Rustdoc: in "Blanket Implementations" list for a given type, show std traits separately from non-std traits

[abonander]

The "Blanket Implementations" section has a high noise level since it lists a lot of traits that you tend to assume every type implements:

• Any
• Borrow<T> for T
• BorrowMut<T> for T
• From<T> for T
• Into<T> for T
• ToOwned (for types implementing Clone)
• TryFrom<T> for T
• TryInto<T> for T

For example, blake2::Blake2b has this issue where the important blanket impl of Digest is easy to miss amongst all the std traits: https://www.reddit.com/r/rust/comments/lpgw1n/hey_rustaceans_got_an_easy_question_ask_here_82021/gohdi07/?context=10000

For this reason, I'd like to see blanket impls of std traits listed separately, preferably below traits that aren't from std. Perhaps this shouldn't just apply to the "Blanket Implementations" section but the main "Trait Implementations" section as well.

[jyn514]

cc @RDambrosio016, I think you suggested something like this in the Rustdoc discord a while back.

[RDambrosio016]

My suggestion applied to separating derived traits, not this, but i like this change too :)

[Stupremee]

Should this also apply to normal trait implementations, as suggested by @abonander?

I feel like they should only be separated by either "from std or not" or "derived or not", but not both at the same time since that would add too many sections IMO. If we separate by "from std or not", it's also partially separated by "derived or not" since most std-traits are implemented by derive

[jyn514]

I think "from std" is maybe more useful - you can derive Serialize and Deserialize (and even Error/Display), but those are still important to show near the top.

[Stupremee]

@rustbot claim

[jyn514]

Note the PR implementing this was closed: #84921 (comment)

[abonander]

@GuillaumeGomez here is the issue summed up:

https://docs.rs/blake2/0.9.1/blake2/struct.Blake2b.html#blanket-implementations

All the blanket impls outlined in red are from std, implemented by every type and so are noise. ToOwned is semi-meaningful because it's not implemented by every type, but it's still from std and implemented by a large number of types so it's not as important.

The impls outlined in green are important for understanding how to use the type being documented here. However, because they're buried in "uninteresting" blanket impls they're very easy to miss. They should be visually separate from the others.

How we separate "uninteresting" impls from "interesting" ones is open to bikeshedding, but that's the primary problem to solve. I figured an easy way to separate these would be "from std or core" vs "not from std or core".

And as shown in the OP, this is actually confusing people in the wild so I think it's worth addressing.

[GuillaumeGomez]

It is, but the issue is the same with any of the dependencies having blanket implementations. Focusing only on std and core ones doesn't seem like the right approach, which is why I wanted to discuss about a more global improvement.

For example, we could list/group blanket implementations by crate. Like: "Blanket implementations from std", "Blanket implementations from X". And we would start listing the blanket implementations from the current crate.

Does that approach sounds good to you? If not, what would you suggest?