Index: include/clang-c/Index.h
===================================================================
--- include/clang-c/Index.h
+++ include/clang-c/Index.h
@@ -1332,7 +1332,17 @@
    *
    * The function bodies of the main file are not skipped.
    */
-  CXTranslationUnit_LimitSkipFunctionBodiesToPreamble = 0x800
+  CXTranslationUnit_LimitSkipFunctionBodiesToPreamble = 0x800,
+
+  /**
+   * Used to indicate that non-errors from included files should be ignored.
+   *
+   * If set, clang_getDiagnosticSetFromTU() will not report e.g. warnings from
+   * included files anymore. This speeds up clang_getDiagnosticSetFromTU() for
+   * the case where these warnings are not of interest, as for an IDE for
+   * example, which typically shows only the diagnostics in the main file.
+   */
+  CXTranslationUnit_IgnoreNonErrorsFromIncludedFiles = 0x1000
 };
 
 /**
Index: include/clang/Basic/Diagnostic.h
===================================================================
--- include/clang/Basic/Diagnostic.h
+++ include/clang/Basic/Diagnostic.h
@@ -213,6 +213,9 @@
   // Suppress all diagnostics.
   bool SuppressAllDiagnostics = false;
 
+  // Suppress non-errors from all included files.
+  bool SuppressNonErrorsFromIncludedFiles = false;
+
   // Elide common types of templates.
   bool ElideType = true;
 
@@ -634,6 +637,10 @@
   }
   bool getSuppressAllDiagnostics() const { return SuppressAllDiagnostics; }
 
+  void setSuppressNonErrorsFromIncludedFiles(bool Val = true) {
+    SuppressNonErrorsFromIncludedFiles = Val;
+  }
+
   /// Set type eliding, to skip outputting same types occurring in
   /// template types.
   void setElideType(bool Val = true) { ElideType = Val; }
Index: lib/Basic/DiagnosticIDs.cpp
===================================================================
--- lib/Basic/DiagnosticIDs.cpp
+++ lib/Basic/DiagnosticIDs.cpp
@@ -477,6 +477,14 @@
       Result = diag::Severity::Fatal;
   }
 
+  // If requested, ignore non-errors from all included files.
+  if (Diag.SuppressNonErrorsFromIncludedFiles &&
+      Result <= diag::Severity::Warning && Loc.isValid() &&
+      !Diag.getSourceManager().isInMainFile(
+          Diag.getSourceManager().getExpansionLoc(Loc))) {
+    return diag::Severity::Ignored;
+  }
+
   // Custom diagnostics always are emitted in system headers.
   bool ShowInSystemHeader =
       !GetDiagInfo(DiagID) || GetDiagInfo(DiagID)->WarnShowInSystemHeader;
Index: test/Index/ignore-warnings-from-headers.h
===================================================================
--- /dev/null
+++ test/Index/ignore-warnings-from-headers.h
@@ -0,0 +1 @@
+void f(int unusedInHeader) {}
Index: test/Index/ignore-warnings-from-headers.cpp
===================================================================
--- /dev/null
+++ test/Index/ignore-warnings-from-headers.cpp
@@ -0,0 +1,7 @@
+#include "ignore-warnings-from-headers.h"
+
+void g(int unusedInMainFile) {}
+
+// RUN: env CINDEXTEST_IGNORE_NONERRORS_FROM_INCLUDED_FILES=1 c-index-test -test-load-source function %s -Wunused-parameter 2>&1 | FileCheck %s
+// CHECK-NOT: warning: unused parameter 'unusedInHeader'
+// CHECK: warning: unused parameter 'unusedInMainFile'
Index: tools/c-index-test/c-index-test.c
===================================================================
--- tools/c-index-test/c-index-test.c
+++ tools/c-index-test/c-index-test.c
@@ -84,6 +84,8 @@
     options |= CXTranslationUnit_KeepGoing;
   if (getenv("CINDEXTEST_LIMIT_SKIP_FUNCTION_BODIES_TO_PREAMBLE"))
     options |= CXTranslationUnit_LimitSkipFunctionBodiesToPreamble;
+  if (getenv("CINDEXTEST_IGNORE_NONERRORS_FROM_INCLUDED_FILES"))
+    options |= CXTranslationUnit_IgnoreNonErrorsFromIncludedFiles;
 
   return options;
 }
Index: tools/libclang/CIndex.cpp
===================================================================
--- tools/libclang/CIndex.cpp
+++ tools/libclang/CIndex.cpp
@@ -3397,6 +3397,9 @@
   if (options & CXTranslationUnit_KeepGoing)
     Diags->setSuppressAfterFatalError(false);
 
+  if (options & CXTranslationUnit_IgnoreNonErrorsFromIncludedFiles)
+      Diags->setSuppressNonErrorsFromIncludedFiles(true);
+
   // Recover resources if we crash before exiting this function.
   llvm::CrashRecoveryContextCleanupRegistrar<DiagnosticsEngine,
     llvm::CrashRecoveryContextReleaseRefCleanup<DiagnosticsEngine> >