Add copy code to clipboard button in rustdoc

[grey4owl]

A very simple button that was missing in the upper right corner to copy the code blocks to the clipboard. It is hidden by default, but when you hover over the code block (<pre>) element then it appears.. It also works on mobile devices and it will appears after you tap on the code block.

It looks like this:

r? @fmease

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @jsha (or someone else) soon.

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[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

[fmease]

Before I can do a full review, could you please make CI pass (run ./x.py fmt again to make ./x.py test tidy pass) and address the ESLint errors:

EsLint errors

  1739:9   error  'preElements' is never reassigned. Use 'const' instead                   prefer-const
  1740:9   error  'filterElements' is never reassigned. Use 'const' instead                prefer-const
  1741:28  error  Unexpected function expression                                           prefer-arrow-callback
  1741:42  error  'index' is defined but never used. Allowed unused args must match /^_/u  no-unused-vars
  1743:13  error  'copyBtn' is never reassigned. Use 'const' instead                       prefer-const
  1744:13  error  'icon' is never reassigned. Use 'const' instead                          prefer-const
  1745:43  error  Unexpected function expression                                           prefer-arrow-callback
  1746:17  error  'code' is never reassigned. Use 'const' instead                          prefer-const
  1747:17  error  'summary' is never reassigned. Use 'const' instead                       prefer-const
  1752:57  error  Unexpected function expression                                           prefer-arrow-callback
  1757:43  error  Unexpected function expression                                           prefer-arrow-callback

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[fmease]

@grey4owl Any updates on this? If you need any assistance with the CI failures, you can contact me on https://rust-lang.zulipchat.com/ for specific questions.

[grey4owl]

@fmease Sorry i just doesn't have so much free time right now.. I made a new commit, maybe it will go through CI now 🥲

[GuillaumeGomez]

We still need to debate over this UI change: is it worth if there is code below it? Shouldn't we instead add a "space" at the top of the code block to put extra buttons there? For example, a while ago it was proposed to add an "expand" button to show the hidden lines (here). In addition, we could also move the "run" button into this "space".

Overall, having overlaying buttons isn't great I think.

[grey4owl]

@GuillaumeGomez I don't know, it's useful for me when I want to quickly copy the multi-line code (crate example) rather than manually selecting everything and maybe left out some lines. In terms of how the button is displayed, rust book and crates.io uses this hover effect approach as well. And I think it will be more useful here than it is used on the crates.io where you need to copy just one short line of code which you can easily remember..

[GuillaumeGomez]

Didn't say it was not useful, I'd like for rustdoc to have this feature. I'm saying that the hover approach is likely not the right one.

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor