feat(derive): add unit test and documentation

Author: maxpowa

README.md

@@ -457,6 +457,41 @@ pub enum Gender {
 }
 ```
 
+### Skipping Fields of Existing Types
+You may skip encoding or decoding fields in existing Rust types using the `skip`
+attribute. This will prevent the field from being encoded or decoded as part of
+the Protobuf message but otherwise allow the field to be part of the Rust type.
+
+```rust,ignore
+use prost;
+use prost::Message;
+
+#[derive(Clone, PartialEq, Message)]
+struct Person {
+    #[prost(string, tag = "1")]
+    pub id: String,
+    #[prost(skip)]
+    pub temp_data: String, // This field will be skipped
+}
+```
+
+If the skipped field type does not implement `Default`, you must provide a
+default value for the field using the `default` attribute.
+
+```rust,ignore
+use prost;
+use prost::Message;
+use std::collections::HashMap;
+
+#[derive(Clone, PartialEq, Message)]
+struct Person {
+    #[prost(string, tag = "1")]
+    pub id: String,
+    #[prost(skip, default = "HashMap::new")]
+    pub temp_data: HashMap<String, String>, // This field will be skipped
+}
+```
+
 ## Nix
 
 The prost project maintains flakes support for local development. Once you have

tests/src/lib.rs

@@ -107,6 +107,8 @@ pub mod oneof_attributes {
 
 #[cfg(test)]
 mod proto3_presence;
+#[cfg(test)]
+mod skipped_fields;
 
 use core::fmt::Debug;
 

tests/src/skipped_fields.rs

@@ -0,0 +1,46 @@
+//! Tests for skipping fields when using prost-derive.
+
+use crate::alloc::string::ToString;
+use crate::check_serialize_equivalent;
+use alloc::collections::BTreeMap;
+use alloc::string::String;
+use prost::Message;
+
+/// A struct with the same data as another, but with a skipped field, should be equal when encoded.
+#[test]
+fn skipped_field_serial_equality() {
+    #[derive(Clone, PartialEq, prost::Message)]
+    struct TypeWithoutSkippedField {
+        #[prost(string, tag = "1")]
+        value: String,
+    }
+
+    fn create_hashmap() -> BTreeMap<String, String> {
+        let mut map = BTreeMap::new();
+        map.insert("key".to_string(), "value".to_string());
+        map
+    }
+
+    #[derive(Clone, PartialEq, prost::Message)]
+    struct TypeWithSkippedField {
+        #[prost(string, tag = "1")]
+        value: String,
+        #[prost(skip, default = "create_hashmap")]
+        pub temp_data: BTreeMap<String, String>, // This field will be skipped
+    }
+
+    let a = TypeWithoutSkippedField {
+        value: "hello".to_string(),
+    };
+    let b = TypeWithSkippedField {
+        value: "hello".to_string(),
+        temp_data: create_hashmap(),
+    };
+
+    // Encoded forms should be equal
+    check_serialize_equivalent(&a, &b);
+
+    // Decoded forms should be equal, with the skipped field initialized using the default attribute
+    let decoded = TypeWithSkippedField::decode(a.encode_to_vec().as_slice()).unwrap();
+    assert_eq!(b, decoded);
+}