Rust-clippy: Performance: Suggest String::new() over "".to_string()

[phansch]

From https://github.com/rust-lang/rust/pull/52805#discussion_r205945647. 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

[mikerite]

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.