[Tooling][Format] Treat compound extensions (foo.bar.cc) as matching foo.h

Motivating use case is ".cu.cc" extensions used in some bazel projects.

Alternative is to work around this with IncludeIsMainRegex in styles.
I proposed this approach because it seems like a better default.

Differential Revision: https://reviews.llvm.org/D86597

clang/lib/Tooling/Inclusions/HeaderIncludes.cpp

@@ -12,6 +12,7 @@
 #include "clang/Lex/Lexer.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/Support/FormatVariadic.h"
+#include "llvm/Support/Path.h"
 
 namespace clang {
 namespace tooling {
@@ -174,12 +175,22 @@ inline StringRef trimInclude(StringRef IncludeName) {
 const char IncludeRegexPattern[] =
     R"(^[\t\ ]*#[\t\ ]*(import|include)[^"<]*(["<][^">]*[">]))";
 
+// The filename of Path excluding extension.
+// Used to match implementation with headers, this differs from sys::path::stem:
+//  - in names with multiple dots (foo.cu.cc) it terminates at the *first*
+//  - an empty stem is never returned: /foo/.bar.x => .bar
+//  - we don't bother to handle . and .. specially
+StringRef matchingStem(llvm::StringRef Path) {
+  StringRef Name = llvm::sys::path::filename(Path);
+  return Name.substr(0, Name.find('.', 1));
+}
+
 } // anonymous namespace
 
 IncludeCategoryManager::IncludeCategoryManager(const IncludeStyle &Style,
                                                StringRef FileName)
     : Style(Style), FileName(FileName) {
-  FileStem = llvm::sys::path::stem(FileName);
+  FileStem = matchingStem(FileName);
   for (const auto &Category : Style.IncludeCategories)
     CategoryRegexs.emplace_back(Category.Regex, llvm::Regex::IgnoreCase);
   IsMainFile = FileName.endswith(".c") || FileName.endswith(".cc") ||
@@ -222,8 +233,7 @@ int IncludeCategoryManager::getSortIncludePriority(StringRef IncludeName,
 bool IncludeCategoryManager::isMainHeader(StringRef IncludeName) const {
   if (!IncludeName.startswith("\""))
     return false;
-  StringRef HeaderStem =
-      llvm::sys::path::stem(IncludeName.drop_front(1).drop_back(1));
+  StringRef HeaderStem = matchingStem(IncludeName.drop_front(1).drop_back(1));
   if (FileStem.startswith(HeaderStem) ||
       FileStem.startswith_lower(HeaderStem)) {
     llvm::Regex MainIncludeRegex(HeaderStem.str() + Style.IncludeIsMainRegex,

clang/unittests/Tooling/HeaderIncludesTest.cpp

@@ -40,7 +40,7 @@ protected:
     return *Result;
   }
 
-  const std::string FileName = "fix.cpp";
+  std::string FileName = "fix.cpp";
   IncludeStyle Style = format::getLLVMStyle().IncludeStyle;
 };
 
@@ -102,6 +102,15 @@ TEST_F(HeaderIncludesTest, InsertAfterMainHeader) {
   Style = format::getGoogleStyle(format::FormatStyle::LanguageKind::LK_Cpp)
               .IncludeStyle;
   EXPECT_EQ(Expected, insert(Code, "<a>"));
+
+  FileName = "fix.cu.cpp";
+  EXPECT_EQ(Expected, insert(Code, "<a>"));
+
+  FileName = "fix_test.cu.cpp";
+  EXPECT_EQ(Expected, insert(Code, "<a>"));
+
+  FileName = "bar.cpp";
+  EXPECT_NE(Expected, insert(Code, "<a>")) << "Not main header";
 }
 
 TEST_F(HeaderIncludesTest, InsertBeforeSystemHeaderLLVM) {