Add a parent to ParametersContext so we can chain parameters contexts for use with the select helper.

src/renderer/parameters_context.rs

@@ -21,6 +21,7 @@ use std::collections::HashMap;
 
 #[derive(Debug)]
 pub struct ParametersContext<'a> {
+    parent: Option<&'a ParametersContext<'a>>,
     params: HashMap<&'a str, (&'a RValue<'a>, Option<BreadcrumbTreeElement<'a>>)>,
 }
 
@@ -29,6 +30,7 @@ impl<'a> ParametersContext<'a> {
         renderer: &DustRenderer,
         breadcrumbs: &'a Vec<BreadcrumbTreeElement<'a>>,
         params: &'a Vec<KVPair>,
+        parent: Option<&'a ParametersContext<'a>>,
     ) -> Self {
         // If the parameter is a Path, then we resolve it immediately
         // to a context element because those are resolved using the
@@ -60,12 +62,17 @@ impl<'a> ParametersContext<'a> {
                 .collect();
 
         ParametersContext {
+            parent: parent,
             params: rendered_params,
         }
     }
 
     pub fn contains_key(&self, segment: &str) -> bool {
         self.params.contains_key(segment)
+            || self
+                .parent
+                .map(|p| p.contains_key(segment))
+                .unwrap_or(false)
     }
 }
 
@@ -83,7 +90,11 @@ impl<'a> Walkable for ParametersContext<'a> {
     fn walk(&self, segment: &str) -> Result<&dyn IntoContextElement, WalkError> {
         match self.params.get(segment).map(|(_rvalue, bte)| bte) {
             Some(Some(bte)) => Ok(bte.borrow()),
-            _ => Err(WalkError::CantWalk),
+            Some(None) => Err(WalkError::CantWalk),
+            None => self
+                .parent
+                .map(|p| p.walk(segment))
+                .unwrap_or(Err(WalkError::CantWalk)),
         }
     }