[clangd] Improve global code completion when scope specifier is unresolved.

[ioeric]

Summary

Suppose clangd:: is unresolved in the following example. Currently,
we simply use "clangd::" as the query scope. We can do better by combining with
accessible scopes in the context. The query scopes can be {clangd::, clang::clangd::}.

namespace clang { clangd::^ }

[kadircet]

LG

Do we have any metrics regarding change in completion quality?

[ioeric]

• Rebase and minor cleanup

clangd/CodeComplete.cpp

 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
 #include "clang/Sema/Sema.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include <algorithm>
 #include <iterator>
 
 // We log detailed candidate here if you run with -debug-only=codecomplete.
struct SpecifiedScope {
   llvm::Optional<std::string> UnresolvedQualifier;
 
   // 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::set<std::string> Results;
     for (llvm::StringRef AS : AccessibleScopes)
       Results.insert(
-          ((UnresolvedQualifier ? *UnresolvedQualifier : "") + AS).str());
+          (AS + (UnresolvedQualifier ? *UnresolvedQualifier : "")).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,
 
   // Qualified completion ("std::vec^"), we have two cases depending on whether
   // the qualifier can be resolved by Sema.
   if ((*SS)->isValid()) { // Resolved qualifier.
     return {GetAllAccessibleScopes(CCContext).scopesForIndexQuery(), false};
   }
 
   // Unresolved qualifier.
-  // FIXME: When Sema can resolve part of a scope chain (e.g.
-  // "known::unknown::id"), we should expand the known part ("known::") rather
-  // than treating the whole thing as unknown.
-  SpecifiedScope Info;
-  Info.AccessibleScopes.push_back(""); // global namespace
+  SpecifiedScope Info = GetAllAccessibleScopes(CCContext);
+  if (Info.AccessibleScopes.empty())
+    Info.AccessibleScopes.push_back(""); // Fallback to global namespace.
 
-  Info.UnresolvedQualifier =
+  llvm::StringRef SpelledSpecifier =
       Lexer::getSourceText(CharSourceRange::getCharRange((*SS)->getRange()),
-                           CCSema.SourceMgr, clang::LangOptions())
-          .ltrim("::");
+                           CCSema.SourceMgr, clang::LangOptions());
+  if (SpelledSpecifier.consume_front("::"))
+    Info.AccessibleScopes = {""};
+  Info.UnresolvedQualifier = SpelledSpecifier;
   // Sema excludes the trailing "::".
   if (!Info.UnresolvedQualifier->empty())
     *Info.UnresolvedQualifier += "::";
 
   return {Info.scopesForIndexQuery(), false};
 }
 
 // Should we perform index-based completion in a context of the specified kind?

unittests/clangd/CodeCompleteTests.cpp

auto Requests = captureIndexRequests(R"cpp(
       using namespace a;
       namespace ns {
       void f() {
       bar::^
       }
       } // namespace ns
   )cpp");
 
-  EXPECT_THAT(Requests, ElementsAre(Field(&FuzzyFindRequest::Scopes,
-                                          UnorderedElementsAre("bar::"))));
+  EXPECT_THAT(Requests,
+              ElementsAre(Field(
+                  &FuzzyFindRequest::Scopes,
+                  UnorderedElementsAre("a::bar::", "ns::bar::", "bar::"))));
 }
 
 TEST(CompletionTest, UnresolvedNestedQualifierIdQuery) {
   auto Requests = captureIndexRequests(R"cpp(
       namespace a {}
       using namespace a;
       namespace ns {
       void f() {
int main() {
       for (auto [loopVar] : y ) { // y has to be unresolved.
         int z = loopV^;
       }
     }
   )cpp");
   EXPECT_THAT(R.Completions, ElementsAre(Named("loopVar")));
 }
 
+TEST(CompletionTest, ScopeIsUnresolved) {
+  clangd::CodeCompleteOptions Opts = {};
+  Opts.AllScopes = true;
+
+  auto Results = completions(R"cpp(
+    namespace a {
+    void f() { b::X^ }
+    }
+  )cpp",
+                             {cls("a::b::XYZ")}, Opts);
+  EXPECT_THAT(Results.Completions,
+              UnorderedElementsAre(AllOf(Qualifier(""), Named("XYZ"))));
+}
+
+TEST(CompletionTest, NestedScopeIsUnresolved) {
+  clangd::CodeCompleteOptions Opts = {};
+  Opts.AllScopes = true;
+
+  auto Results = completions(R"cpp(
+    namespace a {
+    namespace b {}
+    void f() { b::c::X^ }
+    }
+  )cpp",
+                             {cls("a::b::c::XYZ")}, Opts);
+  EXPECT_THAT(Results.Completions,
+              UnorderedElementsAre(AllOf(Qualifier(""), Named("XYZ"))));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang