New lint: adding a char to a String through to_string()

[jakubdabek]

What it does

Warns when turning a char into a String before pushing it to another String.

Advantage

• It's more clear that we're pushing only a single char
• It doesn't allocate a new String

Drawbacks

No response

Example

let mut s = String::from("...");
let c = 'a';
// or: let c = &'a';
s.push_str(&c.to_string());
// or: s = s + &c.to_string();
// or: s += &c.to_string();

Could be written as:

let mut s = String::from("...");
let c = 'a';
s.push(c);
// or: s.push(*c); for the reference case

[J-ZhengLi]

could be an extension of clippy::single_char_add_str, which is a lot easier

[belyakov-am]

@rustbot claim