Add xmlAttribute trait support

Author: JordonPhillips

smithy-docgen-core/src/main/java/software/amazon/smithy/docgen/core/integrations/BuiltinsIntegration.java

@@ -34,6 +34,7 @@
 import software.amazon.smithy.docgen.core.interceptors.TimestampFormatInterceptor;
 import software.amazon.smithy.docgen.core.interceptors.UniqueItemsInterceptor;
 import software.amazon.smithy.docgen.core.interceptors.UnstableInterceptor;
+import software.amazon.smithy.docgen.core.interceptors.XmlAttributeInterceptor;
 import software.amazon.smithy.docgen.core.interceptors.XmlNameInterceptor;
 import software.amazon.smithy.docgen.core.writers.DocWriter;
 import software.amazon.smithy.docgen.core.writers.MarkdownWriter;
@@ -80,6 +81,7 @@ public List<? extends CodeInterceptor<? extends CodeSection, DocWriter>> interce
                 new ApiKeyAuthInterceptor(),
                 new TimestampFormatInterceptor(),
                 new JsonNameInterceptor(),
+                new XmlAttributeInterceptor(),
                 new XmlNameInterceptor(),
                 new PaginationInterceptor(),
                 new RequestCompressionInterceptor(),

smithy-docgen-core/src/main/java/software/amazon/smithy/docgen/core/interceptors/XmlAttributeInterceptor.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.docgen.core.interceptors;
+
+import software.amazon.smithy.docgen.core.sections.ProtocolSection;
+import software.amazon.smithy.docgen.core.writers.DocWriter;
+import software.amazon.smithy.docgen.core.writers.DocWriter.AdmonitionType;
+import software.amazon.smithy.model.shapes.ShapeId;
+import software.amazon.smithy.model.traits.XmlAttributeTrait;
+import software.amazon.smithy.utils.SmithyInternalApi;
+
+/**
+ * Notes that a member is an
+ * <a href="https://smithy.io/2.0/spec/protocol-traits.html#xmlnattribute-trait">
+ * xml attribute</a> in the {@link ProtocolSection} if the protocol supports it.
+ */
+@SmithyInternalApi
+public final class XmlAttributeInterceptor extends ProtocolTraitInterceptor<XmlAttributeTrait> {
+    @Override
+    protected Class<XmlAttributeTrait> getTraitClass() {
+        return XmlAttributeTrait.class;
+    }
+
+    @Override
+    protected ShapeId getTraitId() {
+        return XmlAttributeTrait.ID;
+    }
+
+    @Override
+    void write(DocWriter writer, String previousText, ProtocolSection section, XmlAttributeTrait trait) {
+        writer.openAdmonition(AdmonitionType.IMPORTANT);
+        writer.write("This member represents an XML attribute rather than a nested tag.");
+        writer.closeAdmonition();
+    }
+}

smithy-docgen-test/model/main.smithy

@@ -147,12 +147,29 @@ structure DocumentedStructure {
 
     @recommended(reason: "Because unions are cool")
     union: DocumentedUnion
+
+    xmlTraits: XmlTraits
 }
 
 /// Timestamp in RFC3339 format
 @timestampFormat("date-time")
 timestamp DateTime
 
+/// This structure showcases various XML traits
+@xmlName("foo")
+structure XmlTraits {
+    /// This shows that the xml name isn't inherited from the target.
+    nested: XmlTraits
+
+    /// This shows an xml name targeting a normal shape.
+    @xmlName("bar")
+    xmlName: String
+
+    /// This shows how xml attributes are displayed.
+    @xmlAttribute
+    xmlAttribute: String
+}
+
 /// This in an enum that can have one of the following values:
 ///
 /// - `FOO`