Rust第三方库之Serde：反序列化架构分析

╰+攻爆jí腚メ 2021-09-20 02:48 201阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

serde的反序列化架构主要涉及三个特性：Deserialize、Deserializer和Visitor，各自承担的责任不同。

## *Deserialize* Trait ##

pub trait Deserialize<'de>: Sized {
        /// Deserialize this value from the given Serde deserializer.
        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where
           D: Deserializer<'de>;
    }

该 Trait 定义了反序列化一个值的接口，使用某个的反序列化器（Deserializer）创建自身对象，如果出错返回错误。

带有一个泛型生存期 `'de` 指使用的反序列化器的生存期，返回的错误是反序列化器定义的错误类型。

## *Deserializer* Trait ##

pub trait Deserializer<'de>: Sized {
        /// The error type that can be returned if some error occurs during
        /// deserialization.
        type Error: Error;
    
        /// 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.
        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `bool` value.
        fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting an `i8` value.
        fn deserialize_i8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting an `i16` value.
        fn deserialize_i16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting an `i32` value.
        fn deserialize_i32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting an `i64` value.
        fn deserialize_i64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `u8` value.
        fn deserialize_u8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `u16` value.
        fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `u32` value.
        fn deserialize_u32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `u64` value.
        fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `f32` value.
        fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `f64` value.
        fn deserialize_f64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a `char` value.
        fn deserialize_char<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// 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,
        /// indiciate this to the `Deserializer` by using `deserialize_string`
        /// instead.
        fn deserialize_str<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// 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.
        fn deserialize_string<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// 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.
        fn deserialize_bytes<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// 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.
        fn deserialize_byte_buf<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// 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)`.
        fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a unit value.
        fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a unit struct with a
        /// particular name.
        fn deserialize_unit_struct<V>(
            self,
            name: &'static str,
            visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a newtype struct with a
        /// particular name.
        fn deserialize_newtype_struct<V>(
            self,
            name: &'static str,
            visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a sequence of values.
        fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a sequence of values and
        /// knows how many values there are without looking at the serialized data.
        fn deserialize_tuple<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a tuple struct with a
        /// particular name and number of fields.
        fn deserialize_tuple_struct<V>(
            self,
            name: &'static str,
            len: usize,
            visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a map of key-value pairs.
        fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting a struct with a particular
        /// name and fields.
        fn deserialize_struct<V>(
            self,
            name: &'static str,
            fields: &'static [&'static str],
            visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting an enum value with a
        /// particular name and possible variants.
        fn deserialize_enum<V>(
            self,
            name: &'static str,
            variants: &'static [&'static str],
            visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// Hint that the `Deserialize` type is expecting the name of a struct
        /// field or the discriminant of an enum variant.
        fn deserialize_identifier<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// 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.
        fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    
        /// 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 std::ops::Add;
        /// # use std::str::FromStr;
        /// #
        /// # struct Timestamp;
        /// #
        /// # impl Timestamp {
        /// #     const EPOCH: Timestamp = Timestamp;
        /// # }
        /// #
        /// # impl FromStr for Timestamp {
        /// #     type Err = String;
        /// #     fn from_str(_: &str) -> Result<Self, Self::Err> {
        /// #         unimplemented!()
        /// #     }
        /// # }
        /// #
        /// # struct Duration;
        /// #
        /// # impl Duration {
        /// #     fn seconds(_: u64) -> Self { unimplemented!() }
        /// # }
        /// #
        /// # impl Add<Duration> for Timestamp {
        /// #     type Output = Timestamp;
        /// #     fn add(self, _: Duration) -> Self::Output {
        /// #         unimplemented!()
        /// #     }
        /// # }
        /// #
        /// 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.
        #[inline]
        fn is_human_readable(&self) -> bool {
            true
        }
    }

反序列化器提供了一系列的方法，可以供 Deserialize 使用，每个方法都需要一个 Visitor，方法返回的值由Visitor决定，在实现某个数据类型的 Deserialize 特性时，需要调用反序列化器的方法表达我需要一个什么类型的数据，并给方法一个合适的 Visitor，值得注意的是：调用 deserialize\_u8 方法不一定返回u8，返回的实际类型由 Visitor::Value 决定。

## *Visitor* Trait ##

pub trait Visitor<'de>: Sized {
        /// The value produced by this visitor.
        type Value;
    
        /// Format a message stating what data this Visitor expects to receive.
        ///
        /// This is used in error messages. The message should complete the sentence
        /// "This Visitor expects to receive ...", for example the message could be
        /// "an integer between 0 and 64". The message should not be capitalized and
        /// should not end with a period.
        ///
        /// ```rust
        /// # use std::fmt;
        /// #
        /// # struct S {
        /// #     max: usize,
        /// # }
        /// #
        /// # impl<'de> serde::de::Visitor<'de> for S {
        /// #     type Value = ();
        /// #
        /// fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///     write!(formatter, "an integer between 0 and {}", self.max)
        /// }
        /// # }
        /// ```
        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result;
    
        /// The input contains a boolean.
        ///
        /// The default implementation fails with a type error.
        fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
        where
            E: Error,
        {
            Err(Error::invalid_type(Unexpected::Bool(v), &self))
        }
    
        /// The input contains an `i8`.
        ///
        /// The default implementation forwards to [`visit_i64`].
        ///
        /// [`visit_i64`]: #method.visit_i64
        fn visit_i8<E>(self, v: i8) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_i64(v as i64)
        }
    
        /// The input contains an `i16`.
        ///
        /// The default implementation forwards to [`visit_i64`].
        ///
        /// [`visit_i64`]: #method.visit_i64
        fn visit_i16<E>(self, v: i16) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_i64(v as i64)
        }
    
        /// The input contains an `i32`.
        ///
        /// The default implementation forwards to [`visit_i64`].
        ///
        /// [`visit_i64`]: #method.visit_i64
        fn visit_i32<E>(self, v: i32) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_i64(v as i64)
        }
    
        /// The input contains an `i64`.
        ///
        /// The default implementation fails with a type error.
        fn visit_i64<E>(self, v: i64) -> Result<Self::Value, E>
        where
            E: Error,
        {
            Err(Error::invalid_type(Unexpected::Signed(v), &self))
        }
    
        /// The input contains a `u8`.
        ///
        /// The default implementation forwards to [`visit_u64`].
        ///
        /// [`visit_u64`]: #method.visit_u64
        fn visit_u8<E>(self, v: u8) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_u64(v as u64)
        }
    
        /// The input contains a `u16`.
        ///
        /// The default implementation forwards to [`visit_u64`].
        ///
        /// [`visit_u64`]: #method.visit_u64
        fn visit_u16<E>(self, v: u16) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_u64(v as u64)
        }
    
        /// The input contains a `u32`.
        ///
        /// The default implementation forwards to [`visit_u64`].
        ///
        /// [`visit_u64`]: #method.visit_u64
        fn visit_u32<E>(self, v: u32) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_u64(v as u64)
        }
    
        /// The input contains a `u64`.
        ///
        /// The default implementation fails with a type error.
        fn visit_u64<E>(self, v: u64) -> Result<Self::Value, E>
        where
            E: Error,
        {
            Err(Error::invalid_type(Unexpected::Unsigned(v), &self))
        }
    
        /// The input contains an `f32`.
        ///
        /// The default implementation forwards to [`visit_f64`].
        ///
        /// [`visit_f64`]: #method.visit_f64
        fn visit_f32<E>(self, v: f32) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_f64(v as f64)
        }
    
        /// The input contains an `f64`.
        ///
        /// The default implementation fails with a type error.
        fn visit_f64<E>(self, v: f64) -> Result<Self::Value, E>
        where
            E: Error,
        {
            Err(Error::invalid_type(Unexpected::Float(v), &self))
        }
    
        /// The input contains a `char`.
        ///
        /// The default implementation forwards to [`visit_str`] as a one-character
        /// string.
        ///
        /// [`visit_str`]: #method.visit_str
        #[inline]
        fn visit_char<E>(self, v: char) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_str(utf8::encode(v).as_str())
        }
    
        /// The input contains a string. The lifetime of the string is ephemeral and
        /// it may be destroyed after this method returns.
        ///
        /// This method allows the `Deserializer` to avoid a copy by retaining
        /// ownership of any buffered data. `Deserialize` implementations that do
        /// not benefit from taking ownership of `String` data should indicate that
        /// to the deserializer by using `Deserializer::deserialize_str` rather than
        /// `Deserializer::deserialize_string`.
        ///
        /// It is never correct to implement `visit_string` without implementing
        /// `visit_str`. Implement neither, both, or just `visit_str`.
        fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
        where
            E: Error,
        {
            Err(Error::invalid_type(Unexpected::Str(v), &self))
        }
    
        /// The input contains a string that lives at least as long as the
        /// `Deserializer`.
        ///
        /// This enables zero-copy deserialization of strings in some formats. For
        /// example JSON input containing the JSON string `"borrowed"` can be
        /// deserialized with zero copying into a `&'a str` as long as the input
        /// data outlives `'a`.
        ///
        /// The default implementation forwards to `visit_str`.
        #[inline]
        fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_str(v)
        }
    
        /// The input contains a string and ownership of the string is being given
        /// to the `Visitor`.
        ///
        /// This method allows the `Visitor` to avoid a copy by taking ownership of
        /// a string created by the `Deserializer`. `Deserialize` implementations
        /// that benefit from taking ownership of `String` data should indicate that
        /// to the deserializer by using `Deserializer::deserialize_string` rather
        /// than `Deserializer::deserialize_str`, although not every deserializer
        /// will honor such a request.
        ///
        /// It is never correct to implement `visit_string` without implementing
        /// `visit_str`. Implement neither, both, or just `visit_str`.
        ///
        /// The default implementation forwards to `visit_str` and then drops the
        /// `String`.
        #[inline]
        #[cfg(any(feature = "std", feature = "alloc"))]
        fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_str(&v)
        }
    
        /// The input contains a byte array. The lifetime of the byte array is
        /// ephemeral and it may be destroyed after this method returns.
        ///
        /// This method allows the `Deserializer` to avoid a copy by retaining
        /// ownership of any buffered data. `Deserialize` implementations that do
        /// not benefit from taking ownership of `Vec<u8>` data should indicate that
        /// to the deserializer by using `Deserializer::deserialize_bytes` rather
        /// than `Deserializer::deserialize_byte_buf`.
        ///
        /// It is never correct to implement `visit_byte_buf` without implementing
        /// `visit_bytes`. Implement neither, both, or just `visit_bytes`.
        fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
        where
            E: Error,
        {
            let _ = v;
            Err(Error::invalid_type(Unexpected::Bytes(v), &self))
        }
    
        /// The input contains a byte array that lives at least as long as the
        /// `Deserializer`.
        ///
        /// This enables zero-copy deserialization of bytes in some formats. For
        /// example Bincode data containing bytes can be deserialized with zero
        /// copying into a `&'a [u8]` as long as the input data outlives `'a`.
        ///
        /// The default implementation forwards to `visit_bytes`.
        #[inline]
        fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_bytes(v)
        }
    
        /// The input contains a byte array and ownership of the byte array is being
        /// given to the `Visitor`.
        ///
        /// This method allows the `Visitor` to avoid a copy by taking ownership of
        /// a byte buffer created by the `Deserializer`. `Deserialize`
        /// implementations that benefit from taking ownership of `Vec<u8>` data
        /// should indicate that to the deserializer by using
        /// `Deserializer::deserialize_byte_buf` rather than
        /// `Deserializer::deserialize_bytes`, although not every deserializer will
        /// honor such a request.
        ///
        /// It is never correct to implement `visit_byte_buf` without implementing
        /// `visit_bytes`. Implement neither, both, or just `visit_bytes`.
        ///
        /// The default implementation forwards to `visit_bytes` and then drops the
        /// `Vec<u8>`.
        #[cfg(any(feature = "std", feature = "alloc"))]
        fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
        where
            E: Error,
        {
            self.visit_bytes(&v)
        }
    
        /// The input contains an optional that is absent.
        ///
        /// The default implementation fails with a type error.
        fn visit_none<E>(self) -> Result<Self::Value, E>
        where
            E: Error,
        {
            Err(Error::invalid_type(Unexpected::Option, &self))
        }
    
        /// The input contains an optional that is present.
        ///
        /// The default implementation fails with a type error.
        fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
        where
            D: Deserializer<'de>,
        {
            let _ = deserializer;
            Err(Error::invalid_type(Unexpected::Option, &self))
        }
    
        /// The input contains a unit `()`.
        ///
        /// The default implementation fails with a type error.
        fn visit_unit<E>(self) -> Result<Self::Value, E>
        where
            E: Error,
        {
            Err(Error::invalid_type(Unexpected::Unit, &self))
        }
    
        /// The input contains a newtype struct.
        ///
        /// The content of the newtype struct may be read from the given
        /// `Deserializer`.
        ///
        /// The default implementation fails with a type error.
        fn visit_newtype_struct<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
        where
            D: Deserializer<'de>,
        {
            let _ = deserializer;
            Err(Error::invalid_type(Unexpected::NewtypeStruct, &self))
        }
    
        /// The input contains a sequence of elements.
        ///
        /// The default implementation fails with a type error.
        fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
        where
            A: SeqAccess<'de>,
        {
            let _ = seq;
            Err(Error::invalid_type(Unexpected::Seq, &self))
        }
    
        /// The input contains a key-value map.
        ///
        /// The default implementation fails with a type error.
        fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>
        where
            A: MapAccess<'de>,
        {
            let _ = map;
            Err(Error::invalid_type(Unexpected::Map, &self))
        }
    
        /// The input contains an enum.
        ///
        /// The default implementation fails with a type error.
        fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
        where
            A: EnumAccess<'de>,
        {
            let _ = data;
            Err(Error::invalid_type(Unexpected::Enum, &self))
        }
    }

Visitor 的作用就是一个类型转换器，它定义了各种数据类型转换为 Self::Value 的方法。

## 反序列化过程 ##

Deserialize 负责告诉 Deserializer 我想要一个什么类型的值以及它可以由哪些数据类型转换而来（Visitor），Deserializer 解析特定格式的原数据得到具体的数据以及该数据的类型，然后调用Visitor的相关类型方法将数据转换为Deserialize需要的数据类型。解析出错或者转换出错都会导致反序列化出错。

## 职责分工与解耦 ##

*  serde库可以提供常见数据类型的转换方法，即实现常用数据类型的Visitor。如果没有满足需要的Visitor，那么Deserialize的实现者可以实现自己的Visitor。
 *  某种原数据格式的解析方法由第三方来提供，即由第三方来实现通用的Deserializer，如json、xml等。
 *  由Deserialize的实现者来提供特定数据结构构造方法，也可以由serde库提供常见数据类型的构造方法。

**三者职责不同，却紧密配合，完成反序列化与文档解析的解耦合，让担任不同角色的程序员可以独立工作，不用考虑其它角色的工作，达到最大化的代码共享和复用**

## 附注 ##

/// The input contains a sequence of elements.
        ///
        /// The default implementation fails with a type error.
        fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
        where
            A: SeqAccess<'de>,
        {
            let _ = seq;
            Err(Error::invalid_type(Unexpected::Seq, &self))
        }
    
        /// The input contains a key-value map.
        ///
        /// The default implementation fails with a type error.
        fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>
        where
            A: MapAccess<'de>,
        {
            let _ = map;
            Err(Error::invalid_type(Unexpected::Map, &self))
        }
    
        /// The input contains an enum.
        ///
        /// The default implementation fails with a type error.
        fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
        where
            A: EnumAccess<'de>,
        {
            let _ = data;
            Err(Error::invalid_type(Unexpected::Enum, &self))
        }

Visitor中有三个特殊的方法，使用到了三个 Trait：SeqAccess、MapAccess、EnumAccess，为什么需要这三个 Trait？ 有些 Self::Value 可以从一些复杂的数据结构转换而来，比如列表、映射表或枚举类型，而这些数据是由 Deserializer 创建的，Visitor 不知道如何来访问其中的数值，所以需要 Deserializer 的实现者提供访问的方法，这三个 Trait 由 Deserializer 的实现者来实现，并在需要的时候提供给 Visitor 使用。

转载于:https://my.oschina.net/zengsai/blog/1595708

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20210726/70f76953fb944465bc4cb7f074227433.png