Index: clang/lib/Format/FormatTokenLexer.h
===================================================================
--- clang/lib/Format/FormatTokenLexer.h
+++ clang/lib/Format/FormatTokenLexer.h
@@ -54,6 +54,7 @@
   bool tryMergeCSharpNullConditionals();
   bool tryMergeCSharpDoubleQuestion();
   bool tryTransformCSharpForEach();
+  bool tryMergeCSharpAttributeAndTarget();
 
   bool tryMergeTokens(ArrayRef<tok::TokenKind> Kinds, TokenType NewType);
 
Index: clang/lib/Format/FormatTokenLexer.cpp
===================================================================
--- clang/lib/Format/FormatTokenLexer.cpp
+++ clang/lib/Format/FormatTokenLexer.cpp
@@ -76,6 +76,8 @@
     return;
 
   if (Style.isCSharp()) {
+    if (tryMergeCSharpAttributeAndTarget())
+      return;
     if (tryMergeCSharpKeywordVariables())
       return;
     if (tryMergeCSharpStringLiteral())
@@ -275,6 +277,39 @@
   return true;
 }
 
+bool FormatTokenLexer::tryMergeCSharpAttributeAndTarget() {
+  // Treat '[assembly:' and '[field:' as tokens in their own right.
+  if (Tokens.size() < 3)
+    return false;
+
+  auto &SquareBracket = *(Tokens.end() - 3);
+  auto &Target = *(Tokens.end() - 2);
+  auto &Colon = *(Tokens.end() - 1);
+
+  if (!SquareBracket->Tok.is(tok::l_square))
+    return false;
+
+  // Valid C# attribute targets:
+  // https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/#attribute-targets
+  if (!(Target->TokenText == "assembly" || Target->TokenText == "module" ||
+        Target->TokenText == "field" || Target->TokenText == "event" ||
+        Target->TokenText == "method" || Target->TokenText == "param" ||
+        Target->TokenText == "property" || Target->TokenText == "return" ||
+        Target->TokenText == "type"))
+    return false;
+
+  if (!Colon->Tok.is(tok::colon))
+    return false;
+
+  SquareBracket->TokenText =
+      StringRef(SquareBracket->TokenText.begin(),
+                Colon->TokenText.end() - SquareBracket->TokenText.begin());
+  SquareBracket->ColumnWidth += (Target->ColumnWidth + Colon->ColumnWidth);
+  Tokens.erase(Tokens.end() - 2);
+  Tokens.erase(Tokens.end() - 1);
+  return true;
+}
+
 bool FormatTokenLexer::tryMergeCSharpDoubleQuestion() {
   if (Tokens.size() < 2)
     return false;
Index: clang/unittests/Format/FormatTestCSharp.cpp
===================================================================
--- clang/unittests/Format/FormatTestCSharp.cpp
+++ clang/unittests/Format/FormatTestCSharp.cpp
@@ -233,6 +233,15 @@
       "[DllImport(\"Hello\", EntryPoint = \"hello_world\")]\n"
       "// The const char* returned by hello_world must not be deleted.\n"
       "private static extern IntPtr HelloFromCpp();)");
+
+  //  Unwrappable lines go on a line of their own.
+  // 'target:' is not treated as a label.
+  // Modify Style to enforce a column limit.
+  FormatStyle Style = getGoogleStyle(FormatStyle::LK_CSharp);
+  Style.ColumnLimit = 10;
+  verifyFormat(R"([assembly:InternalsVisibleTo(
+    "SomeAssembly, PublicKey=SomePublicKeyThatExceedsTheColumnLimit")])",
+               Style);
 }
 
 TEST_F(FormatTestCSharp, CSharpUsing) {