[clangd] Drop namespace references in the index.

[hokein]

Summary

Namespace references is less useful compared with other symbols, and
they contribute large part of the index. This patch drops them.
The number of refs is reduced from 5.4 million to 4.7 million.

	Before	After
file size	78 MB	71MB
memory	330MB	300MB

[hokein]

Fix a typo.

[kadircet]

clangd/index/SymbolCollector.cpp

359

Why not handle in shouldCollectSymbol ?

clangd/index/SymbolCollector.h

60

another typo anbd -> and

[hokein]

Fix another typo.

[hokein]

clangd/index/SymbolCollector.cpp

359

shouldCollectSymbol is shared between the code collecting symbols and the code collecting references, we still want to collect namespace symbols.

clangd/index/SymbolCollector.h

struct Options {
     bool CollectIncludePath = false;
     /// If set, this is used to map symbol #include path to a potentially
     /// different #include path.
     const CanonicalIncludes *Includes = nullptr;
     // Populate the Symbol.References field.
     bool CountReferences = false;
     /// The symbol ref kinds that will be collected.
     /// If not set, SymbolCollector will not collect refs.
+    /// Note that references of namespace decls are not collected, as they
+    /// contribute large part of the index, and they are less useful compared
+    /// with other decls.
     RefKind RefFilter = RefKind::Unknown;
     /// If set to true, SymbolCollector will collect all refs (from main file
     /// and included headers); otherwise, only refs from main file will be
     /// collected.
     /// This flag is only meaningful when RefFilter is set.
     bool RefsInHeaders = false;
     // Every symbol collected will be stamped with this origin.
     SymbolOrigin Origin = SymbolOrigin::Unknown;

clangd/index/SymbolCollector.cpp

bool SymbolCollector::handleDeclOccurence(
   bool IsOnlyRef =
       !(Roles & (static_cast<unsigned>(index::SymbolRole::Declaration) |
                  static_cast<unsigned>(index::SymbolRole::Definition)));
 
   if (IsOnlyRef && !CollectRef)
     return true;
   if (!shouldCollectSymbol(*ND, *ASTCtx, Opts))
     return true;
-  if (CollectRef &&
+  if (CollectRef && !isa<NamespaceDecl>(ND) &&
       (Opts.RefsInHeaders || SM.getFileID(SpellingLoc) == SM.getMainFileID()))
     DeclRefs[ND].emplace_back(SpellingLoc, Roles);
   // Don't continue indexing if this is a mere reference.
   if (IsOnlyRef)
     return true;
 
   auto ID = getSymbolID(ND);
   if (!ID)

unittests/clangd/SymbolCollectorTests.cpp

TEST_F(SymbolCollectorTest, Refs) {
   Annotations Header(R"(
   class $foo[[Foo]] {
   public:
     $foo[[Foo]]() {}
     $foo[[Foo]](int);
   };
   class $bar[[Bar]];
   void $func[[func]]();
+
+  namespace $ns[[NS]] {} // namespace ref is ignored
   )");
   Annotations Main(R"(
   class $bar[[Bar]] {};
 
   void $func[[func]]();
 
   void fff() {
     $foo[[Foo]] foo;
TEST_F(SymbolCollectorTest, Refs) {
   auto HeaderSymbols = TestTU::withHeaderCode(Header.code()).headerSymbols();
 
   EXPECT_THAT(Refs, Contains(Pair(findSymbol(Symbols, "Foo").ID,
                                   HaveRanges(Main.ranges("foo")))));
   EXPECT_THAT(Refs, Contains(Pair(findSymbol(Symbols, "Bar").ID,
                                   HaveRanges(Main.ranges("bar")))));
   EXPECT_THAT(Refs, Contains(Pair(findSymbol(Symbols, "func").ID,
                                   HaveRanges(Main.ranges("func")))));
+  EXPECT_THAT(Refs, Not(Contains(Pair(findSymbol(Symbols, "NS").ID, _))));
   // Symbols *only* in the main file (a, b, c) had no refs collected.
   auto MainSymbols =
       TestTU::withHeaderCode(SymbolsOnlyInMainCode.code()).headerSymbols();
   EXPECT_THAT(Refs, Not(Contains(Pair(findSymbol(MainSymbols, "a").ID, _))));
   EXPECT_THAT(Refs, Not(Contains(Pair(findSymbol(MainSymbols, "b").ID, _))));
   EXPECT_THAT(Refs, Not(Contains(Pair(findSymbol(MainSymbols, "c").ID, _))));
 }