new lint: if bool {} else if !bool {}

#[cfg_attr(feature = "cargo-clippy", warn(clippy_pedantic))]

fn main() {
    println!("Hello, world!");
    foo(true);
}

fn foo(var: bool) {
    if var {
        println!("yay");
    } else if !var {
        println!("nay");
    }
}

if we already checked the trueness of a bool, we can simply use "else" instead of "else if !", since bools can only be either true or false:

    if var {
        println!("yay");
    } else {
        println!("nay");
    }

Start by locating the existing Clippy lint implementations and their tests for conditional expressions involving booleans; the issue does not name specific files or entry points. Confirm the expected diagnostic and test coverage for replacing else if !var after if var with else, then run the relevant lint tests to verify the behavior.