Make sure newline starts with /// when splitting doc comment

The existing behaviour where splitting a /// line results in the next
line starting with // is a vscode issue. Other examples follow a similar
approach as this PR to accomplish the desired behaviour.

Issue: #1648

Author: award999

src/editor/CommentCompletion.ts

@@ -15,6 +15,14 @@
 import * as vscode from "vscode";
 import { DocumentParser } from "./DocumentParser";
 
+function isLineComment(document: vscode.TextDocument, line: number): boolean {
+    // test if line consists of just '///'
+    if (/^\s*\/\/\//.test(document.lineAt(line).text)) {
+        return true;
+    }
+    return false;
+}
+
 /** CompletionItem for Swift Comments */
 class CommentCompletion extends vscode.CompletionItem {
     constructor(
@@ -40,20 +48,26 @@ class CommentCompletionProvider implements vscode.CompletionItemProvider {
         position: vscode.Position
     ): Promise<vscode.CompletionItem[] | undefined> {
         // Is line a '///' comment
-        if (position.line === 0 || this.isLineComment(document, position.line - 1) === false) {
+        if (position.line === 0 || isLineComment(document, position.line - 1) === false) {
+            return undefined;
+        }
+        // Fixes https://github.com/swiftlang/vscode-swift/issues/1648
+        const match = /^(\s*)\/\/\s(.+)/.exec(document.lineAt(position.line).text);
+        if (match) {
+            void vscode.window.activeTextEditor?.edit(
+                edit => {
+                    void edit.replace(
+                        new vscode.Range(position.line, 0, position.line, match[0].length),
+                        `${match[1]}///${match[2]}`
+                    );
+                },
+                { undoStopBefore: false, undoStopAfter: true }
+            );
             return undefined;
         }
         const completion = new CommentCompletion("/// ", "///", "Documentation comment");
         return [completion];
     }
-
-    private isLineComment(document: vscode.TextDocument, line: number): boolean {
-        // test if line starts with '///'
-        if (/^\s*\/\/\//.test(document.lineAt(line).text)) {
-            return true;
-        }
-        return false;
-    }
 }
 
 interface FunctionDetails {
@@ -73,7 +87,7 @@ class FunctionDocumentationCompletionProvider implements vscode.CompletionItemPr
         position: vscode.Position
     ): Promise<vscode.CompletionItem[] | undefined> {
         // Is line a '///' comment
-        const isComment = this.isLineComment(document, position.line);
+        const isComment = isLineComment(document, position.line);
         if (isComment === false) {
             return undefined;
         }
@@ -117,14 +131,6 @@ class FunctionDocumentationCompletionProvider implements vscode.CompletionItemPr
         }
     }
 
-    private isLineComment(document: vscode.TextDocument, line: number): boolean {
-        // test if line consists of just '///'
-        if (/^\s*\/\/\/\s*$/.test(document.lineAt(line).text)) {
-            return true;
-        }
-        return false;
-    }
-
     /**
      * Extract function details from line below. Inspiration for this code can be found
      * here https://github.com/fappelman/swift-add-documentation