Trait serde::Deserializer

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pub trait Deserializer<'de>: Sized {
    type Error: Error;

Show 32 methods    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_i8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_i16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_i32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_i64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_u8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_u32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_f64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_char<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_str<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_string<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_bytes<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_byte_buf<V>(
        self, 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_unit_struct<V>(
        self, 
        name: &'static str, 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_newtype_struct<V>(
        self, 
        name: &'static str, 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_tuple<V>(
        self, 
        len: usize, 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_tuple_struct<V>(
        self, 
        name: &'static str, 
        len: usize, 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_struct<V>(
        self, 
        name: &'static str, 
        fields: &'static [&'static str], 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_enum<V>(
        self, 
        name: &'static str, 
        variants: &'static [&'static str], 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_identifier<V>(
        self, 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_ignored_any<V>(
        self, 
        visitor: V
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    fn deserialize_i128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    { ... }

    fn deserialize_u128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    { ... }

    fn is_human_readable(&self) -> bool { ... }

}
Expand description

A data format that can deserialize any data structure supported by Serde.

The role of this trait is to define the deserialization half of the Serde data model, which is a way to categorize every Rust data type into one of 29 possible types. Each method of the Deserializer trait corresponds to one of the types of the data model.

Implementations of Deserialize map themselves into this data model by passing to the Deserializer a Visitor implementation that can receive these various types.

The types that make up the Serde data model are:

  • 14 primitive types
    • bool
    • i8, i16, i32, i64, i128
    • u8, u16, u32, u64, u128
    • f32, f64
    • char
  • string
    • UTF-8 bytes with a length and no null terminator.
    • When serializing, all strings are handled equally. When deserializing, there are three flavors of strings: transient, owned, and borrowed.
  • byte array - [u8]
    • Similar to strings, during deserialization byte arrays can be transient, owned, or borrowed.
  • option
    • Either none or some value.
  • unit
    • The type of () in Rust. It represents an anonymous value containing no data.
  • unit_struct
    • For example struct Unit or PhantomData<T>. It represents a named value containing no data.
  • unit_variant
    • For example the E::A and E::B in enum E { A, B }.
  • newtype_struct
    • For example struct Millimeters(u8).
  • newtype_variant
    • For example the E::N in enum E { N(u8) }.
  • seq
    • A variably sized heterogeneous sequence of values, for example Vec<T> or HashSet<T>. When serializing, the length may or may not be known before iterating through all the data. When deserializing, the length is determined by looking at the serialized data.
  • tuple
    • A statically sized heterogeneous sequence of values for which the length will be known at deserialization time without looking at the serialized data, for example (u8,) or (String, u64, Vec<T>) or [u64; 10].
  • tuple_struct
    • A named tuple, for example struct Rgb(u8, u8, u8).
  • tuple_variant
    • For example the E::T in enum E { T(u8, u8) }.
  • map
    • A heterogeneous key-value pairing, for example BTreeMap<K, V>.
  • struct
    • A heterogeneous key-value pairing in which the keys are strings and will be known at deserialization time without looking at the serialized data, for example struct S { r: u8, g: u8, b: u8 }.
  • struct_variant
    • For example the E::S in enum E { S { r: u8, g: u8, b: u8 } }.

The Deserializer trait supports two entry point styles which enables different kinds of deserialization.

  1. The deserialize method. Self-describing data formats like JSON are able to look at the serialized data and tell what it represents. For example the JSON deserializer may see an opening curly brace ({) and know that it is seeing a map. If the data format supports Deserializer::deserialize_any, it will drive the Visitor using whatever type it sees in the input. JSON uses this approach when deserializing serde_json::Value which is an enum that can represent any JSON document. Without knowing what is in a JSON document, we can deserialize it to serde_json::Value by going through Deserializer::deserialize_any.

  2. The various deserialize_* methods. Non-self-describing formats like Bincode need to be told what is in the input in order to deserialize it. The deserialize_* methods are hints to the deserializer for how to interpret the next piece of input. Non-self-describing formats are not able to deserialize something like serde_json::Value which relies on Deserializer::deserialize_any.

When implementing Deserialize, you should avoid relying on Deserializer::deserialize_any unless you need to be told by the Deserializer what type is in the input. Know that relying on Deserializer::deserialize_any means your data type will be able to deserialize from self-describing formats only, ruling out Bincode and many others.

Lifetime

The 'de lifetime of this trait is the lifetime of data that may be borrowed from the input when deserializing. See the page Understanding deserializer lifetimes for a more detailed explanation of these lifetimes.

Example implementation

The example data format presented on the website contains example code for a basic JSON Deserializer.

Associated Types

The error type that can be returned if some error occurs during deserialization.

Required methods

Require the Deserializer to figure out how to drive the visitor based on what data type is in the input.

When implementing Deserialize, you should avoid relying on Deserializer::deserialize_any unless you need to be told by the Deserializer what type is in the input. Know that relying on Deserializer::deserialize_any means your data type will be able to deserialize from self-describing formats only, ruling out Bincode and many others.

Hint that the Deserialize type is expecting a bool value.

Hint that the Deserialize type is expecting an i8 value.

Hint that the Deserialize type is expecting an i16 value.

Hint that the Deserialize type is expecting an i32 value.

Hint that the Deserialize type is expecting an i64 value.

Hint that the Deserialize type is expecting a u8 value.

Hint that the Deserialize type is expecting a u16 value.

Hint that the Deserialize type is expecting a u32 value.

Hint that the Deserialize type is expecting a u64 value.

Hint that the Deserialize type is expecting a f32 value.

Hint that the Deserialize type is expecting a f64 value.

Hint that the Deserialize type is expecting a char value.

Hint that the Deserialize type is expecting a string value and does not benefit from taking ownership of buffered data owned by the Deserializer.

If the Visitor would benefit from taking ownership of String data, indicate this to the Deserializer by using deserialize_string instead.

Hint that the Deserialize type is expecting a string value and would benefit from taking ownership of buffered data owned by the Deserializer.

If the Visitor would not benefit from taking ownership of String data, indicate that to the Deserializer by using deserialize_str instead.

Hint that the Deserialize type is expecting a byte array and does not benefit from taking ownership of buffered data owned by the Deserializer.

If the Visitor would benefit from taking ownership of Vec<u8> data, indicate this to the Deserializer by using deserialize_byte_buf instead.

Hint that the Deserialize type is expecting a byte array and would benefit from taking ownership of buffered data owned by the Deserializer.

If the Visitor would not benefit from taking ownership of Vec<u8> data, indicate that to the Deserializer by using deserialize_bytes instead.

Hint that the Deserialize type is expecting an optional value.

This allows deserializers that encode an optional value as a nullable value to convert the null value into None and a regular value into Some(value).

Hint that the Deserialize type is expecting a unit value.

Hint that the Deserialize type is expecting a unit struct with a particular name.

Hint that the Deserialize type is expecting a newtype struct with a particular name.

Hint that the Deserialize type is expecting a sequence of values.

Hint that the Deserialize type is expecting a sequence of values and knows how many values there are without looking at the serialized data.

Hint that the Deserialize type is expecting a tuple struct with a particular name and number of fields.

Hint that the Deserialize type is expecting a map of key-value pairs.

Hint that the Deserialize type is expecting a struct with a particular name and fields.

Hint that the Deserialize type is expecting an enum value with a particular name and possible variants.

Hint that the Deserialize type is expecting the name of a struct field or the discriminant of an enum variant.

Hint that the Deserialize type needs to deserialize a value whose type doesn’t matter because it is ignored.

Deserializers for non-self-describing formats may not support this mode.

Provided methods

Hint that the Deserialize type is expecting an i128 value.

This method is available only on Rust compiler versions >=1.26. The default behavior unconditionally returns an error.

Hint that the Deserialize type is expecting an u128 value.

This method is available only on Rust compiler versions >=1.26. The default behavior unconditionally returns an error.

Determine whether Deserialize implementations should expect to deserialize their human-readable form.

Some types have a human-readable form that may be somewhat expensive to construct, as well as a binary form that is compact and efficient. Generally text-based formats like JSON and YAML will prefer to use the human-readable one and binary formats like Bincode will prefer the compact one.

ⓘ
use serde::de::{self, Deserialize, Deserializer};

impl<'de> Deserialize<'de> for Timestamp {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        if deserializer.is_human_readable() {
            // Deserialize from a human-readable string like "2015-05-15T17:01:00Z".
            let s = String::deserialize(deserializer)?;
            Timestamp::from_str(&s).map_err(de::Error::custom)
        } else {
            // Deserialize from a compact binary representation, seconds since
            // the Unix epoch.
            let n = u64::deserialize(deserializer)?;
            Ok(Timestamp::EPOCH + Duration::seconds(n))
        }
    }
}

The default implementation of this method returns true. Data formats may override this to false to request a compact form for types that support one. Note that modifying this method to change a format from human-readable to compact or vice versa should be regarded as a breaking change, as a value serialized in human-readable mode is not required to deserialize from the same data in compact mode.

Implementors