When returning the outcome of consuming a StdinLock, why was the borrow to stdin retained?

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This seems to be a bug in the compiler. You can make the compiler happy by using an explicit return statement:

use std::io::{stdin, BufRead};

fn foo() -> usize {
    let stdin = stdin();
    let stdinlock = stdin.lock();
    return stdinlock
        .lines()
        .count();
}

fn main() {}

playground

As mentioned in the comments, there are multiple Rust issues related to this:

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