Move completions to new compiler testing infrastructure

[olafurpg]

In #51 and #55 we moved signatureHelp and hover to a new testing infrastructure. This new infrastructure makes it very productive to polish the output from the presentation compiler using ~metaserver/testOnly -- tests.compiler. This workflow is way more productive than doing publishLocal, open vscode, manually type code and see what it produces.

The completion provider can also benefit from some polishing in how the completions are ordered, for example that fields of a class take prioritity over defaults like toString/finalize

[olafurpg]

PS. I've started working on this.

[olafurpg]

Actually, would someone be interested in working on this? I have a WIP diff here https://github.com/scalameta/language-server/compare/master...olafurpg:completions?expand=1 that can be used as a base. There are other issues I can work on instead, for example building semanticdbs on the fly with the presentation compiler.

[gabro]

I can work on this. I've already started to hack around with completion priorities a few days ago, so I have a vague idea of how to tackle this.

[olafurpg]

Awesome! Just realised why completions started crashing a lot now, I removed the _CURSOR_ trick in the signatureHelp PR. Adding _CURSOR_ avoids many parse errors and enables completions on parts like User.<COMPLETE>

fe14433#diff-be76cbaabee1e6378859da7ffe659ab1L60

We should probably add _CURSOR_ in Compiler.addCompilationUnit, even if that's used for more than completions.

I suspect with this change we can avoid this hacky catch here

https://github.com/scalameta/language-server/blob/b160931177b6d9d0d351ec364cfbca70669fba08/metaserver/src/main/scala/scala/meta/languageserver/compiler/CompilerUtils.scala#L15

[gabro]

I'm not sure I understand the _CURSOR_ trick, but is it just to make sure the parser doesn't fail when completing at the end of a line?

We should probably add _CURSOR_ in Compiler.addCompilationUnit, even if that's used for more than completions.

addCompilationUnit as no offset in input, where should we add the __CURSOR__?

[gabro]

Ah, nvm, got it. I think your intuition is right, I've added it back and it seems to be working. I'll push it to a branch soon.

[gabro]

Scratch that, it seems to have no effect :(