Fix #421: Unsupported PowerShell LanguageMode should be reported

This change adds an additional check in the Start-EditorServices.ps1
script to see whether the LanguageMode has been locked to an unsupported
mode like ConstrainedLanguage.  The language server cannot be loaded and
used in this case so the user should be notified.

scripts/Start-EditorServices.ps1

@@ -89,6 +89,16 @@ function WriteSessionFile($sessionInfo) {
     ConvertTo-Json -InputObject $sessionInfo -Compress | Set-Content -Force -Path "$SessionDetailsPath" -ErrorAction Stop
 }
 
+if ($host.Runspace.LanguageMode -eq 'ConstrainedLanguage') {
+    WriteSessionFile @{
+        "status" = "failed"
+        "reason" = "languageMode"
+        "detail" = $host.Runspace.LanguageMode.ToString()
+    }
+
+    ExitWithError "PowerShell is configured with an unsupported LanguageMode (ConstrainedLanguage), language features are disabled."
+}
+
 # Are we running in PowerShell 5 or later?
 $isPS5orLater = $PSVersionTable.PSVersion.Major -ge 5
 

src/session.ts

@@ -327,6 +327,10 @@ export class SessionManager {
                                 this.setSessionFailure(
                                     `PowerShell language features are only supported on PowerShell version 3 and above.  The current version is ${sessionDetails.powerShellVersion}.`)
                             }
+                            else if (sessionDetails.reason === "languageMode") {
+                                this.setSessionFailure(
+                                    `PowerShell language features are disabled due to an unsupported LanguageMode: ${sessionDetails.detail}`);
+                            }
                             else {
                                 this.setSessionFailure(`PowerShell could not be started for an unknown reason '${sessionDetails.reason}'`)
                             }

src/utils.ts

@@ -57,6 +57,7 @@ export function getPipePath(pipeName: string) {
 export interface EditorServicesSessionDetails {
     status: string;
     reason: string;
+    detail: string;
     powerShellVersion: string;
     channel: string;
     languageServicePort: number;