Add a Loopable trait for dust sections.

src/renderer/context_element.rs

@@ -2,7 +2,7 @@ use crate::parser::Filter;
 use crate::renderer::errors::RenderError;
 use std::fmt::Debug;
 
-pub trait ContextElement: Walkable + Renderable + Debug {}
+pub trait ContextElement: Debug + Walkable + Renderable + Loopable {}
 
 pub trait Walkable {
     fn walk(&self, segment: &str) -> Result<&dyn ContextElement, RenderError>;
@@ -11,3 +11,18 @@ pub trait Walkable {
 pub trait Renderable {
     fn render(&self, filters: &Vec<Filter>) -> Result<String, RenderError>;
 }
+
+pub trait Loopable {
+    /// Return the elements for a Dust section
+    ///
+    /// Sections in dust are accomplished with the {#path} syntax. A
+    /// section has a truthiness check performed on it. If that
+    /// truthiness check fails, then it will render the
+    /// else-block. Otherwise if its a scalar value it will render
+    /// once with the context being the element at that path. Finally,
+    /// if its an array-like value then it will render n-times, once
+    /// for each element of the array.
+    ///
+    /// TODO: Should this return an iterator instead of a vec?
+    fn get_loop_elements(&self) -> Result<Vec<&dyn ContextElement>, RenderError>;
+}