Add decimal scalar

Cargo.toml

@@ -20,6 +20,7 @@ unblock = ["blocking"]
 string_number = ["num-traits"]
 dataloader = ["futures-timer", "futures-channel", "lru"]
 tracing = ["tracinglib", "tracing-futures"]
+decimal = ["rust_decimal"]
 
 [dependencies]
 async-graphql-derive = { path = "derive", version = "=2.9.5" }
@@ -53,6 +54,7 @@ tracing-futures = { version = "0.2.5", optional = true, features = ["std-future"
 opentelemetry = { version = "0.13.0", optional = true }
 url = { version = "2.2.1", optional = true }
 uuid = { version = "0.8.2", optional = true, features = ["v4", "serde"] }
+rust_decimal = { version = "1.14.3", optional = true }
 
 # Non-feature optional dependencies
 blocking = { version = "1.0.2", optional = true }

src/lib.rs

@@ -71,6 +71,7 @@
 //! - `uuid`: Integrate with the [`uuid` crate](https://crates.io/crates/uuid).
 //! - `string_number`: Enable the [StringNumber](types/struct.StringNumber.html).
 //! - `dataloader`: Support [DataLoader](dataloader/struct.DataLoader.html).
+//! - `decimal`: Integrate with the [`rust_decimal` crate](https://crates.io/crates/rust_decimal).
 //!
 //! ## Integrations
 //!

src/types/external/decimal.rs

@@ -0,0 +1,19 @@
+use std::str::FromStr;
+
+use rust_decimal::Decimal;
+
+use crate::{InputValueError, InputValueResult, Scalar, ScalarType, Value};
+
+#[Scalar(internal, name = "Decimal")]
+impl ScalarType for Decimal {
+    fn parse(value: Value) -> InputValueResult<Self> {
+        match &value {
+            Value::String(s) => Ok(Decimal::from_str(s)?),
+            _ => Err(InputValueError::expected_type(value)),
+        }
+    }
+
+    fn to_value(&self) -> Value {
+        Value::String(self.to_string())
+    }
+}

src/types/external/mod.rs

@@ -17,6 +17,8 @@ mod bson;
 mod chrono_tz;
 #[cfg(feature = "chrono")]
 mod datetime;
+#[cfg(feature = "decimal")]
+mod decimal;
 #[cfg(feature = "chrono")]
 mod naive_time;
 #[cfg(feature = "secrecy")]