alloc::sync::Weak<T, A> is missing core::fmt::Debug

[matthewpipie]

I'm fairly new to Rust so I'm not sure if this is intended, but I am running into this issue while storing a Weak inside a derive(Debug) struct. I checked the Rust source and found this line:

#[stable(feature = "arc_weak", since = "1.4.0")]
impl<T: ?Sized> fmt::Debug for Weak<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "(Weak)")
    }
}

However, this only implements Debug for Weak, not Weak<T, A>.

Should this be changed to say Weak<T, A> instead? I'm happy to make a PR to rust-lang that changes it to:

#[stable(feature = "arc_weak", since = "1.4.0")]
impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "(Weak)")
    }
}

Additionally, though out of the scope of this issue, is there a good reason that this debug function does not try to display the Option that it implicitly contains?

[Skgland]

Is this also applicable to the Weak of Rc?

Additionally, though out of the scope of this issue, is there a good reason that this debug function does not try to display the Option that it implicitly contains?

In Short: Reference Counted pointers often end up in recursive structures, so printing the contents of Weak would likely lead to recursing infinitely (until the stack overflows).

The PR that removed the Debug bound explains this
rust-lang/rust#90291

It also can't be readded as adding such a bound would be a breaking change.

[matthewpipie]

Is this also applicable to the Weak of Rc?

It appears that Rc's Weak correctly implements this:

#[stable(feature = "rc_weak", since = "1.4.0")]
impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "(Weak)")
    }
}

(src: https://doc.rust-lang.org/1.80.1/src/alloc/rc.rs.html#3348 )

In Short: Reference Counted pointers often end up in recursive structures, so printing the contents of Weak would likely lead to recursing infinitely (until the stack overflows).

This makes a ton of sense, thanks!

[matthewpipie]

Made a PR! rust-lang/rust#129673

[matthewpipie]

Merged!