Colour cfg'ed code grey

[nrc]

I'm not sure how to implement this, since the syntax highlighter is not connected to the RLS. We need to know from the compiler what code is compiled and what is not, then inform the highlighter some how. We might only be able to do that in rustw.

[HighCommander4]

cquery (a C++ language server) does this by implementing a new server -> client message called publishInactiveRegions as a protocol extension. The message contains an array of Ranges identifying the inactive regions in the file.

[Xanewok]

I think this is in the semantic highlighting area: microsoft/vscode-languageserver-node#368

[HighCommander4]

I think it's distinct from semantic highlighting. Semantic highlighting involves coloring individual tokens based on their role (or the role of the entity they resolve to). This feature involves taking entire sections of code (entire lines) and greying them out, possibly combining the grey filter with the semantic colors of the tokens inside.

[Xanewok]

Relevant implementation in VSCode extension - https://github.com/cquery-project/vscode-cquery/blob/4aac325dcf0735aa5d5f98960f28d6af6ca83d50/src/extension.ts#L523-L552