diff --git a/scripts/did.sh b/scripts/did.sh
index 7197fa0..458f77f 100755
--- a/scripts/did.sh
+++ b/scripts/did.sh
@@ -1,37 +1,37 @@
 #!/usr/bin/env bash
 
 did_file_location_from_cargo() {
-	# Warning: Does no _- conversion.
-	cargo metadata --format-version 1 | jq -r --arg v "$1" '.packages[] | select(.name==$v) | .manifest_path | sub("Cargo.toml";"\($v).did")'
+  # Warning: Does no _- conversion.
+  cargo metadata --format-version 1 | jq -r --arg v "$1" '.packages[] | select(.name==$v) | .manifest_path | sub("Cargo.toml";"\($v).did")'
 }
 cargo_manifest_path() {
-	# Warning: Does no _- conversion.
-	cargo metadata --format-version 1 | jq -r --arg v "$1" '.packages[] | select(.name==$v) | .manifest_path")'
+  # Warning: Does no _- conversion.
+  cargo metadata --format-version 1 | jq -r --arg v "$1" '.packages[] | select(.name==$v) | .manifest_path")'
 }
 did_file_location_from_dfx_json() {
-	# Warning: Does no _- conversion.
-	jq -r --arg v "$1" '.canisters[$v].candid' dfx.json
+  # Warning: Does no _- conversion.
+  jq -r --arg v "$1" '.canisters[$v].candid' dfx.json
 }
 
 function generate_did() {
-	local canister=$1
-	echo "Deriving candid file from Rust for $canister"
-	#local manifest_path="$(cargo_manifest_path "$canister")"
-	#local candid_file="${manifest_path%Cargo.toml}$canister.did"
-	local candid_file="$(did_file_location_from_dfx_json "$canister")"
+  local canister=$1
+  echo "Deriving candid file from Rust for $canister"
+  #local manifest_path="$(cargo_manifest_path "$canister")"
+  #local candid_file="${manifest_path%Cargo.toml}$canister.did"
+  local candid_file="$(did_file_location_from_dfx_json "$canister")"
 
-	test -e "target/wasm32-unknown-unknown/release/$canister.wasm" ||
-		cargo build -p "$canister" \
-			--target wasm32-unknown-unknown \
-			--release --package "$canister"
+  test -e "target/wasm32-unknown-unknown/release/$canister.wasm" ||
+    cargo build -p "$canister" \
+      --target wasm32-unknown-unknown \
+      --release --package "$canister"
 
-	# cargo install candid-extractor
-	candid-extractor "target/wasm32-unknown-unknown/release/$canister.wasm" >"$candid_file"
-	echo "Written: $candid_file"
+  # cargo install candid-extractor
+  candid-extractor "target/wasm32-unknown-unknown/release/$canister.wasm" >"$candid_file"
+  echo "Written: $candid_file"
 }
 
 CANISTERS=(example_app_backend example_paid_service)
 
 for canister in ${CANISTERS[@]}; do
-	generate_did "$canister"
+  generate_did "$canister"
 done
diff --git a/scripts/pic-install b/scripts/pic-install
index b796a0c..0edf992 100755
--- a/scripts/pic-install
+++ b/scripts/pic-install
@@ -13,7 +13,7 @@ else
   exit 1
 fi
 
-if "$POCKET_IC_SERVER_PATH" --version &>/dev/null | grep -wq  "$POCKET_IC_SERVER_VERSION" ; then
+if "$POCKET_IC_SERVER_PATH" --version &>/dev/null | grep -wq "$POCKET_IC_SERVER_VERSION"; then
   echo "PocketIC server already exists at: $POCKET_IC_SERVER_PATH"
   echo "Skipping download."
 else
diff --git a/scripts/test-rs b/scripts/test-rs
new file mode 100755
index 0000000..2bd8931
--- /dev/null
+++ b/scripts/test-rs
@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+print_help() {
+  echo "Runs rust unit tests."
+}
+
+[[ "${1:-}" != "--help" ]] || print_help
+
+cargo test --lib --bins "${@}"
diff --git a/src/api/src/lib.rs b/src/api/src/lib.rs
index a5bff2f..aacde2e 100644
--- a/src/api/src/lib.rs
+++ b/src/api/src/lib.rs
@@ -9,6 +9,7 @@ pub use caller::PaymentType;
 pub use error::PaymentError;
 pub use vendor::Icrc2Payer;
 
+#[must_use]
 pub fn principal2account(principal: &Principal) -> ByteBuf {
     // TODO: This is NOT the right way.
     let mut ans = principal.as_slice().to_vec();
diff --git a/src/example/paid_service/src/lib.rs b/src/example/paid_service/src/lib.rs
index 9dda2a5..e21ad2b 100644
--- a/src/example/paid_service/src/lib.rs
+++ b/src/example/paid_service/src/lib.rs
@@ -18,7 +18,7 @@ fn init(init_args: Option<InitArgs>) {
 }
 
 #[update()]
-async fn free() -> String {
+fn free() -> String {
     "Yes, I am free!".to_string()
 }
 
diff --git a/src/example/paid_service/src/state.rs b/src/example/paid_service/src/state.rs
index aca6cad..e027b6d 100644
--- a/src/example/paid_service/src/state.rs
+++ b/src/example/paid_service/src/state.rs
@@ -14,7 +14,7 @@ where
     INIT_ARGS.with(|init_args| f(init_args.borrow().as_ref().expect("No init args provided")))
 }
 
-/// Provides the canister_id of the ledger used for payments.
+/// Provides the canister id of the ledger used for payments.
 pub fn payment_ledger() -> Principal {
     init_element(|init_args| init_args.ledger.expect("Init args specify no ledger"))
 }
diff --git a/src/guard/src/guards/icrc2_cycles.rs b/src/guard/src/guards/icrc2_cycles.rs
index 243e097..c34b23e 100644
--- a/src/guard/src/guards/icrc2_cycles.rs
+++ b/src/guard/src/guards/icrc2_cycles.rs
@@ -16,17 +16,28 @@ pub struct Icrc2CyclesPaymentGuard {
     pub own_canister_id: Principal,
 }
 impl Icrc2CyclesPaymentGuard {
+    #[must_use]
     pub fn default_account() -> Account {
         Account {
             owner: ic_cdk::caller(),
             subaccount: None,
         }
     }
+    /// The normal cycles ledger canister ID.
+    ///
+    /// - If the cycles ledger is listed in `dfx.json`, a normal `dfx build` will set the
+    ///   environment variable `CANISTER_ID_CYCLES_LEDGER` and we use this to obtain the canister ID.
+    /// - Otherwise, we use the mainnet cycled ledger canister ID, which is `um5iw-rqaaa-aaaaq-qaaba-cai`.
+    ///
+    /// # Panics
+    /// - If the `CANISTER_ID_CYCLES_LEDGER` environment variable is not a valid canister ID at
+    ///   build time.
+    #[must_use]
     pub fn default_cycles_ledger() -> Principal {
         Principal::from_text(
-            option_env!("DFX_CYCLES_LEDGER_CANISTER_ID").unwrap_or("um5iw-rqaaa-aaaaq-qaaba-cai"),
+            option_env!("CANISTER_ID_CYCLES_LEDGER").unwrap_or("um5iw-rqaaa-aaaaq-qaaba-cai"),
         )
-        .expect("Failed to parse cycles ledger canister ID")
+        .expect("Compile error: Failed to parse build env var 'CANISTER_ID_CYCLES_LEDGER' as a canister ID.")
     }
 }