rustdoc: add header map to the table of contents
## Summary
Add header sections to the sidebar TOC.
### Preview
* http://notriddle.com/rustdoc-html-demo-9/toc/rust/std/index.html
* http://notriddle.com/rustdoc-html-demo-9/toc/rust-derive-builder/derive_builder/index.html
## Motivation
Some pages are very wordy, like these.
| crate | word count |
|--|--|
| [std::option](https://doc.rust-lang.org/stable/std/option/index.html) | 2,138
| [derive_builder](https://docs.rs/derive_builder/0.13.0/derive_builder/index.html) | 2,403
| [tracing](https://docs.rs/tracing/0.1.40/tracing/index.html) | 3,912
| [regex](https://docs.rs/regex/1.10.3/regex/index.html) | 8,412
This kind of very long document is more navigable with a table of contents, like Wikipedia's or the one [GitHub recently added](https://github.blog/changelog/2021-04-13-table-of-contents-support-in-markdown-files/) for READMEs.
In fact, the use case is so compelling, that it's been requested multiple times and implemented in an extension:
* https://github.com/rust-lang/rust/issues/80858
* https://github.com/rust-lang/rust/issues/28056
* https://github.com/rust-lang/rust/issues/14475
* https://rust.extension.sh/#show-table-of-content
(Some of these issues ask for more than this, so don’t close them.)
It's also been implemented by hand in some crates, because the author really thought it was needed. Protip: for a more exhaustive list, run [`site:docs.rs table of contents`](https://duckduckgo.com/?t=ffab&q=site%3Adocs.rs+table+of+contents&ia=web), though some of them are false positives.
* https://docs.rs/figment/0.10.14/figment/index.html#table-of-contents
* https://docs.rs/csv/1.3.0/csv/tutorial/index.html#table-of-contents
* https://docs.rs/axum/0.7.4/axum/response/index.html#table-of-contents
* https://docs.rs/regex-automata/0.4.5/regex_automata/index.html#table-of-contents
Unfortunately for these hand-built ToCs, because they're just part of the docs, there's no consistent way to turn them off if the reader doesn't want them. It's also more complicated to ensure they stay in sync with the docs they're supposed to describe, and they don't stay with you when you scroll like Wikipedia's [does now](https://uxdesign.cc/design-notes-on-the-2023-wikipedia-redesign-d6573b9af28d).
## Guide-level explanation
When writing docs for a top-level item, the first and second level of headers will be shown in an outline in the sidebar. In this context, "top level" means "not associated".
This means, if you're writing very long guides or explanations, and you want it to have a table of contents in the sidebar for its headings, the ideal place to attach it is usually the *module* or *crate*, because this page has fewer other things on it (and is the ideal place to describe "cross-cutting concerns" for its child items).
If you're reading documentation, and want to get rid of the table of contents, open the  Settings panel and checkmark "Hide table of contents."
## Reference-level explanation
Top-level items have an outline generated. This works for potentially-malformed header trees by pairing a header with the nearest header with a higher level. For example:
```markdown
## A
# B
# C
## D
## E
```
A, B, and C are all siblings, and D and E are children of C.
Rustdoc only presents two layers of tree, but it tracks up to the full depth of 6 while preparing it.
That means that these two doc comment both generate the same outline:
```rust
/// # First
/// ## Second
struct One;
/// ## First
/// ### Second
struct Two;
```
## Drawbacks
The biggest drawback is adding more stuff to the sidebar.
My crawl through docs.rs shows this to, surprisingly, be less of a problem than I thought. The manually-built tables of contents, and the pages with dozens of headers, usually seem to be modules or crates, not types (where extreme scrolling would become a problem, since they already have methods to deal with).
The best example of a type with many headers is [vec::Vec](https://doc.rust-lang.org/1.75.0/std/vec/struct.Vec.html), which still only has five headers, not dozens like [axum::extract](https://docs.rs/axum/0.7.4/axum/extract/index.html).
## Rationale and alternatives
### Why in the existing sidebar?
The method links and the top-doc header links have more in common with each other than either of them do with the "In [parent module]" links, and should go together.
### Why limited to two levels?
The sidebar is pretty narrow, and I don't want too much space used by indentation. Making the sidebar wider, while it has some upsides, also takes up more space on middling-sized screens or tiled WMs.
### Why not line wrap?
That behaves strangely when resizing.
## Prior art
### Doc generators that have TOC for headers
https://hexdocs.pm/phoenix/Phoenix.Controller.html is very close, in the sense that it also has header sections directly alongside functions and types.
Another example, referenced as part of the [early sidebar discussion](https://github.com/rust-lang/rust/issues/37856) that added methods, Ruby will show a table of contents in the sidebar (for example, on the [ARGF](https://docs.ruby-lang.org/en/master/ARGF.html) class). According to their changelog, [they added it in 2013](06137bde8c/History.rdoc (400--2013-02-24-)).
Haskell seems to mix text and functions even more freely than Elixir. For example, this [Naming conventions](https://hackage.haskell.org/package/base-4.19.0.0/docs/Control-Monad.html#g:3) is plain text, and is immediately followed by functions. And the [Pandoc top level](https://hackage.haskell.org/package/pandoc-3.1.11.1/docs/Text-Pandoc.html) has items split up by function, rather than by kind. Their TOC matches exactly with the contents of the page.
### Doc generators that don't have header TOC, but still have headers
Elm, interestingly enough, seems to have the same setup that Rust used to have: sibling navigation between modules, and no index within a single page. [They keep Haskell's habit of named sections with machine-generated type signatures](https://package.elm-lang.org/packages/elm/browser/latest/Browser-Dom), though.
[PHP](https://www.php.net/manual/en/book.datetime.php), like elm, also has a right-hand sidebar with sibling navigation. However, PHP has a single page for a single method, unlike Rust's page for an entire "class." So even though these pages have headers, it's never more than ten at most. And when they have guides, those guides are also multi-page.
## Unresolved questions
* Writing recommendations for anyone who wants to take advantage of this.
* Right now, it does not line wrap. That might be a bad idea: a lot of these are getting truncated.
* Split sidebars, which I [tried implementing](https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/Table.20of.20contents), are not required. The TOC can be turned off, if it's really a problem. Implemented in https://github.com/rust-lang/rust/pull/120818, but needs more, separate, discussion.
## Future possibilities
I would like to do a better job of distinguishing global navigation from local navigation. Rustdoc has a pretty reasonable information architecture, if only we did a better job of communicating it.
This PR aims, mostly, to help doc authors help their users by writing docs that can be more effectively skimmed. But it doesn't do anything to make it easier to tell the TOC and the Module Nav apart.
This commit adds the headers for the top level documentation to
rustdoc's existing table of contents, along with associated items.
It only show two levels of headers. Going further would require the
sidebar to be wider, and that seems unnecessary (the crates that
have manually-built TOCs usually don't need deeply nested headers).
This approach is, roughly, based on how Discourse does it.
It came up while discussing some other possible sidebar changes,
as a design that made rapid scanning easier while avoiding the
inherent trade-offs in summarizing.
[rustdoc] Make the buttons remain when code example is clicked
Follow-up of https://github.com/rust-lang/rust/pull/125779.
One current issue we have with "run" button and the newly added copy code button is that if you're on mobile devices, you can't use them. I took a look at how `mdbook` is handling it and when you click on a code example, they show the buttons. I think it's a really good idea as if you want to copy the code on your mobile device, you will click on it, showing the buttons.
Feature can be tested [here](https://rustdoc.crud.net/imperio/click-code-example/foo/struct.Bar.html).
r? `@notriddle`
[rustdoc] Add copy code feature
This PR adds a "copy code" to code blocks. Since this is a JS only feature, the HTML is generated with JS when the user hovers the code block to prevent generating DOM unless needed.
Two things to note:
1. I voluntarily kept the current behaviour of the run button (only when hovering a code block with a mouse) so it doesn't do anything on mobile. I plan to send a follow-up where the buttons would "expandable" or something. Still need to think which approach would be the best.
2. I used a picture and not text like the run button to remain consistent with the "copy path" button. I'd also prefer for the run button to use a picture (like what is used in mdbook) but again, that's something to be discussed later on.
The rendering looks like this:
It can be tested [here](https://guillaume-gomez.fr/rustdoc/bar/struct.Bar.html) (without the run button) and [here](https://guillaume-gomez.fr/rustdoc/foo/struct.Bar.html) (with the run button).
Fixes#86851.
r? ``@notriddle``
rustdoc: dedup search form HTML
This change constructs the search form HTML using JavaScript, instead of plain HTML. It uses a custom element because
- the [parser]'s insert algorithm runs the connected callback synchronously, so we won't get layout jank
- it requires very little HTML, so it's a real win in size
[parser]: https://html.spec.whatwg.org/multipage/parsing.html#create-an-element-for-the-token
This shrinks the standard library by about 60MiB, by my test.
There should be no visible changes. Just use less disk space.
