Add example of parsing JSON with serde

examples/serde_complex/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "serde_complex"
+version = "0.1.0"
+edition = "2021"
+publish = false
+authors = ["Serhii Potapov <[email protected]>"]
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+nutype = { path = "../../nutype", features = ["serde"] }
+serde = { version = "1.0.192", features = ["derive"] }
+serde_json = "1.0.108"

examples/serde_complex/src/main.rs

@@ -0,0 +1,96 @@
+use nutype::nutype;
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+struct Product {
+    name: Name,
+    image_url: ImageUrl,
+    price: Price,
+}
+
+#[nutype(
+    sanitize(trim),
+    validate(not_empty, char_len_max = 50),
+    derive(Debug, Clone, PartialEq, AsRef, Serialize, Deserialize)
+)]
+struct Name(String);
+
+#[nutype(
+    sanitize(trim),
+    validate(
+        predicate = |url| url.starts_with("https://") && url.ends_with(".jpg")
+    ),
+    derive(Debug, Clone, PartialEq, AsRef, Serialize, Deserialize)
+)]
+struct ImageUrl(String);
+
+// Note: in the real world, you should use decimal instead of float to represent price.
+#[nutype(
+    validate(greater = 0.0, less = 1000_000.0),
+    derive(Debug, Clone, PartialEq, AsRef, Serialize, Deserialize)
+)]
+struct Price(f64);
+
+fn main() {
+    {
+        // Invalid because name is empty
+        let json = r#"
+            {
+                "name": " ",
+                "image_url": "https://example.com/image.jpg",
+                "price": 9.99
+            }
+        "#;
+        let res: Result<Product, _> = serde_json::from_str(json);
+        let err = res.unwrap_err();
+        assert!(err.to_string().contains("empty, expected valid Name"));
+    }
+
+    {
+        // Invalid because image_url does not end with ".jpg"
+        let json = r#"
+            {
+                "name": "FlySniper",
+                "image_url": "https://example.com/image.png",
+                "price": 9.99
+            }
+        "#;
+        let res: Result<Product, _> = serde_json::from_str(json);
+        let err = res.unwrap_err();
+        assert!(err.to_string().contains("invalid, expected valid ImageUrl"));
+    }
+
+    {
+        // Invalid because the price is negative
+        let json = r#"
+            {
+                "name": "FlySniper",
+                "image_url": "https://example.com/image.jpg",
+                "price": -0.1
+            }
+        "#;
+        let res: Result<Product, _> = serde_json::from_str(json);
+        let err = res.unwrap_err();
+        assert!(err.to_string().contains("too small, expected valid Price"));
+    }
+
+    {
+        // Valid product
+        let json = r#"
+            {
+                "name": "FlySniper\n",
+                "image_url": "https://example.com/image.jpg",
+                "price": 9.99
+            }
+        "#;
+        let product: Product = serde_json::from_str(json).unwrap();
+        assert_eq!(
+            product,
+            Product {
+                name: Name::new("FlySniper").unwrap(),
+                image_url: ImageUrl::new("https://example.com/image.jpg").unwrap(),
+                price: Price::new(9.99).unwrap(),
+            }
+        )
+    }
+}

nutype_macros/src/common/gen/traits.rs

@@ -229,8 +229,13 @@ pub fn gen_impl_trait_serde_deserialize(
 ) -> TokenStream {
     let inner_type: InnerType = inner_type.into();
     let raw_value_to_result: TokenStream = if maybe_error_type_name.is_some() {
+        let type_name_str = type_name.to_string();
         quote! {
-            #type_name::new(raw_value).map_err(<DE::Error as serde::de::Error>::custom)
+            #type_name::new(raw_value).map_err(|validation_error| {
+                // Add a hint about which type is causing the error,
+                let err_msg = format!("{validation_error}, expected valid {}", #type_name_str);
+                <DE::Error as serde::de::Error>::custom(err_msg)
+            })
         }
     } else {
         quote! {

test_suite/tests/any.rs

@@ -284,7 +284,7 @@ mod traits {
 
                 {
                     let err = serde_json::from_str::<LinePoint>("{\"x\":7,\"y\":9}").unwrap_err();
-                    assert_eq!(err.to_string(), "invalid");
+                    assert_eq!(err.to_string(), "invalid, expected valid LinePoint");
                 }
 
                 {