# Custom de/serialization functions for Rust's [serde](https://serde.rs)
[](https://crates.io/crates/serde_with/)
[](https://github.com/jonasbb/serde_with)
[](https://codecov.io/gh/jonasbb/serde_with)
[](https://bestpractices.coreinfrastructure.org/projects/4322)
[](https://www.rustexplorer.com/b/py7ida)
---
This crate provides custom de/serialization helpers to use in combination with [serde's with-annotation][with-annotation] and with the improved [`serde_as`][as-annotation]-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](#large-and-const-generic-arrays).
* Skip serializing all empty `Option` types with [`#[skip_serializing_none]`][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::StringWithSeparator::<CommaSeparator, T>`][StringWithSeparator].
### Getting Help
**Check out the [user guide][user guide] to find out more tips and tricks about this crate.**
For further help using this crate you can [open a new discussion](https://github.com/jonasbb/serde_with/discussions/new) or ask on [users.rust-lang.org](https://users.rust-lang.org/).
For bugs, please open a [new issue](https://github.com/jonasbb/serde_with/issues/new) on GitHub.
## Use `serde_with` in your Project
```bash
# Add the current version to your Cargo.toml
cargo add serde_with
```
The crate contains different features for integration with other common crates.
Check the [feature flags][] section for information about all available features.
## Examples
Annotate your struct or enum to enable the custom de/serializer.
The `#[serde_as]` attribute must be placed *before* the `#[derive]`.
The `as` is analogous to the `with` attribute of serde.
You mirror the type structure of the field you want to de/serialize.
You can specify converters for the inner types of a field, e.g., `Vec<DisplayFromStr>`.
The default de/serialization behavior can be restored by using `_` as a placeholder, e.g., `BTreeMap<_, DisplayFromStr>`.
### `DisplayFromStr`
[](https://www.rustexplorer.com/b/py7ida)
```rust
#[serde_as]
#[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.
On top of it, `[u8; N]` (aka, bytes) can use the specialized `"Bytes"` for efficiency much like the `serde_bytes` crate.
[](https://www.rustexplorer.com/b/um0xyi)
```rust
#[serde_as]
#[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]>,
#[serde_as(as = "Bytes")]
bytes: [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]),
bytes: [0x42; 128],
};
assert!(serde_json::to_string(&arrays).is_ok());
```
### `skip_serializing_none`
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]`.
[](https://www.rustexplorer.com/b/xr1tm0)
```rust
#[skip_serializing_none]
#[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][with-annotation].
More details about `serde_as` can be found in the [user guide].
```rust
use std::time::Duration;
#[serde_as]
#[derive(Deserialize, Serialize)]
enum Foo {
Durations(
// Serialize them into a list of number as seconds
#[serde_as(as = "Vec<DurationSeconds>")]
Vec<Duration>,
),
Bytes {
// 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 = "Map<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)]
)
// into this JSON
{
"Durations": [5, 3600, 0]
}
// and serializes
Foo::Bytes {
bytes: vec![
(1, vec![0, 1, 2]),
(-100, vec![100, 200, 255]),
(1, vec![0, 111, 222]),
],
}
// into this JSON
{
"Bytes": {
"bytes": {
"1": "000102",
"-100": "64c8ff",
"1": "006fde"
}
}
}
```
[`DisplayFromStr`]: https://docs.rs/serde_with/3.4.0/serde_with/struct.DisplayFromStr.html
[`with_prefix!`]: https://docs.rs/serde_with/3.4.0/serde_with/macro.with_prefix.html
[feature flags]: https://docs.rs/serde_with/3.4.0/serde_with/guide/feature_flags/index.html
[skip_serializing_none]: https://docs.rs/serde_with/3.4.0/serde_with/attr.skip_serializing_none.html
[StringWithSeparator]: https://docs.rs/serde_with/3.4.0/serde_with/struct.StringWithSeparator.html
[user guide]: https://docs.rs/serde_with/3.4.0/serde_with/guide/index.html
[with-annotation]: https://serde.rs/field-attrs.html#with
[as-annotation]: https://docs.rs/serde_with/3.4.0/serde_with/guide/serde_as/index.html
## License
Licensed under either of
* Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
* MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
at your option.
## Contribution
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.
[`CONTRIBUTING.md`]: https://github.com/jonasbb/serde_with/blob/master/CONTRIBUTING.md