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>
rustdoc: Fix implementors list javascript
* Use a different loop variable, `i` was already taken. This caused
missing items in the implementors list.
* Use `.getAttribute('href')` rather than `.href` to get the relative
URL which is what it needs to actually fix the links.
More fallout from #41307.
r? @GuillaumeGomez
* Use a different loop variable, `i` was already taken. This caused
missing items in the implementors list.
* Use `.getAttribute('href')` rather than `.href` to get the relative
URL which is what it needs to actually fix the links.
