Fix unknown_method_callback error reporting (#743)

Author: mitsuhiko

CHANGELOG.md

@@ -7,6 +7,8 @@ All notable changes to MiniJinja are documented here.
 - Do not panic if too large templates (too many lines or too many
   columns) are loaded.  The error reporting will be wrong in those
   cases but the templates will load.  #742
+- Fixed a bug that caused unknown method callbacks to not get
+  proper error reporting if they cannot find a method.  #743
 
 ## 2.8.0
 

minijinja/src/value/mod.rs

@@ -1590,8 +1590,21 @@ impl Value {
             Err(mut err) => {
                 if err.kind() == ErrorKind::UnknownMethod {
                     if let Some(ref callback) = state.env().unknown_method_callback {
-                        return callback(state, self, name, args);
-                    } else if err.detail().is_none() {
+                        match callback(state, self, name, args) {
+                            Ok(result) => return Ok(result),
+                            Err(callback_err) => {
+                                // if the callback fails with the same error, we
+                                // want to also attach the default detail if
+                                // it's missing
+                                if callback_err.kind() == ErrorKind::UnknownMethod {
+                                    err = callback_err;
+                                } else {
+                                    return Err(err);
+                                }
+                            }
+                        }
+                    }
+                    if err.detail().is_none() {
                         err.set_detail(format!("{} has no method named {}", self.kind(), name));
                     }
                 }

minijinja/tests/test_environment.rs

@@ -173,6 +173,13 @@ fn test_unknown_method_callback() {
 
     let rv = env.render_str("{{ {'x': 42}.items() }}", ()).unwrap();
     assert_snapshot!(rv, @r###"[["x", 42]]"###);
+
+    let err = env.render_str("{{ [].does_not_exist() }}", ()).unwrap_err();
+    assert_eq!(err.kind(), ErrorKind::UnknownMethod);
+    assert_eq!(
+        err.detail(),
+        Some("sequence has no method named does_not_exist")
+    );
 }
 
 #[test]