# mod.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.125/src/de/mod.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Generic data structure deserialization framework.
    //!
    //! The two most important traits in this module are [`Deserialize`] and
    //! [`Deserializer`].
    //!
    //!  - **A type that implements `Deserialize` is a data structure** that can be
    //!    deserialized from any data format supported by Serde, and conversely
    //!  - **A type that implements `Deserializer` is a data format** that can
    //!    deserialize any data structure supported by Serde.
    //!
    //! # The Deserialize trait
    //!
    //! Serde provides [`Deserialize`] implementations for many Rust primitive and
    //! standard library types. The complete list is below. All of these can be
    //! deserialized using Serde out of the box.
    //!
    //! Additionally, Serde provides a procedural macro called [`serde_derive`] to
    //! automatically generate [`Deserialize`] implementations for structs and enums
    //! in your program. See the [derive section of the manual] for how to use this.
    //!
    //! In rare cases it may be necessary to implement [`Deserialize`] manually for
    //! some type in your program. See the [Implementing `Deserialize`] section of
    //! the manual for more about this.
    //!
    //! Third-party crates may provide [`Deserialize`] implementations for types
    //! that they expose. For example the [`linked-hash-map`] crate provides a
    //! [`LinkedHashMap<K, V>`] type that is deserializable by Serde because the
    //! crate provides an implementation of [`Deserialize`] for it.
    //!
    //! # The Deserializer trait
    //!
    //! [`Deserializer`] implementations are provided by third-party crates, for
    //! example [`serde_json`], [`serde_yaml`] and [`bincode`].
    //!
    //! A partial list of well-maintained formats is given on the [Serde
    //! website][data formats].
    //!
    //! # Implementations of Deserialize provided by Serde
    //!
    //! This is a slightly different set of types than what is supported for
    //! serialization. Some types can be serialized by Serde but not deserialized.
    //! One example is `OsStr`.
    //!
    //!  - **Primitive types**:
    //!    - bool
    //!    - i8, i16, i32, i64, i128, isize
    //!    - u8, u16, u32, u64, u128, usize
    //!    - f32, f64
    //!    - char
    //!  - **Compound types**:
    //!    - \[T; 0\] through \[T; 32\]
    //!    - tuples up to size 16
    //!  - **Common standard library types**:
    //!    - String
    //!    - Option\<T\>
    //!    - Result\<T, E\>
    //!    - PhantomData\<T\>
    //!  - **Wrapper types**:
    //!    - Box\<T\>
    //!    - Box\<\[T\]\>
    //!    - Box\<str\>
    //!    - Cow\<'a, T\>
    //!    - Cell\<T\>
    //!    - RefCell\<T\>
    //!    - Mutex\<T\>
    //!    - RwLock\<T\>
    //!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
    //!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
    //!  - **Collection types**:
    //!    - BTreeMap\<K, V\>
    //!    - BTreeSet\<T\>
    //!    - BinaryHeap\<T\>
    //!    - HashMap\<K, V, H\>
    //!    - HashSet\<T, H\>
    //!    - LinkedList\<T\>
    //!    - VecDeque\<T\>
    //!    - Vec\<T\>
    //!  - **Zero-copy types**:
    //!    - &str
    //!    - &\[u8\]
    //!  - **FFI types**:
    //!    - CString
    //!    - Box\<CStr\>
    //!    - OsString
    //!  - **Miscellaneous standard library types**:
    //!    - Duration
    //!    - SystemTime
    //!    - Path
    //!    - PathBuf
    //!    - Range\<T\>
    //!    - RangeInclusive\<T\>
    //!    - Bound\<T\>
    //!    - num::NonZero*
    //!    - `!` *(unstable)*
    //!  - **Net types**:
    //!    - IpAddr
    //!    - Ipv4Addr
    //!    - Ipv6Addr
    //!    - SocketAddr
    //!    - SocketAddrV4
    //!    - SocketAddrV6
    //!
    //! [Implementing `Deserialize`]: https://serde.rs/impl-deserialize.html
    //! [`Deserialize`]: ../trait.Deserialize.html
    //! [`Deserializer`]: ../trait.Deserializer.html
    //! [`LinkedHashMap<K, V>`]: https://docs.rs/linked-hash-map/*/linked_hash_map/struct.LinkedHashMap.html
    //! [`bincode`]: https://github.com/servo/bincode
    //! [`linked-hash-map`]: https://crates.io/crates/linked-hash-map
    //! [`serde_derive`]: https://crates.io/crates/serde_derive
    //! [`serde_json`]: https://github.com/serde-rs/json
    //! [`serde_yaml`]: https://github.com/dtolnay/serde-yaml
    //! [derive section of the manual]: https://serde.rs/derive.html
    //! [data formats]: https://serde.rs/#data-formats

    use lib::*;

    ////////////////////////////////////////////////////////////////////////////////

    pub mod value;

    mod ignored_any;
    mod impls;
    mod utf8;

    pub use self::ignored_any::IgnoredAny;

    #[cfg(feature = "std")]
    #[doc(no_inline)]
    pub use std::error::Error as StdError;
    #[cfg(not(feature = "std"))]
    #[doc(no_inline)]
    pub use std_error::Error as StdError;

    ////////////////////////////////////////////////////////////////////////////////

    macro_rules! declare_error_trait {
        (Error: Sized $(+ $($supertrait:ident)::+)*) => {
            /// The `Error` trait allows `Deserialize` implementations to create descriptive
            /// error messages belonging to the `Deserializer` against which they are
            /// currently running.
            ///
            /// Every `Deserializer` declares an `Error` type that encompasses both
            /// general-purpose deserialization errors as well as errors specific to the
            /// particular deserialization format. For example the `Error` type of
            /// `serde_json` can represent errors like an invalid JSON escape sequence or an
            /// unterminated string literal, in addition to the error cases that are part of
            /// this trait.
            ///
            /// Most deserializers should only need to provide the `Error::custom` method
            /// and inherit the default behavior for the other methods.
            ///
            /// # Example implementation
            ///
            /// The [example data format] presented on the website shows an error
            /// type appropriate for a basic JSON data format.
            ///
            /// [example data format]: https://serde.rs/data-format.html
            pub trait Error: Sized $(+ $($supertrait)::+)* {
                /// Raised when there is general error when deserializing a type.
                ///
                /// The message should not be capitalized and should not end with a period.
                ///
                /// ```edition2018
                /// # use std::str::FromStr;
                /// #
                /// # struct IpAddr;
                /// #
                /// # impl FromStr for IpAddr {
                /// #     type Err = String;
                /// #
                /// #     fn from_str(_: &str) -> Result<Self, String> {
                /// #         unimplemented!()
                /// #     }
                /// # }
                /// #
                /// use serde::de::{self, Deserialize, Deserializer};
                ///
                /// impl<'de> Deserialize<'de> for IpAddr {
                ///     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
                ///     where
                ///         D: Deserializer<'de>,
                ///     {
                ///         let s = String::deserialize(deserializer)?;
                ///         s.parse().map_err(de::Error::custom)
                ///     }
                /// }
                /// ```
                fn custom<T>(msg: T) -> Self
                where
                    T: Display;

                /// Raised when a `Deserialize` receives a type different from what it was
                /// expecting.
                ///
                /// The `unexp` argument provides information about what type was received.
                /// This is the type that was present in the input file or other source data
                /// of the Deserializer.
                ///
                /// The `exp` argument provides information about what type was being
                /// expected. This is the type that is written in the program.
                ///
                /// For example if we try to deserialize a String out of a JSON file
                /// containing an integer, the unexpected type is the integer and the
                /// expected type is the string.
                #[cold]
                fn invalid_type(unexp: Unexpected, exp: &Expected) -> Self {
                    Error::custom(format_args!("invalid type: {}, expected {}", unexp, exp))
                }

                /// Raised when a `Deserialize` receives a value of the right type but that
                /// is wrong for some other reason.
                ///
                /// The `unexp` argument provides information about what value was received.
                /// This is the value that was present in the input file or other source
                /// data of the Deserializer.
                ///
                /// The `exp` argument provides information about what value was being
                /// expected. This is the type that is written in the program.
                ///
                /// For example if we try to deserialize a String out of some binary data
                /// that is not valid UTF-8, the unexpected value is the bytes and the
                /// expected value is a string.
                #[cold]
                fn invalid_value(unexp: Unexpected, exp: &Expected) -> Self {
                    Error::custom(format_args!("invalid value: {}, expected {}", unexp, exp))
                }

                /// Raised when deserializing a sequence or map and the input data contains
                /// too many or too few elements.
                ///
                /// The `len` argument is the number of elements encountered. The sequence
                /// or map may have expected more arguments or fewer arguments.
                ///
                /// The `exp` argument provides information about what data was being
                /// expected. For example `exp` might say that a tuple of size 6 was
                /// expected.
                #[cold]
                fn invalid_length(len: usize, exp: &Expected) -> Self {
                    Error::custom(format_args!("invalid length {}, expected {}", len, exp))
                }

                /// Raised when a `Deserialize` enum type received a variant with an
                /// unrecognized name.
                #[cold]
                fn unknown_variant(variant: &str, expected: &'static [&'static str]) -> Self {
                    if expected.is_empty() {
                        Error::custom(format_args!(
                            "unknown variant `{}`, there are no variants",
                            variant
                        ))
                    } else {
                        Error::custom(format_args!(
                            "unknown variant `{}`, expected {}",
                            variant,
                            OneOf { names: expected }
                        ))
                    }
                }

                /// Raised when a `Deserialize` struct type received a field with an
                /// unrecognized name.
                #[cold]
                fn unknown_field(field: &str, expected: &'static [&'static str]) -> Self {
                    if expected.is_empty() {
                        Error::custom(format_args!(
                            "unknown field `{}`, there are no fields",
                            field
                        ))
                    } else {
                        Error::custom(format_args!(
                            "unknown field `{}`, expected {}",
                            field,
                            OneOf { names: expected }
                        ))
                    }
                }

                /// Raised when a `Deserialize` struct type expected to receive a required
                /// field with a particular name but that field was not present in the
                /// input.
                #[cold]
                fn missing_field(field: &'static str) -> Self {
                    Error::custom(format_args!("missing field `{}`", field))
                }

                /// Raised when a `Deserialize` struct type received more than one of the
                /// same field.
                #[cold]
                fn duplicate_field(field: &'static str) -> Self {
                    Error::custom(format_args!("duplicate field `{}`", field))
                }
            }
        }
    }

    #[cfg(feature = "std")]
    declare_error_trait!(Error: Sized + StdError);

    #[cfg(not(feature = "std"))]
    declare_error_trait!(Error: Sized + Debug + Display);

    /// `Unexpected` represents an unexpected invocation of any one of the `Visitor`
    /// trait methods.
    ///
    /// This is used as an argument to the `invalid_type`, `invalid_value`, and
    /// `invalid_length` methods of the `Error` trait to build error messages.
    ///
    /// ```edition2018
    /// # use std::fmt;
    /// #
    /// # use serde::de::{self, Unexpected, Visitor};
    /// #
    /// # struct Example;
    /// #
    /// # impl<'de> Visitor<'de> for Example {
    /// #     type Value = ();
    /// #
    /// #     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    /// #         write!(formatter, "definitely not a boolean")
    /// #     }
    /// #
    /// fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
    /// where
    ///     E: de::Error,
    /// {
    ///     Err(de::Error::invalid_type(Unexpected::Bool(v), &self))
    /// }
    /// # }
    /// ```
    #[derive(Copy, Clone, PartialEq, Debug)]
    pub enum Unexpected<'a> {
        /// The input contained a boolean value that was not expected.
        Bool(bool),

        /// The input contained an unsigned integer `u8`, `u16`, `u32` or `u64` that
        /// was not expected.
        Unsigned(u64),

        /// The input contained a signed integer `i8`, `i16`, `i32` or `i64` that
        /// was not expected.
        Signed(i64),

        /// The input contained a floating point `f32` or `f64` that was not
        /// expected.
        Float(f64),

        /// The input contained a `char` that was not expected.
        Char(char),

        /// The input contained a `&str` or `String` that was not expected.
        Str(&'a str),

        /// The input contained a `&[u8]` or `Vec<u8>` that was not expected.
        Bytes(&'a [u8]),

        /// The input contained a unit `()` that was not expected.
        Unit,

        /// The input contained an `Option<T>` that was not expected.
        Option,

        /// The input contained a newtype struct that was not expected.
        NewtypeStruct,

        /// The input contained a sequence that was not expected.
        Seq,

        /// The input contained a map that was not expected.
        Map,

        /// The input contained an enum that was not expected.
        Enum,

        /// The input contained a unit variant that was not expected.
        UnitVariant,

        /// The input contained a newtype variant that was not expected.
        NewtypeVariant,

        /// The input contained a tuple variant that was not expected.
        TupleVariant,

        /// The input contained a struct variant that was not expected.
        StructVariant,

        /// A message stating what uncategorized thing the input contained that was
        /// not expected.
        ///
        /// The message should be a noun or noun phrase, not capitalized and without
        /// a period. An example message is "unoriginal superhero".
        Other(&'a str),
    }

    impl<'a> fmt::Display for Unexpected<'a> {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            use self::Unexpected::*;
            match *self {
                Bool(b) => write!(formatter, "boolean `{}`", b),
                Unsigned(i) => write!(formatter, "integer `{}`", i),
                Signed(i) => write!(formatter, "integer `{}`", i),
                Float(f) => write!(formatter, "floating point `{}`", f),
                Char(c) => write!(formatter, "character `{}`", c),
                Str(s) => write!(formatter, "string {:?}", s),
                Bytes(_) => write!(formatter, "byte array"),
                Unit => write!(formatter, "unit value"),
                Option => write!(formatter, "Option value"),
                NewtypeStruct => write!(formatter, "newtype struct"),
                Seq => write!(formatter, "sequence"),
                Map => write!(formatter, "map"),
                Enum => write!(formatter, "enum"),
                UnitVariant => write!(formatter, "unit variant"),
                NewtypeVariant => write!(formatter, "newtype variant"),
                TupleVariant => write!(formatter, "tuple variant"),
                StructVariant => write!(formatter, "struct variant"),
                Other(other) => formatter.write_str(other),
            }
        }
    }

    /// `Expected` represents an explanation of what data a `Visitor` was expecting
    /// to receive.
    ///
    /// This is used as an argument to the `invalid_type`, `invalid_value`, and
    /// `invalid_length` methods of the `Error` trait to build error messages. The
    /// message should be a noun or noun phrase that completes 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.
    ///
    /// Within the context of a `Visitor` implementation, the `Visitor` itself
    /// (`&self`) is an implementation of this trait.
    ///
    /// ```edition2018
    /// # use std::fmt;
    /// #
    /// # use serde::de::{self, Unexpected, Visitor};
    /// #
    /// # struct Example;
    /// #
    /// # impl<'de> Visitor<'de> for Example {
    /// #     type Value = ();
    /// #
    /// #     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    /// #         write!(formatter, "definitely not a boolean")
    /// #     }
    /// #
    /// fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
    /// where
    ///     E: de::Error,
    /// {
    ///     Err(de::Error::invalid_type(Unexpected::Bool(v), &self))
    /// }
    /// # }
    /// ```
    ///
    /// Outside of a `Visitor`, `&"..."` can be used.
    ///
    /// ```edition2018
    /// # use serde::de::{self, Unexpected};
    /// #
    /// # fn example<E>() -> Result<(), E>
    /// # where
    /// #     E: de::Error,
    /// # {
    /// #     let v = true;
    /// return Err(de::Error::invalid_type(Unexpected::Bool(v), &"a negative integer"));
    /// # }
    /// ```
    pub trait Expected {
        /// Format an explanation of what data was being expected. Same signature as
        /// the `Display` and `Debug` traits.
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result;
    }

    impl<'de, T> Expected for T
    where
        T: Visitor<'de>,
    {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.expecting(formatter)
        }
    }

    impl<'a> Expected for &'a str {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.write_str(self)
        }
    }

    impl<'a> Display for Expected + 'a {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            Expected::fmt(self, formatter)
        }
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// A **data structure** that can be deserialized from any data format supported
    /// by Serde.
    ///
    /// Serde provides `Deserialize` implementations for many Rust primitive and
    /// standard library types. The complete list is [here][de]. All of these can
    /// be deserialized using Serde out of the box.
    ///
    /// Additionally, Serde provides a procedural macro called `serde_derive` to
    /// automatically generate `Deserialize` implementations for structs and enums
    /// in your program. See the [derive section of the manual][derive] for how to
    /// use this.
    ///
    /// In rare cases it may be necessary to implement `Deserialize` manually for
    /// some type in your program. See the [Implementing
    /// `Deserialize`][impl-deserialize] section of the manual for more about this.
    ///
    /// Third-party crates may provide `Deserialize` implementations for types that
    /// they expose. For example the `linked-hash-map` crate provides a
    /// `LinkedHashMap<K, V>` type that is deserializable by Serde because the crate
    /// provides an implementation of `Deserialize` for it.
    ///
    /// [de]: https://docs.serde.rs/serde/de/index.html
    /// [derive]: https://serde.rs/derive.html
    /// [impl-deserialize]: https://serde.rs/impl-deserialize.html
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the lifetime of data that may be
    /// borrowed by `Self` when deserialized. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    pub trait Deserialize<'de>: Sized {
        /// Deserialize this value from the given Serde deserializer.
        ///
        /// See the [Implementing `Deserialize`][impl-deserialize] section of the
        /// manual for more information about how to implement this method.
        ///
        /// [impl-deserialize]: https://serde.rs/impl-deserialize.html
        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where
            D: Deserializer<'de>;

        /// Deserializes a value into `self` from the given Deserializer.
        ///
        /// The purpose of this method is to allow the deserializer to reuse
        /// resources and avoid copies. As such, if this method returns an error,
        /// `self` will be in an indeterminate state where some parts of the struct
        /// have been overwritten. Although whatever state that is will be
        /// memory-safe.
        ///
        /// This is generally useful when repeatedly deserializing values that
        /// are processed one at a time, where the value of `self` doesn't matter
        /// when the next deserialization occurs.
        ///
        /// If you manually implement this, your recursive deserializations should
        /// use `deserialize_in_place`.
        ///
        /// This method is stable and an official public API, but hidden from the
        /// documentation because it is almost never what newbies are looking for.
        /// Showing it in rustdoc would cause it to be featured more prominently
        /// than it deserves.
        #[doc(hidden)]
        fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
        where
            D: Deserializer<'de>,
        {
            // Default implementation just delegates to `deserialize` impl.
            *place = Deserialize::deserialize(deserializer)?;
            Ok(())
        }
    }

    /// A data structure that can be deserialized without borrowing any data from
    /// the deserializer.
    ///
    /// This is primarily useful for trait bounds on functions. For example a
    /// `from_str` function may be able to deserialize a data structure that borrows
    /// from the input string, but a `from_reader` function may only deserialize
    /// owned data.
    ///
    /// ```edition2018
    /// # use serde::de::{Deserialize, DeserializeOwned};
    /// # use std::io::{Read, Result};
    /// #
    /// # trait Ignore {
    /// fn from_str<'a, T>(s: &'a str) -> Result<T>
    /// where
    ///     T: Deserialize<'a>;
    ///
    /// fn from_reader<R, T>(rdr: R) -> Result<T>
    /// where
    ///     R: Read,
    ///     T: DeserializeOwned;
    /// # }
    /// ```
    ///
    /// # Lifetime
    ///
    /// The relationship between `Deserialize` and `DeserializeOwned` in trait
    /// bounds is explained in more detail on the page [Understanding deserializer
    /// lifetimes].
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    pub trait DeserializeOwned: for<'de> Deserialize<'de> {}
    impl<T> DeserializeOwned for T where T: for<'de> Deserialize<'de> {}

    /// `DeserializeSeed` is the stateful form of the `Deserialize` trait. If you
    /// ever find yourself looking for a way to pass data into a `Deserialize` impl,
    /// this trait is the way to do it.
    ///
    /// As one example of stateful deserialization consider deserializing a JSON
    /// array into an existing buffer. Using the `Deserialize` trait we could
    /// deserialize a JSON array into a `Vec<T>` but it would be a freshly allocated
    /// `Vec<T>`; there is no way for `Deserialize` to reuse a previously allocated
    /// buffer. Using `DeserializeSeed` instead makes this possible as in the
    /// example code below.
    ///
    /// The canonical API for stateless deserialization looks like this:
    ///
    /// ```edition2018
    /// # use serde::Deserialize;
    /// #
    /// # enum Error {}
    /// #
    /// fn func<'de, T: Deserialize<'de>>() -> Result<T, Error>
    /// # {
    /// #     unimplemented!()
    /// # }
    /// ```
    ///
    /// Adjusting an API like this to support stateful deserialization is a matter
    /// of accepting a seed as input:
    ///
    /// ```edition2018
    /// # use serde::de::DeserializeSeed;
    /// #
    /// # enum Error {}
    /// #
    /// fn func_seed<'de, T: DeserializeSeed<'de>>(seed: T) -> Result<T::Value, Error>
    /// # {
    /// #     let _ = seed;
    /// #     unimplemented!()
    /// # }
    /// ```
    ///
    /// In practice the majority of deserialization is stateless. An API expecting a
    /// seed can be appeased by passing `std::marker::PhantomData` as a seed in the
    /// case of stateless deserialization.
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the lifetime of data that may be
    /// borrowed by `Self::Value` when deserialized. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example
    ///
    /// Suppose we have JSON that looks like `[[1, 2], [3, 4, 5], [6]]` and we need
    /// to deserialize it into a flat representation like `vec![1, 2, 3, 4, 5, 6]`.
    /// Allocating a brand new `Vec<T>` for each subarray would be slow. Instead we
    /// would like to allocate a single `Vec<T>` and then deserialize each subarray
    /// into it. This requires stateful deserialization using the `DeserializeSeed`
    /// trait.
    ///
    /// ```edition2018
    /// use std::fmt;
    /// use std::marker::PhantomData;
    ///
    /// use serde::de::{Deserialize, DeserializeSeed, Deserializer, SeqAccess, Visitor};
    ///
    /// // A DeserializeSeed implementation that uses stateful deserialization to
    /// // append array elements onto the end of an existing vector. The preexisting
    /// // state ("seed") in this case is the Vec<T>. The `deserialize` method of
    /// // `ExtendVec` will be traversing the inner arrays of the JSON input and
    /// // appending each integer into the existing Vec.
    /// struct ExtendVec<'a, T: 'a>(&'a mut Vec<T>);
    ///
    /// impl<'de, 'a, T> DeserializeSeed<'de> for ExtendVec<'a, T>
    /// where
    ///     T: Deserialize<'de>,
    /// {
    ///     // The return type of the `deserialize` method. This implementation
    ///     // appends onto an existing vector but does not create any new data
    ///     // structure, so the return type is ().
    ///     type Value = ();
    ///
    ///     fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
    ///     where
    ///         D: Deserializer<'de>,
    ///     {
    ///         // Visitor implementation that will walk an inner array of the JSON
    ///         // input.
    ///         struct ExtendVecVisitor<'a, T: 'a>(&'a mut Vec<T>);
    ///
    ///         impl<'de, 'a, T> Visitor<'de> for ExtendVecVisitor<'a, T>
    ///         where
    ///             T: Deserialize<'de>,
    ///         {
    ///             type Value = ();
    ///
    ///             fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    ///                 write!(formatter, "an array of integers")
    ///             }
    ///
    ///             fn visit_seq<A>(self, mut seq: A) -> Result<(), A::Error>
    ///             where
    ///                 A: SeqAccess<'de>,
    ///             {
    ///                 // Visit each element in the inner array and push it onto
    ///                 // the existing vector.
    ///                 while let Some(elem) = seq.next_element()? {
    ///                     self.0.push(elem);
    ///                 }
    ///                 Ok(())
    ///             }
    ///         }
    ///
    ///         deserializer.deserialize_seq(ExtendVecVisitor(self.0))
    ///     }
    /// }
    ///
    /// // Visitor implementation that will walk the outer array of the JSON input.
    /// struct FlattenedVecVisitor<T>(PhantomData<T>);
    ///
    /// impl<'de, T> Visitor<'de> for FlattenedVecVisitor<T>
    /// where
    ///     T: Deserialize<'de>,
    /// {
    ///     // This Visitor constructs a single Vec<T> to hold the flattened
    ///     // contents of the inner arrays.
    ///     type Value = Vec<T>;
    ///
    ///     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(formatter, "an array of arrays")
    ///     }
    ///
    ///     fn visit_seq<A>(self, mut seq: A) -> Result<Vec<T>, A::Error>
    ///     where
    ///         A: SeqAccess<'de>,
    ///     {
    ///         // Create a single Vec to hold the flattened contents.
    ///         let mut vec = Vec::new();
    ///
    ///         // Each iteration through this loop is one inner array.
    ///         while let Some(()) = seq.next_element_seed(ExtendVec(&mut vec))? {
    ///             // Nothing to do; inner array has been appended into `vec`.
    ///         }
    ///
    ///         // Return the finished vec.
    ///         Ok(vec)
    ///     }
    /// }
    ///
    /// # fn example<'de, D>(deserializer: D) -> Result<(), D::Error>
    /// # where
    /// #     D: Deserializer<'de>,
    /// # {
    /// let visitor = FlattenedVecVisitor(PhantomData);
    /// let flattened: Vec<u64> = deserializer.deserialize_seq(visitor)?;
    /// #     Ok(())
    /// # }
    /// ```
    pub trait DeserializeSeed<'de>: Sized {
        /// The type produced by using this seed.
        type Value;

        /// Equivalent to the more common `Deserialize::deserialize` method, except
        /// with some initial piece of data (the seed) passed in.
        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
        where
            D: Deserializer<'de>;
    }

    impl<'de, T> DeserializeSeed<'de> for PhantomData<T>
    where
        T: Deserialize<'de>,
    {
        type Value = T;

        #[inline]
        fn deserialize<D>(self, deserializer: D) -> Result<T, D::Error>
        where
            D: Deserializer<'de>,
        {
            T::deserialize(deserializer)
        }
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// 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.
    ///
    /// [Serde data model]: https://serde.rs/data-model.html
    ///
    /// # 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.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example implementation
    ///
    /// The [example data format] presented on the website contains example code for
    /// a basic JSON `Deserializer`.
    ///
    /// [example data format]: https://serde.rs/data-format.html
    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>;

        serde_if_integer128! {
            /// 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.
            fn deserialize_i128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
            where
                V: Visitor<'de>
            {
                let _ = visitor;
                Err(Error::custom("i128 is not supported"))
            }
        }

        /// 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>;

        serde_if_integer128! {
            /// 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.
            fn deserialize_u128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
            where
                V: Visitor<'de>
            {
                let _ = visitor;
                Err(Error::custom("u128 is not supported"))
            }
        }

        /// 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.
        ///
        /// ```edition2018
        /// # 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
        }
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// This trait represents a visitor that walks through a deserializer.
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the requirement for lifetime of data
    /// that may be borrowed by `Self::Value`. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example
    ///
    /// ```edition2018
    /// # use std::fmt;
    /// #
    /// # use serde::de::{self, Unexpected, Visitor};
    /// #
    /// /// A visitor that deserializes a long string - a string containing at least
    /// /// some minimum number of bytes.
    /// struct LongString {
    ///     min: usize,
    /// }
    ///
    /// impl<'de> Visitor<'de> for LongString {
    ///     type Value = String;
    ///
    ///     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(formatter, "a string containing at least {} bytes", self.min)
    ///     }
    ///
    ///     fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
    ///     where
    ///         E: de::Error,
    ///     {
    ///         if s.len() >= self.min {
    ///             Ok(s.to_owned())
    ///         } else {
    ///             Err(de::Error::invalid_value(Unexpected::Str(s), &self))
    ///         }
    ///     }
    /// }
    /// ```
    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.
        ///
        /// ```edition2018
        /// # 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))
        }

        serde_if_integer128! {
            /// The input contains a `i128`.
            ///
            /// This method is available only on Rust compiler versions >=1.26. The
            /// default implementation fails with a type error.
            fn visit_i128<E>(self, v: i128) -> Result<Self::Value, E>
            where
                E: Error,
            {
                let _ = v;
                Err(Error::invalid_type(Unexpected::Other("i128"), &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))
        }

        serde_if_integer128! {
            /// The input contains a `u128`.
            ///
            /// This method is available only on Rust compiler versions >=1.26. The
            /// default implementation fails with a type error.
            fn visit_u128<E>(self, v: u128) -> Result<Self::Value, E>
            where
                E: Error,
            {
                let _ = v;
                Err(Error::invalid_type(Unexpected::Other("u128"), &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))
        }

        // Used when deserializing a flattened Option field. Not public API.
        #[doc(hidden)]
        fn __private_visit_untagged_option<D>(self, _: D) -> Result<Self::Value, ()>
        where
            D: Deserializer<'de>,
        {
            Err(())
        }
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// Provides a `Visitor` access to each element of a sequence in the input.
    ///
    /// This is a trait that a `Deserializer` passes to a `Visitor` implementation,
    /// which deserializes each item in a sequence.
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the lifetime of data that may be
    /// borrowed by deserialized sequence elements. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example implementation
    ///
    /// The [example data format] presented on the website demonstrates an
    /// implementation of `SeqAccess` for a basic JSON data format.
    ///
    /// [example data format]: https://serde.rs/data-format.html
    pub trait SeqAccess<'de> {
        /// The error type that can be returned if some error occurs during
        /// deserialization.
        type Error: Error;

        /// This returns `Ok(Some(value))` for the next value in the sequence, or
        /// `Ok(None)` if there are no more remaining items.
        ///
        /// `Deserialize` implementations should typically use
        /// `SeqAccess::next_element` instead.
        fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
        where
            T: DeserializeSeed<'de>;

        /// This returns `Ok(Some(value))` for the next value in the sequence, or
        /// `Ok(None)` if there are no more remaining items.
        ///
        /// This method exists as a convenience for `Deserialize` implementations.
        /// `SeqAccess` implementations should not override the default behavior.
        #[inline]
        fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
        where
            T: Deserialize<'de>,
        {
            self.next_element_seed(PhantomData)
        }

        /// Returns the number of elements remaining in the sequence, if known.
        #[inline]
        fn size_hint(&self) -> Option<usize> {
            None
        }
    }

    impl<'de, 'a, A> SeqAccess<'de> for &'a mut A
    where
        A: SeqAccess<'de>,
    {
        type Error = A::Error;

        #[inline]
        fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
        where
            T: DeserializeSeed<'de>,
        {
            (**self).next_element_seed(seed)
        }

        #[inline]
        fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
        where
            T: Deserialize<'de>,
        {
            (**self).next_element()
        }

        #[inline]
        fn size_hint(&self) -> Option<usize> {
            (**self).size_hint()
        }
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// Provides a `Visitor` access to each entry of a map in the input.
    ///
    /// This is a trait that a `Deserializer` passes to a `Visitor` implementation.
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the lifetime of data that may be
    /// borrowed by deserialized map entries. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example implementation
    ///
    /// The [example data format] presented on the website demonstrates an
    /// implementation of `MapAccess` for a basic JSON data format.
    ///
    /// [example data format]: https://serde.rs/data-format.html
    pub trait MapAccess<'de> {
        /// The error type that can be returned if some error occurs during
        /// deserialization.
        type Error: Error;

        /// This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)`
        /// if there are no more remaining entries.
        ///
        /// `Deserialize` implementations should typically use
        /// `MapAccess::next_key` or `MapAccess::next_entry` instead.
        fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
        where
            K: DeserializeSeed<'de>;

        /// This returns a `Ok(value)` for the next value in the map.
        ///
        /// `Deserialize` implementations should typically use
        /// `MapAccess::next_value` instead.
        ///
        /// # Panics
        ///
        /// Calling `next_value_seed` before `next_key_seed` is incorrect and is
        /// allowed to panic or return bogus results.
        fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
        where
            V: DeserializeSeed<'de>;

        /// This returns `Ok(Some((key, value)))` for the next (key-value) pair in
        /// the map, or `Ok(None)` if there are no more remaining items.
        ///
        /// `MapAccess` implementations should override the default behavior if a
        /// more efficient implementation is possible.
        ///
        /// `Deserialize` implementations should typically use
        /// `MapAccess::next_entry` instead.
        #[inline]
        fn next_entry_seed<K, V>(
            &mut self,
            kseed: K,
            vseed: V,
        ) -> Result<Option<(K::Value, V::Value)>, Self::Error>
        where
            K: DeserializeSeed<'de>,
            V: DeserializeSeed<'de>,
        {
            match try!(self.next_key_seed(kseed)) {
                Some(key) => {
                    let value = try!(self.next_value_seed(vseed));
                    Ok(Some((key, value)))
                }
                None => Ok(None),
            }
        }

        /// This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)`
        /// if there are no more remaining entries.
        ///
        /// This method exists as a convenience for `Deserialize` implementations.
        /// `MapAccess` implementations should not override the default behavior.
        #[inline]
        fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error>
        where
            K: Deserialize<'de>,
        {
            self.next_key_seed(PhantomData)
        }

        /// This returns a `Ok(value)` for the next value in the map.
        ///
        /// This method exists as a convenience for `Deserialize` implementations.
        /// `MapAccess` implementations should not override the default behavior.
        ///
        /// # Panics
        ///
        /// Calling `next_value` before `next_key` is incorrect and is allowed to
        /// panic or return bogus results.
        #[inline]
        fn next_value<V>(&mut self) -> Result<V, Self::Error>
        where
            V: Deserialize<'de>,
        {
            self.next_value_seed(PhantomData)
        }

        /// This returns `Ok(Some((key, value)))` for the next (key-value) pair in
        /// the map, or `Ok(None)` if there are no more remaining items.
        ///
        /// This method exists as a convenience for `Deserialize` implementations.
        /// `MapAccess` implementations should not override the default behavior.
        #[inline]
        fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error>
        where
            K: Deserialize<'de>,
            V: Deserialize<'de>,
        {
            self.next_entry_seed(PhantomData, PhantomData)
        }

        /// Returns the number of entries remaining in the map, if known.
        #[inline]
        fn size_hint(&self) -> Option<usize> {
            None
        }
    }

    impl<'de, 'a, A> MapAccess<'de> for &'a mut A
    where
        A: MapAccess<'de>,
    {
        type Error = A::Error;

        #[inline]
        fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
        where
            K: DeserializeSeed<'de>,
        {
            (**self).next_key_seed(seed)
        }

        #[inline]
        fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
        where
            V: DeserializeSeed<'de>,
        {
            (**self).next_value_seed(seed)
        }

        #[inline]
        fn next_entry_seed<K, V>(
            &mut self,
            kseed: K,
            vseed: V,
        ) -> Result<Option<(K::Value, V::Value)>, Self::Error>
        where
            K: DeserializeSeed<'de>,
            V: DeserializeSeed<'de>,
        {
            (**self).next_entry_seed(kseed, vseed)
        }

        #[inline]
        fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error>
        where
            K: Deserialize<'de>,
            V: Deserialize<'de>,
        {
            (**self).next_entry()
        }

        #[inline]
        fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error>
        where
            K: Deserialize<'de>,
        {
            (**self).next_key()
        }

        #[inline]
        fn next_value<V>(&mut self) -> Result<V, Self::Error>
        where
            V: Deserialize<'de>,
        {
            (**self).next_value()
        }

        #[inline]
        fn size_hint(&self) -> Option<usize> {
            (**self).size_hint()
        }
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// Provides a `Visitor` access to the data of an enum in the input.
    ///
    /// `EnumAccess` is created by the `Deserializer` and passed to the
    /// `Visitor` in order to identify which variant of an enum to deserialize.
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the lifetime of data that may be
    /// borrowed by the deserialized enum variant. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example implementation
    ///
    /// The [example data format] presented on the website demonstrates an
    /// implementation of `EnumAccess` for a basic JSON data format.
    ///
    /// [example data format]: https://serde.rs/data-format.html
    pub trait EnumAccess<'de>: Sized {
        /// The error type that can be returned if some error occurs during
        /// deserialization.
        type Error: Error;
        /// The `Visitor` that will be used to deserialize the content of the enum
        /// variant.
        type Variant: VariantAccess<'de, Error = Self::Error>;

        /// `variant` is called to identify which variant to deserialize.
        ///
        /// `Deserialize` implementations should typically use `EnumAccess::variant`
        /// instead.
        fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self::Variant), Self::Error>
        where
            V: DeserializeSeed<'de>;

        /// `variant` is called to identify which variant to deserialize.
        ///
        /// This method exists as a convenience for `Deserialize` implementations.
        /// `EnumAccess` implementations should not override the default behavior.
        #[inline]
        fn variant<V>(self) -> Result<(V, Self::Variant), Self::Error>
        where
            V: Deserialize<'de>,
        {
            self.variant_seed(PhantomData)
        }
    }

    /// `VariantAccess` is a visitor that is created by the `Deserializer` and
    /// passed to the `Deserialize` to deserialize the content of a particular enum
    /// variant.
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the lifetime of data that may be
    /// borrowed by the deserialized enum variant. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example implementation
    ///
    /// The [example data format] presented on the website demonstrates an
    /// implementation of `VariantAccess` for a basic JSON data format.
    ///
    /// [example data format]: https://serde.rs/data-format.html
    pub trait VariantAccess<'de>: Sized {
        /// The error type that can be returned if some error occurs during
        /// deserialization. Must match the error type of our `EnumAccess`.
        type Error: Error;

        /// Called when deserializing a variant with no values.
        ///
        /// If the data contains a different type of variant, the following
        /// `invalid_type` error should be constructed:
        ///
        /// ```edition2018
        /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
        /// #
        /// # struct X;
        /// #
        /// # impl<'de> VariantAccess<'de> for X {
        /// #     type Error = value::Error;
        /// #
        /// fn unit_variant(self) -> Result<(), Self::Error> {
        ///     // What the data actually contained; suppose it is a tuple variant.
        ///     let unexp = Unexpected::TupleVariant;
        ///     Err(de::Error::invalid_type(unexp, &"unit variant"))
        /// }
        /// #
        /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
        /// #     where
        /// #         T: DeserializeSeed<'de>,
        /// #     { unimplemented!() }
        /// #
        /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
        /// #     where
        /// #         V: Visitor<'de>,
        /// #     { unimplemented!() }
        /// #
        /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
        /// #     where
        /// #         V: Visitor<'de>,
        /// #     { unimplemented!() }
        /// # }
        /// ```
        fn unit_variant(self) -> Result<(), Self::Error>;

        /// Called when deserializing a variant with a single value.
        ///
        /// `Deserialize` implementations should typically use
        /// `VariantAccess::newtype_variant` instead.
        ///
        /// If the data contains a different type of variant, the following
        /// `invalid_type` error should be constructed:
        ///
        /// ```edition2018
        /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
        /// #
        /// # struct X;
        /// #
        /// # impl<'de> VariantAccess<'de> for X {
        /// #     type Error = value::Error;
        /// #
        /// #     fn unit_variant(self) -> Result<(), Self::Error> {
        /// #         unimplemented!()
        /// #     }
        /// #
        /// fn newtype_variant_seed<T>(self, _seed: T) -> Result<T::Value, Self::Error>
        /// where
        ///     T: DeserializeSeed<'de>,
        /// {
        ///     // What the data actually contained; suppose it is a unit variant.
        ///     let unexp = Unexpected::UnitVariant;
        ///     Err(de::Error::invalid_type(unexp, &"newtype variant"))
        /// }
        /// #
        /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
        /// #     where
        /// #         V: Visitor<'de>,
        /// #     { unimplemented!() }
        /// #
        /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
        /// #     where
        /// #         V: Visitor<'de>,
        /// #     { unimplemented!() }
        /// # }
        /// ```
        fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, Self::Error>
        where
            T: DeserializeSeed<'de>;

        /// Called when deserializing a variant with a single value.
        ///
        /// This method exists as a convenience for `Deserialize` implementations.
        /// `VariantAccess` implementations should not override the default
        /// behavior.
        #[inline]
        fn newtype_variant<T>(self) -> Result<T, Self::Error>
        where
            T: Deserialize<'de>,
        {
            self.newtype_variant_seed(PhantomData)
        }

        /// Called when deserializing a tuple-like variant.
        ///
        /// The `len` is the number of fields expected in the tuple variant.
        ///
        /// If the data contains a different type of variant, the following
        /// `invalid_type` error should be constructed:
        ///
        /// ```edition2018
        /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
        /// #
        /// # struct X;
        /// #
        /// # impl<'de> VariantAccess<'de> for X {
        /// #     type Error = value::Error;
        /// #
        /// #     fn unit_variant(self) -> Result<(), Self::Error> {
        /// #         unimplemented!()
        /// #     }
        /// #
        /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
        /// #     where
        /// #         T: DeserializeSeed<'de>,
        /// #     { unimplemented!() }
        /// #
        /// fn tuple_variant<V>(
        ///     self,
        ///     _len: usize,
        ///     _visitor: V,
        /// ) -> Result<V::Value, Self::Error>
        /// where
        ///     V: Visitor<'de>,
        /// {
        ///     // What the data actually contained; suppose it is a unit variant.
        ///     let unexp = Unexpected::UnitVariant;
        ///     Err(de::Error::invalid_type(unexp, &"tuple variant"))
        /// }
        /// #
        /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
        /// #     where
        /// #         V: Visitor<'de>,
        /// #     { unimplemented!() }
        /// # }
        /// ```
        fn tuple_variant<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;

        /// Called when deserializing a struct-like variant.
        ///
        /// The `fields` are the names of the fields of the struct variant.
        ///
        /// If the data contains a different type of variant, the following
        /// `invalid_type` error should be constructed:
        ///
        /// ```edition2018
        /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
        /// #
        /// # struct X;
        /// #
        /// # impl<'de> VariantAccess<'de> for X {
        /// #     type Error = value::Error;
        /// #
        /// #     fn unit_variant(self) -> Result<(), Self::Error> {
        /// #         unimplemented!()
        /// #     }
        /// #
        /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
        /// #     where
        /// #         T: DeserializeSeed<'de>,
        /// #     { unimplemented!() }
        /// #
        /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
        /// #     where
        /// #         V: Visitor<'de>,
        /// #     { unimplemented!() }
        /// #
        /// fn struct_variant<V>(
        ///     self,
        ///     _fields: &'static [&'static str],
        ///     _visitor: V,
        /// ) -> Result<V::Value, Self::Error>
        /// where
        ///     V: Visitor<'de>,
        /// {
        ///     // What the data actually contained; suppose it is a unit variant.
        ///     let unexp = Unexpected::UnitVariant;
        ///     Err(de::Error::invalid_type(unexp, &"struct variant"))
        /// }
        /// # }
        /// ```
        fn struct_variant<V>(
            self,
            fields: &'static [&'static str],
            visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>;
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// Converts an existing value into a `Deserializer` from which other values can
    /// be deserialized.
    ///
    /// # Lifetime
    ///
    /// The `'de` lifetime of this trait is the lifetime of data that may be
    /// borrowed from the resulting `Deserializer`. See the page [Understanding
    /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
    ///
    /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
    ///
    /// # Example
    ///
    /// ```edition2018
    /// use std::str::FromStr;
    /// use serde::Deserialize;
    /// use serde::de::{value, IntoDeserializer};
    ///
    /// #[derive(Deserialize)]
    /// enum Setting {
    ///     On,
    ///     Off,
    /// }
    ///
    /// impl FromStr for Setting {
    ///     type Err = value::Error;
    ///
    ///     fn from_str(s: &str) -> Result<Self, Self::Err> {
    ///         Self::deserialize(s.into_deserializer())
    ///     }
    /// }
    /// ```
    pub trait IntoDeserializer<'de, E: Error = value::Error> {
        /// The type of the deserializer being converted into.
        type Deserializer: Deserializer<'de, Error = E>;

        /// Convert this value into a deserializer.
        fn into_deserializer(self) -> Self::Deserializer;
    }

    ////////////////////////////////////////////////////////////////////////////////

    /// Used in error messages.
    ///
    /// - expected `a`
    /// - expected `a` or `b`
    /// - expected one of `a`, `b`, `c`
    ///
    /// The slice of names must not be empty.
    struct OneOf {
        names: &'static [&'static str],
    }

    impl Display for OneOf {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            match self.names.len() {
                0 => panic!(), // special case elsewhere
                1 => write!(formatter, "`{}`", self.names[0]),
                2 => write!(formatter, "`{}` or `{}`", self.names[0], self.names[1]),
                _ => {
                    try!(write!(formatter, "one of "));
                    for (i, alt) in self.names.iter().enumerate() {
                        if i > 0 {
                            try!(write!(formatter, ", "));
                        }
                        try!(write!(formatter, "`{}`", alt));
                    }
                    Ok(())
                }
            }
        }
    }