Suggest to change std::iter::repeat(s).take(n).collect::<String>() to s.repeat(n)

[wooster0]

What it does

It suggests the user to change std::iter::repeat(string).take(n).collect::<String>() to string.repeat(n). It should work without the String type argument to collect too and clippy has to find out based on context whether it does collect to a String.
Additionally, when the argument to std::iter::repeat is a constant char like 'a' it should suggest to change that to "a".repeat(n). I said only constant chars because I'm not sure how well it will work if it does that for variable chars as well.

Categories (optional)

• Kind: clippy::perf

use criterion::{criterion_group, criterion_main, Criterion};

fn repeat1(count: usize) -> String {
    std::iter::repeat("a").take(count).collect()
}

fn repeat2(count: usize) -> String {
    "a".repeat(count)
}


fn criterion_benchmark(c: &mut Criterion) {
    c.bench_function("std::iter::repeat", |b| b.iter(|| repeat1(100)));
    c.bench_function("str::repeat", |b| b.iter(|| repeat2(100)));
}

criterion_group!(benches, criterion_benchmark);
criterion_main!(benches);

std::iter::repeat       time:   [433.62 ns 434.36 ns 435.09 ns]                              
Found 10 outliers among 100 measurements (10.00%)
  4 (4.00%) low mild
  5 (5.00%) high mild
  1 (1.00%) high severe

str::repeat             time:   [52.540 ns 52.658 ns 52.857 ns]                        
Found 3 outliers among 100 measurements (3.00%)
  1 (1.00%) high mild
  2 (2.00%) high severe

It's faster and generates a lot less instructions, in the char case too.
It's also easier to read.

Drawbacks

None.

Example

std::iter::repeat("hello").take(10).collect::<String>();
std::iter::repeat('x').take(10).collect::<String>();

Could be written as:

"hello".repeat(10);
"x".repeat(10);

See also: rust-lang/rust#85538.

[ghost]

https://doc.rust-lang.org/std/primitive.str.html#method.repeat
MSRV 1.16
There will be code that uses iter::repeat because it was written before str::repeat was added.

[wooster0]

Then I suppose Clippy should check if the version is >=1.16.0 if that's a thing.

[ghost]

Yes. My last comment was supposed to be a note for the implementer to add that config.