Turns out I was wrong, the @size helper attempts to cast to a number regardless of the input and it uses that. Fixed the test.

src/renderer/renderer.rs

@@ -730,17 +730,37 @@ impl<'a> DustRenderer<'a> {
                     maybe_ice
                         .as_ref()
                         .map(|ice| ice.get_context_element_reference())
-                        .map(|ce| ce.get_size())
+                    // .map(|ce| ce.get_size())
                 });
                 match value_ce {
-                    // If we found the key but could not get the size from it, render nothing.
-                    Some(Ok(None)) => return Ok("".to_owned()),
                     // If "key" is not on the @size tag at all, render 0.
                     None => return Ok("0".to_owned()),
                     // If the key value could not be found in the context, render 0.
                     Some(Err(_)) => return Ok("0".to_owned()),
-                    Some(Ok(Some(ce_size))) => {
-                        return ce_size.get_context_element_reference().render(&Vec::new())
+                    Some(Ok(ce)) => {
+                        // The @size helper attempts to cast values to
+                        // numbers, and if that succeeds it uses the
+                        // number, otherwise we'll get the size of the
+                        // original type.
+                        match ce.cast_to_type("number") {
+                            Some(ice) => {
+                                return ice
+                                    .get_context_element_reference()
+                                    .get_size()
+                                    .map(|ce_size| {
+                                        ce_size.get_context_element_reference().render(&Vec::new())
+                                    })
+                                    .unwrap_or(Ok("".to_owned()))
+                            }
+                            None => {
+                                return ce
+                                    .get_size()
+                                    .map(|ce_size| {
+                                        ce_size.get_context_element_reference().render(&Vec::new())
+                                    })
+                                    .unwrap_or(Ok("".to_owned()))
+                            }
+                        }
                     }
                 }
             }