rustdoc: Add option to hide imports from rendered examples

[lcmgh]

Problem

Adding imports to code examples are necessary to make them compile however it makes code examples more hard to read. That's why many developers choose to hide them from the generated output. This leads to the problem that when copying code examples users have to manually choose the correct imports which can be error-prone.

Proposed Solution

Add an UI option to each Rust code block that allows to dynamically hide and show the imports.

Notes

This may raise the question if we also want an option to show/hide all hidden code fragments.

[ehuss]

Transferred to rust-lang/rust as this sounds like a rustdoc issue, and that is where rustdoc issues are tracked.

[jyn514]

you can already do this today: https://doc.rust-lang.org/beta/rustdoc/write-documentation/documentation-tests.html#hiding-portions-of-the-example

if you want a button that users can click to show or hide it, that's harder, but i'm not sure why it would be useful?

[jsha]

This sounds like a Curse of Knowledge issue :-D. Doc authors hide imports because the needed imports are obvious to the author, but they are sometimes not obvious to the reader.

It would probably be good to add to the rustdoc book, under the "How to write documentation > What to include (and exclude) > Examples" section, a recommendation to include any necessary use statements in the first example for a given item (including for each method), and optionally hide them for brevity in subsequent examples.

[lcmgh]

if you want a button that users can click to show or hide it, that's harder, but i'm not sure why it would be useful?

This helps users to understand examples quickly without getting distracted by imports while still providing an option to copy a fully working code example if they decide to use it.

[GuillaumeGomez]

I had a PR which proposed this change a while ago: #86892