Rust-clippy: Warn on `push_str` with a single-character string literal

[camelid]

What it does

Warns when using push_str with a single-character string literal, and push with a char would work fine.

Categories (optional)

• Kind: clippy::style and clippy::perf (maybe clippy::pedantic?)

What is the advantage of the recommended code over the original code?

• It's more obvious what's going on; after all, you're not trying to push a string, you're trying to push a character
• It should be more performant and reduce binary size to push a char, since it's just a number and will be inlined, than to push a &'static str, which must be stored in the binary and accessed via a pointer

Drawbacks

None.

Example

let mut string = String::new();
string.push_str("R");

Could be written as:

let mut string = String::new();
string.push('R');

[camelid]

@rustbot modify labels to +A-style +A-performance

[camelid]

Hmm, rustbot's not responding...

[Mark-Simulacrum]

@rustbot modify labels: -A-style

Okay, seems to work, just had to add rustbot with write permissions to this repo.

[camelid]

Correct me if I'm wrong, but this lint does not seem to working on the latest nightly. With this code:

// src/main.rs

fn main() {
    let mut string = String::new();
    string.push_str("R");
}

When I run nightly clippy, I don't get any warnings:

$ cargo +nightly clippy
    Checking clippy-issue-5875 v0.1.0 (~/clippy-issue-5875)
    Finished dev [unoptimized + debuginfo] target(s) in 0.11s

Output of cargo clippy --version:

clippy 0.0.212 (04488afe3 2020-08-24)

[ebroto]

@camelid if I'm not mistaken, the clippy component is built now from the rust-lang/rust repo copy and not from this repo. The latest sync was merged yesterday (see here), so I think it should be available on the next nightly.

cc @flip1995 for any imprecision with the component release process, I don't want to lie to anyone :)