Warn on push_str(&format!())

[kangalio]

What it does

Emits a warning on code like ```rust
some_string.push_str(&format!(...));
// or
some_string += &format!(...);

and suggests rewriting it to ```rust
write!(&mut some_string, ...);

Categories

• Kind: Perf

What is the advantage of the recommended code over the original code
It avoids one extra heap allocation by writing directly to the existing String instead of creating a new temporary String.

Drawbacks

The user has to use std::io::Write for the write! macro to work (which I don't get tbh, why can't write! just import the trait on its own...)

Example

let mut string = String::new();
for foo in foo_list {
    string += &format!("{:?}", foo);
}

Could be written as:

use std::io::Write;

let mut string = String::new();
for foo in foo_list {
    write!(&mut string, "{:?}", foo);
}

[Urcra]

I had actually thought this would be optimized away but seems like the slowdown is quite significant in release mode see example here https://play.rust-lang.org/?version=nightly&mode=release&edition=2018&gist=daed734101a8d8bc0c6a7bd811154504

There are some additional drawbacks in addition to the ones you mentioned however. write! returns a result so you would need to write it as:

use std::fmt::Write;

let mut string = String::new();
for foo in foo_list {
    write!(&mut string, "{:?}", foo).expect("This can never fail");
}

Which might cause other lints to give you trouble specifically lints such as unwrap_in_result if using this piece of code in a function that returns a Result

You also need to import either/both std::fmt:Write and std::io::Write and in the case of importing both of them you need to give them an alias so they don't clash such as:

use std::fmt::Write as FmtWrite;
use std::io::Write as IoWrite;

I also feel the readability of string += &format!("{:?}", foo); is better than write!(&mut string, "{:?}", foo).expect("This can never fail"); Though that is more of a subjective opinion

All that said I still feel like the performance probably makes up for the drawbacks.

I'll try to implement a lint for this over weekend

[camsteffen]

If anyone asks about using write! with String, just change the subject to how amazing Rust's ownership system is or something like that.

[pitaj]

@rustbot claim