Derived traits in the standard library are missing stability information

[jethrogb]

impl Default for Duration was added in 1.15 but the current documentation doesn't indicate it wasn't available before that.

[steveklabnik]

We never talk about old versions for documentation; if you want to see that version's docs, check out the docs for that version.

Thanks, but this is something we just don't do.

[jethrogb]

I'm not sure what you mean, there are version numbers all over the page I linked.

[steveklabnik]

I don't see "version" anywhere there, could you send me a screenshot or something? It's possible I'm just misunderstanding you.

[steveklabnik]

Ohhhh I see now, you're not talking about in text, you're talking about the stability flags.

I believe that implementations are auto-stable, and therefore, don't have the flag, and therefore, don't have this. So this is more of a feature request.

Sorry for the misunderstanding here.

[jethrogb]

[jethrogb]

The XyzAssign implementations have a version number though.

[codyps]

Likely because the trait itself was stabilized on that version number (trait XyzAssign was added in 1.9.0).

Agreed that having a marker on impls would be useful.

[jethrogb]

Ok, looking at the source, it seems the cause is that you can't specify a stability attribute for a #[derive]. Related #29152

[steveklabnik]

Triage: no changes I'm aware of.

[jyn514]

I don't think this is really a rustdoc bug - the information just isn't available in the source code. I'm not sure who's in charge of #[stable] and #[unstable], maybe T-compiler?

That said, I think a syntax like #[derive(Clone(stable = "1.15.0"))] might be nice.

[petrochenkov]

the information just isn't available in the source code

It should be available, starting from #29152 stability attributes are copied from the item to derived impls.
If they are not displayed in the doc, then something goes wrong along the way.

[petrochenkov]

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9ef33271d10b380c800731eb31835323

#[stable(feature = "name", since = "version")]
#[derive(Clone)]
struct S;

#[stable(feature = "name", since = "version")]
struct S;
#[automatically_derived]
#[allow(unused_qualifications)]
#[stable(feature = "name", since = "version")]
impl ::core::clone::Clone for S {
    #[inline]
    fn clone(&self) -> S { match *self { S => S, } }
}

[jyn514]

Hmm, that seems like the wrong behavior though? Duration was stabilized in 1.3.0 but impl Default for Duration is only present since 1.15.0. I would rather not show the stability than show the wrong stability.

[petrochenkov]

Ah, I see what is the issue now, the reported stabilization version for the impl is incorrect exactly because it's inherited from the item.

I don't think it's something important enough to extend derive syntax for (or do anything else really).