[clangd] Deduplicate query scopes.

[ioeric]

Summary

For example, when anonymous namespace is present, duplicated namespaces might be
generated for the enclosing namespace.

[ilya-biryukov]

LGTM

clangd/CodeComplete.cpp

struct SpecifiedScope {
   //   "using namespace std; namespace ns { vec^ }" => {"ns::", "std::", ""}
   //
   // "" for global namespace, "ns::" for normal namespace.
   std::vector<std::string> AccessibleScopes;
   // The full scope qualifier as typed by the user (without the leading "::").
   // Set if the qualifier is not fully resolved by Sema.
   Optional<std::string> UnresolvedQualifier;
 
-  // Construct scopes being queried in indexes.
+  // Construct scopes being queried in indexes. The results are deduplicated.
   // This method format the scopes to match the index request representation.
   std::vector<std::string> scopesForIndexQuery() {
-    std::vector<std::string> Results;
-    for (StringRef AS : AccessibleScopes) {
-      Results.push_back(AS);
-      if (UnresolvedQualifier)
-        Results.back() += *UnresolvedQualifier;
-    }
-    return Results;
+    std::set<std::string> Results;
+    for (StringRef AS : AccessibleScopes)
+      Results.insert(
+          ((UnresolvedQualifier ? *UnresolvedQualifier : "") + AS).str());
+    return {Results.begin(), Results.end()};
   }
 };
 
 // Get all scopes that will be queried in indexes and whether symbols from
 // any scope is allowed. The first scope in the list is the preferred scope
 // (e.g. enclosing namespace).
 std::pair<std::vector<std::string>, bool>
 getQueryScopes(CodeCompletionContext &CCContext, const Sema &CCSema,

unittests/clangd/CodeCompleteTests.cpp

auto Requests = captureIndexRequests(R"cpp(
       }
       } // namespace ns
   )cpp");
 
   EXPECT_THAT(Requests, ElementsAre(Field(&FuzzyFindRequest::Scopes,
                                           UnorderedElementsAre(""))));
 }
 
+TEST(CompletionTest, NoDuplicatedQueryScopes) {
+  auto Requests = captureIndexRequests(R"cpp(
+      namespace {}
+
+      namespace na {
+      namespace {}
+      namespace nb {
+      ^
+      } // namespace nb
+      } // namespace na
+  )cpp");
+
+  EXPECT_THAT(Requests,
+              ElementsAre(Field(&FuzzyFindRequest::Scopes,
+                                UnorderedElementsAre("na::", "na::nb::", ""))));
+}
+
 TEST(CompletionTest, NoIndexCompletionsInsideClasses) {
   auto Completions = completions(
       R"cpp(
     struct Foo {
       int SomeNameOfField;
       typedef int SomeNameOfTypedefField;
     };