JsonAdapter.nonNull() forbids explicit nulls in the JSON body

This adapter modifier throws exceptions if an unexpected null is
encountered. This may pair nicely with Kotlin.

kotlin-codegen/compiler/src/main/java/com/squareup/moshi/JsonClassCodeGenProcessor.kt

@@ -196,7 +196,7 @@ class JsonClassCodeGenProcessor : KotlinAbstractProcessor(), KotlinMetadataUtils
 
       val hasDefault = parameter?.declaresDefaultValue ?: true
 
-      if (enclosedElement.modifiers.contains(Modifier.TRANSIENT)) {
+      if (Modifier.TRANSIENT in enclosedElement.modifiers) {
         if (!hasDefault) {
           throw IllegalArgumentException("No default value for transient property $name")
         }

moshi/src/main/java/com/squareup/moshi/JsonAdapter.java

@@ -153,6 +153,39 @@ public abstract class JsonAdapter<T> {
     };
   }
 
+  /**
+   * Returns a JSON adapter equal to this JSON adapter, but that refuses null values. If null is
+   * read or written this will throw a {@link JsonDataException}.
+   *
+   * <p>Note that this adapter will not usually be invoked for absent values and so those must be
+   * handled elsewhere. This should only be used to fail on explicit nulls.
+   */
+  @CheckReturnValue public final JsonAdapter<T> nonNull() {
+    final JsonAdapter<T> delegate = this;
+    return new JsonAdapter<T>() {
+      @Override public @Nullable T fromJson(JsonReader reader) throws IOException {
+        if (reader.peek() == JsonReader.Token.NULL) {
+          throw new JsonDataException("Unexpected null at " + reader.getPath());
+        } else {
+          return delegate.fromJson(reader);
+        }
+      }
+      @Override public void toJson(JsonWriter writer, @Nullable T value) throws IOException {
+        if (value == null) {
+          throw new JsonDataException("Unexpected null at " + writer.getPath());
+        } else {
+          delegate.toJson(writer, value);
+        }
+      }
+      @Override boolean isLenient() {
+        return delegate.isLenient();
+      }
+      @Override public String toString() {
+        return delegate + ".nonNull()";
+      }
+    };
+  }
+
   /** Returns a JSON adapter equal to this, but is lenient when reading and writing. */
   @CheckReturnValue public final JsonAdapter<T> lenient() {
     final JsonAdapter<T> delegate = this;

moshi/src/test/java/com/squareup/moshi/JsonAdapterTest.java

@@ -95,6 +95,45 @@ public final class JsonAdapterTest {
     assertThat(factory.json()).isEqualTo("[\"A\",null,\"C\"]");
   }
 
+  @Test public void nonNull() throws Exception {
+    JsonAdapter<String> toUpperCase = new JsonAdapter<String>() {
+      @Override public String fromJson(JsonReader reader) throws IOException {
+        return reader.nextString().toUpperCase(Locale.US);
+      }
+
+      @Override public void toJson(JsonWriter writer, String value) throws IOException {
+        writer.value(value.toUpperCase(Locale.US));
+      }
+    }.nonNull();
+
+    JsonReader reader = factory.newReader("[\"a\", null, \"c\"]");
+    reader.beginArray();
+    assertThat(toUpperCase.fromJson(reader)).isEqualTo("A");
+    try {
+      toUpperCase.fromJson(reader);
+      fail();
+    } catch (JsonDataException expected) {
+      assertThat(expected).hasMessage("Unexpected null at $[1]");
+      assertThat(reader.nextNull()).isNull();
+    }
+    assertThat(toUpperCase.fromJson(reader)).isEqualTo("C");
+    reader.endArray();
+
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    toUpperCase.toJson(writer, "a");
+    try {
+      toUpperCase.toJson(writer, null);
+      fail();
+    } catch (JsonDataException expected) {
+      assertThat(expected).hasMessage("Unexpected null at $[1]");
+      writer.nullValue();
+    }
+    toUpperCase.toJson(writer, "c");
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"A\",null,\"C\"]");
+  }
+
   @Test public void failOnUnknown() throws Exception {
     JsonAdapter<String> alwaysSkip = new JsonAdapter<String>() {
       @Override public String fromJson(JsonReader reader) throws IOException {