Disallow adt on mixins

Author: kubukoz

modules/protocol-tests/test/src/smithy4s/api/validation/AdtTraitValidatorSpec.scala

@@ -24,6 +24,41 @@ import ModelUtils._
 
 object AdtTraitValidatorSpec extends FunSuite {
 
+  test("AdtTrait - not allowed on mixins") {
+
+    val events = eventsWithoutLocations(
+      assembleModel(
+        """$version: "2"
+          |namespace test
+          |
+          |use smithy4s.meta#adt
+          |
+          |structure struct {
+          |  testing: String
+          |}
+          |
+          |@adt
+          |@mixin
+          |union MyUnion {
+          |  unionMember: struct
+          |}
+          |""".stripMargin
+      )
+    )
+
+    val expected = ValidationEvent
+      .builder()
+      .id("TraitTarget")
+      .shapeId(ShapeId.fromParts("test", "MyUnion"))
+      .severity(Severity.ERROR)
+      .message(
+        "Trait `smithy4s.meta#adt` cannot be applied to `test#MyUnion`. This trait may only be applied to shapes that match the following selector: :test(union :not([trait|mixin]))"
+      )
+      .build()
+
+    assert(events.contains(expected))
+  }
+
   test("AdtTrait - return no error when union targets the structure") {
     assembleModel(
       """$version: "2"

modules/protocol-tests/test/src/smithy4s/api/validation/ModelUtils.scala

@@ -22,6 +22,7 @@ private object ModelUtils {
   def eventsWithoutLocations(
       result: ValidatedResult[?]
   ): List[ValidationEvent] = {
+    if (!result.isBroken()) sys.error("Expected a broken result")
     result.getValidationEvents.asScala.toList.map(e =>
       e.toBuilder.sourceLocation(SourceLocation.NONE).build()
     )

modules/protocol/resources/META-INF/smithy/smithy4s.meta.smithy

@@ -30,7 +30,7 @@ string adtMember
 /// Implies that all members of the union are annotated with the `adtMember` trait.
 /// Further signals that the `sealed trait` for this adt will extend the traits
 /// defined by any mixins that are present on all of the adt members.
-@trait(selector: ":test(union, :not([trait|mixin]))")
+@trait(selector: ":test(union :not([trait|mixin]))")
 structure adt {}
 
 // the indexedSeq trait can be added to list shapes in order for the generated collection