Support serde serializing Amount as string

Sometimes JSON parsers may munge floats. Instead of using `f64` we can
serialize BTC amounts as strings.

Includes addition of `alloc` feature gate to `DisplayFullError` to
remove lint warnings when building with `--no-default-features`.

Close: #894

units/Cargo.toml

@@ -15,7 +15,7 @@ exclude = ["tests", "contrib"]
 [features]
 default = ["std"]
 std = ["alloc", "internals/std"]
-alloc = ["internals/alloc"]
+alloc = ["internals/alloc","serde?/alloc"]
 
 [dependencies]
 internals = { package = "bitcoin-internals", version = "0.4.0" }

units/src/amount/serde.rs

@@ -20,13 +20,16 @@
 //! }
 //! ```
 
+#[cfg(feature = "alloc")]
 use core::fmt;
 
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 #[cfg(feature = "alloc")] // This is because `to_float_in` uses `to_string`.
 use super::Denomination;
-use super::{Amount, ParseAmountError, SignedAmount};
+#[cfg(feature = "alloc")]
+use super::ParseAmountError;
+use super::{Amount, SignedAmount};
 
 /// This trait is used only to avoid code duplication and naming collisions
 /// of the different serde serialization crates.
@@ -37,6 +40,10 @@ pub trait SerdeAmount: Copy + Sized {
     fn ser_btc<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error>;
     #[cfg(feature = "alloc")]
     fn des_btc<'d, D: Deserializer<'d>>(d: D, _: private::Token) -> Result<Self, D::Error>;
+    #[cfg(feature = "alloc")]
+    fn ser_str<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error>;
+    #[cfg(feature = "alloc")]
+    fn des_str<'d, D: Deserializer<'d>>(d: D, _: private::Token) -> Result<Self, D::Error>;
 }
 
 mod private {
@@ -50,8 +57,11 @@ pub trait SerdeAmountForOpt: Copy + Sized + SerdeAmount {
     fn ser_sat_opt<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error>;
     #[cfg(feature = "alloc")]
     fn ser_btc_opt<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error>;
+    #[cfg(feature = "alloc")]
+    fn ser_str_opt<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error>;
 }
 
+#[cfg(feature = "alloc")]
 struct DisplayFullError(ParseAmountError);
 
 #[cfg(feature = "std")]
@@ -70,6 +80,7 @@ impl fmt::Display for DisplayFullError {
 }
 
 #[cfg(not(feature = "std"))]
+#[cfg(feature = "alloc")]
 impl fmt::Display for DisplayFullError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Display::fmt(&self.0, f) }
 }
@@ -90,6 +101,18 @@ impl SerdeAmount for Amount {
         use serde::de::Error;
         Amount::from_btc(f64::deserialize(d)?).map_err(DisplayFullError).map_err(D::Error::custom)
     }
+    #[cfg(feature = "alloc")]
+    fn ser_str<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error> {
+        s.serialize_str(&self.to_string_in(Denomination::Bitcoin))
+    }
+    #[cfg(feature = "alloc")]
+    fn des_str<'d, D: Deserializer<'d>>(d: D, _: private::Token) -> Result<Self, D::Error> {
+        use serde::de::Error;
+        let s: alloc::string::String = Deserialize::deserialize(d)?;
+        Amount::from_str_in(&s, Denomination::Bitcoin)
+            .map_err(DisplayFullError)
+            .map_err(D::Error::custom)
+    }
 }
 
 impl SerdeAmountForOpt for Amount {
@@ -101,6 +124,10 @@ impl SerdeAmountForOpt for Amount {
     fn ser_btc_opt<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error> {
         s.serialize_some(&self.to_btc())
     }
+    #[cfg(feature = "alloc")]
+    fn ser_str_opt<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error> {
+        s.serialize_some(&self.to_string_in(Denomination::Bitcoin))
+    }
 }
 
 impl SerdeAmount for SignedAmount {
@@ -121,6 +148,18 @@ impl SerdeAmount for SignedAmount {
             .map_err(DisplayFullError)
             .map_err(D::Error::custom)
     }
+    #[cfg(feature = "alloc")]
+    fn ser_str<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error> {
+        s.serialize_str(self.to_string_in(Denomination::Bitcoin).as_str())
+    }
+    #[cfg(feature = "alloc")]
+    fn des_str<'d, D: Deserializer<'d>>(d: D, _: private::Token) -> Result<Self, D::Error> {
+        use serde::de::Error;
+        let s: alloc::string::String = Deserialize::deserialize(d)?;
+        SignedAmount::from_str_in(&s, Denomination::Bitcoin)
+            .map_err(DisplayFullError)
+            .map_err(D::Error::custom)
+    }
 }
 
 impl SerdeAmountForOpt for SignedAmount {
@@ -132,6 +171,10 @@ impl SerdeAmountForOpt for SignedAmount {
     fn ser_btc_opt<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error> {
         s.serialize_some(&self.to_btc())
     }
+    #[cfg(feature = "alloc")]
+    fn ser_str_opt<S: Serializer>(self, s: S, _: private::Token) -> Result<S::Ok, S::Error> {
+        s.serialize_some(&self.to_string_in(Denomination::Bitcoin))
+    }
 }
 
 pub mod as_sat {
@@ -272,3 +315,73 @@ pub mod as_btc {
         }
     }
 }
+
+#[cfg(feature = "alloc")]
+pub mod as_str {
+    //! Serialize and deserialize [`Amount`](crate::Amount) as a JSON string denominated in BTC.
+    //! Use with `#[serde(with = "amount::serde::as_str")]`.
+
+    use serde::{Deserializer, Serializer};
+
+    use super::private;
+    use crate::amount::serde::SerdeAmount;
+
+    pub fn serialize<A: SerdeAmount, S: Serializer>(a: &A, s: S) -> Result<S::Ok, S::Error> {
+        a.ser_str(s, private::Token)
+    }
+
+    pub fn deserialize<'d, A: SerdeAmount, D: Deserializer<'d>>(d: D) -> Result<A, D::Error> {
+        A::des_str(d, private::Token)
+    }
+
+    pub mod opt {
+        //! Serialize and deserialize `Option<Amount>` as a JSON string denominated in BTC.
+        //! Use with `#[serde(default, with = "amount::serde::as_str::opt")]`.
+
+        use core::fmt;
+        use core::marker::PhantomData;
+
+        use serde::{de, Deserializer, Serializer};
+
+        use super::private;
+        use crate::amount::serde::SerdeAmountForOpt;
+
+        pub fn serialize<A: SerdeAmountForOpt, S: Serializer>(
+            a: &Option<A>,
+            s: S,
+        ) -> Result<S::Ok, S::Error> {
+            match *a {
+                Some(a) => a.ser_str_opt(s, private::Token),
+                None => s.serialize_none(),
+            }
+        }
+
+        pub fn deserialize<'d, A: SerdeAmountForOpt, D: Deserializer<'d>>(
+            d: D,
+        ) -> Result<Option<A>, D::Error> {
+            struct VisitOptAmt<X>(PhantomData<X>);
+
+            impl<'de, X: SerdeAmountForOpt> de::Visitor<'de> for VisitOptAmt<X> {
+                type Value = Option<X>;
+
+                fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                    write!(formatter, "An Option<String>")
+                }
+
+                fn visit_none<E>(self) -> Result<Self::Value, E>
+                where
+                    E: de::Error,
+                {
+                    Ok(None)
+                }
+                fn visit_some<D>(self, d: D) -> Result<Self::Value, D::Error>
+                where
+                    D: Deserializer<'de>,
+                {
+                    Ok(Some(X::des_str(d, private::Token)?))
+                }
+            }
+            d.deserialize_option(VisitOptAmt::<A>(PhantomData))
+        }
+    }
+}

units/src/amount/tests.rs

@@ -741,6 +741,31 @@ fn serde_as_btc() {
     assert!(t.unwrap_err().to_string().contains(&OutOfRangeError::negative().to_string()));
 }
 
+#[cfg(feature = "serde")]
+#[cfg(feature = "alloc")]
+#[test]
+fn serde_as_str() {
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    struct T {
+        #[serde(with = "crate::amount::serde::as_str")]
+        pub amt: Amount,
+        #[serde(with = "crate::amount::serde::as_str")]
+        pub samt: SignedAmount,
+    }
+
+    serde_test::assert_tokens(
+        &T { amt: Amount::from_sat(123456789), samt: SignedAmount::from_sat(-123456789) },
+        &[
+            serde_test::Token::Struct { name: "T", len: 2 },
+            serde_test::Token::String("amt"),
+            serde_test::Token::String("1.23456789"),
+            serde_test::Token::String("samt"),
+            serde_test::Token::String("-1.23456789"),
+            serde_test::Token::StructEnd,
+        ],
+    );
+}
+
 #[cfg(feature = "serde")]
 #[cfg(feature = "alloc")]
 #[test]
@@ -825,6 +850,49 @@ fn serde_as_sat_opt() {
     assert_eq!(without, serde_json::from_value(value_without).unwrap());
 }
 
+#[cfg(feature = "serde")]
+#[cfg(feature = "alloc")]
+#[test]
+#[allow(clippy::inconsistent_digit_grouping)] // Group to show 100,000,000 sats per bitcoin.
+fn serde_as_str_opt() {
+    use serde_json;
+
+    #[derive(Serialize, Deserialize, PartialEq, Debug, Eq)]
+    struct T {
+        #[serde(default, with = "crate::amount::serde::as_str::opt")]
+        pub amt: Option<Amount>,
+        #[serde(default, with = "crate::amount::serde::as_str::opt")]
+        pub samt: Option<SignedAmount>,
+    }
+
+    let with = T {
+        amt: Some(Amount::from_sat(123456789)),
+        samt: Some(SignedAmount::from_sat(-123456789)),
+    };
+    let without = T { amt: None, samt: None };
+
+    // Test Roundtripping
+    for s in [&with, &without].iter() {
+        let v = serde_json::to_string(s).unwrap();
+        let w: T = serde_json::from_str(&v).unwrap();
+        assert_eq!(w, **s);
+    }
+
+    let t: T =
+        serde_json::from_str("{\"amt\": \"1.23456789\", \"samt\": \"-1.23456789\"}").unwrap();
+    assert_eq!(t, with);
+
+    let t: T = serde_json::from_str("{}").unwrap();
+    assert_eq!(t, without);
+
+    let value_with: serde_json::Value =
+        serde_json::from_str("{\"amt\": \"1.23456789\", \"samt\": \"-1.23456789\"}").unwrap();
+    assert_eq!(with, serde_json::from_value(value_with).unwrap());
+
+    let value_without: serde_json::Value = serde_json::from_str("{}").unwrap();
+    assert_eq!(without, serde_json::from_value(value_without).unwrap());
+}
+
 #[test]
 fn sum_amounts() {
     assert_eq!(Amount::from_sat(0), [].iter().sum::<Amount>());