Emit events when traits are changed

smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ModifiedTrait.java

@@ -101,6 +101,29 @@ public List<ValidationEvent> evaluate(Differences differences) {
                             changedShape.getNewShape().getType(),
                             Node.prettyPrintJson(oldTrait.toNode()),
                             Node.prettyPrintJson(newTrait.toNode()))));
+                } else if (oldTrait == null) {
+                    events.add(note(changedShape.getNewShape(), newTrait, String.format(
+                            "The `%s` trait was added to the `%s` %s shape with the value: %s",
+                            traitName,
+                            changedShape.getNewShape().getId(),
+                            changedShape.getNewShape().getType(),
+                            Node.prettyPrintJson(newTrait.toNode()))));
+                } else if (newTrait == null) {
+                    events.add(warning(changedShape.getNewShape(), String.format(
+                            "The `%s` trait was removed from the `%s` %s shape. The removed trait value was: %s",
+                            traitName,
+                            changedShape.getNewShape().getId(),
+                            changedShape.getNewShape().getType(),
+                            Node.prettyPrintJson(oldTrait.toNode()))));
+                } else {
+                    events.add(note(changedShape.getNewShape(), newTrait, String.format(
+                            "The `%s` trait was changed on the `%s` %s shape. The old trait value was: %s. "
+                            + "The new trait value is: %s",
+                            traitName,
+                            changedShape.getNewShape().getId(),
+                            changedShape.getNewShape().getType(),
+                            Node.prettyPrintJson(oldTrait.toNode()),
+                            Node.prettyPrintJson(newTrait.toNode()))));
                 }
             });
         });

smithy-diff/src/test/java/software/amazon/smithy/diff/evaluators/ChangedEnumTraitTest.java

@@ -49,7 +49,7 @@ public void detectsAddedEnums() {
         List<ValidationEvent> events = ModelDiff.compare(modelA, modelB);
 
         assertThat(TestHelper.findEvents(events, "ChangedEnumTrait").size(), equalTo(1));
-        assertThat(TestHelper.findEvents(events, Severity.NOTE).size(), equalTo(1));
+        assertThat(TestHelper.findEvents(events, "ChangedEnumTrait").get(0).getSeverity(), equalTo(Severity.NOTE));
     }
 
     @Test

smithy-diff/src/test/java/software/amazon/smithy/diff/evaluators/ModifiedTraitTest.java

@@ -18,10 +18,14 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.hasItem;
+import static org.hamcrest.Matchers.hasSize;
 
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.List;
+import java.util.stream.Collectors;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 import software.amazon.smithy.diff.ModelDiff;
@@ -33,6 +37,7 @@
 import software.amazon.smithy.model.traits.DynamicTrait;
 import software.amazon.smithy.model.traits.TagsTrait;
 import software.amazon.smithy.model.traits.TraitDefinition;
+import software.amazon.smithy.model.validation.Severity;
 import software.amazon.smithy.model.validation.ValidationEvent;
 
 public class ModifiedTraitTest {
@@ -98,4 +103,28 @@ private static Shape createDefinition(String tag) {
                 .addTrait(TraitDefinition.builder().build())
                 .build();
     }
+
+    @Test
+    public void modifiedShapeNoTag() {
+        Model modelA = Model.assembler()
+                .addImport(getClass().getResource("trait-test-a.smithy"))
+                .assemble()
+                .unwrap();
+        Model modelB = Model.assembler()
+                .addImport(getClass().getResource("trait-test-b.smithy"))
+                .assemble()
+                .unwrap();
+        List<ValidationEvent> events = TestHelper.findEvents(ModelDiff.compare(modelA, modelB), "ModifiedTrait");
+        List<String> messages = events.stream()
+                .map(ValidationEvent::getMessage)
+                .collect(Collectors.toList());
+
+        assertThat(events, hasSize(3));
+        assertThat(events.stream().filter(e -> e.getSeverity() == Severity.WARNING).count(), equalTo(1L));
+        assertThat(events.stream().filter(e -> e.getSeverity() == Severity.NOTE).count(), equalTo(2L));
+
+        assertThat(messages, hasItem("The `smithy.example#c` trait was added to the `smithy.example#Foo` string shape with the value: \"foo\""));
+        assertThat(messages, hasItem("The `smithy.example#a` trait was removed from the `smithy.example#Foo` string shape. The removed trait value was: {}"));
+        assertThat(messages, hasItem("The `smithy.example#b` trait was changed on the `smithy.example#Foo` string shape. The old trait value was: \"hello\". The new trait value is: \"hello!\""));
+    }
 }

smithy-diff/src/test/resources/software/amazon/smithy/diff/evaluators/trait-test-a.smithy

@@ -0,0 +1,14 @@
+namespace smithy.example
+
+@a
+@b("hello")
+string Foo
+
+@trait
+structure a {}
+
+@trait
+string b
+
+@trait
+string c

smithy-diff/src/test/resources/software/amazon/smithy/diff/evaluators/trait-test-b.smithy

@@ -0,0 +1,14 @@
+namespace smithy.example
+
+@b("hello!")
+@c("foo")
+string Foo
+
+@trait
+structure a {}
+
+@trait
+string b
+
+@trait
+string c