[lldb] Fix missing characters when autocompleting LLDB commands in REPL

[poya]

Summary

When tabbing to complete LLDB commands in REPL, characters would at best be missing but at worst cause the REPL to crash due to out of range string access.
This patch appends the command character to the completion results to fulfill the assumption that all matches are prefixed by the request's cursor argument prefix.

Bug report for the Swift REPL
https://bugs.swift.org/browse/SR-12867

[poya]

Tests for the Swift REPL in https://github.com/apple/llvm-project/pull/1388

[teemperor]

LGTM, thanks!

lldb/source/Expression/REPL.cpp

460

Nit: Missing . at the end of the comment, but I'll fix that when landing.

[poya]

lldb/source/Expression/REPL.cpp

460

Thanks, I wasn't sure about the convention since other comments in the same file had it both ways

[poya]

Don't believe I have the necessary permissions to land this myself, so would need some help to get this committed to the repo. Thanks.

lldb/source/Expression/REPL.cpp

 //===-- REPL.cpp ----------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "lldb/Expression/REPL.h"
 #include "lldb/Core/Debugger.h"
 #include "lldb/Core/PluginManager.h"
 #include "lldb/Core/StreamFile.h"
 #include "lldb/Expression/ExpressionVariable.h"
 #include "lldb/Expression/UserExpression.h"
 #include "lldb/Host/HostInfo.h"
 #include "lldb/Interpreter/CommandInterpreter.h"
 #include "lldb/Interpreter/CommandReturnObject.h"
if (request.GetRawLine().startswith(":")) {
     // auto complete LLDB commands
     llvm::StringRef new_line = request.GetRawLine().drop_front();
     CompletionResult sub_result;
     CompletionRequest sub_request(new_line, request.GetRawCursorPos() - 1,
                                   sub_result);
     debugger.GetCommandInterpreter().HandleCompletion(sub_request);
     StringList matches, descriptions;
     sub_result.GetMatches(matches);
+    // Prepend command prefix that was excluded in the completion request
+    if (request.GetCursorIndex() == 0) {
+      for (auto &match : matches) {
+        match.insert(0, 1, ':');
+      }
+    }
     sub_result.GetDescriptions(descriptions);
     request.AddCompletions(matches, descriptions);
     return;
   }
 
   // Strip spaces from the line and see if we had only spaces
   if (request.GetRawLine().trim().empty()) {
     // Only spaces on this line, so just indent