Using `format!` to add a prefix/suffix to `String` and then never using the original `String` again

What it does

Detects uses of format! to add a prefix or suffix to a String, and then never accessing the original String again.

Advantage

• Reduces allocations and memory copies

Drawbacks

No response

Example

pub fn example1(val: String) -> String {
    format!("a{val}")
}

pub fn example2(val: String) -> String {
    format!("abcd{val}")
}

pub fn example3(val: String) -> String {
    format!("{val}a")
}

pub fn example4(val: String) -> String {
    format!("{val}abcd")
}

Could be written as:

pub fn example1(mut val: String) -> String {
    val.insert(0, 'a');
    val
}

pub fn example2(mut val: String) -> String {
    val.insert_str(0, "abcd");
    val
}

pub fn example3(mut val: String) -> String {
    val.push('a');
    val
}

pub fn example4(mut val: String) -> String {
    val.push_str("abcd");
    val
}