diff --git a/llvm/include/llvm/Support/JSON.h b/llvm/include/llvm/Support/JSON.h
--- a/llvm/include/llvm/Support/JSON.h
+++ b/llvm/include/llvm/Support/JSON.h
@@ -909,6 +909,17 @@
     Contents();
     objectEnd();
   }
+  /// Emit an externally-serialized value.
+  /// The caller must write exactly one valid JSON value to the provided stream.
+  /// No validation or formatting of this value occurs.
+  void rawValue(llvm::function_ref<void(raw_ostream &)> Contents) {
+    rawValueBegin();
+    Contents(OS);
+    rawValueEnd();
+  }
+  void rawValue(llvm::StringRef Contents) {
+    rawValue([&](raw_ostream &OS) { OS << Contents; });
+  }
   /// Emit a JavaScript comment associated with the next printed value.
   /// The string must be valid until the next attribute or value is emitted.
   /// Comments are not part of standard JSON, and many parsers reject them!
@@ -939,8 +950,10 @@
   void objectEnd();
   void attributeBegin(llvm::StringRef Key);
   void attributeEnd();
+  raw_ostream &rawValueBegin();
+  void rawValueEnd();
 
- private:
+private:
   void attributeImpl(llvm::StringRef Key, Block Contents) {
     attributeBegin(Key);
     Contents();
@@ -955,6 +968,7 @@
     Singleton, // Top level, or object attribute.
     Array,
     Object,
+    RawValue, // External code writing a value to OS directly.
   };
   struct State {
     Context Ctx = Singleton;
diff --git a/llvm/lib/Support/JSON.cpp b/llvm/lib/Support/JSON.cpp
--- a/llvm/lib/Support/JSON.cpp
+++ b/llvm/lib/Support/JSON.cpp
@@ -251,20 +251,13 @@
 // Prints a one-line version of a value that isn't our main focus.
 // We interleave writes to OS and JOS, exploiting the lack of extra buffering.
 // This is OK as we own the implementation.
-// FIXME: once we have a "write custom serialized value" API, use it here.
-void abbreviate(const Value &V, OStream &JOS, raw_ostream &OS) {
+void abbreviate(const Value &V, OStream &JOS) {
   switch (V.kind()) {
   case Value::Array:
-    JOS.array([&] {
-      if (!V.getAsArray()->empty())
-        OS << " ... ";
-    });
+    JOS.rawValue(V.getAsArray()->empty() ? "[]" : "[ ... ]");
     break;
   case Value::Object:
-    JOS.object([&] {
-      if (!V.getAsObject()->empty())
-        OS << " ... ";
-    });
+    JOS.rawValue(V.getAsObject()->empty() ? "{}" : "{ ... }");
     break;
   case Value::String: {
     llvm::StringRef S = *V.getAsString();
@@ -284,19 +277,19 @@
 
 // Prints a semi-expanded version of a value that is our main focus.
 // Array/Object entries are printed, but not recursively as they may be huge.
-void abbreviateChildren(const Value &V, OStream &JOS, raw_ostream &OS) {
+void abbreviateChildren(const Value &V, OStream &JOS) {
   switch (V.kind()) {
   case Value::Array:
     JOS.array([&] {
       for (const auto &I : *V.getAsArray())
-        abbreviate(I, JOS, OS);
+        abbreviate(I, JOS);
     });
     break;
   case Value::Object:
     JOS.object([&] {
       for (const auto *KV : sortedElements(*V.getAsObject())) {
         JOS.attributeBegin(KV->first);
-        abbreviate(KV->second, JOS, OS);
+        abbreviate(KV->second, JOS);
         JOS.attributeEnd();
       }
     });
@@ -322,7 +315,7 @@
       std::string Comment = "error: ";
       Comment.append(ErrorMessage.data(), ErrorMessage.size());
       JOS.comment(Comment);
-      abbreviateChildren(V, JOS, OS);
+      abbreviateChildren(V, JOS);
     };
     if (Path.empty()) // We reached our target.
       return HighlightCurrent();
@@ -339,7 +332,7 @@
           if (FieldName.equals(KV->first))
             Recurse(KV->second, Path.drop_back(), Recurse);
           else
-            abbreviate(KV->second, JOS, OS);
+            abbreviate(KV->second, JOS);
           JOS.attributeEnd();
         }
       });
@@ -354,7 +347,7 @@
           if (Current++ == S.index())
             Recurse(V, Path.drop_back(), Recurse);
           else
-            abbreviate(V, JOS, OS);
+            abbreviate(V, JOS);
         }
       });
     }
@@ -893,6 +886,18 @@
   assert(Stack.back().Ctx == Object);
 }
 
+raw_ostream &llvm::json::OStream::rawValueBegin() {
+  valueBegin();
+  Stack.emplace_back();
+  Stack.back().Ctx = RawValue;
+  return OS;
+}
+
+void llvm::json::OStream::rawValueEnd() {
+  assert(Stack.back().Ctx == RawValue);
+  Stack.pop_back();
+}
+
 } // namespace json
 } // namespace llvm
 
diff --git a/llvm/unittests/Support/JSONTest.cpp b/llvm/unittests/Support/JSONTest.cpp
--- a/llvm/unittests/Support/JSONTest.cpp
+++ b/llvm/unittests/Support/JSONTest.cpp
@@ -479,6 +479,7 @@
         J.arrayBegin();
         J.value(43);
         J.arrayEnd();
+        J.rawValue([](raw_ostream &OS) { OS << "'unverified\nraw value'"; });
       });
       J.comment("attribute");
       J.attributeBegin("bar");
@@ -492,7 +493,8 @@
   };
 
   const char *Plain =
-      R"(/*top* /level*/{"foo":[null,/*element*/42.5,[43]],/*attribute*/"bar":/*attribute value*/{},"baz":"xyz"})";
+      R"(/*top* /level*/{"foo":[null,/*element*/42.5,[43],'unverified
+raw value'],/*attribute*/"bar":/*attribute value*/{},"baz":"xyz"})";
   EXPECT_EQ(Plain, StreamStuff(0));
   const char *Pretty = R"(/* top* /level */
 {
@@ -502,7 +504,9 @@
     42.5,
     [
       43
-    ]
+    ],
+    'unverified
+raw value'
   ],
   /* attribute */
   "bar": /* attribute value */ {},