diff --git a/tools/calyx-ffi/src/prelude.rs b/tools/calyx-ffi/src/prelude.rs
index d1f95663c..425a29c2e 100644
--- a/tools/calyx-ffi/src/prelude.rs
+++ b/tools/calyx-ffi/src/prelude.rs
@@ -11,8 +11,12 @@ macro_rules! declare_interface {
     ($name:ident($($input:ident: $input_width:literal),*)
     -> ($($output:ident: $output_width:literal),*)
     $(impl {
+        $(fn $fn2:ident(& $self2:ident $(, $arg2:ident: $argty2:ty)* $(,)?) $(-> $ret2:ty)? $body2:block)*
+    })?
+    $(mut impl {
         $(fn $fn:ident(&mut $self:ident $(, $arg:ident: $argty:ty)* $(,)?) $(-> $ret:ty)? $body:block)*
-    })? ) => {
+    })?
+    ) => {
         calyx_ffi::prelude::paste::paste! {
             pub trait $name: CalyxFFIComponent {
                 $(
@@ -25,6 +29,9 @@ macro_rules! declare_interface {
 
                     fn $output(&self) -> u64;
                 )*
+                $($(
+                    fn $fn2(&$self2, $($arg2: $argty2),*) $(-> $ret2)* {$body2}
+                )*)*
                 $($(
                     fn $fn(&mut $self, $($arg: $argty),*) $(-> $ret)* {$body}
                 )*)*
diff --git a/tools/calyx-ffi/tests/fifo.rs b/tools/calyx-ffi/tests/fifo.rs
index 76ffd1159..f1199f540 100644
--- a/tools/calyx-ffi/tests/fifo.rs
+++ b/tools/calyx-ffi/tests/fifo.rs
@@ -14,10 +14,10 @@ enum QueueStatus {
 
 calyx_ffi::declare_interface! {
     Queue(cmd: 1, value: 32) -> (ans: 32, err: 1) impl {
-        fn status(&mut self) -> QueueStatus {
+        fn status(&self) -> QueueStatus {
             if self.err() == 0 { QueueStatus::Ok } else { QueueStatus::Err }
         }
-
+    } mut impl {
         fn assert_no_error(&mut self) {
             assert_eq!(QueueStatus::Ok, self.status(), "queue underflowed or overflowed");
         }
diff --git a/tools/calyx-ffi/tests/stack.rs b/tools/calyx-ffi/tests/stack.rs
index 8009f5143..8e5b0d60a 100644
--- a/tools/calyx-ffi/tests/stack.rs
+++ b/tools/calyx-ffi/tests/stack.rs
@@ -9,8 +9,8 @@ enum StackCommand {
 const STACK_CAPACITY: u64 = 16;
 
 calyx_ffi::declare_interface! {
-    Stack(cmd: 1, value: 32) -> (out: 32, length: 4) impl {
-        fn push(&mut self, value: u32) {
+    Stack(cmd: 1, value: 32) -> (out: 32, length: 4) mut impl {
+        fn push(&mut self, value: u32,) {
             assert!(self.length() < STACK_CAPACITY, "tried to push when length={}", STACK_CAPACITY);
             println!("stack has length {} before push", self.length());
             let old_length = self.length();