str::get_mut doc example should demonstrate mutability

[frewsxcv]

https://doc.rust-lang.org/nightly/std/primitive.str.html#method.get_mut

The doc example should demonstrate the effects of mutating the resulting string slice.

The easiest way I've found to demonstrate mutability with doc examples for str is to use AsciiExt: https://doc.rust-lang.org/nightly/std/primitive.str.html#method.split_at_mut

[toidiu]

I would like to attempt this.

[frewsxcv]

Long time no see @toidiu 👋 Let me know if you need any help with this or have any questions

[toidiu]

@frewsxcv 🖖

I had a thought about how to implement this better. The current example uses the string 🗻∈🌏 where the characters have a variable byte length. This was a source of confusing when I first looked at the example (its not super obvious that 🗻 is 4 bytes).

I was thinking of changing the string to something more uniform like hello which is 1 byte per char and then applying make_ascii_uppercase(). This could also be applied to other examples.

[frewsxcv]

I was thinking of changing the string to something more uniform like hello which is 1 byte per char and then applying make_ascii_uppercase(). This could also be applied to other examples.

yep, this makes sense to me 👌

[frewsxcv]

Completed in #44467