Don't assume lookaheads are always fields

Fixes https://github.com/graphql-rust/juniper/issues/398.

integration_tests/juniper_tests/src/issue_398.rs

@@ -0,0 +1,67 @@
+// Original author of this test is <https://github.com/davidpdrsn>.
+use juniper::*;
+
+struct Query;
+
+#[juniper::object]
+impl Query {
+    fn users(executor: &Executor) -> Vec<User> {
+        // This doesn't cause a panic
+        executor.look_ahead();
+
+        vec![User {
+            country: Country { id: 1 },
+        }]
+    }
+}
+
+struct User {
+    country: Country,
+}
+
+#[juniper::object]
+impl User {
+    fn country(&self, executor: &Executor) -> &Country {
+        // This panics!
+        executor.look_ahead();
+
+        &self.country
+    }
+}
+
+struct Country {
+    id: i32,
+}
+
+#[juniper::object]
+impl Country {
+    fn id(&self) -> i32 {
+        self.id
+    }
+}
+
+type Schema = juniper::RootNode<'static, Query, EmptyMutation<()>>;
+
+#[test]
+fn it_works() {
+    let _ = juniper::execute(
+        r#"
+            query Query {
+                users {
+                    ...userFields
+                }
+            }
+
+            fragment userFields on User {
+                country {
+                    id
+                }
+            }
+        "#,
+        None,
+        &Schema::new(Query, EmptyMutation::new()),
+        &Variables::new(),
+        &(),
+    )
+    .unwrap();
+}

integration_tests/juniper_tests/src/lib.rs

@@ -4,3 +4,5 @@ mod codegen;
 mod custom_scalar;
 #[cfg(test)]
 mod issue_371;
+#[cfg(test)]
+mod issue_398;

juniper/src/executor/mod.rs

@@ -518,22 +518,23 @@ where
         };
         self.parent_selection_set
             .map(|p| {
-                let p = p
-                    .iter()
-                    .find(|&x| {
-                        match *x {
-                            Selection::Field(ref field) => {
-                                let field = &field.item;
-                                // TODO: support excludes.
-                                let name = field.name.item;
-                                let alias = field.alias.as_ref().map(|a| a.item);
-                                alias.unwrap_or(name) == field_name
-                            }
-                            _ => false,
+                let found_field = p.iter().find(|&x| {
+                    match *x {
+                        Selection::Field(ref field) => {
+                            let field = &field.item;
+                            // TODO: support excludes.
+                            let name = field.name.item;
+                            let alias = field.alias.as_ref().map(|a| a.item);
+                            alias.unwrap_or(name) == field_name
                         }
-                    })
-                    .expect("lookahead to find the field");
-                LookAheadSelection::build_from_selection(&p, self.variables, self.fragments)
+                        _ => false,
+                    }
+                });
+                if let Some(p) = found_field {
+                    LookAheadSelection::build_from_selection(&p, self.variables, self.fragments)
+                } else {
+                    None
+                }
             })
             .filter(|s| s.is_some())
             .unwrap_or_else(|| {