diff --git a/src/rcore/modes.rs b/src/rcore/modes.rs
index f16a7a7..38a6bd0 100644
--- a/src/rcore/modes.rs
+++ b/src/rcore/modes.rs
@@ -7,6 +7,8 @@ pub trait Mode {
 
     type ExtensionResult<A, E>;
 
+    type ExtensionSource<A, E>;
+
     fn map_err<A, E0, E1, I>(
         result: Self::ParseResult<A, E0, I>,
         f: impl FnOnce(E0) -> E1,
@@ -106,6 +108,8 @@ impl Mode for RegularMode {
 
     type ExtensionResult<A, E> = Result<A, E>;
 
+    type ExtensionSource<A, E> = A;
+
     fn map_err<A, E0, E1, I>(
         result: Self::ParseResult<A, E0, I>,
         f: impl FnOnce(E0) -> E1,
diff --git a/src/rstd/inlining/modes.rs b/src/rstd/inlining/modes.rs
index 5840288..5402be9 100644
--- a/src/rstd/inlining/modes.rs
+++ b/src/rstd/inlining/modes.rs
@@ -7,6 +7,8 @@ impl Mode for InliningMode {
 
     type ExtensionResult<A, E> = E;
 
+    type ExtensionSource<A, E> = ();
+
     fn map_err<A, E0, E1, I>(
         result: Self::ParseResult<A, E0, I>,
         f: impl FnOnce(E0) -> E1,