Add SmartIndent capability

[IntelOrca]

Visual Studio has an extensibility point called SmartIndent:
https://msdn.microsoft.com/en-us/library/microsoft.visualstudio.text.editor.ismartindent.aspx

This allows a language service to provide the editor, given a line, what the indentation level should be. If you click on a blank line in Visual Studio, it will automatically place the caret at the correct indent level based on the code above. This also works when using the arrow keys to move the caret. Currently in VSCode, it will place the caret at column 0 for blank lines.

This may come hand-in-hand with implementing virtual space (see #13960) as Visual Studio will not insert spaces, but just place the caret at the correct column in virtual space.

One thing to note is that in Visual Studio, when clicking on a blank line, the X position that you click at must be greater than the expected indentation column in order for this to work, otherwise it will place the caret at column 0. With the arrow keys, unless you are at column 0, your caret will always move to the correct indentation level for that blank line.

[sharwell]

This is a good feature to have. I was disappointed to learn that ISmartIndent was quite limited in its abilities. In particular, the interface doesn't allow a language to define something like the Comment Continuation Mode I describe in Smart Indent (Draft).

📝 Since publishing that paper, I've managed to define behavior for Smart Indent in more generalized scenarios. However, the paper remains a good overview for the general functionality that a ISmartIndent interface needs to support.

[IntelOrca]

@sharwell Your paper talks about splitting a line which suggests you expect SmartIndent to define not just the indent for blank lines but also on what it does when pressing RETURN. This sounds like its also related to automatic doc-comment continuation and automatic generation of end constructs (for example end if in VB.NET). When developing this logic for a Visual Studio extension, I had to use a command filter to override the RETURN behaviour. This is what Roslyn uses as well.

In Visual Studio, the ISmartIndent only works with blank lines, but perhaps if implemented in VSCODE it could also allow overriding the behaviour for non-blank lines so that the right caret position can be set inside comment blocks.

And if special logic is needed for the RETURN action, then another method to override that behaviour.

[sharwell]

💭 I thought in Visual Studio, ISmartIndent works both on blank lines and in the case of splitting a line in the middle, but only for the purpose of determining the indentation. I could be wrong though, it's been a while since I worked on that component.

To me, Smart Indent deals with two things:

1. What happens when you move the caret to a blank line, either by clicking on a blank line or by hitting the up or down arrows (or page up/page down)?
2. What happens when you press Enter?

I don't necessarily have an obvious idea for what the interface should look like, but I can tell you what should happen when Enter is pressed provided the interface exists for extensions. In Visual Studio, the answer could probably be provided in terms of a ITextEdit (for the modifications) and a VirtualSnapshotPoint (for the caret placement). But that's just one possibility. 😄

[IntelOrca]

I thought in Visual Studio, ISmartIndent works both on blank lines and in the case of splitting a line in the middle, but only for the purpose of determining the indentation.

That's correct, I thought you also expected it to perform a text edit like how it trims the line or inserts a new comment prefix etc.

[sharwell]

Ah, no. I'm saying that I like your idea of supporting Smart Indent in new languages in VS Code, but I'd like to see it go one step farther than ISmartIndent because that interface isn't able to express some pretty useful behavior of a smart indent feature.

[claybford]

Hi, are there any updates on this issue? It makes vscode much harder to use with typical mouse interaction.