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: include/clang/Basic/DiagnosticOptions.def
===================================================================
--- include/clang/Basic/DiagnosticOptions.def
+++ include/clang/Basic/DiagnosticOptions.def
@@ -45,6 +45,7 @@
 #endif
 
 SEMANTIC_DIAGOPT(IgnoreWarnings, 1, 0)   /// -w
+DIAGOPT(SuppressNonErrorsFromIncludedFiles, 1, 0)  /// -fsuppress-non-errors-from-included-files
 DIAGOPT(NoRewriteMacros, 1, 0)  /// -Wno-rewrite-macros
 DIAGOPT(Pedantic, 1, 0)         /// -pedantic
 DIAGOPT(PedanticErrors, 1, 0)   /// -pedantic-errors
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -1742,6 +1742,8 @@
 def fdebug_prefix_map_EQ
   : Joined<["-"], "fdebug-prefix-map=">, Group<f_Group>, Flags<[CC1Option]>,
     HelpText<"remap file source paths in debug info">;
+def fsuppress_non_errors_from_included_files : Flag<["-"], "fsuppress-non-errors-from-included-files">,
+  Group<f_Group>, Flags<[CC1Option]>;
 def g_Flag : Flag<["-"], "g">, Group<g_Group>,
   HelpText<"Generate source-level debug information">;
 def gline_tables_only : Flag<["-"], "gline-tables-only">, Group<gN_Group>,
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: lib/Basic/Warnings.cpp
===================================================================
--- lib/Basic/Warnings.cpp
+++ lib/Basic/Warnings.cpp
@@ -46,6 +46,7 @@
                                   bool ReportDiags) {
   Diags.setSuppressSystemWarnings(true);  // Default to -Wno-system-headers
   Diags.setIgnoreAllWarnings(Opts.IgnoreWarnings);
+  Diags.setSuppressNonErrorsFromIncludedFiles(Opts.SuppressNonErrorsFromIncludedFiles);
   Diags.setShowOverloads(Opts.getShowOverloads());
 
   Diags.setElideType(Opts.ElideType);
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1223,6 +1223,8 @@
           Args.getLastArg(OPT_diagnostic_serialized_file, OPT__serialize_diags))
     Opts.DiagnosticSerializationFile = A->getValue();
   Opts.IgnoreWarnings = Args.hasArg(OPT_w);
+  Opts.SuppressNonErrorsFromIncludedFiles =
+      Args.hasArg(OPT_fsuppress_non_errors_from_included_files);
   Opts.NoRewriteMacros = Args.hasArg(OPT_Wno_rewrite_macros);
   Opts.Pedantic = Args.hasArg(OPT_pedantic);
   Opts.PedanticErrors = Args.hasArg(OPT_pedantic_errors);
Index: test/Frontend/Inputs/header-with-warning.h
===================================================================
--- /dev/null
+++ test/Frontend/Inputs/header-with-warning.h
@@ -0,0 +1 @@
+static void f0(void) {}
Index: test/Frontend/warnings-ignore-from-includes.cpp
===================================================================
--- /dev/null
+++ test/Frontend/warnings-ignore-from-includes.cpp
@@ -0,0 +1,7 @@
+#include "header-with-warning.h"
+
+static void f1(void) {}
+
+// RUN: %clang_cc1 -I%S/Inputs/ -Wunused-function -fsuppress-non-errors-from-included-files %s 2>&1 | FileCheck %s
+// CHECK-NOT: warning: unused function 'f0'
+// CHECK: warning: unused function 'f1'
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
@@ -3398,6 +3398,9 @@
   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> >