[clangd] Respect IWYU keep pragma for standard headers.

[hokein]

Summary

see the issue https://github.com/llvm/llvm-project/issues/64191

[kadircet]

thanks!

clang-tools-extra/clangd/IncludeCleaner.cpp

86–87

can you unify this and the previous check and just have a single:

if (PI && PI->shouldKeep(Inc.HashLine + 1))
  return false;

at the top of the function?

[kadircet]

also we should get this cherry-picked too. keep pragmas on includes are not common, but people do have export pragmas often enough to cause some annoyance here.

[hokein]

address review comment

[hokein]

In D156650#4545867, @kadircet wrote:

also we should get this cherry-picked too. keep pragmas on includes are not common, but people do have export pragmas often enough to cause some annoyance here.

Filed https://github.com/llvm/llvm-project/issues/64260.

clang-tools-extra/clangd/IncludeCleaner.cpp

if (Filter(NormalizedPath))
       return true;
   }
   return false;
 }
 
 bool mayConsiderUnused(
     const Inclusion &Inc, ParsedAST &AST,
     const include_cleaner::PragmaIncludes *PI) {
+  if (PI && PI->shouldKeep(Inc.HashLine + 1))
+      return false;
   // FIXME(kirillbobyrev): We currently do not support the umbrella headers.
   // System headers are likely to be standard library headers.
   // Until we have good support for umbrella headers, don't warn about them.
   if (Inc.Written.front() == '<')
     return tooling::stdlib::Header::named(Inc.Written).has_value();
   assert(Inc.HeaderID);
   auto HID = static_cast<IncludeStructure::HeaderID>(*Inc.HeaderID);
   auto FE = AST.getSourceManager().getFileManager().getFileRef(
       AST.getIncludeStructure().getRealPath(HID));
   assert(FE);
   if (PI) {
-    if (PI->shouldKeep(Inc.HashLine + 1))
-      return false;
     // Check if main file is the public interface for a private header. If so we
     // shouldn't diagnose it as unused.
     if (auto PHeader = PI->getPublic(*FE); !PHeader.empty()) {
       PHeader = PHeader.trim("<>\"");
       // Since most private -> public mappings happen in a verbatim way, we
       // check textually here. This might go wrong in presence of symlinks or
       // header mappings. But that's not different than rest of the places.
       if (AST.tuPath().endswith(PHeader))
         return false;

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

if (arg.Written != Written)
     *result_listener << arg.Written;
   return arg.Written == Written;
 }
 
 TEST(IncludeCleaner, StdlibUnused) {
   auto TU = TestTU::withCode(R"cpp(
     #include <list>
     #include <queue>
+    #include <vector> // IWYU pragma: keep
+    #include <string> // IWYU pragma: export
     std::list<int> x;
   )cpp");
   // Layout of std library impl is not relevant.
   TU.AdditionalFiles["bits"] = R"cpp(
     #pragma once
     namespace std {
       template <typename> class list {};
       template <typename> class queue {};
+      template <typename> class vector {};
     }
   )cpp";
   TU.AdditionalFiles["list"] = "#include <bits>";
   TU.AdditionalFiles["queue"] = "#include <bits>";
+  TU.AdditionalFiles["vector"] = "#include <bits>";
+  TU.AdditionalFiles["string"] = "#include <bits>";
   TU.ExtraArgs = {"-isystem", testRoot()};
   auto AST = TU.build();
   IncludeCleanerFindings Findings = computeIncludeCleanerFindings(AST);
   EXPECT_THAT(Findings.UnusedIncludes,
               ElementsAre(Pointee(writtenInclusion("<queue>"))));
 }
 
 TEST(IncludeCleaner, GetUnusedHeaders) {