Fix const_add_assign for 32-bit platforms

Author: kayabaNerve

curve25519-dalek/src/backend/serial/fiat_u32/field.rs

@@ -71,7 +71,7 @@ impl Zeroize for FieldElement2625 {
 }
 
 impl FieldElement2625 {
-    pub(crate) const fn const_add_assign(&mut self, _rhs: &FieldElement2625) -> FieldElement2625 {
+    pub(crate) const fn const_add_assign(&mut self, _rhs: &FieldElement2625) {
         let mut result_loose = fiat_25519_loose_field_element([0; 10]);
         fiat_25519_add(&mut result_loose, &self.0, &rhs.0);
         fiat_25519_carry(&mut self.0, &result_loose);

curve25519-dalek/src/backend/serial/u32/field.rs

@@ -67,7 +67,7 @@ impl Zeroize for FieldElement2625 {
 }
 
 impl FieldElement2625 {
-    pub(crate) const fn const_add_assign(&mut self, _rhs: &FieldElement2625) -> FieldElement2625 {
+    pub(crate) const fn const_add_assign(&mut self, _rhs: &FieldElement2625) {
         let mut i = 0;
         while i < 10 {
             self.0[i] += _rhs.0[i];