Deserr

Introduction

Deserr is a crate for deserializing data, with the ability to return custom, type-specific errors upon failure. It was also designed with user-facing APIs in mind and thus provides better defaults than serde for this use case.

Unlike serde, deserr does not parse the data in its serialization format itself but offloads the work to other crates. Instead, it deserializes the already-parsed serialized data into the final type. For example:

// bytes of the serialized value
let s: &str = ".." ;
// parse serialized data using another crate, such as `serde_json`
let json: serde_json::Value = serde_json::from_str(s).unwrap();
// finally deserialize with deserr
let data = T::deserialize_from_value(json.into_value()).unwrap();
// `T` must implement `Deserr`.

Why would I use it

The main place where you should use deserr is on your user-facing API, especially if it's supposed to be read by a human. Since deserr gives you full control over your error types, you can improve the quality of your error messages. Here is a little preview of what you can do with deserr:

Let's say I sent this payload to update my Meilisearch settings:

{
  "filterableAttributes": ["doggo.age", "catto.age"],
  "sortableAttributes": ["uploaded_at"],
  "typoTolerance": {
    "minWordSizeForTypos": {
      "oneTypo": 1000, "twoTypo": 80
    },
    "enabled": true
  },
  "displayedAttributes": ["*"],
  "searchableAttributes": ["doggo.name", "catto.name"]
}

With serde

With serde, we don't have much customization; this is the typical kind of message we would get in return:

{
  "message": "Json deserialize error: invalid value: integer `1000`, expected u8 at line 6 column 21",
  "code": "bad_request",
  "type": "invalid_request",
  "link": "https://docs.meilisearch.com/errors#bad_request"
}

The message

Json deserialize error: invalid value: integer 1000, expected u8 at line 6 column 21

• The message uses the word u8, which definitely won't help a user who doesn't know rust or is unfamiliar with types.
• The location is provided in terms of lines and columns. While this is generally good, when most of your users read this message in their terminal, it doesn't actually help much.

The rest of the payload

Since serde returned this error, we cannot know what happened or on which field it happened. Thus, the best we can do is generate a code bad_request that is common for our whole API. We then use this code to generate a link to our documentation to help our users. But such a generic link does not help our users because it can be thrown by every single route of Meilisearch.

With deserr

{
  "message": "Invalid value at `.typoTolerance.minWordSizeForTypos.oneTypo`: value: `1000` is too large to be deserialized, maximum value authorized is `255`",
  "code": "invalid_settings_typo_tolerance",
  "type": "invalid_request",
  "link": "https://docs.meilisearch.com/errors#invalid-settings-typo-tolerance"
}

The message

Invalid value at .typoTolerance.minWordSizeForTypos.oneTypo: value: 1000 is too large to be deserialized, maximum value authorized is 255

• We get a more human-readable location; .typoTolerance.minWordSizeForTypos.oneTypo. It gives us the faulty field.
• We also get a non-confusing and helpful message this time; it explicitly tells us that the maximum value authorized is 255.

The rest of the payload

Since deserr called one of our functions in the process, we were able to use a custom error code + link to redirect our user to the documentation specific to this feature and this field.

More possibilities with deserr that were impossible with serde

Adding constraints on multiples fields

In Meilisearch, there is another constraint on this minWordSizeForTypos, the twoTypo field must be greater than the oneType field.

Serde doesn't provide any feature to do that. You could write your own implementation of Deserialize for the entire sub-object minWordSizeForTypos, but that's generally hard and wouldn't even let you customize the error type. Thus, that's the kind of thing you're going to check by hand in your code later on. This is error-prone and may bring inconsistencies between most of the deserialization error messages and your error message.

With deserr, we provide attributes that allow you to validate your structure once it's deserialized.

When a field is missing

It's possible to provide your own function when a field is missing.

pub fn missing_field<E: DeserializeError>(field: &str, location: ValuePointerRef) -> E {
    todo!()
}

At Meilisearch, we use this function to specify a custom error code, but we keep the default error message which is pretty accurate.

When an unknown field is encountered

It's possible to provide your own function when a field is missing.

fn unknown_field<E: DeserializeError>(
    field: &str,
    accepted: &[&str],
    location: ValuePointerRef,
) -> E {
    todo!()
}

Here is a few ideas we have or would like to implement at Meilisearch;

• In the case of a resource you can PUT with some fields, but can't PATCH all its fields. We can throw a special immutable field x error instead of an unknown field x.
• Detecting when you use the field name of an alternative; for example, we use q to make a query while some Meilisearch alternatives use query. We could help our users with a did you mean? message that corrects the field to its proper name in Meilisearch.
• Trying to guess what the user was trying to say by computing the levenstein distance between what the user typed and what is accepted to provide a did you mean? message that attempts to correct typos.

When multiple errors are encountered

Deserr lets you accumulate multiple errors with its MergeWithError trait while trying to deserialize the value into your type. This is a good way to improve your user experience by reducing the number of interactions a user needs to have to fix an invalid payload.

---

The main parts of deserr are:

1. Deserr<E> is the main trait for deserialization, unlike Serde, it's very easy to deserialize this trait manually, see the implements_deserr_manually.rs file in our examples directory.
2. IntoValue and Value describes the shape that the parsed serialized data must have
3. DeserializeError is the trait that all deserialization errors must conform to
4. MergeWithError<E> describe how to combine multiple errors together. It allows deserr to return multiple deserialization errors at once.
5. ValuePointerRef and ValuePointer point to locations within the value. They are used to locate the origin of an error.
6. deserialize<Ret, Val, E> is the main function to use to deserialize a value.
   • Ret is the returned value or the structure you want to deserialize.
   • Val is the value type you want to deserialize from. Currently, only an implementation for serde_json::Value is provided in this crate, but you could add your own! Feel free to look into our serde_json module.
   • E is the error type that should be used if an error happens during the deserialization.
7. The Deserr derive proc macro

Example

Implementing deserialize for a custom type with a custom error

In the following example, we're going to deserialize a structure containing a bunch of fields and uses a custom error type that accumulates all the errors encountered while deserializing the structure.

use deserr::{deserialize, DeserializeError, Deserr, ErrorKind, errors::JsonError, Value, ValueKind, IntoValue, take_cf_content, MergeWithError, ValuePointerRef, ValuePointer};
use serde_json::json;
use std::str::FromStr;
use std::ops::ControlFlow;
use std::fmt;
use std::convert::Infallible;

/// This is our custom error type. It'll accumulate multiple `JsonError`.
#[derive(Debug)]
struct MyError(Vec<JsonError>);

impl DeserializeError for MyError {
    /// Create a new error with the custom message.
    ///
    /// Return `ControlFlow::Continue` to continue deserializing even though an error was encountered.
    /// We could return `ControlFlow::Break` as well to stop right here.
    fn error<V: IntoValue>(self_: Option<Self>, error: ErrorKind<V>, location: ValuePointerRef) -> ControlFlow<Self, Self> {
        /// The `take_cf_content` return the inner error in a `ControlFlow<E, E>`.
        let error = take_cf_content(JsonError::error(None, error, location));

        let errors = if let Some(MyError(mut errors)) = self_ {
            errors.push(error);
            errors
        } else {
            vec![error]
        };
        ControlFlow::Continue(MyError(errors))
    }
}

/// We have to implements `MergeWithError` between our error type _aaand_ our error type.
impl MergeWithError<MyError> for MyError {
    fn merge(self_: Option<Self>, mut other: MyError, _merge_location: ValuePointerRef) -> ControlFlow<Self, Self> {
        if let Some(MyError(mut errors)) = self_ {
                other.0.append(&mut errors);
        }
        ControlFlow::Continue(other)
    }
}

#[derive(Debug, Deserr, PartialEq, Eq)]
#[deserr(deny_unknown_fields)]
struct Search {
    #[deserr(default = String::new())]
    query: String,
    #[deserr(try_from(&String) = FromStr::from_str -> IndexUidError)]
    index: IndexUid,
    #[deserr(from(String) = From::from)]
    field: Wildcard,
    #[deserr(default)]
    filter: Option<serde_json::Value>,
    // Even though this field is an `Option` it IS mandatory.
    limit: Option<usize>,
    #[deserr(default)]
    offset: usize,
}

/// An `IndexUid` can only be composed of ascii characters.
#[derive(Debug, PartialEq, Eq)]
struct IndexUid(String);
/// If we encounter a non-ascii character this is the error type we're going to throw.
struct IndexUidError(char);

impl FromStr for IndexUid {
    type Err = IndexUidError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        if let Some(c) = s.chars().find(|c| !c.is_ascii()) {
            Err(IndexUidError(c))
        } else {
            Ok(Self(s.to_string()))
        }
    }
}

impl fmt::Display for IndexUidError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "Encountered invalid character: `{}`, only ascii characters are accepted in the index",
            self.0
        )
    }
}

/// We need to define how the `IndexUidError` error is going to be merged with our
/// custom error type.
impl MergeWithError<IndexUidError> for MyError {
    fn merge(self_: Option<Self>, other: IndexUidError, merge_location: ValuePointerRef) -> ControlFlow<Self, Self> {
            // To be consistent with the other error and automatically get the position of the error we re-use the `JsonError`
            // type and simply define ourself as an `Unexpected` error.
        let error = take_cf_content(JsonError::error::<Infallible>(None, ErrorKind::Unexpected { msg: other.to_string() }, merge_location));
        let errors = if let Some(MyError(mut errors)) = self_ {
            errors.push(error);
            errors
        } else {
            vec![error]
        };
        ControlFlow::Continue(MyError(errors))
    }
}

/// A `Wildcard` can either contains a normal value or be a unit wildcard.
#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(from(String) = From::from)]
enum Wildcard {
    Wildcard,
    Value(String),
}

impl From<String> for Wildcard {
    fn from(s: String) -> Self {
        if s == "*" {
            Wildcard::Wildcard
        } else {
            Wildcard::Value(s)
        }
    }
}

// Here is an example of a typical payload we could deserialize:
let data = deserialize::<Search, _, MyError>(
    json!({ "index": "mieli", "field": "doggo", "filter": ["id = 1", ["catto = jorts"]], "limit": null }),
).unwrap();
assert_eq!(data, Search {
    query: String::new(),
    index: IndexUid(String::from("mieli")),
    field: Wildcard::Value(String::from("doggo")),
    filter: Some(json!(["id = 1", ["catto = jorts"]])),
    limit: None,
    offset: 0,
});

// And here is what happens when everything goes wrong at the same time:
let error = deserialize::<Search, _, MyError>(
    json!({ "query": 12, "index": "mieli 🍯", "field": true, "offset": "🔢"  }),
).unwrap_err();
// We're going to stringify all the error so it's easier to read
assert_eq!(error.0.into_iter().map(|error| error.to_string()).collect::<Vec<String>>().join("\n"),
"\
Invalid value type at `.query`: expected a string, but found a positive integer: `12`
Invalid value type at `.offset`: expected a positive integer, but found a string: `\"🔢\"`
Invalid value at `.index`: Encountered invalid character: `🍯`, only ascii characters are accepted in the index
Invalid value type at `.field`: expected a string, but found a boolean: `true`
Missing field `limit`\
");

Supported features

rename_all

Rename all the fields of the struct according to the given case convention. The possible values are lowercase, camelCase. If you need more case conventions, please open an issue; adding more is trivial.

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(rename_all = camelCase)]
struct Search {
    query: String,
    attributes_to_retrieve: Vec<String>,
}

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "attributesToRetrieve": ["age", "name"] }),
)
.unwrap();
assert_eq!(data, Search {
    query: String::from("doggo"),
    attributes_to_retrieve: vec![String::from("age"), String::from("name")],
});

deny_unknown_fields

Throw an error when encountering unknown fields. When this attribute is absent, unknown fields are ignored by default.

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug)]
#[deserr(deny_unknown_fields)]
struct Search {
    query: String,
}

let err = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "doggo": "bork" }),
)
.unwrap_err();

assert_eq!(err.to_string(), "Unknown field `doggo`: expected one of `query`");

It is also possible to provide a custom function to handle the error.

use deserr::{Deserr, deserialize, ErrorKind, DeserializeError, ValuePointerRef, take_cf_content, errors::JsonError};
use std::convert::Infallible;
use serde_json::json;

#[derive(Deserr, Debug)]
#[deserr(deny_unknown_fields = unknown_fields_search)]
struct Search {
    query: String,
}

fn unknown_fields_search<E: DeserializeError>(
    field: &str,
    accepted: &[&str],
    location: ValuePointerRef,
) -> E {
    // `E::error` returns a `ControlFlow<E, E>`, which returns the error and indicates
    // whether we should keep accumulating errors or not. However, here we simply
    // want to retrieve the error's value. This is what `take_cf_content` does.
    match field {
        "doggo" => take_cf_content(E::error::<Infallible>(
                None,
                ErrorKind::Unexpected {
                    msg: String::from("can I pet the doggo? uwu")
                },
                location,
            )),
        _ => take_cf_content(E::error::<Infallible>(
            None,
            deserr::ErrorKind::UnknownKey { key: field, accepted },
            location,
        )),
    }
}

let err = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "doggo": "bork" }),
)
.unwrap_err();

assert_eq!(err.to_string(), "Invalid value: can I pet the doggo? uwu");

let err = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "catto": "jorts" }),
)
.unwrap_err();

assert_eq!(err.to_string(), "Unknown field `catto`: expected one of `query`");

tag

Externally tag an enum. Deserr does not support internally tagging your enum yet, which means you'll always need to use this attribute if you're deserializing an enum. For complete unit enums, deserr can deserialize their value from a string, though.

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search {
    query: Query,
}

#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(tag = "type")]
enum Query {
    Single {
        search: String,
    },
    Multi {
        searches: Vec<String>,
    }
}

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": { "type": "Single", "search": "bork" } }),
)
.unwrap();
assert_eq!(data, Search {
    query: Query::Single {
        search: String::from("bork"),
    },
});

from

Deserializing a type from a function instead of a Value. You need to provide the following information;

1. The input type of the function (here &String)
2. The path of the function (here, we're simply using the std FromStr implementation)

deserr will first try to deserialize the given type using its Deserr<E> implementation. That means the input type of the from can be complex. Then deserr will call your function.

See also try_from if your function can fail.

It can be used as a container attribute

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(from(String) = From::from)]
enum Wildcard {
    Wildcard,
    Value(String),
}

impl From<String> for Wildcard {
    fn from(s: String) -> Self {
        if s == "*" {
            Wildcard::Wildcard
        } else {
            Wildcard::Value(s)
        }
    }
}

let data = deserialize::<Wildcard, _, JsonError>(
    json!("doggo"),
)
.unwrap();
assert_eq!(data, Wildcard::Value(String::from("doggo")));

let data = deserialize::<Wildcard, _, JsonError>(
    json!("*"),
)
.unwrap();
assert_eq!(data, Wildcard::Wildcard);

Or as a field attribute

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(from(String) = From::from)]
enum Wildcard {
    Wildcard,
    Value(String),
}

impl From<String> for Wildcard {
    fn from(s: String) -> Self {
        if s == "*" {
            Wildcard::Wildcard
        } else {
            Wildcard::Value(s)
        }
    }
}

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search {
    query: String,
    #[deserr(from(String) = From::from)]
    field: Wildcard,
}

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "field": "catto" }),
)
.unwrap();
assert_eq!(data, Search { query: String::from("doggo"), field: Wildcard::Value(String::from("catto")) });

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "field": "*" }),
)
.unwrap();
assert_eq!(data, Search { query: String::from("doggo"), field: Wildcard::Wildcard });

try_from

Try deserializing a type from a function instead of a Value. You need to provide the following information;

1. The input type of the function (here &String)
2. The path of the function (here, we're simply using the std FromStr implementation)
3. The error type that this function can return (here Infallible)

deserr will first try to deserialize the given type using its Deserr<E> implementation. That means the input type of the try_from can be complex. Then deserr will call your function and accumulate the specified error against the error type of the caller.

It can be used as a container attribute

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;
use std::str::FromStr;
use std::fmt;

// Notice how the `try_from` allows us to leverage the deserr limitation on tuple struct.
#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(try_from(&String) = FromStr::from_str -> AsciiStringError)]
struct AsciiString(String);

#[derive(Debug)]
struct AsciiStringError(char);

impl fmt::Display for AsciiStringError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "Encountered invalid character: `{}`, only ascii characters are accepted",
            self.0
        )
    }
}
impl std::error::Error for AsciiStringError {}

impl FromStr for AsciiString {
    type Err = AsciiStringError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        if let Some(c) = s.chars().find(|c| !c.is_ascii()) {
            Err(AsciiStringError(c))
        } else {
            Ok(Self(s.to_string()))
        }
    }
}

let data = deserialize::<AsciiString, _, JsonError>(
    json!("doggo"),
)
.unwrap();
assert_eq!(data, AsciiString(String::from("doggo")));

let error = deserialize::<AsciiString, _, JsonError>(
    json!("👉👈"),
)
.unwrap_err();
assert_eq!(error.to_string(), "Invalid value: Encountered invalid character: `👉`, only ascii characters are accepted");

Or as a field attribute

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;
use std::convert::Infallible;
use std::str::FromStr;
use std::num::ParseIntError;

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search {
    query: String,
    #[deserr(try_from(&String) = FromStr::from_str -> ParseIntError)]
    limit: usize,

}

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "limit": "12" }),
)
.unwrap();
assert_eq!(data, Search { query: String::from("doggo"), limit: 12 });

let error = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "limit": 12 }),
)
.unwrap_err();
assert_eq!(error.to_string(), "Invalid value type at `.limit`: expected a string, but found a positive integer: `12`");

validate

Validate a structure after it has been deserialized. This is typically useful when your validation logic needs to take multiple fields into account.

use deserr::{Deserr, DeserializeError, ErrorKind, ValuePointerRef, deserialize, errors::JsonError};
use serde_json::json;
use std::convert::Infallible;

// `__Deserr_E` represents the Error returned by the generated `Deserr` implementation.
#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(validate = validate_range -> __Deserr_E)]
struct Range {
    min: u8,
    max: u8,
}

fn validate_range<E: DeserializeError>(
    range: Range,
    location: ValuePointerRef,
) -> Result<Range, E> {
    if range.min > range.max {
        Err(deserr::take_cf_content(E::error::<Infallible>(
            None,
            ErrorKind::Unexpected {
                msg: format!(
                    "`max` (`{}`) should be greater than `min` (`{}`)",
                    range.max, range.min
                ),
            },
            location,
        )))
    } else {
        Ok(range)
    }
}

let data = deserialize::<Range, _, JsonError>(
    json!({ "min": 2, "max": 4 }),
)
.unwrap();
assert_eq!(data, Range { min: 2, max: 4 });

let error = deserialize::<Range, _, JsonError>(
    json!({ "min": 4, "max": 2 }),
)
.unwrap_err();
assert_eq!(error.to_string(), "Invalid value: `max` (`2`) should be greater than `min` (`4`)");

default

Allows you to specify a default value for a field.

Note that, unlike serde, by default, Option doesn't automatically use this attribute. Here you need to explicitly define whether your type can get a default value. This makes it less error-prone and easier to make an optional field mandatory.

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search {
    #[deserr(default)]
    query: Option<String>,
    #[deserr(default = 20)]
    limit: usize,
}

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "limit": 4 }),
)
.unwrap();
assert_eq!(data, Search { query: Some(String::from("doggo")), limit: 4 });

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo" }),
)
.unwrap();
assert_eq!(data, Search { query: Some(String::from("doggo")), limit: 20 });

skip

Allows you to skip the deserialization of a field. It won't show up in the list of fields generated by deny_unknown_fields or in the UnknownKey variant of the ErrorKind type.

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search {
    query: String,
    // A field can be skipped if it implements `Default` or if the `default` attribute is specified.
    #[deserr(skip)]
    hidden: usize,
}

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo" }),
)
.unwrap();
assert_eq!(data, Search { query: String::from("doggo"), hidden: 0 });

// if you try to specify the field, it is ignored
let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "hidden": 2 }),
)
.unwrap();
assert_eq!(data, Search { query: String::from("doggo"), hidden: 0 });

// Here, we're going to see how skip interacts with `deny_unknown_fields`

#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(deny_unknown_fields)]
struct Search2 {
    query: String,
    // A field can be skipped if it implements `Default`.
    #[deserr(skip)]
    hidden: usize,
}

let error = deserialize::<Search2, _, JsonError>(
    json!({ "query": "doggo", "hidden": 1 }),
)
.unwrap_err();
// NOTE: `hidden` isn't in the list of expected fields + `hidden` is effectively considered as a non-existing field.
assert_eq!(error.to_string(), "Unknown field `hidden`: expected one of `query`");

map

Map a field after it has been deserialized.

use deserr::{Deserr, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search {
    query: String,
    #[deserr(map = add_one)]
    limit: usize,
}

fn add_one(n: usize) -> usize {
    n.saturating_add(1)
}

let data = deserialize::<Search, _, JsonError>(
    json!({ "query": "doggo", "limit": 0 }),
)
.unwrap();
assert_eq!(data, Search { query: String::from("doggo"), limit: 1 });

// Let's see how `map` interacts with the `default` attributes.
#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search2 {
    query: String,
    #[deserr(default, map = add_one)]
    limit: usize,
}

let data = deserialize::<Search2, _, JsonError>(
    json!({ "query": "doggo" }),
)
.unwrap();
// As we can see, the `map` attribute is applied AFTER the `default`.
assert_eq!(data, Search2 { query: String::from("doggo"), limit: 1 });

missing_field_error

Gives you the opportunity to customize the error message if this specific field is missing.

use deserr::{Deserr, DeserializeError, ValuePointerRef, ErrorKind, deserialize, errors::JsonError};
use serde_json::json;
use std::convert::Infallible;

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search {
    #[deserr(missing_field_error = missing_query_field)]
    query: String,
    limit: usize,
}

fn missing_query_field<E: DeserializeError>(_field_name: &str, location: ValuePointerRef) -> E {
    deserr::take_cf_content(E::error::<Infallible>(
        None,
        ErrorKind::Unexpected {
            msg: String::from("I really need the query field, please give it to me uwu"),
        },
        location,
    ))
}

let error = deserialize::<Search, _, JsonError>(
    json!({ "limit": 0 }),
)
.unwrap_err();
assert_eq!(error.to_string(), "Invalid value: I really need the query field, please give it to me uwu");

error

Customize the error type that can be returned when deserializing this structure instead of keeping it generic.

use deserr::{Deserr, DeserializeError, ValuePointerRef, ErrorKind, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(error = JsonError)]
struct Search {
    query: String,
    limit: usize,
}

// As we can see, rust is able to infer the error type.
let data = deserialize::<Search, _, _>(
    json!({ "query": "doggo", "limit": 1 }),
)
.unwrap();
assert_eq!(data, Search { query: String::from("doggo"), limit: 1 });

It can also be used as a field attribute;

use deserr::{Deserr, DeserializeError, ValuePointerRef, ErrorKind, deserialize, errors::JsonError};
use serde_json::json;

// Since the error returned by the `Search` structure needs to implements `MergeWithError<JsonError>`
// we also need to specify the `error` attribute as a `JsonError`. But as you will see later there are
// other solutions.
#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(error = JsonError)]
struct Search<A> {
    #[deserr(error = JsonError)]
    query: A,
    limit: usize,
}

where_predicate

Let you add where clauses to the Deserr implementation that deserr will generate.

use deserr::{Deserr, DeserializeError, MergeWithError, deserialize, errors::JsonError};
use serde_json::json;

// Here we can constraint the generic `__Deserr_E` type used by deserr to implements `MergeWithError`.
// Now instead of constraining the final error type it stays generic if it's able to accumulate with
// with a `JsonError`.
#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(where_predicate = __Deserr_E: MergeWithError<JsonError>, where_predicate = A: Deserr<JsonError>)]
struct Search<A> {
    #[deserr(error = JsonError)]
    query: A,
    limit: usize,
}

needs_predicate

Automatically adds where_predicate = FieldType: Deserr<ErrType> for each field with this attribute.

use deserr::{Deserr, DeserializeError, MergeWithError, deserialize, errors::JsonError};
use serde_json::json;

#[derive(Deserr, Debug, PartialEq, Eq)]
struct Search<A> {
    #[deserr(needs_predicate)]
    query: A,
    limit: usize,
}

Is strictly equivalent to the following:

use deserr::{Deserr, DeserializeError, MergeWithError, deserialize, errors::JsonError};
use serde_json::json;

// `__Deserr_E` represents the Error returned by the generated `Deserr` implementation.
#[derive(Deserr, Debug, PartialEq, Eq)]
#[deserr(where_predicate = A: Deserr<__Deserr_E>)]
struct Search<A> {
    query: A,
    limit: usize,
}

Comparison with serde

Since deserr needs to first deserialize the payload into a generic Value that allocates a lot of memory before creating your structure, it's a lot slower than serde.

For example, at Meilisearch for our search route, in case of a valid payload, we observed a 400% slowdown (4 times slower). That made our search request deserialize in 2µs instead of 500ns. This is fast enough for most use cases but could be an issue if most of your time is spent deserializing.

Datastructure support

datastructure	serde	deserr	note
Struct	yes	yes
Tuple struct	yes	no
Untagged Enum	yes	no
Untagged unit Enum	yes	yes
Tagged Enum	yes	yes

Container attributes

features	serde	deserr	note
rename	yes	no
rename_all	yes	yes
deny_unknown_fields	yes	yes	With deserr you can call a custom function when an unknown field is encountered
tag	yes	yes
tag+content	yes	no
untagged	yes	no	it's only supported for unit enums
bound	yes	no	Can be emulated with where_predicate
default	yes	no
remote	yes	no
transparent	yes	no
from	yes	yes
try_from	yes	yes
into	yes	no
crate	yes	no
validate	no	yes	Allows you to validate the content of struct after it has been deserialized
error	no	yes	Specify the error type that should be used while deserializing this structure
where_predicate	no	yes	Let you add where clauses to the generated Deserr implementation

Field attributes

features	serde	deserr	note
rename	yes	no
alias	yes	no
default	yes	yes
flatten	yes	no	serde doesn't support flattening + denying unknown field
skip	yes	yes
deserialize_with	yes	no	But it's kinda emulated with from and try_from
with	yes	no
borrow	yes	no	deserr does not support types with references
bound	yes	no
map	no	yes	Allows you to map the value after it was deserialized
from	no	yes	Deserialize this field from an infallible function
try_from	no	yes	Deserialize this field from a fallible function
missing_field_error	no	yes	Allows you to return a custom error if this field is missing
error	no	yes	Specify the error type that should be used while deserializing this field

Feature flags

serde-json

Import serde_json and provide;

• An implementation of deserr::IntoValue for serde_json::Value which make it easy to use both crate together.
• A default implementation of the JsonError type that provide the best generic error messages possible.

serde-cs

Import serde-cs and provide;

• An implementation of Deserr for serde_cs::CS<R>.

actix-web

Import actix-web and futures and provide;

• An implementation of a json actix-web extractor if used with the serde-json feature.

• An implementation of ResponseError for the JsonError type if used with the serde-json feature.

FAQ

But why?

At Meilisearch, we wanted to customize the error code we return when we fail the deserialization of a specific field. Some error messages were also not clear at all and impossible to edit.

What about the maintenance?

At Meilisearch we're already using deserr in production; thus, it's well maintained.

Where can I see more examples of usage of this crate?

Currently, you can read our examples in the examples directory of this repository. You can also look at our integration test; each attribute has a simple-to-read test.

And obviously, you can read the code of Meilisearch where deserr is used on all our routes.

My question is not listed

Please, if you think there is a bug in this lib or would like a new feature, open an issue or a discussion. If you would like to chat more directly with us, you can join us on discord at https://discord.com/invite/meilisearch

License

^{Licensed under either of Apache License, Version 2.0 or MIT license at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.}