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.