Renable -Wimplicit-fallthrough for PDFium.

Didn't find any bugs, but also doesn't require much boilerplate.

Bug: TODO
Change-Id: I5f30183c55216f1333c79fcb46476d281720873e
Reviewed-on: https://pdfium-review.googlesource.com/26210
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>

9 files changed, 49 insertions, 2 deletions

diff --git a/core/fpdfapi/page/cpdf_allstates.cpp b/core/fpdfapi/page/cpdf_allstates.cpp
index afc87a2b5a..d2491bbee7 100644
--- a/core/fpdfapi/page/cpdf_allstates.cpp
+++ b/core/fpdfapi/page/cpdf_allstates.cpp
@@ -12,6 +12,7 @@
 #include "core/fpdfapi/page/cpdf_streamcontentparser.h"
 #include "core/fpdfapi/parser/cpdf_array.h"
 #include "core/fpdfapi/parser/cpdf_dictionary.h"
+#include "core/fxcrt/fx_fallthrough.h"
 #include "core/fxge/cfx_graphstatedata.h"
 #include "third_party/base/stl_util.h"
 
@@ -89,6 +90,7 @@ void CPDF_AllStates::ProcessExtGS(CPDF_Dictionary* pGS,
         if (pGS->KeyExist("TR2")) {
           continue;
         }
+        FX_FALLTHROUGH;
       case FXBSTR_ID('T', 'R', '2', 0):
         m_GeneralState.SetTR(pObject && !pObject->IsName() ? pObject : nullptr);
         break;
@@ -131,6 +133,7 @@ void CPDF_AllStates::ProcessExtGS(CPDF_Dictionary* pGS,
         if (pGS->KeyExist("BG2")) {
           continue;
         }
+        FX_FALLTHROUGH;
       case FXBSTR_ID('B', 'G', '2', 0):
         m_GeneralState.SetBG(pObject);
         break;
@@ -138,6 +141,7 @@ void CPDF_AllStates::ProcessExtGS(CPDF_Dictionary* pGS,
         if (pGS->KeyExist("UCR2")) {
           continue;
         }
+        FX_FALLTHROUGH;
       case FXBSTR_ID('U', 'C', 'R', '2'):
         m_GeneralState.SetUCR(pObject);
         break;
diff --git a/core/fpdfapi/parser/cpdf_data_avail.cpp b/core/fpdfapi/parser/cpdf_data_avail.cpp
index af80fd96ab..e871e0a578 100644
--- a/core/fpdfapi/parser/cpdf_data_avail.cpp
+++ b/core/fpdfapi/parser/cpdf_data_avail.cpp
@@ -26,6 +26,7 @@
 #include "core/fpdfapi/parser/fpdf_parser_utility.h"
 #include "core/fxcrt/cfx_memorystream.h"
 #include "core/fxcrt/fx_extension.h"
+#include "core/fxcrt/fx_fallthrough.h"
 #include "core/fxcrt/fx_safe_types.h"
 #include "third_party/base/numerics/safe_conversions.h"
 #include "third_party/base/ptr_util.h"
@@ -133,6 +134,7 @@ bool CPDF_DataAvail::CheckDocStatus() {
       return LoadAllFile();
     case PDF_DATAAVAIL_PAGE_LATERLOAD:
       m_docStatus = PDF_DATAAVAIL_PAGE;
+      FX_FALLTHROUGH;
     default:
       m_bDocAvail = true;
       return true;
diff --git a/core/fpdfapi/render/cpdf_dibtransferfunc.cpp b/core/fpdfapi/render/cpdf_dibtransferfunc.cpp
index 3b0e8b25a9..959fec383d 100644
--- a/core/fpdfapi/render/cpdf_dibtransferfunc.cpp
+++ b/core/fpdfapi/render/cpdf_dibtransferfunc.cpp
@@ -10,6 +10,7 @@
 
 #include "core/fpdfapi/parser/cpdf_dictionary.h"
 #include "core/fpdfapi/render/cpdf_transferfunc.h"
+#include "core/fxcrt/fx_fallthrough.h"
 
 CPDF_DIBTransferFunc::CPDF_DIBTransferFunc(
     const RetainPtr<CPDF_TransferFunc>& pTransferFunc)
@@ -119,6 +120,7 @@ void CPDF_DIBTransferFunc::TranslateScanline(
     }
     case FXDIB_Rgb32:
       bSkip = true;
+      FX_FALLTHROUGH;
     case FXDIB_Argb: {
       int index = 0;
       for (int i = 0; i < m_Width; i++) {
diff --git a/core/fpdfapi/render/cpdf_renderstatus.cpp b/core/fpdfapi/render/cpdf_renderstatus.cpp
index 66688a2d4c..6525164547 100644
--- a/core/fpdfapi/render/cpdf_renderstatus.cpp
+++ b/core/fpdfapi/render/cpdf_renderstatus.cpp
@@ -49,6 +49,7 @@
 #include "core/fpdfdoc/cpdf_occontext.h"
 #include "core/fxcrt/autorestorer.h"
 #include "core/fxcrt/cfx_fixedbufgrow.h"
+#include "core/fxcrt/fx_fallthrough.h"
 #include "core/fxcrt/fx_safe_types.h"
 #include "core/fxcrt/maybe_owned.h"
 #include "core/fxge/cfx_defaultrenderdevice.h"
@@ -1765,6 +1766,7 @@ bool CPDF_RenderStatus::ProcessText(CPDF_TextObject* textobj,
         // Already handled above, but the compiler is not smart enough to
         // realize it. Fall through.
         NOTREACHED();
+        FX_FALLTHROUGH;
       case TextRenderingMode::MODE_CLIP:
         return true;
     }
diff --git a/core/fpdfdoc/cpdf_variabletext.cpp b/core/fpdfdoc/cpdf_variabletext.cpp
index 38c631d820..7da1b2f946 100644
--- a/core/fpdfdoc/cpdf_variabletext.cpp
+++ b/core/fpdfdoc/cpdf_variabletext.cpp
@@ -16,6 +16,7 @@
 #include "core/fpdfdoc/csection.h"
 #include "core/fpdfdoc/ipvt_fontmap.h"
 #include "core/fxcrt/fx_codepage.h"
+#include "core/fxcrt/fx_fallthrough.h"
 #include "third_party/base/ptr_util.h"
 #include "third_party/base/stl_util.h"
 
@@ -317,6 +318,7 @@ void CPDF_VariableText::SetText(const WideString& swText) {
         break;
       case 0x09:
         word = 0x20;
+        FX_FALLTHROUGH;
       default:
         wp = InsertWord(wp, word, FX_CHARSET_Default);
         break;
diff --git a/core/fxcrt/fx_extension.cpp b/core/fxcrt/fx_extension.cpp
index 62cf8a0e04..371f1b5211 100644
--- a/core/fxcrt/fx_extension.cpp
+++ b/core/fxcrt/fx_extension.cpp
@@ -6,6 +6,8 @@
 
 #include "core/fxcrt/fx_extension.h"
 
+#include "core/fxcrt/fx_fallthrough.h"
+
 #include <algorithm>
 #include <cwctype>
 
@@ -21,6 +23,7 @@ float FXSYS_wcstof(const wchar_t* pwsStr, int32_t iLength, int32_t* pUsedLen) {
   switch (pwsStr[iUsedLen]) {
     case '-':
       bNegtive = true;
+      FX_FALLTHROUGH;
     case '+':
       iUsedLen++;
       break;
diff --git a/core/fxcrt/fx_fallthrough.h b/core/fxcrt/fx_fallthrough.h
new file mode 100644
index 0000000000..5587ee2295
--- /dev/null
+++ b/core/fxcrt/fx_fallthrough.h
@@ -0,0 +1,20 @@
+// Copyright 2018 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CORE_FXCRT_FX_FALLTHROUGH_H_
+#define CORE_FXCRT_FX_FALLTHROUGH_H_
+
+// When clang suggests inserting [[clang::fallthrough]], it first checks if
+// it knows of a macro expanding to it, and if so suggests inserting the
+// macro.  This means that this macro must be used only in code internal
+// to PDFium, so that PDFium's user code doesn't end up getting suggestions
+// for FX_FALLTHROUGH instead of the user-specific fallthrough macro.
+// So do not include this header in any of PDFium's public headers.
+#if defined(__clang__)
+#define FX_FALLTHROUGH [[clang::fallthrough]]
+#else
+#define FX_FALLTHROUGH
+#endif
+
+#endif  // CORE_FXCRT_FX_FALLTHROUGH_H_
diff --git a/core/fxcrt/xml/cxml_parser.cpp b/core/fxcrt/xml/cxml_parser.cpp
index ae5fd6d0fc..64bb0ec530 100644
--- a/core/fxcrt/xml/cxml_parser.cpp
+++ b/core/fxcrt/xml/cxml_parser.cpp
@@ -14,6 +14,7 @@
 #include "core/fxcrt/cfx_utf8decoder.h"
 #include "core/fxcrt/cfx_widetextbuf.h"
 #include "core/fxcrt/fx_extension.h"
+#include "core/fxcrt/fx_fallthrough.h"
 #include "core/fxcrt/xml/cxml_content.h"
 #include "core/fxcrt/xml/cxml_element.h"
 #include "core/fxcrt/xml/cxml_parser.h"
@@ -200,6 +201,7 @@ uint32_t CXML_Parser::GetCharRef() {
             break;
           }
           iState = 1;
+          FX_FALLTHROUGH;
         case 1:
           m_dwIndex++;
           if (ch == ';') {
@@ -226,6 +228,7 @@ uint32_t CXML_Parser::GetCharRef() {
             break;
           }
           iState = 3;
+          FX_FALLTHROUGH;
         case 3:
           m_dwIndex++;
           if (ch == ';') {
diff --git a/core/fxge/agg/fx_agg_driver.cpp b/core/fxge/agg/fx_agg_driver.cpp
index b394facace..922f39efcd 100644
--- a/core/fxge/agg/fx_agg_driver.cpp
+++ b/core/fxge/agg/fx_agg_driver.cpp
@@ -15,6 +15,14 @@
 #include "core/fxge/cfx_pathdata.h"
 #include "core/fxge/dib/cfx_imagerenderer.h"
 #include "core/fxge/dib/cfx_imagestretcher.h"
+#include "third_party/base/ptr_util.h"
+#include "third_party/base/stl_util.h"
+
+// Ignore fallthrough warnings in agg23 headers.
+#if defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+#endif
 #include "third_party/agg23/agg_conv_dash.h"
 #include "third_party/agg23/agg_conv_stroke.h"
 #include "third_party/agg23/agg_curves.h"
@@ -23,8 +31,9 @@
 #include "third_party/agg23/agg_rasterizer_scanline_aa.h"
 #include "third_party/agg23/agg_renderer_scanline.h"
 #include "third_party/agg23/agg_scanline_u.h"
-#include "third_party/base/ptr_util.h"
-#include "third_party/base/stl_util.h"
+#if defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
 
 namespace {