Use List<const Whatever> for encodable lists. (#11006)

We should not be requiring creation of non-const data to encode a list
if it can be backed by const data instead.

Another option would be to make List<T> inherit from Span<const T>,
but that makes some assumptions about whether people will want to
write to a buffer via its List representation.

The change to the List::operator= is because importing _all_ operator=
from the super-class, including the implicitly-defined ones, causes
assignments from List<T> to List<const T> to be ambiguous.  Forwarding
just the one operator= we want to forward fixes that.

Author: bzbarsky-apple

src/app/data-model/List.h

@@ -48,7 +48,13 @@ struct List : public Span<T>
     // List<T> instances as though they were just Spans.
     //
     using Span<T>::Span;
-    using Span<T>::operator=;
+
+    template <size_t N>
+    constexpr List & operator=(T (&databuf)[N])
+    {
+        Span<T>::operator=(databuf);
+        return (*this);
+    }
 };
 
 template <typename X>

src/app/zap-templates/templates/app/helper.js

@@ -375,11 +375,19 @@ async function zapTypeToClusterObjectType(type, isDecodable, options)
   let promise = templateUtil.ensureZclPackageId(this).then(fn.bind(this));
   if ((this.isList || this.isArray || this.entryType) && !options.hash.forceNotList) {
     passByReference = true;
-    let listType    = isDecodable ? "DecodableList" : "List";
     // If we did not have a namespace provided, we can assume we're inside
     // chip::app.
     let listNamespace = options.hash.ns ? "chip::app::" : ""
-    promise           = promise.then(typeStr => `${listNamespace}DataModel::${listType}<${typeStr}>`);
+    if (isDecodable)
+    {
+      promise = promise.then(typeStr => `${listNamespace}DataModel::DecodableList<${typeStr}>`);
+    }
+    else
+    {
+      // Use const ${typeStr} so that consumers don't have to create non-const
+      // data to encode.
+      promise = promise.then(typeStr => `${listNamespace}DataModel::List<const ${typeStr}>`);
+    }
   }
   if (this.isNullable && !options.hash.forceNotNullable) {
     passByReference = true;