Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2300 insertions, 0 deletions

diff --git a/vendor/serde/src/de/mod.rs b/vendor/serde/src/de/mod.rs
new file mode 100644
index 000000000..6100815f7
--- /dev/null
+++ b/vendor/serde/src/de/mod.rs
@@ -0,0 +1,2300 @@
+//! 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/bincode-org/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;
+
+#[cfg(not(no_integer128))]
+mod format;
+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>,
+///             {
+///                 // Decrease the number of reallocations if there are many elements
+///                 if let Some(size_hint) = seq.size_hint() {
+///                    self.0.reserve(size_hint);
+///                 }
+///
+///                 // 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,
+    /// indicate 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
+    }
+
+    // Not public API.
+    #[cfg(all(not(no_serde_derive), any(feature = "std", feature = "alloc")))]
+    #[doc(hidden)]
+    fn __deserialize_content<V>(
+        self,
+        _: ::actually_private::T,
+        visitor: V,
+    ) -> Result<::private::de::Content<'de>, Self::Error>
+    where
+        V: Visitor<'de, Value = ::private::de::Content<'de>>,
+    {
+        self.deserialize_any(visitor)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// 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 mut buf = [0u8; 58];
+            let mut writer = format::Buf::new(&mut buf);
+            fmt::Write::write_fmt(&mut writer, format_args!("integer `{}` as i128", v)).unwrap();
+            Err(Error::invalid_type(Unexpected::Other(writer.as_str()), &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 mut buf = [0u8; 57];
+            let mut writer = format::Buf::new(&mut buf);
+            fmt::Write::write_fmt(&mut writer, format_args!("integer `{}` as u128", v)).unwrap();
+            Err(Error::invalid_type(Unexpected::Other(writer.as_str()), &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: ?Sized> 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: ?Sized> 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(())
+            }
+        }
+    }
+}