Polish sqrt in fr.rs: use pattern matching with Legendre enums.

src/bls12_381/fr.rs

@@ -555,14 +555,10 @@ impl SqrtField for Fr {
     fn sqrt(&self) -> Option<Self> {
         // Tonelli-Shank's algorithm for q mod 16 = 1
         // https://eprint.iacr.org/2012/685.pdf (page 12, algorithm 5)
-
-        if self.is_zero() {
-            return Some(*self);
-        }
-
-        if let QNonResidue = self.legendre() {
-            None
-        } else {
+        match self.legendre() {
+            Zero => Some(*self),
+            QNonResidue => None,
+            QResidue => {
                 let mut c = Fr(ROOT_OF_UNITY);
                 // r = self^((t + 1) // 2)
                 let mut r = self.pow([0x7fff2dff80000000, 0x4d0ec02a9ded201, 0x94cebea4199cec04, 0x39f6d3a9]);
@@ -597,6 +593,7 @@ impl SqrtField for Fr {
             }
         }
     }
+}
 
 impl LegendreField for Fr {
     fn legendre(&self) -> ::LegendreSymbol {
@@ -606,6 +603,7 @@ impl LegendreField for Fr {
         else { QNonResidue }
     }
 }
+
 #[cfg(test)]
 use rand::{SeedableRng, XorShiftRng, Rand};