1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
//! Generic data structure serialization framework.
//!
//! The two most important traits in this module are `Serialize` and
//! `Serializer`.
//!
//!  - **A type that implements `Serialize` is a data structure** that can be
//!    serialized to any data format supported by Serde, and conversely
//!  - **A type that implements `Serializer` is a data format** that can
//!    serialize any data structure supported by Serde.
//!
//! # The Serialize trait
//!
//! Serde provides `Serialize` implementations for many Rust primitive and
//! standard library types. The complete list is below. All of these can be
//! serialized using Serde out of the box.
//!
//! Additionally, Serde provides a procedural macro called `serde_derive` to
//! automatically generate `Serialize` implementations for structs and enums in
//! your program. See the [codegen section of the manual][codegen] for how to
//! use this.
//!
//! In rare cases it may be necessary to implement `Serialize` manually for some
//! type in your program. See the [Implementing `Serialize`][impl-serialize]
//! section of the manual for more about this.
//!
//! Third-party crates may provide `Serialize` implementations for types that
//! they expose. For example the `linked-hash-map` crate provides a
//! `LinkedHashMap<K, V>` type that is serializable by Serde because the crate
//! provides an implementation of `Serialize` for it.
//!
//! # The Serializer trait
//!
//! `Serializer` implementations are provided by third-party crates, for example
//! [`serde_json`][serde_json], [`serde_yaml`][serde_yaml] and
//! [`bincode`][bincode].
//!
//! A partial list of well-maintained formats is given on the [Serde
//! website][data-formats].
//!
//! # Implementations of Serialize provided by Serde
//!
//!  - **Primitive types**:
//!    - bool
//!    - isize, i8, i16, i32, i64
//!    - usize, u8, u16, u32, u64
//!    - f32, f64
//!    - char
//!    - str
//!    - &T and &mut T
//!  - **Compound types**:
//!    - [T]
//!    - [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\>
//!    - Rc\<T\>
//!    - Arc\<T\>
//!    - Cow\<'a, T\>
//!  - **Collection types**:
//!    - BTreeMap\<K, V\>
//!    - BTreeSet\<T\>
//!    - BinaryHeap\<T\>
//!    - HashMap\<K, V, H\>
//!    - HashSet\<T, H\>
//!    - LinkedList\<T\>
//!    - VecDeque\<T\>
//!    - Vec\<T\>
//!    - EnumSet\<T\> (unstable)
//!    - Range\<T\> (unstable)
//!    - RangeInclusive\<T\> (unstable)
//!  - **Miscellaneous standard library types**:
//!    - Duration
//!    - Path
//!    - PathBuf
//!    - NonZero\<T\> (unstable)
//!  - **Net types**:
//!    - IpAddr
//!    - Ipv4Addr
//!    - Ipv6Addr
//!    - SocketAddr
//!    - SocketAddrV4
//!    - SocketAddrV6
//!
//! [codegen]: https://serde.rs/codegen.html
//! [impl-serialize]: https://serde.rs/impl-serialize.html
//! [serde_json]: https://github.com/serde-rs/json
//! [serde_yaml]: https://github.com/dtolnay/serde-yaml
//! [bincode]: https://github.com/TyOverby/bincode
//! [data-formats]: https://serde.rs/#data-formats

#[cfg(feature = "std")]
use std::error;
#[cfg(not(feature = "std"))]
use error;

#[cfg(feature = "unstable")]
use core::cell::RefCell;

use core::fmt::Display;

mod impls;
mod impossible;

// Helpers used by generated code. Not public API.
#[doc(hidden)]
pub mod private;

pub use self::impossible::Impossible;

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

/// Trait used by `Serialize` implementations to generically construct errors
/// belonging to the `Serializer` against which they are currently running.
pub trait Error: Sized + error::Error {
    /// Raised when a `Serialize` implementation encounters a general error
    /// while serializing a type.
    ///
    /// The message should not be capitalized and should not end with a period.
    ///
    /// For example, a filesystem `Path` may refuse to serialize itself if it
    /// contains invalid UTF-8 data.
    ///
    /// ```rust
    /// # use serde::ser::{Serialize, Serializer, Error};
    /// # struct Path;
    /// # impl Path { fn to_str(&self) -> Option<&str> { unimplemented!() } }
    /// impl Serialize for Path {
    ///     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    ///         where S: Serializer
    ///     {
    ///         match self.to_str() {
    ///             Some(s) => s.serialize(serializer),
    ///             None => Err(Error::custom("path contains invalid UTF-8 characters")),
    ///         }
    ///     }
    /// }
    /// ```
    fn custom<T: Display>(msg: T) -> Self;
}

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

/// A **data structure** that can be serialized into any data format supported
/// by Serde.
///
/// Serde provides `Serialize` implementations for many Rust primitive and
/// standard library types. The complete list is [here][ser]. All of these can
/// be serialized using Serde out of the box.
///
/// Additionally, Serde provides a procedural macro called `serde_derive` to
/// automatically generate `Serialize` implementations for structs and enums in
/// your program. See the [codegen section of the manual][codegen] for how to
/// use this.
///
/// In rare cases it may be necessary to implement `Serialize` manually for some
/// type in your program. See the [Implementing `Serialize`][impl-serialize]
/// section of the manual for more about this.
///
/// Third-party crates may provide `Serialize` implementations for types that
/// they expose. For example the `linked-hash-map` crate provides a
/// `LinkedHashMap<K, V>` type that is serializable by Serde because the crate
/// provides an implementation of `Serialize` for it.
///
/// [ser]: https://docs.serde.rs/serde/ser/index.html
/// [codegen]: https://serde.rs/codegen.html
/// [impl-serialize]: https://serde.rs/impl-serialize.html
pub trait Serialize {
    /// Serialize this value into the given Serde serializer.
    ///
    /// See the [Implementing `Serialize`][impl-serialize] section of the manual
    /// for more information about how to implement this method.
    ///
    /// [impl-serialize]: https://serde.rs/impl-serialize.html
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
        where S: Serializer;
}

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

/// A **data format** that can serialize any data structure supported by Serde.
///
/// The role of this trait is to define the serialization half of the Serde data
/// model, which is a way to categorize every Rust data structure into one of 28
/// possible types. Each method of the `Serializer` trait corresponds to one of
/// the types of the data model.
///
/// Implementations of `Serialize` map themselves into this data model by
/// invoking exactly one of the `Serializer` methods.
///
/// The types that make up the Serde data model are:
///
///  - 12 primitive types:
///    - bool
///    - i8, i16, i32, i64
///    - u8, u16, u32, u64
///    - f32, f64
///    - char
///  - string
///  - byte array - [u8]
///  - option
///    - either none or some value
///  - unit
///    - unit is the type of () in Rust
///  - unit_struct
///    - for example `struct Unit` or `PhantomData<T>`
///  - unit_variant
///    - the `E::A` and `E::B` in `enum E { A, B }`
///  - newtype_struct
///    - for example `struct Millimeters(u8)`
///  - newtype_variant
///    - the `E::N` in `enum E { N(u8) }`
///  - seq
///    - a dynamically sized sequence of values, for example `Vec<T>` or
///      `HashSet<T>`
///  - seq_fixed_size
///    - a statically sized sequence of values for which the size will be known
///      at deserialization time without looking at the serialized data, for
///      example `[u64; 10]`
///  - tuple
///    - for example `(u8,)` or `(String, u64, Vec<T>)`
///  - tuple_struct
///    - for example `struct Rgb(u8, u8, u8)`
///  - tuple_variant
///    - the `E::T` in `enum E { T(u8, u8) }`
///  - map
///    - for example `BTreeMap<K, V>`
///  - struct
///    - a key-value pairing in which the keys will be known at deserialization
///      time without looking at the serialized data, for example `struct S { r:
///      u8, g: u8, b: u8 }`
///  - struct_variant
///    - the `E::S` in `enum E { S { r: u8, g: u8, b: u8 } }`
///
/// Many Serde serializers produce text or binary data as output, for example
/// JSON or Bincode. This is not a requirement of the `Serializer` trait, and
/// there are serializers that do not produce text or binary output. One example
/// is the `serde_json::value::Serializer` (distinct from the main `serde_json`
/// serializer) that produces a `serde_json::Value` data structure in memory as
/// output.
pub trait Serializer {
    /// The output type produced by this `Serializer` during successful
    /// serialization. Most serializers that produce text or binary output
    /// should set `Ok = ()` and serialize into an `io::Write` or buffer
    /// contained within the `Serializer` instance. Serializers that build
    /// in-memory data structures may be simplified by using `Ok` to propagate
    /// the data structure around.
    type Ok;

    /// The error type when some error occurs during serialization.
    type Error: Error;

    /// Type returned from `serialize_seq` and `serialize_seq_fixed_size` for
    /// serializing the content of the sequence.
    type SerializeSeq: SerializeSeq<Ok=Self::Ok, Error=Self::Error>;

    /// Type returned from `serialize_tuple` for serializing the content of the
    /// tuple.
    type SerializeTuple: SerializeTuple<Ok=Self::Ok, Error=Self::Error>;

    /// Type returned from `serialize_tuple_struct` for serializing the content
    /// of the tuple struct.
    type SerializeTupleStruct: SerializeTupleStruct<Ok=Self::Ok, Error=Self::Error>;

    /// Type returned from `serialize_tuple_variant` for serializing the content
    /// of the tuple variant.
    type SerializeTupleVariant: SerializeTupleVariant<Ok=Self::Ok, Error=Self::Error>;

    /// Type returned from `serialize_map` for serializing the content of the
    /// map.
    type SerializeMap: SerializeMap<Ok=Self::Ok, Error=Self::Error>;

    /// Type returned from `serialize_struct` for serializing the content of the
    /// struct.
    type SerializeStruct: SerializeStruct<Ok=Self::Ok, Error=Self::Error>;

    /// Type returned from `serialize_struct_variant` for serializing the
    /// content of the struct variant.
    type SerializeStructVariant: SerializeStructVariant<Ok=Self::Ok, Error=Self::Error>;

    /// Serialize a `bool` value.
    fn serialize_bool(self, v: bool) -> Result<Self::Ok, Self::Error>;

    /// Serialize an `i8` value.
    ///
    /// If the format does not differentiate between `i8` and `i64`, a
    /// reasonable implementation would be to cast the value to `i64` and
    /// forward to `serialize_i64`.
    fn serialize_i8(self, v: i8) -> Result<Self::Ok, Self::Error>;

    /// Serialize an `i16` value.
    ///
    /// If the format does not differentiate between `i16` and `i64`, a
    /// reasonable implementation would be to cast the value to `i64` and
    /// forward to `serialize_i64`.
    fn serialize_i16(self, v: i16) -> Result<Self::Ok, Self::Error>;

    /// Serialize an `i32` value.
    ///
    /// If the format does not differentiate between `i32` and `i64`, a
    /// reasonable implementation would be to cast the value to `i64` and
    /// forward to `serialize_i64`.
    fn serialize_i32(self, v: i32) -> Result<Self::Ok, Self::Error>;

    /// Serialize an `i64` value.
    fn serialize_i64(self, v: i64) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `u8` value.
    ///
    /// If the format does not differentiate between `u8` and `u64`, a
    /// reasonable implementation would be to cast the value to `u64` and
    /// forward to `serialize_u64`.
    fn serialize_u8(self, v: u8) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `u16` value.
    ///
    /// If the format does not differentiate between `u16` and `u64`, a
    /// reasonable implementation would be to cast the value to `u64` and
    /// forward to `serialize_u64`.
    fn serialize_u16(self, v: u16) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `u32` value.
    ///
    /// If the format does not differentiate between `u32` and `u64`, a
    /// reasonable implementation would be to cast the value to `u64` and
    /// forward to `serialize_u64`.
    fn serialize_u32(self, v: u32) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `u64` value.
    fn serialize_u64(self, v: u64) -> Result<Self::Ok, Self::Error>;

    /// Serialize an `f32` value.
    ///
    /// If the format does not differentiate between `f32` and `f64`, a
    /// reasonable implementation would be to cast the value to `f64` and
    /// forward to `serialize_f64`.
    fn serialize_f32(self, v: f32) -> Result<Self::Ok, Self::Error>;

    /// Serialize an `f64` value.
    fn serialize_f64(self, v: f64) -> Result<Self::Ok, Self::Error>;

    /// Serialize a character.
    ///
    /// If the format does not support characters, it is reasonable to serialize
    /// it as a single element `str` or a `u32`.
    fn serialize_char(self, v: char) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `&str`.
    fn serialize_str(self, value: &str) -> Result<Self::Ok, Self::Error>;

    /// Serialize a chunk of raw byte data.
    ///
    /// Enables serializers to serialize byte slices more compactly or more
    /// efficiently than other types of slices. If no efficient implementation
    /// is available, a reasonable implementation would be to forward to
    /// `serialize_seq`. If forwarded, the implementation looks usually just
    /// like this:
    ///
    /// ```rust,ignore
    /// let mut seq = self.serialize_seq(Some(value.len()))?;
    /// for b in value {
    ///     seq.serialize_element(b)?;
    /// }
    /// seq.end()
    /// ```
    fn serialize_bytes(self, value: &[u8]) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `None` value.
    fn serialize_none(self) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `Some(T)` value.
    fn serialize_some<T: ?Sized + Serialize>(
        self,
        value: &T,
    ) -> Result<Self::Ok, Self::Error>;

    /// Serialize a `()` value.
    fn serialize_unit(self) -> Result<Self::Ok, Self::Error>;

    /// Serialize a unit struct like `struct Unit` or `PhantomData<T>`.
    ///
    /// A reasonable implementation would be to forward to `serialize_unit`.
    fn serialize_unit_struct(
        self,
        name: &'static str,
    ) -> Result<Self::Ok, Self::Error>;

    /// Serialize a unit variant like `E::A` in `enum E { A, B }`.
    ///
    /// The `name` is the name of the enum, the `variant_index` is the index of
    /// this variant within the enum, and the `variant` is the name of the
    /// variant.
    ///
    /// A reasonable implementation would be to forward to `serialize_unit`.
    ///
    /// ```rust,ignore
    /// match *self {
    ///     E::A => serializer.serialize_unit_variant("E", 0, "A"),
    ///     E::B => serializer.serialize_unit_variant("E", 1, "B"),
    /// }
    /// ```
    fn serialize_unit_variant(
        self,
        name: &'static str,
        variant_index: usize,
        variant: &'static str,
    ) -> Result<Self::Ok, Self::Error>;

    /// Serialize a newtype struct like `struct Millimeters(u8)`.
    ///
    /// Serializers are encouraged to treat newtype structs as insignificant
    /// wrappers around the data they contain. A reasonable implementation would
    /// be to forward to `value.serialize(self)`.
    ///
    /// ```rust,ignore
    /// serializer.serialize_newtype_struct("Millimeters", &self.0)
    /// ```
    fn serialize_newtype_struct<T: ?Sized + Serialize>(
        self,
        name: &'static str,
        value: &T,
    ) -> Result<Self::Ok, Self::Error>;

    /// Serialize a newtype variant like `E::N` in `enum E { N(u8) }`.
    ///
    /// The `name` is the name of the enum, the `variant_index` is the index of
    /// this variant within the enum, and the `variant` is the name of the
    /// variant. The `value` is the data contained within this newtype variant.
    ///
    /// ```rust,ignore
    /// match *self {
    ///     E::N(ref n) => serializer.serialize_newtype_variant("E", 0, "N", n),
    /// }
    /// ```
    fn serialize_newtype_variant<T: ?Sized + Serialize>(
        self,
        name: &'static str,
        variant_index: usize,
        variant: &'static str,
        value: &T,
    ) -> Result<Self::Ok, Self::Error>;

    /// Begin to serialize a dynamically sized sequence. This call must be
    /// followed by zero or more calls to `serialize_element`, then a call to
    /// `end`.
    ///
    /// The argument is the number of elements in the sequence, which may or may
    /// not be computable before the sequence is iterated. Some serializers only
    /// support sequences whose length is known up front.
    ///
    /// ```rust,ignore
    /// let mut seq = serializer.serialize_seq(Some(self.len()))?;
    /// for element in self {
    ///     seq.serialize_element(element)?;
    /// }
    /// seq.end()
    /// ```
    fn serialize_seq(
        self,
        len: Option<usize>,
    ) -> Result<Self::SerializeSeq, Self::Error>;

    /// Begin to serialize a statically sized sequence whose length will be
    /// known at deserialization time without looking at the serialized data.
    /// This call must be followed by zero or more calls to `serialize_element`,
    /// then a call to `end`.
    ///
    /// ```rust,ignore
    /// let mut seq = serializer.serialize_seq_fixed_size(self.len())?;
    /// for element in self {
    ///     seq.serialize_element(element)?;
    /// }
    /// seq.end()
    /// ```
    fn serialize_seq_fixed_size(
        self,
        size: usize,
    ) -> Result<Self::SerializeSeq, Self::Error>;

    /// Begin to serialize a tuple. This call must be followed by zero or more
    /// calls to `serialize_field`, then a call to `end`.
    ///
    /// ```rust,ignore
    /// let mut tup = serializer.serialize_tuple(3)?;
    /// tup.serialize_field(&self.0)?;
    /// tup.serialize_field(&self.1)?;
    /// tup.serialize_field(&self.2)?;
    /// tup.end()
    /// ```
    fn serialize_tuple(
        self,
        len: usize,
    ) -> Result<Self::SerializeTuple, Self::Error>;

    /// Begin to serialize a tuple struct like `struct Rgb(u8, u8, u8)`. This
    /// call must be followed by zero or more calls to `serialize_field`, then a
    /// call to `end`.
    ///
    /// The `name` is the name of the tuple struct and the `len` is the number
    /// of data fields that will be serialized.
    ///
    /// ```rust,ignore
    /// let mut ts = serializer.serialize_tuple_struct("Rgb", 3)?;
    /// ts.serialize_field(&self.0)?;
    /// ts.serialize_field(&self.1)?;
    /// ts.serialize_field(&self.2)?;
    /// ts.end()
    /// ```
    fn serialize_tuple_struct(
        self,
        name: &'static str,
        len: usize,
    ) -> Result<Self::SerializeTupleStruct, Self::Error>;

    /// Begin to serialize a tuple variant like `E::T` in `enum E { T(u8, u8)
    /// }`. This call must be followed by zero or more calls to
    /// `serialize_field`, then a call to `end`.
    ///
    /// The `name` is the name of the enum, the `variant_index` is the index of
    /// this variant within the enum, the `variant` is the name of the variant,
    /// and the `len` is the number of data fields that will be serialized.
    ///
    /// ```rust,ignore
    /// match *self {
    ///     E::T(ref a, ref b) => {
    ///         let mut tv = serializer.serialize_tuple_variant("E", 0, "T", 2)?;
    ///         tv.serialize_field(a)?;
    ///         tv.serialize_field(b)?;
    ///         tv.end()
    ///     }
    /// }
    /// ```
    fn serialize_tuple_variant(
        self,
        name: &'static str,
        variant_index: usize,
        variant: &'static str,
        len: usize,
    ) -> Result<Self::SerializeTupleVariant, Self::Error>;

    /// Begin to serialize a map. This call must be followed by zero or more
    /// calls to `serialize_key` and `serialize_value`, then a call to `end`.
    ///
    /// The argument is the number of elements in the map, which may or may not
    /// be computable before the map is iterated. Some serializers only support
    /// maps whose length is known up front.
    ///
    /// ```rust,ignore
    /// let mut map = serializer.serialize_map(Some(self.len()))?;
    /// for (k, v) in self {
    ///     map.serialize_entry(k, v)?;
    /// }
    /// map.end()
    /// ```
    fn serialize_map(
        self,
        len: Option<usize>,
    ) -> Result<Self::SerializeMap, Self::Error>;

    /// Begin to serialize a struct like `struct Rgb { r: u8, g: u8, b: u8 }`.
    /// This call must be followed by zero or more calls to `serialize_field`,
    /// then a call to `end`.
    ///
    /// The `name` is the name of the struct and the `len` is the number of
    /// data fields that will be serialized.
    ///
    /// ```rust,ignore
    /// let mut struc = serializer.serialize_struct("Rgb", 3)?;
    /// struc.serialize_field("r", &self.r)?;
    /// struc.serialize_field("g", &self.g)?;
    /// struc.serialize_field("b", &self.b)?;
    /// struc.end()
    /// ```
    fn serialize_struct(
        self,
        name: &'static str,
        len: usize,
    ) -> Result<Self::SerializeStruct, Self::Error>;

    /// Begin to serialize a struct variant like `E::S` in `enum E { S { r: u8,
    /// g: u8, b: u8 } }`. This call must be followed by zero or more calls to
    /// `serialize_field`, then a call to `end`.
    ///
    /// The `name` is the name of the enum, the `variant_index` is the index of
    /// this variant within the enum, the `variant` is the name of the variant,
    /// and the `len` is the number of data fields that will be serialized.
    ///
    /// ```rust,ignore
    /// match *self {
    ///     E::S { ref r, ref g, ref b } => {
    ///         let mut sv = serializer.serialize_struct_variant("E", 0, "S", 3)?;
    ///         sv.serialize_field("r", r)?;
    ///         sv.serialize_field("g", g)?;
    ///         sv.serialize_field("b", b)?;
    ///         sv.end()
    ///     }
    /// }
    /// ```
    fn serialize_struct_variant(
        self,
        name: &'static str,
        variant_index: usize,
        variant: &'static str,
        len: usize,
    ) -> Result<Self::SerializeStructVariant, Self::Error>;
}

/// Returned from `Serializer::serialize_seq` and
/// `Serializer::serialize_seq_fixed_size`.
///
/// ```rust,ignore
/// let mut seq = serializer.serialize_seq(Some(self.len()))?;
/// for element in self {
///     seq.serialize_element(element)?;
/// }
/// seq.end()
/// ```
pub trait SerializeSeq {
    /// Must match the `Ok` type of our `Serializer`.
    type Ok;

    /// Must match the `Error` type of our `Serializer`.
    type Error: Error;

    /// Serialize a sequence element.
    fn serialize_element<T: ?Sized + Serialize>(&mut self, value: &T) -> Result<(), Self::Error>;

    /// Finish serializing a sequence.
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

/// Returned from `Serializer::serialize_tuple`.
///
/// ```rust,ignore
/// let mut tup = serializer.serialize_tuple(3)?;
/// tup.serialize_field(&self.0)?;
/// tup.serialize_field(&self.1)?;
/// tup.serialize_field(&self.2)?;
/// tup.end()
/// ```
pub trait SerializeTuple {
    /// Must match the `Ok` type of our `Serializer`.
    type Ok;

    /// Must match the `Error` type of our `Serializer`.
    type Error: Error;

    /// Serialize a tuple element.
    fn serialize_element<T: ?Sized + Serialize>(&mut self, value: &T) -> Result<(), Self::Error>;

    /// Finish serializing a tuple.
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

/// Returned from `Serializer::serialize_tuple_struct`.
///
/// ```rust,ignore
/// let mut ts = serializer.serialize_tuple_struct("Rgb", 3)?;
/// ts.serialize_field(&self.0)?;
/// ts.serialize_field(&self.1)?;
/// ts.serialize_field(&self.2)?;
/// ts.end()
/// ```
pub trait SerializeTupleStruct {
    /// Must match the `Ok` type of our `Serializer`.
    type Ok;

    /// Must match the `Error` type of our `Serializer`.
    type Error: Error;

    /// Serialize a tuple struct field.
    fn serialize_field<T: ?Sized + Serialize>(&mut self, value: &T) -> Result<(), Self::Error>;

    /// Finish serializing a tuple struct.
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

/// Returned from `Serializer::serialize_tuple_variant`.
///
/// ```rust,ignore
/// match *self {
///     E::T(ref a, ref b) => {
///         let mut tv = serializer.serialize_tuple_variant("E", 0, "T", 2)?;
///         tv.serialize_field(a)?;
///         tv.serialize_field(b)?;
///         tv.end()
///     }
/// }
/// ```
pub trait SerializeTupleVariant {
    /// Must match the `Ok` type of our `Serializer`.
    type Ok;

    /// Must match the `Error` type of our `Serializer`.
    type Error: Error;

    /// Serialize a tuple variant field.
    fn serialize_field<T: ?Sized + Serialize>(&mut self, value: &T) -> Result<(), Self::Error>;

    /// Finish serializing a tuple variant.
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

/// Returned from `Serializer::serialize_map`.
///
/// ```rust,ignore
/// let mut map = serializer.serialize_map(Some(self.len()))?;
/// for (k, v) in self {
///     map.serialize_entry(k, v)?;
/// }
/// map.end()
/// ```
pub trait SerializeMap {
    /// Must match the `Ok` type of our `Serializer`.
    type Ok;

    /// Must match the `Error` type of our `Serializer`.
    type Error: Error;

    /// Serialize a map key.
    fn serialize_key<T: ?Sized + Serialize>(&mut self, key: &T) -> Result<(), Self::Error>;

    /// Serialize a map value.
    fn serialize_value<T: ?Sized + Serialize>(&mut self, value: &T) -> Result<(), Self::Error>;

    /// Serialize a map entry consisting of a key and a value.
    ///
    /// Some `Serialize` types are not able to hold a key and value in memory at
    /// the same time so `SerializeMap` implementations are required to support
    /// `serialize_key` and `serialize_value` individually. The
    /// `serialize_entry` method allows serializers to optimize for the case
    /// where key and value are both available. `Serialize` implementations are
    /// encouraged to use `serialize_entry` if possible.
    ///
    /// The default implementation delegates to `serialize_key` and
    /// `serialize_value`. This is appropriate for serializers that do not care
    /// about performance or are not able to optimize `serialize_entry` any
    /// better than this.
    fn serialize_entry<K: ?Sized + Serialize, V: ?Sized + Serialize>(
        &mut self,
        key: &K,
        value: &V,
    ) -> Result<(), Self::Error> {
        try!(self.serialize_key(key));
        self.serialize_value(value)
    }

    /// Finish serializing a map.
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

/// Returned from `Serializer::serialize_struct`.
///
/// ```rust,ignore
/// let mut struc = serializer.serialize_struct("Rgb", 3)?;
/// struc.serialize_field("r", &self.r)?;
/// struc.serialize_field("g", &self.g)?;
/// struc.serialize_field("b", &self.b)?;
/// struc.end()
/// ```
pub trait SerializeStruct {
    /// Must match the `Ok` type of our `Serializer`.
    type Ok;

    /// Must match the `Error` type of our `Serializer`.
    type Error: Error;

    /// Serialize a struct field.
    fn serialize_field<T: ?Sized + Serialize>(&mut self, key: &'static str, value: &T) -> Result<(), Self::Error>;

    /// Finish serializing a struct.
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

/// Returned from `Serializer::serialize_struct_variant`.
///
/// ```rust,ignore
/// match *self {
///     E::S { ref r, ref g, ref b } => {
///         let mut sv = serializer.serialize_struct_variant("E", 0, "S", 3)?;
///         sv.serialize_field("r", r)?;
///         sv.serialize_field("g", g)?;
///         sv.serialize_field("b", b)?;
///         sv.end()
///     }
/// }
/// ```
pub trait SerializeStructVariant {
    /// Must match the `Ok` type of our `Serializer`.
    type Ok;

    /// Must match the `Error` type of our `Serializer`.
    type Error: Error;

    /// Serialize a struct variant field.
    fn serialize_field<T: ?Sized + Serialize>(&mut self, key: &'static str, value: &T) -> Result<(), Self::Error>;

    /// Finish serializing a struct variant.
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

/// A wrapper type for iterators that implements `Serialize` for iterators whose
/// items implement `Serialize`. Don't use multiple times. Create new versions
/// of this with the `serde::ser::iterator` function every time you want to
/// serialize an iterator.
#[cfg(feature = "unstable")]
pub struct Iterator<I>
    where <I as IntoIterator>::Item: Serialize,
          I: IntoIterator
{
    data: RefCell<Option<I>>,
}

/// Create a wrapper type that can be passed to any function expecting a
/// `Serialize` and will serialize the given iterator as a sequence.
#[cfg(feature = "unstable")]
pub fn iterator<I>(iter: I) -> Iterator<I>
    where <I as IntoIterator>::Item: Serialize,
          I: IntoIterator
{
    Iterator {
        data: RefCell::new(Some(iter)),
    }
}