false positive in simple example of io::stdin::lines()

[richlowe]

Summary

Taking the basic example from the io::stdin::lines() documentation, I get clippy warnings about significant drops. I'm not sure if I understand certainly that it's a false positive, but it seems likely since I took the example from the rust documentation here: https://doc.rust-lang.org/std/io/struct.Stdin.html#method.lines

Sorry If I've mis-filed this, or just become overly confused.

Lint Name

clippy::significant_drop_in_scrutinee

Reproducer

I tried this code:

fn main() {
    let lines = std::io::stdin().lines();
    for line in lines {
        println!("foo: {}", line.unwrap());
    }
}

I saw this happen:

   Checking playground v0.0.1 (/playground)
warning: temporary with significant `Drop` in `for` loop condition will live until the end of the `for` expression
 --> src/main.rs:3:17
  |
3 |     for line in lines {
  |                 ^^^^^ another value with significant `Drop` created here
4 |         println!("foo: {}", line.unwrap());
5 |     }
  |      - temporary lives until here
  |
  = note: `#[warn(clippy::significant_drop_in_scrutinee)]` on by default
  = note: this might lead to deadlocks or other unexpected behavior
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_in_scrutinee

warning: temporary with significant `Drop` in `for` loop condition will live until the end of the `for` expression
 --> src/main.rs:3:17
  |
3 |     for line in lines {
  |                 ^^^^^- temporary lives until here
  |
  = note: this might lead to deadlocks or other unexpected behavior
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_in_scrutinee

warning: `playground` (bin "playground") generated 2 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 0.41s

I expected to see this happen:
No clippy warning, or a clippy warning that makes more sense in context.

This is also available on the playground, here: https://play.rust-lang.org/?version=beta&mode=debug&edition=2021&gist=52eb4bd0d9fc3e2cace0cd54cd18da5c

It requires the beta clippy.

Version

rustc 1.63.0-beta.4 (94811fdc2 2022-07-06)
binary: rustc
commit-hash: 94811fdc269d23dd9f1dff88f165bc2d6f5f85d4
commit-date: 2022-07-06
host: x86_64-unknown-illumos
release: 1.63.0-beta.4
LLVM version: 14.0.5

Additional Labels

No response

[Jarcho]

The for loop in that example is lowered to (roughly):

match lines {
    lines => loop {
        match lines.next() {
            Some(line) => { /* body */ }
            None => break,
        }
    }
};

It probably shouldn't warn as the drop order is exactly as one would expect.