new perf lint: use conditional Vec::with_capacity() when conditionally pushing elements one by one

original rustc ticket: https://github.com/rust-lang/rust/issues/52495

code:

fn main() {
    foo(false);
}

fn foo(yes: bool) {
    let mut x = Vec::new();
    println!("capacity: {}", x.capacity());
    println!("length : {}", x.len());
    println!("pushing data");
    if yes {
	x.push(1);
        x.push(2);
        x.push(3);
        x.push(4);
    } else {
        x.push(1);
        x.push(2);
        x.push(3);
        x.push(4);
        x.push(5);
        x.push(6);
    }
    println!("capacity: {}", x.capacity());
    println!("length : {}", x.len());
    println!("vec: {:?}", x);
}

current output:

capacity: 0
length : 0
pushing data
capacity: 8
length : 6
vec: [1, 2, 3, 4, 5, 6]

the lint could suggest to use Vec::with_capacity() conditionally:

    let mut x = Vec::with_capacity(if yes { 4 } else { 6 });

fn main() {
    foo(false);
}

fn foo(yes: bool) {
    let mut x = Vec::with_capacity(if yes { 4 } else { 6 });
    println!("capacity: {}", x.capacity());
    println!("length : {}", x.len());
    println!("pushing data");
    if yes {
	x.push(1);
        x.push(2);
        x.push(3);
        x.push(4);
    } else {
        x.push(1);
        x.push(2);
        x.push(3);
        x.push(4);
        x.push(5);
        x.push(6);
    }
    println!("capacity: {}", x.capacity());
    println!("length : {}", x.len());
    println!("vec: {:?}", x);
}

Start by reading the linked rustc ticket and the examples in this issue to understand the conditional push pattern and intended suggestion. Identify the Clippy lint entry points and relevant test location in the repository. Done means the lint recognizes the pattern and suggests conditional Vec::with_capacity() without changing the resulting elements.