diff --git a/linera-witty/src/type_traits/implementations/mod.rs b/linera-witty/src/type_traits/implementations/mod.rs
index 5e7785d50485..bd729252a2cf 100644
--- a/linera-witty/src/type_traits/implementations/mod.rs
+++ b/linera-witty/src/type_traits/implementations/mod.rs
@@ -6,3 +6,5 @@
 mod custom_types;
 mod frunk;
 mod std;
+#[cfg(test)]
+mod tests;
diff --git a/linera-witty/src/type_traits/implementations/tests.rs b/linera-witty/src/type_traits/implementations/tests.rs
new file mode 100644
index 000000000000..24277c20f352
--- /dev/null
+++ b/linera-witty/src/type_traits/implementations/tests.rs
@@ -0,0 +1,104 @@
+// Copyright (c) Zefchain Labs, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+//! Unit tests for implementations of the custom traits for existing types.
+
+use crate::{FakeInstance, InstanceWithMemory, Layout, WitLoad, WitStore};
+use frunk::hlist;
+use std::fmt::Debug;
+
+/// Test roundtrip of a heterogeneous list that doesn't need any internal padding.
+#[test]
+fn hlist_without_padding() {
+    let input = hlist![
+        0x1011_1213_1415_1617_1819_1a1b_1c1d_1e1f_u128,
+        0x2021_2223_2425_2627_i64,
+        0x3031_3233_u32,
+        0x4041_i16,
+        true,
+    ];
+
+    test_memory_roundtrip(
+        input,
+        &[
+            0x1f, 0x1e, 0x1d, 0x1c, 0x1b, 0x1a, 0x19, 0x18, 0x17, 0x16, 0x15, 0x14, 0x13, 0x12,
+            0x11, 0x10, 0x27, 0x26, 0x25, 0x24, 0x23, 0x22, 0x21, 0x20, 0x33, 0x32, 0x31, 0x30,
+            0x41, 0x40, 0x01,
+        ],
+    );
+    test_flattening_roundtrip(
+        input,
+        hlist![
+            0x1819_1a1b_1c1d_1e1f_i64,
+            0x1011_1213_1415_1617_i64,
+            0x2021_2223_2425_2627_i64,
+            0x3031_3233_i32,
+            0x0000_4041_i32,
+            0x0000_0001_i32,
+        ],
+    );
+}
+
+/// Test roundtrip of a heterogeneous list that needs internal padding between some of its elements.
+#[test]
+fn hlist_with_padding() {
+    let input = hlist![
+        true,
+        0x1011_i16,
+        0x2021_u16,
+        0x3031_3233_u32,
+        0x4041_4243_4445_4647_i64,
+    ];
+
+    test_memory_roundtrip(
+        input,
+        &[
+            0x01, 0, 0x11, 0x10, 0x21, 0x20, 0, 0, 0x33, 0x32, 0x31, 0x30, 0, 0, 0, 0, 0x47, 0x46,
+            0x45, 0x44, 0x43, 0x42, 0x41, 0x40,
+        ],
+    );
+    test_flattening_roundtrip(
+        input,
+        hlist![
+            0x0000_0001_i32,
+            0x0000_1011_i32,
+            0x0000_2021_i32,
+            0x3031_3233_i32,
+            0x4041_4243_4445_4647_i64,
+        ],
+    );
+}
+
+/// Test storing an instance of `T` to memory, checking that the `memory_data` bytes are correctly
+/// written, and check that the instance can be loaded from those bytes.
+fn test_memory_roundtrip<T>(input: T, memory_data: &[u8])
+where
+    T: Debug + Eq + WitLoad + WitStore,
+{
+    let mut instance = FakeInstance::default();
+    let mut memory = instance.memory().unwrap();
+    let length = memory_data.len() as u32;
+
+    let address = memory.allocate(length).unwrap();
+
+    input.store(&mut memory, address).unwrap();
+
+    assert_eq!(memory.read(address, length).unwrap(), memory_data);
+    assert_eq!(T::load(&memory, address).unwrap(), input);
+}
+
+/// Test lowering an instance of `T`, checking that the resulting flat layout matches the expected
+/// `flat_layout`, and check that the instance can be lifted from that flat layout.
+fn test_flattening_roundtrip<T>(input: T, flat_layout: <T::Layout as Layout>::Flat)
+where
+    T: Debug + Eq + WitLoad + WitStore,
+    <T::Layout as Layout>::Flat: Debug + Eq,
+{
+    let mut instance = FakeInstance::default();
+    let mut memory = instance.memory().unwrap();
+
+    let lowered_layout = input.lower(&mut memory).unwrap();
+
+    assert_eq!(lowered_layout, flat_layout);
+    assert_eq!(T::lift_from(lowered_layout, &memory).unwrap(), input);
+}