units: Remove serde from amounts

The `Amount` and `SignedAmount` were not supposed to implement `serde`
traits by design because doing so implicitly uses sats. We provide two
modules `as_sat` and `as_btc` to allow users to explicitly serialize in
their preferred format.

In commit: `d57ec019d5 Use Amount type for TxOut value field` derives
were added for `serde` and we did not notice it during review.

bitcoin/src/crypto/sighash.rs

@@ -1862,6 +1862,7 @@ mod tests {
             #[serde(rename = "scriptPubKey")]
             script_pubkey: ScriptBuf,
             #[serde(rename = "amountSats")]
+            #[serde(with = "crate::amount::serde::as_sat")]
             value: Amount,
         }
 

primitives/src/transaction.rs

@@ -343,6 +343,7 @@ impl TxIn {
 #[cfg(feature = "alloc")]
 pub struct TxOut {
     /// The value of the output, in satoshis.
+    #[cfg_attr(feature = "serde", serde(with = "crate::amount::serde::as_sat"))]
     pub value: Amount,
     /// The script which must be satisfied for the output to be spent.
     pub script_pubkey: ScriptBuf,

units/src/amount/signed.rs

@@ -17,9 +17,9 @@ use super::{
 
 /// A signed amount.
 ///
-/// The [`SignedAmount`] type can be used to express Bitcoin amounts that support
-/// arithmetic and conversion to various denominations.
-///
+/// The [`SignedAmount`] type can be used to express Bitcoin amounts that support arithmetic and
+/// conversion to various denominations. The `Amount` type does not implement `serde` traits but we
+/// do provide modules for serializing as satoshis or bitcoin.
 ///
 /// Warning!
 ///
@@ -29,6 +29,21 @@ use super::{
 /// start with `checked_`.  The operations from [`core::ops`] that [`Amount`]
 /// implements will panic when overflow or underflow occurs.
 ///
+/// # Examples
+///
+/// ```
+/// # #[cfg(feature = "serde")] {
+/// use serde::{Serialize, Deserialize};
+/// use bitcoin_units::SignedAmount;
+///
+/// #[derive(Serialize, Deserialize)]
+/// struct Foo {
+///     // If you are using `rust-bitcoin` then `bitcoin::amount::serde::as_sat` also works.
+///     #[serde(with = "bitcoin_units::amount::serde::as_sat")]  // Also `serde::as_btc`.
+///     amount: SignedAmount,
+/// }
+/// # }
+/// ```
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct SignedAmount(i64);
 

units/src/amount/unsigned.rs

@@ -7,8 +7,6 @@ use alloc::string::{String, ToString};
 use core::str::FromStr;
 use core::{default, fmt, ops};
 
-#[cfg(feature = "serde")]
-use ::serde::{Deserialize, Serialize};
 #[cfg(feature = "arbitrary")]
 use arbitrary::{Arbitrary, Unstructured};
 
@@ -21,9 +19,9 @@ use crate::{FeeRate, Weight};
 
 /// An amount.
 ///
-/// The [`Amount`] type can be used to express Bitcoin amounts that support
-/// arithmetic and conversion to various denominations.
-///
+/// The [`Amount`] type can be used to express Bitcoin amounts that support arithmetic and
+/// conversion to various denominations. The `Amount` type does not implement `serde` traits but we
+/// do provide modules for serializing as satoshis or bitcoin.
 ///
 /// Warning!
 ///
@@ -36,8 +34,22 @@ use crate::{FeeRate, Weight};
 /// zero is considered an underflow and will cause a panic if you're not using
 /// the checked arithmetic methods.
 ///
+/// # Examples
+///
+/// ```
+/// # #[cfg(feature = "serde")] {
+/// use serde::{Serialize, Deserialize};
+/// use bitcoin_units::Amount;
+///
+/// #[derive(Serialize, Deserialize)]
+/// struct Foo {
+///     // If you are using `rust-bitcoin` then `bitcoin::amount::serde::as_sat` also works.
+///     #[serde(with = "bitcoin_units::amount::serde::as_sat")]  // Also `serde::as_btc`.
+///     amount: Amount,
+/// }
+/// # }
+/// ```
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
-#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct Amount(u64);
 
 impl Amount {