specify supported moduli

Author: benbencik

ff-macros/src/small_fp/mod.rs

@@ -22,7 +22,15 @@ pub(crate) fn small_fp_config_helper(
 
     let backend_impl = match backend.as_str() {
         "standard" => standard_backend::backend_impl(&ty, modulus, generator),
-        "montgomery" => montgomery_backend::backend_impl(&ty, modulus, generator),
+        "montgomery" => {
+            if modulus >= 1u128 << 127 {
+                panic!(
+                    "SmallFpConfig montgomery backend supports only moduli < 2^127. Use MontConfig with BigInt instead of SmallFp."
+                )
+            }
+            montgomery_backend::backend_impl(&ty, modulus, generator)
+        },
+
         _ => panic!("Unknown backend type: {}", backend),
     };