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Custom de/serialization functions for Rust's serde

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This crate provides custom de/serialization helpers to use in combination with serde's with-annotation and with the improved serde_as-annotation. Some common use cases are:

  • De/Serializing a type using the Display and FromStr traits, e.g., for u8, url::Url, or mime::Mime. Check DisplayFromStr for details.
  • Support for arrays larger than 32 elements or using const generics. With serde_as large arrays are supported, even if they are nested in other types. [bool; 64], Option<[u8; M]>, and Box<[[u8; 64]; N]> are all supported, as this examples shows.
  • Skip serializing all empty Option types with #[skip_serializing_none].
  • Apply a prefix to each field name of a struct, without changing the de/serialize implementations of the struct using with_prefix!.
  • Deserialize a comma separated list like #hash,#tags,#are,#great into a Vec<String>. Check the documentation for serde_with::rust::StringWithSeparator::<CommaSeparator>.

Getting Help

Check out the user guide to find out more tips and tricks about this crate.

For further help using this crate you can open a new discussion or ask on users.rust-lang.org. For bugs, please open a new issue on GitHub.

Use serde_with in your Project

Add this to your Cargo.toml:

version = "2.0.0-rc.0"
features = [ "..." ]

The crate contains different features for integration with other common crates. Check the feature flags section for information about all available features.


Annotate your struct or enum to enable the custom de/serializer. The #[serde_as] attribute must be placed before the #[derive].


#[derive(Deserialize, Serialize)]
struct Foo {
    // Serialize with Display, deserialize with FromStr
    #[serde_as(as = "DisplayFromStr")]
    bar: u8,

// This will serialize
Foo {bar: 12}

// into this JSON
{"bar": "12"}

Large and const-generic arrays

serde does not support arrays with more than 32 elements or using const-generics. The serde_as attribute allows circumventing this restriction, even for nested types and nested arrays.

#[derive(Deserialize, Serialize)]
struct Arrays<const N: usize, const M: usize> {
    #[serde_as(as = "[_; N]")]
    constgeneric: [bool; N],

    #[serde_as(as = "Box<[[_; 64]; N]>")]
    nested: Box<[[u8; 64]; N]>,

    #[serde_as(as = "Option<[_; M]>")]
    optional: Option<[u8; M]>,

// This allows us to serialize a struct like this
let arrays: Arrays<100, 128> = Arrays {
    constgeneric: [true; 100],
    nested: Box::new([[111; 64]; 100]),
    optional: Some([222; 128])


This situation often occurs with JSON, but other formats also support optional fields. If many fields are optional, putting the annotations on the structs can become tedious. The #[skip_serializing_none] attribute must be placed before the #[derive].

#[derive(Deserialize, Serialize)]
struct Foo {
    a: Option<usize>,
    b: Option<usize>,
    c: Option<usize>,
    d: Option<usize>,
    e: Option<usize>,
    f: Option<usize>,
    g: Option<usize>,

// This will serialize
Foo {a: None, b: None, c: None, d: Some(4), e: None, f: None, g: Some(7)}

// into this JSON
{"d": 4, "g": 7}

Advanced serde_as usage

This example is mainly supposed to highlight the flexibility of the serde_as-annotation compared to serde's with-annotation. More details about serde_as can be found in the user guide.

#[derive(Deserialize, Serialize)]
struct Foo {
     // Serialize them into a list of number as seconds
     #[serde_as(as = "Vec<DurationSeconds>")]
     durations: Vec<Duration>,
     // We can treat a Vec like a map with duplicates.
     // JSON only allows string keys, so convert i32 to strings
     // The bytes will be hex encoded
     #[serde_as(as = "BTreeMap<DisplayFromStr, Hex>")]
     bytes: Vec<(i32, Vec<u8>)>,

// This will serialize
Foo {
    durations: vec![Duration::new(5, 0), Duration::new(3600, 0), Duration::new(0, 0)],
    bytes: vec![
        (1, vec![0, 1, 2]),
        (-100, vec![100, 200, 255]),
        (1, vec![0, 111, 222]),

// into this JSON
    "durations": [5, 3600, 0],
    "bytes": {
        "1": "000102",
        "-100": "64c8ff",
        "1": "006fde"


Licensed under either of

at your option.


For detailed contribution instructions please read CONTRIBUTING.md.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


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