Addresses #40363, #45720, #24483, #23986 and so on
* Expands and refactors collapseDocs and toggleAllDocs
* Adds [-] toggle to all impls (including inherent impl)
* Makes it hiding though main css file, not though element style
May need to be addressed:
* "[-]" and anchor link copier are overlaid a bit
* Inherent methods are also hidden by the global [-] toggle.
* Auto-collapsing "Iterator" and so on by default is not implemented yet
* Tested only shallowly and only in Chromiuim
* No tests. Are there tests for css/js part here?
* The new implementation may be a bit slower.
show in docs whether the return type of a function impls Iterator/Read/Write
Closes#25928
This PR makes it so that when rustdoc documents a function, it checks the return type to see whether it implements a handful of specific traits. If so, it will print the impl and any associated types. Rather than doing this via a whitelist within rustdoc, i chose to do this by a new `#[doc]` attribute parameter, so things like `Future` could tap into this if desired.
### Known shortcomings
~~The printing of impls currently uses the `where` class over the whole thing to shrink the font size relative to the function definition itself. Naturally, when the impl has a where clause of its own, it gets shrunken even further:~~ (This is no longer a problem because the design changed and rendered this concern moot.)
The lookup currently just looks at the top-level type, not looking inside things like Result or Option, which renders the spotlights on Read/Write a little less useful:
<details><summary>`File::{open, create}` don't have spotlight info (pic of old design)</summary>
</details>
All three of the initially spotlighted traits are generically implemented on `&mut` references. Rustdoc currently treats a `&mut T` reference-to-a-generic as an impl on the reference primitive itself. `&mut Self` counts as a generic in the eyes of rustdoc. All this combines to create this lovely scene on `Iterator::by_ref`:
<details><summary>`Iterator::by_ref` spotlights Iterator, Read, and Write (pic of old design)</summary>
</details>
