[clangd] Mark macros from preamble for code completion

[kadircet]

Summary

If the main file is a header, mark the marcos defined in its preamble
section as code-completion ready.

Fixes https://github.com/clangd/clangd/issues/921.

[usaxena95]

clang-tools-extra/clangd/index/SymbolCollector.cpp

567

Please add a comment about this (maybe similar to the parch description)

clang-tools-extra/clangd/index/SymbolCollector.cpp

for (const auto &MacroRef : IDToRefs.second) {
         auto EndLoc = cantFail(sourceLocationInMainFile(SM, Range.end));
         S.Name = toSourceCode(SM, SourceRange(StartLoc, EndLoc));
         S.SymInfo.Kind = index::SymbolKind::Macro;
         S.SymInfo.SubKind = index::SymbolSubKind::None;
         S.SymInfo.Properties = index::SymbolPropertySet();
         S.SymInfo.Lang = index::SymbolLanguage::C;
         S.Origin = Opts.Origin;
         S.CanonicalDeclaration = R.Location;
+        if (!HeaderFileURIs->getIncludeHeader(SM.getMainFileID()).empty()) {
+          S.Flags |= Symbol::IndexedForCodeCompletion;
+          S.Flags |= Symbol::VisibleOutsideFile;
+        }
         Symbols.insert(S);
       }
     }
   }
 }
 
 bool SymbolCollector::handleMacroOccurrence(const IdentifierInfo *Name,
                                             const MacroInfo *MI,

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

ASSERT_THAT(MT.children(), UnorderedElementsAre(Pair("f1", _), Pair("f2", _),
                                                   Pair("f3", _)));
   EXPECT_THAT(MT.child("f1").children(), ElementsAre(Pair("symbols", _)));
   EXPECT_THAT(MT.child("f1").total(), Gt(0U));
   EXPECT_THAT(MT.child("f2").children(), ElementsAre(Pair("references", _)));
   EXPECT_THAT(MT.child("f2").total(), Gt(0U));
   EXPECT_THAT(MT.child("f3").children(), ElementsAre(Pair("relations", _)));
   EXPECT_THAT(MT.child("f3").total(), Gt(0U));
 }
+
+TEST(FileIndexTest, MacrosFromMainFile) {
+  FileIndex Idx;
+  TestTU TU;
+  TU.Code = "#pragma once\n#define FOO";
+  TU.Filename = "foo.h";
+  auto AST = TU.build();
+  Idx.updateMain(testPath(TU.Filename), AST);
+
+  auto &FooSymbol = findSymbol(runFuzzyFind(Idx, ""), "FOO");
+  EXPECT_TRUE(FooSymbol.Flags & Symbol::IndexedForCodeCompletion);
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang