Able to get the missing variable path from error

src/error.rs

@@ -72,6 +72,23 @@ impl From<TemplateError> for RenderError {
     }
 }
 
+/// Template rendering error
+#[derive(Debug, Error)]
+pub enum RenderErrorReason {
+    #[error("missing variable path {0:?}")]
+    MissingVariable(Option<String>),
+}
+
+impl From<RenderErrorReason> for RenderError {
+    fn from(e: RenderErrorReason) -> RenderError {
+        match e {
+            RenderErrorReason::MissingVariable(_) => {
+                RenderError::from_error("Failed to access variable in strict mode.", e)
+            }
+        }
+    }
+}
+
 #[cfg(feature = "script_helper")]
 impl From<Box<EvalAltResult>> for RenderError {
     fn from(e: Box<EvalAltResult>) -> RenderError {
@@ -102,11 +119,7 @@ impl RenderError {
     }
 
     pub fn strict_error(path: Option<&String>) -> RenderError {
-        let msg = match path {
-            Some(path) => format!("Variable {:?} not found in strict mode.", path),
-            None => "Value is missing in strict mode".to_owned(),
-        };
-        RenderError::new(msg)
+        RenderErrorReason::MissingVariable(path.map(|p| p.to_owned())).into()
     }
 
     pub fn from_error<E>(error_info: &str, cause: E) -> RenderError

src/lib.rs

@@ -388,7 +388,7 @@ extern crate serde_json;
 pub use self::block::{BlockContext, BlockParams};
 pub use self::context::Context;
 pub use self::decorators::DecoratorDef;
-pub use self::error::{RenderError, TemplateError};
+pub use self::error::{RenderError, RenderErrorReason, TemplateError};
 pub use self::helpers::{HelperDef, HelperResult};
 pub use self::json::path::Path;
 pub use self::json::value::{to_json, JsonRender, PathAndJson, ScopedJson};

src/registry.rs

@@ -722,13 +722,14 @@ impl<'reg> Registry<'reg> {
 #[cfg(test)]
 mod test {
     use crate::context::Context;
-    use crate::error::RenderError;
+    use crate::error::{RenderError, RenderErrorReason};
     use crate::helpers::HelperDef;
     use crate::output::Output;
     use crate::registry::Registry;
     use crate::render::{Helper, RenderContext, Renderable};
     use crate::support::str::StringWriter;
     use crate::template::Template;
+    use std::error::Error;
     use std::fs::File;
     use std::io::Write;
     use tempfile::tempdir;
@@ -985,6 +986,16 @@ mod test {
             .render_template("accessing non-exists key {{the_key_never_exists}}", &data)
             .unwrap_err();
         assert_eq!(render_error.column_no.unwrap(), 26);
+        assert_eq!(
+            render_error
+                .source()
+                .and_then(|e| e.downcast_ref::<RenderErrorReason>())
+                .and_then(|e| match e {
+                    RenderErrorReason::MissingVariable(path) => path.to_owned(),
+                })
+                .unwrap(),
+            "the_key_never_exists"
+        );
 
         let data2 = json!([1, 2, 3]);
         assert!(r
@@ -997,6 +1008,16 @@ mod test {
             .render_template("accessing invalid array index {{this.[3]}}", &data2)
             .unwrap_err();
         assert_eq!(render_error2.column_no.unwrap(), 31);
+        assert_eq!(
+            render_error2
+                .source()
+                .and_then(|e| e.downcast_ref::<RenderErrorReason>())
+                .and_then(|e| match e {
+                    RenderErrorReason::MissingVariable(path) => path.to_owned(),
+                })
+                .unwrap(),
+            "this.[3]"
+        )
     }
 
     use crate::json::value::ScopedJson;