units: Add pedantic lint return_self_not_must_use

As part of the effort to polish the `units` create ready for the 1.0
release; enable the pedantic lint `return_self_not_must_use`.

units/src/amount/mod.rs

@@ -527,6 +527,7 @@ pub struct Display {
 
 impl Display {
     /// Makes subsequent calls to `Display::fmt` display denomination.
+    #[must_use]
     pub fn show_denomination(mut self) -> Self {
         match &mut self.style {
             DisplayStyle::FixedDenomination { show_denomination, .. } => *show_denomination = true,

units/src/amount/signed.rs

@@ -206,6 +206,7 @@ impl SignedAmount {
     // Some arithmetic that doesn't fit in [`core::ops`] traits.
 
     /// Get the absolute value of this [`SignedAmount`].
+    #[must_use]
     pub fn abs(self) -> SignedAmount { SignedAmount(self.0.abs()) }
 
     /// Gets the absolute value of this [`SignedAmount`] returning [`Amount`].
@@ -307,6 +308,7 @@ impl SignedAmount {
     /// # Panics
     ///
     /// On overflow, panics in debug mode, wraps in release mode.
+    #[must_use]
     pub fn unchecked_add(self, rhs: SignedAmount) -> SignedAmount { Self(self.0 + rhs.0) }
 
     /// Unchecked subtraction.
@@ -316,6 +318,7 @@ impl SignedAmount {
     /// # Panics
     ///
     /// On overflow, panics in debug mode, wraps in release mode.
+    #[must_use]
     pub fn unchecked_sub(self, rhs: SignedAmount) -> SignedAmount { Self(self.0 - rhs.0) }
 
     /// Subtraction that doesn't allow negative [`SignedAmount`]s.

units/src/amount/unsigned.rs

@@ -318,6 +318,7 @@ impl Amount {
     /// # Panics
     ///
     /// On overflow, panics in debug mode, wraps in release mode.
+    #[must_use]
     pub fn unchecked_add(self, rhs: Amount) -> Amount { Self(self.0 + rhs.0) }
 
     /// Unchecked subtraction.
@@ -327,6 +328,7 @@ impl Amount {
     /// # Panics
     ///
     /// On overflow, panics in debug mode, wraps in release mode.
+    #[must_use]
     pub fn unchecked_sub(self, rhs: Amount) -> Amount { Self(self.0 - rhs.0) }
 
     /// Converts to a signed amount.

units/src/lib.rs

@@ -11,6 +11,8 @@
 #![warn(missing_docs)]
 #![warn(deprecated_in_future)]
 #![doc(test(attr(warn(unused))))]
+// Pedantic lints that we enforce.
+#![warn(clippy::return_self_not_must_use)]
 // Exclude lints we don't think are valuable.
 #![allow(clippy::needless_question_mark)] // https://github.com/rust-bitcoin/rust-bitcoin/pull/2134
 #![allow(clippy::manual_range_contains)] // More readable than clippy's format.