Add support for SmolStr via a feature.

SmolStr is an immutable "packed" string that uses the space normally occupied
by a String's header if the String is short enough.

Cargo.toml

@@ -61,6 +61,7 @@ opentelemetry = { version = "0.16.0", optional = true, default-features = false,
 uuid = { version = "0.8.2", optional = true, features = ["v4", "serde"] }
 rust_decimal = { version = "1.14.3", optional = true }
 url = { version = "2.2.1", optional = true }
+smol_str = { version = "0.1.21", optional = true }
 
 # Non-feature optional dependencies
 blocking = { version = "1.0.2", optional = true }

src/types/external/mod.rs

@@ -26,6 +26,8 @@ mod duration;
 mod naive_time;
 #[cfg(feature = "secrecy")]
 mod secrecy;
+#[cfg(feature = "smol_str")]
+mod smol_str;
 #[cfg(feature = "url")]
 mod url;
 #[cfg(feature = "uuid")]

src/types/external/smol_str.rs

@@ -0,0 +1,25 @@
+use smol_str::SmolStr;
+
+use crate::{InputValueError, InputValueResult, Scalar, ScalarType, Value};
+
+#[Scalar(
+    internal,
+    name = "SmolStr",
+    specified_by_url = "https://docs.rs/smol_str/latest/smol_str/struct.SmolStr.html"
+)]
+impl ScalarType for SmolStr {
+    fn parse(value: Value) -> InputValueResult<Self> {
+        match value {
+            Value::String(s) => Ok(SmolStr::new(s)),
+            _ => Err(InputValueError::expected_type(value)),
+        }
+    }
+
+    fn is_valid(value: &Value) -> bool {
+        matches!(value, Value::String(_))
+    }
+
+    fn to_value(&self) -> Value {
+        Value::String(self.to_string())
+    }
+}