Rustdoc: List traits that a trait implements

[icefoxen]

Basically, a struct will list all the methods and traits that it implements, but a trait will not. So while looking at the docs for a specific struct gets you complete info on what you can do with it, looking at the docs for a trait doesn't tell you anything about what other traits it brings along with it.

For example, IntoIter is implemented for Iterator, but there's no mention of this in the docs for the Iterator trait. So if you implement Iterator on a type you get IntoIter for free along with it but there's no way to discover this unless you dig doooooown through the docs for IntoIter.

Sorry if this is a duplicate.

[Stebalien]

Technically, traits don't actually implement anything (trait objects^† can but that's a different story). Instead, you have blanket impls bounded by a set of traits. That is:

 // Note: Implementations on generic type parameters are called "blanket impls"
impl<T> MyTrait for T where T: Trait1 + Trait2 ... { /* ... */ }

So, in this case, I believe you'd want the Trait1 and Trait2 documentation to list the MyTrait implementation.

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^{† I'm probably being a bit imprecise here. I believe trait object actually refers to &Trait. What do we call the unsized type Trait?}

[bluss]

The unsized type Trait is the object type of the trait with the same name. (Sigh mislicked)

[Mark-Simulacrum]

Nominating for T-dev-tools discussion (cc @nrc) since I think that this is something that would be amazing, but also likely quite difficult.

[Mark-Simulacrum]

Closing in favor of #33772, thanks to @QuietMisdreavus who found it. No need to nominate that since it seems we've had sufficient discussion on this that we do want it very much.