Make most of the AssertThat logic testable and write a simple test fo…

…r it.

pkg/test/assertions/assertions.go

@@ -28,6 +28,16 @@ import (
 // the Explain function.
 func AssertThat(t *testing.T, object client.Object, predicates ...Predicate[client.Object]) {
 	t.Helper()
+	message := assertThat(object, predicates...)
+	if message != "" {
+		assert.Fail(t, "some predicates failed to match", message)
+	}
+}
+
+// assertThat contains the actual logic of the AssertThat function. This is separated out into
+// its own testable function because we cannot cannot capture the result of assert.Fail() in
+// another test.
+func assertThat(object client.Object, predicates ...Predicate[client.Object]) string {
 	results := make([]bool, len(predicates))
 	failure := false
 	for i, p := range predicates {
@@ -45,8 +55,9 @@ func AssertThat(t *testing.T, object client.Object, predicates ...Predicate[clie
 				sb.WriteString(Explain(p, object))
 			}
 		}
-		assert.Fail(t, "some predicates failed to match", sb.String())
+		return sb.String()
 	}
+	return ""
 }
 
 // Explain produces a textual explanation for why the provided predicate didn't match. The explanation

pkg/test/assertions/assertions_test.go

@@ -43,6 +43,35 @@ func TestExplain(t *testing.T) {
 	})
 }
 
+func TestAssertThat(t *testing.T) {
+	t.Run("positive case", func(t *testing.T) {
+		// given
+		actual := &corev1.ConfigMap{}
+		actual.SetName("actual")
+		actual.SetLabels(map[string]string{"k": "v"})
+
+		// when
+		message := assertThat(actual, Has(Name("actual")), Has(Labels(map[string]string{"k": "v"})))
+
+		// then
+		assert.Empty(t, message)
+	})
+
+	t.Run("negative case", func(t *testing.T) {
+		// given
+		actual := &corev1.ConfigMap{}
+		actual.SetName("actual")
+		actual.SetLabels(map[string]string{"k": "v"})
+
+		// when
+		message := assertThat(actual, Has(Name("expected")), Has(Labels(map[string]string{"k": "another value"})))
+
+		// then
+		assert.Contains(t, message, "predicate 'assertions.named' didn't match the object because of the following differences")
+		assert.Contains(t, message, "predicate 'assertions.hasLabels' didn't match the object because of the following differences")
+	})
+}
+
 type predicateWithoutFixing struct{}
 
 var _ Predicate[client.Object] = (*predicateWithoutFixing)(nil)