Implement labelDetails for CompletionItem

[JessicaJHee]

Fixes #2476

[JessicaJHee]

Once we decide on the preferred look of this, I will update the tests to reflect the new behavior.

Current implementation:

[rgrunber]

There's no telling how other clients may implements this, but here's what it looks like in VS Code. Some details do seem a bit redundant. For example, I don't think we need to mention the method signature twice. Maybe we need to hide certain things when we know labelDetails is supported.

Before

After

[JessicaJHee]

Updated the behavior to remove the duplicate information and only show the declaring class on the right hand side.

[rgrunber]

Maybe the label detail description (the element on the far right of the list) should incldue the fully qualified name ? Also, I notice isCompletionItemLabelDetailsSupport() isn't being called in any of the places we set the label details. I think we should still use it since we don't know if a client supports it.

[jdneo]

We received some user feedbacks that hope to show the return type of the method at right (That's also the way that IDEA displays the completion list)

The return type will disappear if the item has a long display content in our current implementation. And the situation may become worse if we display declaring type as detail.

[JessicaJHee]

Here is the updated behavior for each use case:

createLabelWithTypeAndDeclaration

createMethodProposalLabel

createSimpleLabelWithType

createTypeProposalLabel

createJavadocTypeProposalLabel

createPackageProposalLabel

createAnonymousTypeLabel

createOverrideMethodProposalLabel

[jdneo]

This looks pretty good!

One more case is about the snippets. We can put descriptions of the snippet as detail maybe.

IDEA example:

There is a list for postfix which might be helpful: #2288 (comment)

[JessicaJHee]

Currently we have the description of the template shown on the right hand side:

For snippets:

For postfix:

I think this is helpful, but I can also make the change so it shows the evaluated snippet pattern instead (like how it works in IJ). What do you think? @rgrunber @jdneo

[jdneo]

Looks good!

IJ shows description for general snippets and evaluated content for postfixes.

I think using descriptions are fine for now and we can change it if users ask.

[jdneo]

Another feedback about UX. Currently the constructors will show the return type as void:

I'm thinking that, since the constructors in Java are always void, is it necessary to show it in the list? Looks like it does not provide much useful information to users. Leave it empty to make the list panel clearer might be better.

[rgrunber]

Change looks pretty good from trying it out. The only thing we are losing is the ability to know the declaring class of a given method declaration, and only when it is selected :

I think this is pretty minor though. I also noticed that the class snippet has no detail set but that's an existing issue that could be fixed outside this change.

[JessicaJHee]

@jdneo @rgrunber Thanks very much for the thorough review!