Multiple doc issues with "Important Traits" feature

[Diggsey]

First example: https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#method.with_capacity

• On the tool-tip the dark red and cyan text on a black background are very hard to read, and the use of very bold white text on a solid black background looks out of place in the documentation.

• The traits shown are not important. Every function which returns a Vec will now have an information icon explaining that Vec<u8> implements Write.

• It's strange that the "information" icon on a function is related specifically to the return type of that function.

Second example: https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#method.into_boxed_slice

• The tool-tip will arbitrarily pick the first type to display in the event that there are multiple possibilities. (In this case it picks Box<I> where the type parameter I means nothing at that point)

• The modal dialog needs more spacing and probably a border.

• Increases the inconsistency in style of the documentation. The docs used to be so simple that they did seem to have a consistent style throughout. Now rustdoc has a lot more features, and the mishmash of different styles is becoming an issue. Adding a whole new model dialog component doesn't help.

Some suggestions for improvements:

• Use a normal tool-tip
• Only show traits which are implemented for a super-set of the types returned by a function, so:

Impl. def.                Return type    Shown?
-----------------------------------------------
impl Write for Vec<u8>    Vec<T>         No
impl Write for Vec<u8>    Vec<u8>        Yes
impl Write for Vec<T>     Vec<T>         Yes
impl Write for Vec<T>     Vec<u8>        Yes

• Show the return type on the tool-tip, not the type which is mentioned in the impl. definition.

Overall, I don't think that putting this information in a modal is the best option. Personally I think it would make more send to just add a "Return value" section, and put the information in there. It will make the documentation for those functions more verbose, but it will reduce the complexity of the documentation as a whole, and keep everything consistent.

[scottmcm]

More: https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#impl-Index%3Cusize%3E

• It seems everything returning a reference explains that &mut T: Iterator
• It's telling me about &mut T for a method that returns &T

[GuillaumeGomez]

Working on it.

[GuillaumeGomez]

Wrote the fix, looks like this:

I'll open the PR tomorrow.

[GuillaumeGomez]

Merged so I suppose it can be closed now. Don't hesitate to tell if I forgot something.

[pravic]

I could not find the source of this feature, so I'll write here.

1. What is the idea behind it?

2. Only one of the original issues were closed in Invert colors in important traits tooltip #46392. Rest are alive still.

3. The exclamation icon looks ugly (not aliased).

4. Font of the tooltip is a bit blurred.

5. And not related here, but anyway: that empty square inside the "Nightly only" warning. I suppose there must be some character or icon (<span class="microscope">🔬</span>).

Chrome 64, Windows 7/10.