diff --git a/halo2_gadgets/src/sha256/table16/message_schedule.rs b/halo2_gadgets/src/sha256/table16/message_schedule.rs
index 4ac644a1..3d769bb1 100644
--- a/halo2_gadgets/src/sha256/table16/message_schedule.rs
+++ b/halo2_gadgets/src/sha256/table16/message_schedule.rs
@@ -435,7 +435,7 @@ mod tests {
                 // Run message_scheduler to get W_[0..64]
                 let (w, _) = config.message_schedule.process(&mut layouter, inputs)?;
                 for (word, test_word) in w.iter().zip(MSG_SCHEDULE_TEST_OUTPUT.iter()) {
-                    let word: u32 = lebs2ip(&word.value().unwrap()) as u32;
+                    let word: u32 = lebs2ip(word.value().unwrap()) as u32;
                     assert_eq!(word, *test_word);
                 }
                 Ok(())
diff --git a/halo2_gadgets/src/sha256/table16/util.rs b/halo2_gadgets/src/sha256/table16/util.rs
index 5e609b25..8a1ba9df 100644
--- a/halo2_gadgets/src/sha256/table16/util.rs
+++ b/halo2_gadgets/src/sha256/table16/util.rs
@@ -17,10 +17,10 @@ pub fn i2lebsp<const NUM_BITS: usize>(int: u64) -> [bool; NUM_BITS] {
 
     fn gen_const_array_with_default<Output: Copy, const LEN: usize>(
         default_value: Output,
-        mut closure: impl FnMut(usize) -> Output,
+        closure: impl FnMut(usize) -> Output,
     ) -> [Output; LEN] {
         let mut ret: [Output; LEN] = [default_value; LEN];
-        for (bit, val) in ret.iter_mut().zip((0..LEN).map(|idx| closure(idx))) {
+        for (bit, val) in ret.iter_mut().zip((0..LEN).map(closure)) {
             *bit = val;
         }
         ret
diff --git a/halo2_gadgets/src/sinsemilla/chip/hash_to_point.rs b/halo2_gadgets/src/sinsemilla/chip/hash_to_point.rs
index 7f98da5c..e323ff02 100644
--- a/halo2_gadgets/src/sinsemilla/chip/hash_to_point.rs
+++ b/halo2_gadgets/src/sinsemilla/chip/hash_to_point.rs
@@ -132,16 +132,14 @@ where
                 // Get message as a bitstring.
                 let bitstring: Vec<bool> = message
                     .iter()
-                    .map(|piece: &MessagePiece<pallas::Base, K>| {
+                    .flat_map(|piece: &MessagePiece<pallas::Base, K>| {
                         piece
                             .field_elem()
                             .unwrap()
                             .to_le_bits()
                             .into_iter()
                             .take(K * piece.num_words())
-                            .collect::<Vec<_>>()
                     })
-                    .flatten()
                     .collect();
 
                 let hasher_S = pallas::Point::hash_to_curve(S_PERSONALIZATION);
@@ -251,7 +249,7 @@ where
         let words: Option<Vec<u32>> = bitstring.map(|bitstring| {
             bitstring
                 .chunks_exact(sinsemilla::K)
-                .map(|word| lebs2ip_k(word))
+                .map(lebs2ip_k)
                 .collect()
         });
 
diff --git a/halo2_gadgets/src/utilities.rs b/halo2_gadgets/src/utilities.rs
index dfed7457..98260874 100644
--- a/halo2_gadgets/src/utilities.rs
+++ b/halo2_gadgets/src/utilities.rs
@@ -167,10 +167,10 @@ pub fn i2lebsp<const NUM_BITS: usize>(int: u64) -> [bool; NUM_BITS] {
     /// Takes in an FnMut closure and returns a constant-length array with elements of
     /// type `Output`.
     fn gen_const_array<Output: Copy + Default, const LEN: usize>(
-        mut closure: impl FnMut(usize) -> Output,
+        closure: impl FnMut(usize) -> Output,
     ) -> [Output; LEN] {
         let mut ret: [Output; LEN] = [Default::default(); LEN];
-        for (bit, val) in ret.iter_mut().zip((0..LEN).map(|idx| closure(idx))) {
+        for (bit, val) in ret.iter_mut().zip((0..LEN).map(closure)) {
             *bit = val;
         }
         ret
diff --git a/halo2_proofs/examples/cost-model.rs b/halo2_proofs/examples/cost-model.rs
index b77fe7b5..158c8c5b 100644
--- a/halo2_proofs/examples/cost-model.rs
+++ b/halo2_proofs/examples/cost-model.rs
@@ -210,8 +210,8 @@ impl From<CostOptions> for Circuit {
             .chain(opts.instance.iter())
             .chain(opts.fixed.iter())
             .cloned()
-            .chain(opts.lookup.iter().map(|l| l.queries()).flatten())
-            .chain(opts.permutation.iter().map(|p| p.queries()).flatten())
+            .chain(opts.lookup.iter().flat_map(|l| l.queries()))
+            .chain(opts.permutation.iter().flat_map(|p| p.queries()))
             .chain(iter::repeat("0".parse().unwrap()).take(max_deg - 1))
             .collect();