Actually highlight the ? operator in rendered code

[est31]

About one year after the ? operator was stabilized with the promise that it will get highlighting when being rendered, work on getting hightlighting of ? into text editors and renderers has stalled.

I've done work on this for rustdoc in #37102. For (current, non rewritten) rustdoc, it looks like this:

But I've found a couple of places where there is still no highlighting:

• Sublime text (This is a screenshot of how its rendered using syntect)
• mdbook (see it in action here, backup)
• kate (screenshot here)
• rust playground
• vim
• RustyCode VSCode plugin

If you stumble across additional cases, please mention them here!

I know these projects are separate from Rust itself, but its important to get ? highlighted consistently so that it can't be overlooked.

cc @Stebalien @eddyb @GuillaumeGomez

[eddyb]

I believe I locally hacked this into my kate syntax highlighting files:

<DetectChar char="?" attribute="QM" context="#stay"/>

<itemData name="QM" defStyleNum="dsError" bold="true" underline="false"/>

[cramertj]

If you stumble across additional cases

The VIM plugin (using 'rust-lang/rust.vim') and the RustyCode VSCode plugin don't highlight ?.

[ishitatsuyuki]

Given that most IDEs have moved to RLS, is this still a thing?

[petrochenkov]

RLS has to do anything with highlighting?

In VSCode neither default syntax scheme for Rust nor plugins highlight ? currently.

[ishitatsuyuki]

IIRC language server protocol has a feature called semantic highlighting.