Add tests for explicit context setting that check failure conditions and implement a helper function to generate a new breadcrumb stack.

src/renderer/renderer.rs

@@ -3,6 +3,7 @@ use crate::parser::Body;
 use crate::parser::DustTag;
 use crate::parser::KVPair;
 use crate::parser::PartialNameElement;
+use crate::parser::Path;
 use crate::parser::RValue;
 use crate::parser::Special;
 use crate::parser::Template;
@@ -551,6 +552,61 @@ impl<'a> DustRenderer<'a> {
         }
         final_filters
     }
+
+    /// Generate a new breadcrumbs object
+    ///
+    /// This function generates a new breadcrumbs object based on the
+    /// new context information provided.
+    ///
+    /// explicit_context is for contexts specified with a `:path`
+    /// inside a dust tag.
+    ///
+    /// injected_context is for any generated context. This includes
+    /// both parameters on a tag and also the handling of $idx and
+    /// $len.
+    ///
+    /// New context element is the element is an element to append to
+    /// the end, generally for use in section tags which walk to a new
+    /// context.
+    ///
+    /// If explicit_context is not None, then the final breadcrumb stack will be:
+    ///
+    /// ```
+    /// breadcrumbs
+    /// injected_context
+    /// new_context_element
+    /// ```
+    ///
+    /// However, if explicit_context is not None, then the old
+    /// breadcrumbs are omitted, leading to the new breadcrumb stack
+    /// as:
+    ///
+    /// ```
+    /// injected_context
+    /// explicit_context
+    /// new_context_element
+    /// ```
+    fn new_breadcrumbs<'b>(
+        breadcrumbs: &'b Vec<&'b dyn ContextElement>,
+        injected_context: Option<&'b dyn ContextElement>,
+        explicit_context: &Option<Path<'b>>,
+        new_context_element: Option<&'b dyn ContextElement>,
+    ) -> Vec<&'b dyn ContextElement> {
+        let mut new_stack = match explicit_context {
+            Some(_) => Vec::with_capacity(3),
+            None => breadcrumbs.clone(),
+        };
+        injected_context.map(|ctx| new_stack.push(ctx));
+        explicit_context.as_ref().map(|path| {
+            walk_path(breadcrumbs, &path.keys).map(|val| {
+                if val.is_truthy() {
+                    new_stack.push(val)
+                }
+            });
+        });
+        new_context_element.map(|ctx| new_stack.push(ctx));
+        new_stack
+    }
 }
 
 #[cfg(test)]