vscode-dotty: Fix hover functionality with old language server

The current unreleased version of vscode-dotty depends on a new version
of vscode-languageclient which parses hover messages in a stricter way,
and Dotty Language Server <= 0.9.x sends incorrect hover messages due to
a bug in lsp4j. The end result is that when using the current
vscode-dotty on a project with an old version of dotty, the hover shows
"[object Object]" instead of showing the type and documentation.

This commit works around this by manually parsing the hover message when
the server version is too old.

vscode-dotty/package.json

@@ -58,10 +58,12 @@
   ],
   "dependencies": {
     "child-process-promise": "^2.2.1",
+    "compare-versions": "^3.4.0",
     "vscode-languageclient": "^5.1.0",
     "vscode-languageserver": "^5.1.0"
   },
   "devDependencies": {
+    "@types/compare-versions": "^3.0.0",
     "@types/mocha": "^5.2.5",
     "@types/node": "^8.10.28",
     "typescript": "^2.9.2",

vscode-dotty/src/compat.ts

@@ -0,0 +1,46 @@
+import * as vscode from 'vscode';
+
+import { HoverRequest } from 'vscode-languageclient';
+import { MarkedString, LanguageClient, LanguageClientOptions, RevealOutputChannelOn,
+         ServerOptions } from 'vscode-languageclient';
+
+// Fix hover functionality when using this version of vscode-dotty with Dotty
+// Language Server 0.9 or earlier.
+// Without this, the displayed hover would be "[object Object]".
+export function enableOldServerWorkaround(client: LanguageClient): void {
+  client.clientOptions.middleware = {
+    provideHover: (document, position, token, next) => {
+      // This code is adapted from HoverFeature#registerLanguageProvider in
+      // https://github.com/Microsoft/vscode-languageserver-node/blob/master/client/src/client.ts
+      return client
+        .sendRequest(HoverRequest.type,
+                     client.code2ProtocolConverter.asTextDocumentPositionParams(document, position), token)
+        .then(
+          hover => {
+            if (!hover) {
+              return undefined
+            }
+            // The server is supposed to return string or { language: string,
+            // value: string } or an array of those things, but instead it returns
+            // an array of { value: string }. This used to work but with a recent
+            // vscode-languageclient the user ends up seeing "[object Object]" as
+            // the hover information.
+            // We work around this by manually parsing the array of { value: string }
+            // into a Hover.
+            const contents = hover.contents as { value: string }[]
+            let result: vscode.MarkdownString[] = []
+            for (let element of contents) {
+              let item = new vscode.MarkdownString()
+              item.appendCodeblock(element.value, "scala")
+              result.push(item)
+            }
+            return new vscode.Hover(result)
+          },
+          error => {
+            client.logFailedRequest(HoverRequest.type, error)
+            return Promise.resolve(null)
+          }
+        )
+    }
+  }
+}

vscode-dotty/src/extension.ts

@@ -4,11 +4,13 @@ import * as fs from 'fs';
 import * as path from 'path';
 
 import * as cpp from 'child-process-promise';
+import * as compareVersions from 'compare-versions';
 
 import { ExtensionContext } from 'vscode';
 import * as vscode from 'vscode';
 import { LanguageClient, LanguageClientOptions, RevealOutputChannelOn,
          ServerOptions } from 'vscode-languageclient';
+import { enableOldServerWorkaround } from './compat'
 
 let extensionContext: ExtensionContext
 let outputChannel: vscode.OutputChannel
@@ -36,7 +38,7 @@ export function activate(context: ExtensionContext) {
       run({
         module: context.asAbsolutePath('out/src/passthrough-server.js'),
         args: [ port.toString() ]
-      })
+      }, false)
     })
 
   } else {
@@ -72,11 +74,13 @@ function runLanguageServer(coursierPath: string, languageServerArtifactFile: str
     if (err) throw err
     else {
       const languageServerArtifact = data.toString().trim()
+      const languageServerVersion = languageServerArtifact.split(":")[2]
+      const isOldServer = compareVersions(languageServerVersion, "0.9.x") <= 0
       fetchWithCoursier(coursierPath, languageServerArtifact).then((languageServerClasspath) => {
         run({
           command: "java",
           args: ["-classpath", languageServerClasspath, "dotty.tools.languageserver.Main", "-stdio"]
-        })
+        }, isOldServer)
       })
     }
   })
@@ -149,7 +153,7 @@ function configureIDE(sbtClasspath: string, languageServerScalaVersion: string,
   })
 }
 
-function run(serverOptions: ServerOptions) {
+function run(serverOptions: ServerOptions, isOldServer: boolean) {
   const clientOptions: LanguageClientOptions = {
     documentSelector: [
       { language: 'scala', scheme: 'file', pattern: '**/*.scala' },
@@ -163,6 +167,8 @@ function run(serverOptions: ServerOptions) {
   }
 
   const client = new LanguageClient("dotty", "Dotty", serverOptions, clientOptions)
+  if (isOldServer)
+    enableOldServerWorkaround(client)
 
   // Push the disposable to the context's subscriptions so that the
   // client can be deactivated on extension deactivation