Rustdoc links can force constructors to run

The following repro makes a documentation link from a unused dependency linked to the final binary ->

dep.rs

#[no_mangle]
pub static mut m: u8 = 0;

#[no_mangle]
extern "C" fn mark_ctor() {
    unsafe {
        m = 1;
    }
    eprintln!("shouldn't be called");
}

#[used]
#[link_section = ".init_array"]
static INIT: extern "C" fn() = mark_ctor;

middle.rs with the doc link

//! [`dep`]

pub fn value() -> u8 {
    7
}

app.rs

fn main() {
    println!("{}", middle::value());
}

$ rustc --crate-type=rlib dep.rs && rustc --crate-type=rlib middle.rs --extern dep -L. && rustc app.rs --extern middle -L. && ./app
shouldn't be called
7

Start by reproducing the behavior with dep.rs, middle.rs, and app.rs using the rustc commands shown. Then trace how the rustdoc link in middle.rs affects dependency inclusion and linking. Done means the documentation link still resolves while the unused dependency's constructor is not linked into or run by app.