sha256: Simplify single-constraint gates

`Option<T>` implements `IntoIterator<Item = T>`, so we don't need to
wrap it in an explicit iterator.

halo2_gadgets/src/sha256/table16/compression/compression_gates.rs

@@ -166,7 +166,7 @@ impl<F: FieldExt> CompressionGate<F> {
         spread_c_mid: Expression<F>,
         spread_c_hi: Expression<F>,
         spread_d: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let spread_witness = spread_r0_even
             + spread_r0_odd * F::from(2)
             + (spread_r1_even + spread_r1_odd * F::from(2)) * F::from(1 << 32);
@@ -191,7 +191,7 @@ impl<F: FieldExt> CompressionGate<F> {
         let xor = xor_0 + xor_1 + xor_2;
         let check = spread_witness + (xor * -F::one());
 
-        std::iter::empty().chain(Some(("s_upper_sigma_0", s_upper_sigma_0 * check)))
+        Some(("s_upper_sigma_0", s_upper_sigma_0 * check))
     }
 
     // s_upper_sigma_1 on efgh words
@@ -209,7 +209,7 @@ impl<F: FieldExt> CompressionGate<F> {
         spread_b_hi: Expression<F>,
         spread_c: Expression<F>,
         spread_d: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let spread_witness = spread_r0_even
             + spread_r0_odd * F::from(2)
             + (spread_r1_even + spread_r1_odd * F::from(2)) * F::from(1 << 32);
@@ -235,7 +235,7 @@ impl<F: FieldExt> CompressionGate<F> {
         let xor = xor_0 + xor_1 + xor_2;
         let check = spread_witness + (xor * -F::one());
 
-        std::iter::empty().chain(Some(("s_upper_sigma_1", s_upper_sigma_1 * check)))
+        Some(("s_upper_sigma_1", s_upper_sigma_1 * check))
     }
 
     // First part of choice gate on (E, F, G), E ∧ F
@@ -250,7 +250,7 @@ impl<F: FieldExt> CompressionGate<F> {
         spread_e_hi: Expression<F>,
         spread_f_lo: Expression<F>,
         spread_f_hi: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let lhs_lo = spread_e_lo + spread_f_lo;
         let lhs_hi = spread_e_hi + spread_f_hi;
         let lhs = lhs_lo + lhs_hi * F::from(1 << 32);
@@ -261,7 +261,7 @@ impl<F: FieldExt> CompressionGate<F> {
 
         let check = lhs + rhs * -F::one();
 
-        std::iter::empty().chain(Some(("s_ch", s_ch * check)))
+        Some(("s_ch", s_ch * check))
     }
 
     // Second part of Choice gate on (E, F, G), ¬E ∧ G
@@ -320,7 +320,7 @@ impl<F: FieldExt> CompressionGate<F> {
         spread_b_hi: Expression<F>,
         spread_c_lo: Expression<F>,
         spread_c_hi: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let maj_even = spread_m_0_even + spread_m_1_even * F::from(1 << 32);
         let maj_odd = spread_m_0_odd + spread_m_1_odd * F::from(1 << 32);
         let maj = maj_even + maj_odd * F::from(2);
@@ -330,7 +330,7 @@ impl<F: FieldExt> CompressionGate<F> {
         let c = spread_c_lo + spread_c_hi * F::from(1 << 32);
         let sum = a + b + c;
 
-        std::iter::empty().chain(Some(("maj", s_maj * (sum - maj))))
+        Some(("maj", s_maj * (sum - maj)))
     }
 
     // s_h_prime to get H' = H + Ch(E, F, G) + s_upper_sigma_1(E) + K + W
@@ -352,7 +352,7 @@ impl<F: FieldExt> CompressionGate<F> {
         k_hi: Expression<F>,
         w_lo: Expression<F>,
         w_hi: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let lo = h_lo + ch_lo + ch_neg_lo + sigma_e_lo + k_lo + w_lo;
         let hi = h_hi + ch_hi + ch_neg_hi + sigma_e_hi + k_hi + w_hi;
 
@@ -361,7 +361,7 @@ impl<F: FieldExt> CompressionGate<F> {
 
         let check = sum - (h_prime_carry * F::from(1 << 32)) - h_prime;
 
-        std::iter::empty().chain(Some(("s_h_prime", s_h_prime * check)))
+        Some(("s_h_prime", s_h_prime * check))
     }
 
     // s_a_new to get A_new = H' + Maj(A, B, C) + s_upper_sigma_0(A)
@@ -377,7 +377,7 @@ impl<F: FieldExt> CompressionGate<F> {
         maj_abc_hi: Expression<F>,
         h_prime_lo: Expression<F>,
         h_prime_hi: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let lo = sigma_a_lo + maj_abc_lo + h_prime_lo;
         let hi = sigma_a_hi + maj_abc_hi + h_prime_hi;
         let sum = lo + hi * F::from(1 << 16);
@@ -385,7 +385,7 @@ impl<F: FieldExt> CompressionGate<F> {
 
         let check = sum - (a_new_carry * F::from(1 << 32)) - a_new;
 
-        std::iter::empty().chain(Some(("s_a_new", s_a_new * check)))
+        Some(("s_a_new", s_a_new * check))
     }
 
     // s_e_new to get E_new = H' + D
@@ -399,7 +399,7 @@ impl<F: FieldExt> CompressionGate<F> {
         d_hi: Expression<F>,
         h_prime_lo: Expression<F>,
         h_prime_hi: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let lo = h_prime_lo + d_lo;
         let hi = h_prime_hi + d_hi;
         let sum = lo + hi * F::from(1 << 16);
@@ -407,7 +407,7 @@ impl<F: FieldExt> CompressionGate<F> {
 
         let check = sum - (e_new_carry * F::from(1 << 32)) - e_new;
 
-        std::iter::empty().chain(Some(("s_e_new", s_e_new * check)))
+        Some(("s_e_new", s_e_new * check))
     }
 
     // s_digest on final round

halo2_gadgets/src/sha256/table16/message_schedule/schedule_gates.rs

@@ -40,9 +40,9 @@ impl<F: FieldExt> ScheduleGate<F> {
         lo: Expression<F>,
         hi: Expression<F>,
         word: Expression<F>,
-    ) -> impl Iterator<Item = (&'static str, Expression<F>)> {
+    ) -> Option<(&'static str, Expression<F>)> {
         let check = lo + hi * F::from(1 << 16) - word;
-        std::iter::empty().chain(Some(("s_decompose_0", s_decompose_0 * check)))
+        Some(("s_decompose_0", s_decompose_0 * check))
     }
 
     /// s_decompose_1 for W_1 to W_13