leonardomso__rust-skills/rules/serde-skip-empty.md

serde-skip-empty

Omit empty fields with skip_serializing_if

Why It Matters

Serializing None values as null and empty collections as [] bloats payloads, clutters logs, and can confuse clients that distinguish between a missing key and an explicit null. #[serde(skip_serializing_if = "predicate")] conditionally drops a field from output when the predicate returns true, keeping the wire format lean. #[serde(skip)] goes further and excludes a field from both serialization and deserialization entirely.

Bad

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
struct ApiResponse {
    id: u64,
    name: String,
    description: Option<String>,  // serializes as null when None
    tags: Vec<String>,            // serializes as [] when empty
    error: Option<String>,        // serializes as null when None
}

Produces: {"id":1,"name":"Alice","description":null,"tags":[],"error":null}

Good

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
struct ApiResponse {
    id: u64,
    name: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    description: Option<String>,
    #[serde(skip_serializing_if = "Vec::is_empty")]
    tags: Vec<String>,
    #[serde(skip_serializing_if = "Option::is_none")]
    error: Option<String>,
    // internal field excluded entirely from the wire format
    #[serde(skip)]
    _cache_key: Option<String>,
}

impl Default for ApiResponse {
    fn default() -> Self {
        ApiResponse {
            id: 0,
            name: String::new(),
            description: None,
            tags: Vec::new(),
            error: None,
            _cache_key: None,
        }
    }
}

Produces: {"id":1,"name":"Alice"} — absent fields are simply omitted.

Key Points

• skip_serializing_if takes any path resolving to fn(&T) -> bool. Common choices:
  • Option::is_none for Option<T>
  • Vec::is_empty / <[T]>::is_empty for collections
  • String::is_empty for strings
  • A custom function for more complex conditions
• #[serde(skip)] removes the field from both directions. The type must implement Default so deserialization can still construct the struct (serde fills it with Default::default()).
• #[serde(skip_serializing)] skips only on the way out; #[serde(skip_deserializing)] skips only on the way in — useful when reading legacy fields you no longer write.
• Pair skip_serializing_if with #[serde(default)] so that a missing key on deserialization also produces the empty/None value rather than a hard error.

See Also

• serde-default-compat - fill missing fields from Default on deserialization
• serde-rename-all - match external naming conventions with rename_all