library/std/src/macros/tests.rs

// ignore-tidy-dbg

use core::fmt::Debug;

/// Test for <https://github.com/rust-lang/rust/issues/153850>:
/// `dbg!` shouldn't drop arguments' temporaries.
#[test]
fn no_dropping_temps() {
    fn temp() {}

    *dbg!(&temp());
    *dbg!(&temp(), 1).0;
    *dbg!(0, &temp()).1;
    *dbg!(0, &temp(), 2).1;
}

/// Test for <https://github.com/rust-lang/rust/issues/154988>:
/// `dbg!` shouldn't create a temporary that lives past its invocation.
#[test]
fn no_leaking_internal_temps_from_dbg() {
    #[derive(Debug)]
    struct Foo;

    #[derive(Debug)]
    struct Bar<'a>(#[allow(unused)] &'a Foo);
    impl Drop for Bar<'_> {
        fn drop(&mut self) {}
    }

    let foo = Foo;
    let bar = Bar(&foo);
    // If `dbg!` creates a `(Bar<'_>,)` temporary that lasts past its expansion, this will fail
    // to compile, because it will be dropped after `foo`, which it borrows from. The tuple
    // mimics the drop order of block tail expressions before Rust 2024: first the result of `dbg!`
    // is dropped, then `foo`, then any temporaries left over from `dbg!` are dropped, if present.
    (drop(dbg!(bar)), drop(foo));
}