Support decoding unicode escapes in JSON strings.

c++/src/capnp/compat/json-test.c++

@@ -282,6 +282,9 @@ KJ_TEST("decode all types") {
   CASE(R"({"structField":{"boolField":true}})", root.getStructField().getBoolField() == true);
   CASE(R"({"enumField":"bar"})", root.getEnumField() == TestEnum::BAR);
 
+  CASE_NO_ROUNDTRIP(R"({"textField":"foo\u1234bar"})",
+      kj::str("foo\u1234bar") == root.getTextField());
+
   CASE_THROW_RECOVERABLE(R"({"structField":null})", "Expected object value");
   CASE_THROW_RECOVERABLE(R"({"structList":null})", "Expected list value");
   CASE_THROW_RECOVERABLE(R"({"boolList":null})", "Expected list value");

c++/src/capnp/compat/json.c++

@@ -828,9 +828,13 @@ private:
       }
     }
 
-    // TODO(soon): Support at least basic multi-lingual plane, ie ignore surrogates.
-    KJ_REQUIRE(codePoint < 128, "non-ASCII unicode escapes are not supported (yet!)");
+    if (codePoint < 128) {
       target.add(0x7f & static_cast<char>(codePoint));
+    } else {
+      // TODO(perf): This is sorta malloc-heavy...
+      char16_t u = codePoint;
+      target.addAll(kj::decodeUtf16(kj::arrayPtr(&u, 1)));
+    }
   }
 
   const size_t maxNestingDepth;