Support for line continuation

[aljazerzen]

Adds support for line continuation in prompts, which can be used to achieve rectangular prompts.

Supersedes #338 & #372

Review notes:

• I suggest reviewing commits separately,
• To check the thing out, run input_multiline example,
• The breaking: commit is the only breaking change that adds continuation param to fn readline. I'm not fully confident that this feature is worth breaking the API over, but it seems most reasonable public API to me. That's because the continuation is usually dependent on the prompt.
• Alternative public API would fn Editor::set_continuation function. In that case, I can revert last two commits.

I'm very invested in landing this PR and I'm willing to make changes.

TODO:

• windows support
• documentation
• tests

[gwenn]

• continuation prompt should be optional (or behind a feature). Most likely a new Prompt trait with a default impl for &str.
• display size <> continuation.len()
• see Refactor highlighting #795 for impact of continuation prompt on highlighting
• see [Draft] Layout impact gwenn/rustyline#2 for impact on layout
• see Hardcoded continuation prompt implementation with continuation_prompt feature gwenn/rustyline#3 (comment) for another impact (and an alternative / similar PR made by @zao111222333)
• and another advice here: User-specified continuation prompt #793 (comment)

This feature is a minefield...

[aljazerzen]

Most likely a new Prompt trait with a default impl for &str.

I like this. It would also avoid the breaking change.

---

Thank you for the linked info. Given that I manage to solve/avoid these problems, is this a wanted feature?

I don't want to spend a lot of time working on a feature that would not be merged due to big-picture reasons.

[gwenn]

Thank you for the linked info. Given that I manage to solve/avoid these problems, is this a wanted feature?

I don't want to spend a lot of time working on a feature that would not be merged due to big-picture reasons.

This is definitely a wanted feature.
But that raises many issues on the current implementation (Highlighting / Layout / Moving cursor without repainting everything / ...).
So you should probably use reedline which already supports continuation prompt instead of rustyline.

[gwenn]

I believe this needs to be fixed, right? And you've attempted to fix it in gwenn#2 ?

Current behavior might be buggy.
And gwenn#2 is just a draft to make this method work with continuation prompt.
We want something that can tell to the LineBuffer, the (line, column) associated to one byte / grapheme offset and vice-versa.

[zao111222333]

You can have a look at gwenn#3 (comment) , however it's mightly that the Layout requires reconstruction, and the continuation feature seems need to implement base on the new Layout.

[aljazerzen]

I've opened a new PR that implements span tracking first: #820