[clang-format] Fix working -assume-filename with .clang-format-ignore

The filename given by the `-assume-filename` option is used to search
for `.clang-format` files, etc., but is not used to match the contents
of the `.clang-format-ignore` file.

Fixed that when the `-assume-filename` option is specified, the
`.clang-format-ignore` file is processed for that filename.

If "ignore" is implemented as "do not output anything," the formatting
result will be empty, so appropriate processing such as outputting
standard input directly to standard output is necessary.

Author: kakkoko

clang/test/Format/clang-format-ignore.cpp

@@ -46,5 +46,16 @@
 // CHECK5-NEXT: {{Formatting \[4/5] .*foo\.c}}
 // CHECK5-NOT: foo.js
 
+// RUN: echo "foo.*" > .clang-format-ignore
+// RUN: touch foo.c
+// RUN: echo "foo,bar" | clang-format -assume-filename=foo.c 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=CHECK6
+// CHECK6: foo,bar
+
+// RUN: touch bar.c
+// RUN: echo "foo,bar" | clang-format -assume-filename=bar.c 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=CHECK7 -allow-empty
+// CHECK7: foo, bar
+
 // RUN: cd ..
 // RUN: rm -r %t.dir

clang/tools/clang-format/ClangFormat.cpp

@@ -707,8 +707,42 @@ int main(int argc, const char **argv) {
     errs() << "Clang-formatting " << LineNo << " files\n";
   }
 
-  if (FileNames.empty())
+  if (FileNames.empty()) {
+    if (!AssumeFileName.empty() && isIgnored(AssumeFileName)) {
+      ErrorOr<std::unique_ptr<MemoryBuffer>> CodeOrErr =
+          MemoryBuffer::getSTDIN();
+      if (std::error_code EC = CodeOrErr.getError()) {
+        errs() << EC.message() << "\n";
+        return 1;
+      }
+      std::unique_ptr<llvm::MemoryBuffer> Code = std::move(CodeOrErr.get());
+      if (Code->getBufferSize() == 0)
+        return 0; // Empty files are formatted correctly.
+
+      StringRef BufStr = Code->getBuffer();
+
+      const char *InvalidBOM =
+          clang::SrcMgr::ContentCache::getInvalidBOM(BufStr);
+
+      if (InvalidBOM) {
+        errs() << "error: encoding with unsupported byte order mark \""
+               << InvalidBOM << "\" detected.\n";
+        return 1;
+      }
+
+      // Output the input to STDIN as is
+      if (OutputXML) {
+        unsigned CursorPosition = Cursor;
+        clang::format::outputXML(Replacements{}, Replacements{},
+                                 clang::format::FormattingAttemptStatus{},
+                                 Cursor, CursorPosition);
+      } else {
+        outs() << BufStr;
+      }
+      return 0;
+    }
     return clang::format::format("-", FailOnIncompleteFormat);
+  }
 
   if (FileNames.size() > 1 &&
       (!Offsets.empty() || !Lengths.empty() || !LineRanges.empty())) {