/concatenation_benchmarks-rs

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Comparing ways to concatenate strings in Rust

Intro

There are many ways to turn a &str into a String in Rust and therefore many ways to concatenate two &strs.

Here I benchmark several different ways to concatenate the strings "2014-11-28", "T" and "12:00:09Z" into "2014-11-28T12:00:09Z".

Thanks to all the comments on and discussion on reddit where I posted these originally only 7 benchmarks. Some go into the details of what is going on in the background of these operations.

How to run?

  • benchmarks: cargo bench
  • tests: cargo test --benches

Results (on my machine)

$ cargo bench

running 34 tests
test array_concat_test ... ignored
test array_join_long_test ... ignored
test array_join_test ... ignored
test collect_from_array_to_string_test ... ignored
test collect_from_vec_to_string_test ... ignored
test format_macro_test ... ignored
test from_bytes_test ... ignored
test mut_string_push_str_test ... ignored
test mut_string_push_string_test ... ignored
test mut_string_with_capacity_push_str_char_test ... ignored
test mut_string_with_capacity_push_str_test ... ignored
test mut_string_with_too_little_capacity_push_str_test ... ignored
test mut_string_with_too_much_capacity_push_str_test ... ignored
test string_from_all_test ... ignored
test string_from_plus_op_test ... ignored
test to_owned_plus_op_test ... ignored
test to_string_plus_op_test ... ignored
test array_concat                                 ... bench:          38 ns/iter (+/- 27)
test array_join                                   ... bench:          38 ns/iter (+/- 22)
test array_join_long                              ... bench:          35 ns/iter (+/- 18)
test collect_from_array_to_string                 ... bench:          80 ns/iter (+/- 17)
test collect_from_vec_to_string                   ... bench:          76 ns/iter (+/- 27)
test format_macro                                 ... bench:         111 ns/iter (+/- 69)
test from_bytes                                   ... bench:           1 ns/iter (+/- 0)
test mut_string_push_str                          ... bench:          75 ns/iter (+/- 35)
test mut_string_push_string                       ... bench:         164 ns/iter (+/- 77)
test mut_string_with_capacity_push_str            ... bench:          32 ns/iter (+/- 14)
test mut_string_with_capacity_push_str_char       ... bench:          29 ns/iter (+/- 12)
test mut_string_with_too_little_capacity_push_str ... bench:          96 ns/iter (+/- 11)
test mut_string_with_too_much_capacity_push_str   ... bench:          37 ns/iter (+/- 8)
test string_from_all                              ... bench:         133 ns/iter (+/- 101)
test string_from_plus_op                          ... bench:          76 ns/iter (+/- 9)
test to_owned_plus_op                             ... bench:          81 ns/iter (+/- 15)
test to_string_plus_op                            ... bench:          83 ns/iter (+/- 17)

test result: ok. 0 passed; 0 failed; 17 ignored; 17 measured; 0 filtered out

Examples explained

array_concat()

let datetime = &[DATE, T, TIME].concat();

array_join()

let datetime = &[DATE, TIME].join(T);

array_join_long()

let datetime = &[DATE, T, TIME].join("");

collect_from_array_to_string()

let list = [DATE, T, TIME];
let datetime: String = list.iter().map(|x| *x).collect();

collect_from_vecto_string()

let list = vec![DATE, T, TIME];
let datetime: String = list.iter().map(|x| *x).collect();

format_macro()

let datetime = &format!("{}{}{}", DATE, T, TIME);

`from_bytes() // ⚠️ don't actually do this

use std::ffi::OsStr;
use std::os::unix::ffi::OsStrExt;
use std::slice;

let bytes = unsafe { slice::from_raw_parts(DATE.as_ptr(), 20) };

let datetime = OsStr::from_bytes(bytes);

mut_string_push_str()

let mut datetime = String::new();
datetime.push_str(DATE);
datetime.push_str(T);
datetime.push_str(TIME);

mut_string_push_string()

let mut datetime = Vec::<String>::new();
datetime.push(String::from(DATE));
datetime.push(String::from(T));
datetime.push(String::from(TIME));
let datetime = datetime.join("");

mut_string_with_capacity_push_str()

let mut datetime = String::with_capacity(20);
datetime.push_str(DATE);
datetime.push_str(T);
datetime.push_str(TIME);

mut_string_with_capacity_push_str_char()

let mut datetime = String::with_capacity(20);
datetime.push_str(DATE);
datetime.push('T');
datetime.push_str(TIME);

mut_string_with_too_little_capacity_push_str()

let mut datetime = String::with_capacity(2);
datetime.push_str(DATE);
datetime.push_str(T);
datetime.push_str(TIME);

mut_string_with_too_much_capacity_push_str()

let mut datetime = String::with_capacity(200);
datetime.push_str(DATE);
datetime.push_str(T);
datetime.push_str(TIME);

string_from_all()

let datetime = &(String::from(DATE) + &String::from(T) + &String::from(TIME));

string_from_plus_op()

let datetime = &(String::from(DATE) + T + TIME);

to_owned_plus_op()

let datetime = &(DATE.to_owned() + T + TIME);

to_string_plus_op()

let datetime = &(DATE.to_string() + T + TIME);