Metals: Switch to incremental sync instead of full sync

[olafurpg]

/**
 * Defines how the host (editor) should sync document changes to the language server.
 */
export namespace TextDocumentSyncKind {
    /**
     * Documents should not be synced at all.
     */
    export const None = 0;

    /**
     * Documents are synced by always sending the full content
     * of the document.
     */
    export const Full = 1;

    /**
     * Documents are synced by sending the full content on open.
     * After that only incremental updates to the document are
     * send.
     */
    export const Incremental = 2;
}

Currently we do the full-sync, which sends the whole file contents on every keystroke

[gabro]

What's the downside of full text sync? Did you observe/suspect any perfect degradation because of this?

[olafurpg]

Haven't observed any problems, just find it unsatisfying that the full source file is moved on every keystroke. I haven't worked with it in larger files however (>7k loc)

[gabro]

It makes sense. Let's only hope we don't suddenly start goin out of sync :)

[ivanopagano]

I'm talking out of the blue (no idea what we're talking about really), but wouldn't it be possible to do "checkpoints" every once in a while, redoing full sync, and use incremental sync in-between checkpoints?

[olafurpg]

@ivanopagano during the initialize handshake, the server communicates if it supports incremental/full text synchronization and it's not possible to change the mode after that.

[olafurpg]

Closing as wontfix for now. We have no evidence so far to suggest that full text synchronization is a performance bottle-neck in Metals.