Add PrimeField::MODULUS

This is an intentionally-opaque string format for debugging purposes.

CHANGELOG.md

@@ -12,7 +12,7 @@ and this library adheres to Rust's notion of
 - `ff::Field::{sqrt_ratio, sqrt_alt}`
 - `core::iter::{Sum, Product}` bounds on `ff::Field`
 - `ff::PrimeField::from_u128`
-- `ff::PrimeField::TWO_INV`
+- `ff::PrimeField::{MODULUS, TWO_INV}`
 - Constants related to multiplicative generators:
   - `ff::PrimeField::MULTIPLICATIVE_GENERATOR`
   - `ff::PrimeField::{ROOT_OF_UNITY, ROOT_OF_UNITY_INV}`

ff_derive/src/lib.rs

@@ -594,6 +594,7 @@ fn prime_field_constants_and_sqrt(
 
     let r = biguint_to_u64_vec(r, limbs);
     let modulus_le_bytes = ReprEndianness::Little.modulus_repr(modulus, limbs * 8);
+    let modulus_str = format!("0x{}", modulus.to_str_radix(16));
     let modulus = biguint_to_real_u64_vec(modulus.clone(), limbs);
 
     // Compute -m^-1 mod 2**64 by exponentiating by totient(2**64) - 1
@@ -615,6 +616,9 @@ fn prime_field_constants_and_sqrt(
             /// This is the modulus m of the prime field in limb form
             const MODULUS_LIMBS: #name = #name([#(#modulus,)*]);
 
+            /// This is the modulus m of the prime field in hex string form
+            const MODULUS_STR: &'static str = #modulus_str;
+
             /// The number of bits needed to represent the modulus.
             const MODULUS_BITS: u32 = #modulus_num_bits;
 
@@ -1237,6 +1241,8 @@ fn prime_field_impl(
                 ::ff::derive::subtle::Choice::from((r.0[0] & 1) as u8)
             }
 
+            const MODULUS: &'static str = MODULUS_STR;
+
             const NUM_BITS: u32 = MODULUS_BITS;
 
             const CAPACITY: u32 = Self::NUM_BITS - 1;

src/lib.rs

@@ -300,6 +300,12 @@ pub trait PrimeField: Field + From<u64> {
         !self.is_odd()
     }
 
+    /// Modulus of the field written as a string for debugging purposes.
+    ///
+    /// The encoding of the modulus is implementation-specific. Generic users of the
+    /// `PrimeField` trait should treat this string as opaque.
+    const MODULUS: &'static str;
+
     /// How many bits are needed to represent an element of this field.
     const NUM_BITS: u32;
 

tests/derive.rs

@@ -41,6 +41,11 @@ mod full_limbs {
 fn constants() {
     use ff::{Field, PrimeField};
 
+    assert_eq!(
+        Bls381K12Scalar::MODULUS,
+        "0x73eda753299d7d483339d80809a1d80553bda402fffe5bfeffffffff00000001",
+    );
+
     assert_eq!(
         Bls381K12Scalar::from(2) * Bls381K12Scalar::TWO_INV,
         Bls381K12Scalar::ONE,