A Live String Slice Restricts Mutation Of Its Source

A slice points into an existing string, so the normal borrowing rules apply to both the slice and its source in source. For example, lets take this:

fn main() {
    let mut line = String::from("INFO Server started");
 
    let level = &line[0..4];
 
    line.clear();
 
    println!("{level}");
}

This won’t compile because level refers to bytes inside line. So, calling line.clear() could invalidate that reference, but level is still used afterwards

Rust rejects the mutation instead of letting us keep a broken slice