Performance: Suggest String::new() over "".to_string()

[phansch]

From rust-lang/rust#52805 (comment). String::new() is faster than ""::to_string()

Benchmark:

#[bench]
fn bench_string_new(b: &mut Bencher) {
    b.iter(|| String::new());
}

#[bench]
fn bench_to_string(b: &mut Bencher) {
    b.iter(|| "".to_string());
}

running 2 tests
test tests::bench_string_new ... bench:           0 ns/iter (+/- 0)
test tests::bench_to_string  ... bench:           8 ns/iter (+/- 0)

[matthiaskrgr]

The lint should probably also catch String::from("")

test tests::bench_string_from ... bench:          11 ns/iter (+/- 0)
test tests::bench_string_new  ... bench:           0 ns/iter (+/- 0)
test tests::bench_to_string   ... bench:          11 ns/iter (+/- 2)

[H2CO3]

as well as "".into::<String>()

[matthiaskrgr]

Hm, interestingly, when looking in godbolt print!("{}", String::new()); seems to generate more instructions than print!("{}", "");

[flip1995]

Probably because String::new() is of type String and "" is of type &str

I vote for slow_empty_string_creation as the name.

The suggestions of useless_format should to be changed as well.

warning: useless use of `format!`
 --> src/main.rs:2:13
  |
2 |     let _ = format!("");
  |             ^^^^^^^^^^^ help: consider using .to_string(): `"".to_string()`
  |
  = note: #[warn(useless_format)] on by default

Should this lint be written as PreExpansionPass?

[nagisa]

We should just fix this in liballoc instead.

[phansch]

That seems like a preferable solution. If the allocation is fixed, this could still be a style lint.

[weiwei]

For people who aren't aware: It appears that the issue has been fixed. This is my test result on an Intel Mac:

test tests::bench_string_from  ... bench:           0 ns/iter (+/- 0)
test tests::bench_string_new   ... bench:           0 ns/iter (+/- 0)
test tests::bench_to_string    ... bench:           0 ns/iter (+/- 0)

[xFrednet]

Thank you for the benchmarks! Even if this no longer affects performance, I think having a lint that suggest String::new() over "".to_string() would be good for code consistency. It should probably be a lint in the pedantic category to be Allow-by-default.