[clangd] Fix filename completion at the end of file

[kadircet]

Summary

Fixes https://github.com/clangd/clangd/issues/433

[sammccall]

Thanks for puzzling this one out, i thought it would be in lexer...

clang-tools-extra/clangd/CodeComplete.cpp

1127

"include the following byte" doesn't match my mental model, which is that we're on the boundary, and are choosing whether to resolve left out right, i.e *guessing* whether inserted text will become part of the preamble or the main file.

(Of course everyone has their own mental model - but if that seems clearer to you too, maybe reword)

clearer might be:

if (Offset < Size || (!EndsInNewline && Offset == Size))

May not even need the comment in this case, up to you

[kadircet]

• Rewrite the condition as suggested

clang-tools-extra/clangd/CodeComplete.cpp

bool semaCodeComplete(std::unique_ptr<CodeCompleteConsumer> Consumer,
   // We reuse the preamble whether it's valid or not. This is a
   // correctness/performance tradeoff: building without a preamble is slow, and
   // completion is latency-sensitive.
   // However, if we're completing *inside* the preamble section of the draft,
   // overriding the preamble will break sema completion. Fortunately we can just
   // skip all includes in this case; these completions are really simple.
   PreambleBounds PreambleRegion =
       ComputePreambleBounds(*CI->getLangOpts(), *ContentsBuffer, 0);
-  bool CompletingInPreamble = PreambleRegion.Size > Input.Offset;
+  bool CompletingInPreamble = Input.Offset < PreambleRegion.Size ||
+                              (!PreambleRegion.PreambleEndsAtStartOfLine &&
+                               Input.Offset == PreambleRegion.Size);
   if (Input.Patch)
     Input.Patch->apply(*CI);
   // NOTE: we must call BeginSourceFile after prepareCompilerInstance. Otherwise
   // the remapped buffers do not get freed.
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
       Input.ParseInput.TFS->view(Input.ParseInput.CompileCommand.Directory);
   if (Input.Preamble.StatCache)
     VFS = Input.Preamble.StatCache->getConsumingFS(std::move(VFS));

clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp

TEST(SignatureHelpTest, ConstructorInitializeFields) {
     )cpp");
     EXPECT_THAT(Results.signatures,
                 UnorderedElementsAre(Sig("A([[int]])"), Sig("A([[A &&]])"),
                                      Sig("A([[const A &]])")));
   }
 }
 
 TEST(CompletionTest, IncludedCompletionKinds) {
-  Annotations Test(R"cpp(#include "^")cpp");
+  Annotations Test(R"cpp(#include "^)cpp");
   auto TU = TestTU::withCode(Test.code());
   TU.AdditionalFiles["sub/bar.h"] = "";
   TU.ExtraArgs.push_back("-I" + testPath("sub"));
 
   auto Results = completions(TU, Test.point());
   EXPECT_THAT(Results.Completions,
               AllOf(Has("sub/", CompletionItemKind::Folder),
                     Has("bar.h\"", CompletionItemKind::File)));