Disallow traits applied to public prelude shapes

Applying a trait to a prelude shape is a recipe for disaster. For
example, if someone marked the smithy.api#String shape as private,
virtually every model would break.

Author: mtdowling

smithy-model/src/main/java/software/amazon/smithy/model/loader/LoaderVisitor.java

@@ -282,7 +282,7 @@ public void onTrait(ShapeId target, ShapeId trait, Node traitValue) {
             onTrait(target, traitDef);
         } else {
             PendingTrait pendingTrait = new PendingTrait(trait, traitValue);
-            pendingTraits.computeIfAbsent(target, targetId -> new ArrayList<>()).add(pendingTrait);
+            addPendingTrait(target, traitValue.getSourceLocation(), trait, pendingTrait);
         }
     }
 
@@ -294,7 +294,22 @@ public void onTrait(ShapeId target, ShapeId trait, Node traitValue) {
      */
     public void onTrait(ShapeId target, Trait trait) {
         PendingTrait pending = new PendingTrait(target, trait);
-        pendingTraits.computeIfAbsent(target, targetId -> new ArrayList<>()).add(pending);
+        addPendingTrait(target, trait.getSourceLocation(), trait.toShapeId(), pending);
+    }
+
+    private void addPendingTrait(ShapeId target, SourceLocation sourceLocation, ShapeId trait, PendingTrait pending) {
+        if (Prelude.isImmutablePublicPreludeShape(target)) {
+            onError(ValidationEvent.builder()
+                    .severity(Severity.ERROR)
+                    .eventId(Validator.MODEL_ERROR)
+                    .sourceLocation(sourceLocation)
+                    .shapeId(target)
+                    .message(String.format(
+                            "Cannot apply `%s` to an immutable prelude shape defined in `smithy.api`.", trait))
+                    .build());
+        } else {
+            pendingTraits.computeIfAbsent(target, targetId -> new ArrayList<>()).add(pending);
+        }
     }
 
     /**

smithy-model/src/main/java/software/amazon/smithy/model/loader/Prelude.java

@@ -235,6 +235,16 @@ public static boolean isPublicPreludeShape(ToShapeId id) {
         return PUBLIC_PRELUDE_SHAPE_IDS.contains(toId) || PRELUDE_TRAITS.contains(toId);
     }
 
+    /**
+     * Checks if the given shape is an immutable public shape.
+     *
+     * @param id Shape to check.
+     * @return Returns true if the shape is immutable.
+     */
+    static boolean isImmutablePublicPreludeShape(ToShapeId id) {
+        return PUBLIC_PRELUDE_SHAPE_IDS.contains(id.toShapeId());
+    }
+
     // Used by the ModelAssembler to load the prelude into another visitor.
     static Model getPreludeModel() {
         return PreludeHolder.PRELUDE;

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/apply-trait-to-prelude.errors

@@ -0,0 +1 @@
+[ERROR] smithy.api#String: Cannot apply `smithy.api#deprecated` to an immutable prelude shape defined in `smithy.api`. | Model

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/apply-trait-to-prelude.smithy

@@ -0,0 +1,3 @@
+namespace com.foo
+
+apply smithy.api#String @deprecated