Don't use `.to_string()` for hardcoded strings where possible.

[MCJOHN974]

What it does

Look at the following code:

fn open(path: impl AsRef<str>) {}

fn main() {
    let path = "test.txt".to_string();
    open(path);
}

Here, we create some string with hard-coded value, and pass it to a function. As we can see, this variable is used only to pass it to open function, and as we see from open signature, it can work both with &str and String, so .to_string() here is unnecessary.

I suggest to add lint which hints that .to_string() after hardcoded &str can be removed if it can be removed.

Advantage

• eliminates unnecessary allocation and copy
• makes code a bit easier to read

Drawbacks

Probably some functions can behave different depending on &str or String was passed (however, I think such functions are bad practice). Thus, this lint maybe should be in pedantic group or so.

Example

fn open(path: impl AsRef<str>) {}

fn main() {
    let path = "test.txt".to_string();
    open(path);
}

Could be written as:

fn open(path: impl AsRef<str>) {}

fn main() {
    let path = "test.txt";
    open(path);
}