…items

This switches from JS-generated toggles to using the HTML <details> tag
for expanding and collapsing entries in the "Implementors" section.

…, r=pnkfelix

Improve diagnostic for when field is never read

Related to (but does not close) rust-lang#81658

This completes the first step of `@pnkfelix's` [mentoring instructions](rust-lang#81658 (comment)) but does not actually improve the diagnostics (yet!). The two tests are heavily reduced versions of code from the original bug report.

I've confirmed that the reduced `field-used-in-ffi` test [fails on nightly](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f0862c89ddca028c55c20a5ed05e679a) but [passes on stable](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f0862c89ddca028c55c20a5ed05e679a). This confirms that the regression is reproduced correctly. The `drop-only-field` test is a case that `@pnkfelix` mentioned in his mentoring instructions. It is not a regression, but will come in handy when we make the diagnostic smarter by looking at whether the field type implements `Drop`.

Per the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/tests/adding.html), each test includes a comment summarizing what it is about.

Add a suggestion when using a type alias instead of trait alias

Fixes rust-lang#43913

r? `@estebank`

…earth,Nemo157,GuillaumeGomez

Use details tag for trait implementors.

Part of rust-lang#83332 and following on from rust-lang#83337 and rust-lang#83355.

This removes one category of JS-generated toggles (implementors), and replaces them with a `<details>` tag. This simplifies the JS, and fixes some bugs where things that were supposed to be hidden by the toggle were not hidden. Compare https://hoffman-andrews.com/rust/details-implementors/std/io/trait.Read.html#impl-Read vs https://doc.rust-lang.org/nightly/std/io/trait.Read.html#implementors.

This introduces a `left: -23px` to put the toggle in the correct place, matching the current style for `.collapse-toggle`.

It's worth noting this introduces a slight behavior change: since the entire line is now a `<summary>`, any part of the line is clickable. So for instance, in `impl Read for File`, clicking `impl` or `for` will collapse / expand the docs. Clicking `Read` or `File` still links to the appropriate documentation as before.

…=jonas-schievink

⬆️ rust-analyzer

…trochenkov

Improve the docstrings of the `Lto` struct.

This change is the result of [this zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Making.20sense.20of.20LTO.20modes.20in.20rustc).

Hopefully it makes things a little clearer. What do you think?