, it leaves something mut, but that's easily fixable with let v = v;.)
Example
let word = word.to_lowercase();
let char_count: HashMap<char, usize> = word.chars().fold(HashMap::new(), |mut chars, c| {
*chars.entry(c).or_default() += 1;
chars
});
Could be written as:
let word = word.to_lowercase();
let mut char_count: HashMap<char, usize> = HashMap::new();
word.chars().for_each(|c| *chars.entry(c).or_default() += 1);