Fast String Concatenation?

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Good point! I've added the following two benches using .concat(). One with a vec and the other with an array:

#[bench]
fn format_url_vec_concat(b: &mut Bencher) {
    let index = "test_idx".to_string();
    let name = "test_alias".to_string();

    b.iter(|| {
        let url = vec![ "/", &index[..], "/_alias/", &name[..] ].concat();
        url
    });
}

#[bench]
fn format_url_array_concat(b: &mut Bencher) {
    let index = "test_idx".to_string();
    let name = "test_alias".to_string();

    b.iter(|| {
        let url = [ "/", &index[..], "/_alias/", &name[..] ].concat();
        url
    });
}

Results on my machine look like:

test format_url_array_concat ... bench:          69 ns/iter (+/- 16)
test format_url_concat       ... bench:         280 ns/iter (+/- 59)
test format_url_macro        ... bench:         287 ns/iter (+/- 36)
test format_url_push         ... bench:          51 ns/iter (+/- 7)
test format_url_vec_concat   ... bench:         109 ns/iter (+/- 4)
test format_url_write        ... bench:          83 ns/iter (+/- 30)

So the array concat is only a fraction slower than the push_str method and the vec method is a bit slower than the write one.

(Note, I'm also getting crazy variance in the benches on Windows... Some runs show the figures above, some look more like this:

test format_url_array_concat ... bench:         181 ns/iter (+/- 7)
test format_url_concat       ... bench:         708 ns/iter (+/- 21)
test format_url_macro        ... bench:         734 ns/iter (+/- 40)
test format_url_push         ... bench:         129 ns/iter (+/- 4)
test format_url_vec_concat   ... bench:         110 ns/iter (+/- 148)
test format_url_write        ... bench:         197 ns/iter (+/- 4)

Not sure why, but taking the variance into account the url_push still looks the fastest.

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