Add support for matrix-style multiple query parameters

This adds support for query strings in which a sequence field of a
`Deserialize` / `Serialize` struct is (de)serialized as
`field=val1&field=val2&field=val3`, instead of the more common
`field[]=val1&field[]=val2&field[]=val3` syntax.

src/de.rs

@@ -1,7 +1,12 @@
 //! Deserialization support for the `application/x-www-form-urlencoded` format.
+
 use std::{borrow::Cow, io::Read};
 
-use serde::de::{self, value::MapDeserializer, Error as de_Error, IntoDeserializer};
+use serde::de::{
+    self,
+    value::{MapDeserializer, SeqDeserializer},
+    Error as de_Error, IntoDeserializer,
+};
 use url::form_urlencoded::{parse, Parse as UrlEncodedParse};
 
 #[doc(inline)]
@@ -65,9 +70,9 @@ where
     from_bytes(&buf)
 }
 
-/// A deserializer for the `application/x-www-form-urlencoded` format.
+/// A deserializer for the Matrix query string format.
 ///
-/// * Supported top-level outputs are structs, maps and sequences of pairs,
+/// * Supported top-level outputs are structs, maps and sequences,
 ///   with or without a given length.
 ///
 /// * Main `deserialize` methods defers to `deserialize_map`.
@@ -75,15 +80,12 @@ where
 /// * Everything else but `deserialize_seq` and `deserialize_seq_fixed_size`
 ///   defers to `deserialize`.
 pub struct Deserializer<'de> {
-    inner: MapDeserializer<'de, PartIterator<'de>, Error>,
+    parser: UrlEncodedParse<'de>,
 }
 
 impl<'de> Deserializer<'de> {
-    /// Returns a new `Deserializer`.
     pub fn new(parser: UrlEncodedParse<'de>) -> Self {
-        Deserializer {
+        Self { parser }
-            inner: MapDeserializer::new(PartIterator(parser)),
-        }
     }
 }
 
@@ -101,21 +103,53 @@ impl<'de> de::Deserializer<'de> for Deserializer<'de> {
     where
         V: de::Visitor<'de>,
     {
-        visitor.visit_map(self.inner)
+        visitor.visit_map(MapDeserializer::new(PartIterator(self.parser)))
     }
 
     fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
     where
         V: de::Visitor<'de>,
     {
-        visitor.visit_seq(self.inner)
+        visitor.visit_seq(MapDeserializer::new(PartIterator(self.parser)))
+    }
+
+    fn deserialize_struct<V>(
+        self,
+        _name: &'static str,
+        fields: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let pairs = self.parser.collect::<Vec<_>>();
+        let mut map = std::collections::VecDeque::new();
+        for field in fields {
+            let values = pairs
+                .iter()
+                .filter(|(f, _)| f == field)
+                .map(|(_f, v)| v.clone())
+                .collect::<Vec<_>>();
+            map.push_back((*field, values));
+        }
+        let parts = fields
+            .into_iter()
+            .map(|f| Part(Cow::Borrowed(f), None))
+            .zip(PartIterator(self.parser).into_iter().map(|(_, mut v)| {
+                if let Some((_, val)) = map.pop_front() {
+                    v.1 = Some(val);
+                }
+                v
+            }))
+            .map(|(field, value)| (field, value));
+        visitor.visit_map(MapDeserializer::new(parts))
     }
 
     fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
     where
         V: de::Visitor<'de>,
     {
-        self.inner.end()?;
+        MapDeserializer::new(PartIterator(self.parser)).end()?;
         visitor.visit_unit()
     }
 
@@ -140,7 +174,6 @@ impl<'de> de::Deserializer<'de> for Deserializer<'de> {
         unit_struct
         newtype_struct
         tuple_struct
-        struct
         identifier
         tuple
         enum
@@ -154,11 +187,11 @@ impl<'de> Iterator for PartIterator<'de> {
     type Item = (Part<'de>, Part<'de>);
 
     fn next(&mut self) -> Option<Self::Item> {
-        self.0.next().map(|(k, v)| (Part(k), Part(v)))
+        self.0.next().map(|(k, v)| (Part(k, None), Part(v, None)))
     }
 }
 
-struct Part<'de>(Cow<'de, str>);
+struct Part<'de>(Cow<'de, str>, Option<Vec<Cow<'de, str>>>);
 
 impl<'de> IntoDeserializer<'de> for Part<'de> {
     type Deserializer = Self;
@@ -226,6 +259,16 @@ impl<'de> de::Deserializer<'de> for Part<'de> {
         visitor.visit_newtype_struct(self)
     }
 
+    fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        let iter = self.1.ok_or(Error::custom("expected sequence"))?;
+        visitor.visit_seq(SeqDeserializer::new(
+            iter.into_iter().map(|v| Part(v, None)),
+        ))
+    }
+
     serde::forward_to_deserialize_any! {
         char
         str
@@ -239,7 +282,6 @@ impl<'de> de::Deserializer<'de> for Part<'de> {
         identifier
         tuple
         ignored_any
-        seq
         map
     }
 

src/error.rs

@@ -0,0 +1,69 @@
+use std::{borrow::Cow, error, fmt, str};
+
+use serde::ser;
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Errors returned during serializing to `application/x-www-form-urlencoded`.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum Error {
+    Custom(Cow<'static, str>),
+    Utf8(str::Utf8Error),
+}
+
+impl Error {
+    pub fn done() -> Self {
+        Error::Custom("this pair has already been serialized".into())
+    }
+
+    pub fn not_done() -> Self {
+        Error::Custom("this pair has not yet been serialized".into())
+    }
+
+    pub fn unsupported_pair() -> Self {
+        Error::Custom("unsupported pair".into())
+    }
+
+    pub fn top_level() -> Self {
+        let msg = "top-level serializer supports only maps and structs";
+        Error::Custom(msg.into())
+    }
+
+    pub fn no_key() -> Self {
+        let msg = "tried to serialize a value before serializing key";
+        Error::Custom(msg.into())
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Error::Custom(ref msg) => msg.fmt(f),
+            Error::Utf8(ref err) => write!(f, "invalid UTF-8: {}", err),
+        }
+    }
+}
+
+impl error::Error for Error {
+    /// The lower-level cause of this error, in the case of a `Utf8` error.
+    fn cause(&self) -> Option<&dyn error::Error> {
+        match *self {
+            Error::Custom(_) => None,
+            Error::Utf8(ref err) => Some(err),
+        }
+    }
+
+    /// The lower-level source of this error, in the case of a `Utf8` error.
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match *self {
+            Error::Custom(_) => None,
+            Error::Utf8(ref err) => Some(err),
+        }
+    }
+}
+
+impl ser::Error for Error {
+    fn custom<T: fmt::Display>(msg: T) -> Self {
+        Error::Custom(format!("{}", msg).into())
+    }
+}

src/lib.rs

@@ -1,6 +1,7 @@
 //! `x-www-form-urlencoded` meets Serde
 
 pub mod de;
+pub mod error;
 pub mod ser;
 
 #[doc(inline)]

src/ser/key.rs

@@ -3,6 +3,7 @@ use std::{borrow::Cow, ops::Deref};
 use serde::Serialize;
 
 use crate::ser::{part::Sink, Error};
+
 pub enum Key<'key> {
     Static(&'static str),
     Dynamic(Cow<'key, str>),

src/ser/mod.rs

@@ -5,10 +5,14 @@ mod pair;
 mod part;
 mod value;
 
-use std::{borrow::Cow, error, fmt, str};
+use std::{borrow::Cow, str};
 
 use serde::ser;
-use url::form_urlencoded::{Serializer as UrlEncodedSerializer, Target as UrlEncodedTarget};
+use url::form_urlencoded::{
+    Serializer as UrlEncodedSerializer, Target as UrlEncodedTarget,
+};
+
+use crate::error::{Error, Result};
 
 /// Serializes a value into a `application/x-www-form-urlencoded` `String` buffer.
 ///
@@ -24,7 +28,7 @@ use url::form_urlencoded::{Serializer as UrlEncodedSerializer, Target as UrlEnco
 ///     serde_urlencoded::to_string(meal),
 ///     Ok("bread=baguette&cheese=comt%C3%A9&meat=ham&fat=butter".to_owned()));
 /// ```
-pub fn to_string<T: ser::Serialize>(input: T) -> Result<String, Error> {
+pub fn to_string<T: ser::Serialize>(input: T) -> Result<String> {
     let mut urlencoder = UrlEncodedSerializer::new("".to_owned());
     input.serialize(Serializer::new(&mut urlencoder))?;
     Ok(urlencoder.finish())
@@ -43,85 +47,50 @@ pub struct Serializer<'input, 'output, Target: 'output + UrlEncodedTarget> {
     urlencoder: &'output mut UrlEncodedSerializer<'input, Target>,
 }
 
-impl<'input, 'output, Target: 'output + UrlEncodedTarget> Serializer<'input, 'output, Target> {
+impl<'input, 'output, Target: 'output + UrlEncodedTarget>
+    Serializer<'input, 'output, Target>
+{
     /// Returns a new `Serializer`.
-    pub fn new(urlencoder: &'output mut UrlEncodedSerializer<'input, Target>) -> Self {
+    pub fn new(
+        urlencoder: &'output mut UrlEncodedSerializer<'input, Target>,
+    ) -> Self {
         Serializer {
             urlencoder: urlencoder,
         }
     }
 }
 
-/// Errors returned during serializing to `application/x-www-form-urlencoded`.
-#[derive(Clone, Debug, PartialEq, Eq)]
-pub enum Error {
-    Custom(Cow<'static, str>),
-    Utf8(str::Utf8Error),
-}
-
-impl fmt::Display for Error {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match *self {
-            Error::Custom(ref msg) => msg.fmt(f),
-            Error::Utf8(ref err) => write!(f, "invalid UTF-8: {}", err),
-        }
-    }
-}
-
-impl error::Error for Error {
-    fn description(&self) -> &str {
-        match *self {
-            Error::Custom(ref msg) => msg,
-            Error::Utf8(ref err) => error::Error::description(err),
-        }
-    }
-
-    /// The lower-level cause of this error, in the case of a `Utf8` error.
-    fn cause(&self) -> Option<&error::Error> {
-        match *self {
-            Error::Custom(_) => None,
-            Error::Utf8(ref err) => Some(err),
-        }
-    }
-
-    /// The lower-level source of this error, in the case of a `Utf8` error.
-    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
-        match *self {
-            Error::Custom(_) => None,
-            Error::Utf8(ref err) => Some(err),
-        }
-    }
-}
-
-impl ser::Error for Error {
-    fn custom<T: fmt::Display>(msg: T) -> Self {
-        Error::Custom(format!("{}", msg).into())
-    }
-}
-
 /// Sequence serializer.
 pub struct SeqSerializer<'input, 'output, Target: 'output + UrlEncodedTarget> {
     urlencoder: &'output mut UrlEncodedSerializer<'input, Target>,
+    key: Option<Cow<'input, str>>,
+    count: usize,
 }
 
 /// Tuple serializer.
 ///
 /// Mostly used for arrays.
-pub struct TupleSerializer<'input, 'output, Target: 'output + UrlEncodedTarget> {
+pub struct TupleSerializer<'input, 'output, Target: 'output + UrlEncodedTarget>
+{
     urlencoder: &'output mut UrlEncodedSerializer<'input, Target>,
 }
 
 /// Tuple struct serializer.
 ///
 /// Never instantiated, tuple structs are not supported.
-pub struct TupleStructSerializer<'input, 'output, T: 'output + UrlEncodedTarget> {
+pub struct TupleStructSerializer<'input, 'output, T: 'output + UrlEncodedTarget>
+{
     inner: ser::Impossible<&'output mut UrlEncodedSerializer<'input, T>, Error>,
 }
 
 /// Tuple variant serializer.
 ///
 /// Never instantiated, tuple variants are not supported.
-pub struct TupleVariantSerializer<'input, 'output, T: 'output + UrlEncodedTarget> {
+pub struct TupleVariantSerializer<
+    'input,
+    'output,
+    T: 'output + UrlEncodedTarget,
+> {
     inner: ser::Impossible<&'output mut UrlEncodedSerializer<'input, T>, Error>,
 }
 
@@ -132,18 +101,24 @@ pub struct MapSerializer<'input, 'output, Target: 'output + UrlEncodedTarget> {
 }
 
 /// Struct serializer.
-pub struct StructSerializer<'input, 'output, Target: 'output + UrlEncodedTarget> {
+pub struct StructSerializer<'input, 'output, Target: 'output + UrlEncodedTarget>
+{
     urlencoder: &'output mut UrlEncodedSerializer<'input, Target>,
 }
 
 /// Struct variant serializer.
 ///
 /// Never instantiated, struct variants are not supported.
-pub struct StructVariantSerializer<'input, 'output, T: 'output + UrlEncodedTarget> {
+pub struct StructVariantSerializer<
+    'input,
+    'output,
+    T: 'output + UrlEncodedTarget,
+> {
     inner: ser::Impossible<&'output mut UrlEncodedSerializer<'input, T>, Error>,
 }
 
-impl<'input, 'output, Target> ser::Serializer for Serializer<'input, 'output, Target>
+impl<'input, 'output, Target> ser::Serializer
+    for Serializer<'input, 'output, Target>
 where
     Target: 'output + UrlEncodedTarget,
 {
@@ -152,91 +127,90 @@ where
     type SerializeSeq = SeqSerializer<'input, 'output, Target>;
     type SerializeTuple = TupleSerializer<'input, 'output, Target>;
     type SerializeTupleStruct = TupleStructSerializer<'input, 'output, Target>;
-    type SerializeTupleVariant = TupleVariantSerializer<'input, 'output, Target>;
+    type SerializeTupleVariant =
+        TupleVariantSerializer<'input, 'output, Target>;
     type SerializeMap = MapSerializer<'input, 'output, Target>;
     type SerializeStruct = StructSerializer<'input, 'output, Target>;
-    type SerializeStructVariant = StructVariantSerializer<'input, 'output, Target>;
+    type SerializeStructVariant =
+        StructVariantSerializer<'input, 'output, Target>;
 
     /// Returns an error.
-    fn serialize_bool(self, _v: bool) -> Result<Self::Ok, Error> {
+    fn serialize_bool(self, _v: bool) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_i8(self, _v: i8) -> Result<Self::Ok, Error> {
+    fn serialize_i8(self, _v: i8) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_i16(self, _v: i16) -> Result<Self::Ok, Error> {
+    fn serialize_i16(self, _v: i16) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_i32(self, _v: i32) -> Result<Self::Ok, Error> {
+    fn serialize_i32(self, _v: i32) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_i64(self, _v: i64) -> Result<Self::Ok, Error> {
+    fn serialize_i64(self, _v: i64) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u8(self, _v: u8) -> Result<Self::Ok, Error> {
+    fn serialize_u8(self, _v: u8) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u16(self, _v: u16) -> Result<Self::Ok, Error> {
+    fn serialize_u16(self, _v: u16) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u32(self, _v: u32) -> Result<Self::Ok, Error> {
+    fn serialize_u32(self, _v: u32) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u64(self, _v: u64) -> Result<Self::Ok, Error> {
+    fn serialize_u64(self, _v: u64) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_f32(self, _v: f32) -> Result<Self::Ok, Error> {
+    fn serialize_f32(self, _v: f32) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_f64(self, _v: f64) -> Result<Self::Ok, Error> {
+    fn serialize_f64(self, _v: f64) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_char(self, _v: char) -> Result<Self::Ok, Error> {
+    fn serialize_char(self, _v: char) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_str(self, _value: &str) -> Result<Self::Ok, Error> {
+    fn serialize_str(self, _value: &str) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_bytes(self, _value: &[u8]) -> Result<Self::Ok, Error> {
+    fn serialize_bytes(self, _value: &[u8]) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns `Ok`.
-    fn serialize_unit(self) -> Result<Self::Ok, Error> {
+    fn serialize_unit(self) -> Result<Self::Ok> {
         Ok(self.urlencoder)
     }
 
     /// Returns `Ok`.
-    fn serialize_unit_struct(
+    fn serialize_unit_struct(self, _name: &'static str) -> Result<Self::Ok> {
-        self,
-        _name: &'static str,
-    ) -> Result<Self::Ok, Error> {
         Ok(self.urlencoder)
     }
 
@@ -246,7 +220,7 @@ where
         _name: &'static str,
         _variant_index: u32,
         _variant: &'static str,
-    ) -> Result<Self::Ok, Error> {
+    ) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
@@ -255,7 +229,7 @@ where
         self,
         _name: &'static str,
         value: &T,
-    ) -> Result<Self::Ok, Error> {
+    ) -> Result<Self::Ok> {
         value.serialize(self)
     }
 
@@ -266,12 +240,12 @@ where
         _variant_index: u32,
         _variant: &'static str,
         _value: &T,
-    ) -> Result<Self::Ok, Error> {
+    ) -> Result<Self::Ok> {
         Err(Error::top_level())
     }
 
     /// Returns `Ok`.
-    fn serialize_none(self) -> Result<Self::Ok, Error> {
+    fn serialize_none(self) -> Result<Self::Ok> {
         Ok(self.urlencoder)
     }
 
@@ -279,25 +253,21 @@ where
     fn serialize_some<T: ?Sized + ser::Serialize>(
         self,
         value: &T,
-    ) -> Result<Self::Ok, Error> {
+    ) -> Result<Self::Ok> {
         value.serialize(self)
     }
 
     /// Serialize a sequence, given length (if any) is ignored.
-    fn serialize_seq(
+    fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq> {
-        self,
-        _len: Option<usize>,
-    ) -> Result<Self::SerializeSeq, Error> {
         Ok(SeqSerializer {
             urlencoder: self.urlencoder,
+            key: None,
+            count: 0,
         })
     }
 
     /// Returns an error.
-    fn serialize_tuple(
+    fn serialize_tuple(self, _len: usize) -> Result<Self::SerializeTuple> {
-        self,
-        _len: usize,
-    ) -> Result<Self::SerializeTuple, Error> {
         Ok(TupleSerializer {
             urlencoder: self.urlencoder,
         })
@@ -308,7 +278,7 @@ where
         self,
         _name: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeTupleStruct, Error> {
+    ) -> Result<Self::SerializeTupleStruct> {
         Err(Error::top_level())
     }
 
@@ -319,15 +289,12 @@ where
         _variant_index: u32,
         _variant: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeTupleVariant, Error> {
+    ) -> Result<Self::SerializeTupleVariant> {
         Err(Error::top_level())
     }
 
     /// Serializes a map, given length is ignored.
-    fn serialize_map(
+    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap> {
-        self,
-        _len: Option<usize>,
-    ) -> Result<Self::SerializeMap, Error> {
         Ok(MapSerializer {
             urlencoder: self.urlencoder,
             key: None,
@@ -339,7 +306,7 @@ where
         self,
         _name: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeStruct, Error> {
+    ) -> Result<Self::SerializeStruct> {
         Ok(StructSerializer {
             urlencoder: self.urlencoder,
         })
@@ -352,12 +319,13 @@ where
         _variant_index: u32,
         _variant: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeStructVariant, Error> {
+    ) -> Result<Self::SerializeStructVariant> {
         Err(Error::top_level())
     }
 }
 
-impl<'input, 'output, Target> ser::SerializeSeq for SeqSerializer<'input, 'output, Target>
+impl<'input, 'output, Target> ser::SerializeSeq
+    for SeqSerializer<'input, 'output, Target>
 where
     Target: 'output + UrlEncodedTarget,
 {
@@ -367,16 +335,21 @@ where
     fn serialize_element<T: ?Sized + ser::Serialize>(
         &mut self,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
-        value.serialize(pair::PairSerializer::new(self.urlencoder))
+        value.serialize(pair::PairSerializer::new(
+            self.urlencoder,
+            self.key.as_ref(),
+            &mut self.count,
+        ))
     }
 
-    fn end(self) -> Result<Self::Ok, Error> {
+    fn end(self) -> Result<Self::Ok> {
         Ok(self.urlencoder)
     }
 }
 
-impl<'input, 'output, Target> ser::SerializeTuple for TupleSerializer<'input, 'output, Target>
+impl<'input, 'output, Target> ser::SerializeTuple
+    for TupleSerializer<'input, 'output, Target>
 where
     Target: 'output + UrlEncodedTarget,
 {
@@ -386,11 +359,15 @@ where
     fn serialize_element<T: ?Sized + ser::Serialize>(
         &mut self,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
-        value.serialize(pair::PairSerializer::new(self.urlencoder))
+        value.serialize(pair::PairSerializer::new(
+            self.urlencoder,
+            None,
+            &mut 0,
+        ))
     }
 
-    fn end(self) -> Result<Self::Ok, Error> {
+    fn end(self) -> Result<Self::Ok> {
         Ok(self.urlencoder)
     }
 }
@@ -406,11 +383,11 @@ where
     fn serialize_field<T: ?Sized + ser::Serialize>(
         &mut self,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         self.inner.serialize_field(value)
     }
 
-    fn end(self) -> Result<Self::Ok, Error> {
+    fn end(self) -> Result<Self::Ok> {
         self.inner.end()
     }
 }
@@ -426,16 +403,17 @@ where
     fn serialize_field<T: ?Sized + ser::Serialize>(
         &mut self,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         self.inner.serialize_field(value)
     }
 
-    fn end(self) -> Result<Self::Ok, Error> {
+    fn end(self) -> Result<Self::Ok> {
         self.inner.end()
     }
 }
 
-impl<'input, 'output, Target> ser::SerializeMap for MapSerializer<'input, 'output, Target>
+impl<'input, 'output, Target> ser::SerializeMap
+    for MapSerializer<'input, 'output, Target>
 where
     Target: 'output + UrlEncodedTarget,
 {
@@ -449,7 +427,7 @@ where
         &mut self,
         key: &K,
         value: &V,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         let key_sink = key::KeySink::new(|key| {
             let value_sink = value::ValueSink::new(self.urlencoder, &key);
             value.serialize(part::PartSerializer::new(value_sink))?;
@@ -463,7 +441,7 @@ where
     fn serialize_key<T: ?Sized + ser::Serialize>(
         &mut self,
         key: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         let key_sink = key::KeySink::new(|key| Ok(key.into()));
         let key_serializer = part::PartSerializer::new(key_sink);
         self.key = Some(key.serialize(key_serializer)?);
@@ -473,7 +451,7 @@ where
     fn serialize_value<T: ?Sized + ser::Serialize>(
         &mut self,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         {
             let key = self.key.as_ref().ok_or_else(|| Error::no_key())?;
             let value_sink = value::ValueSink::new(self.urlencoder, &key);
@@ -483,12 +461,13 @@ where
         Ok(())
     }
 
-    fn end(self) -> Result<Self::Ok, Error> {
+    fn end(self) -> Result<Self::Ok> {
         Ok(self.urlencoder)
     }
 }
 
-impl<'input, 'output, Target> ser::SerializeStruct for StructSerializer<'input, 'output, Target>
+impl<'input, 'output, Target> ser::SerializeStruct
+    for StructSerializer<'input, 'output, Target>
 where
     Target: 'output + UrlEncodedTarget,
 {
@@ -499,12 +478,16 @@ where
         &mut self,
         key: &'static str,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
-        let value_sink = value::ValueSink::new(self.urlencoder, key);
+        let key = Cow::Borrowed(key);
-        value.serialize(part::PartSerializer::new(value_sink))
+        let mut count = 0;
+        let value_sink =
+            pair::PairSerializer::new(self.urlencoder, Some(&key), &mut count);
+        value.serialize(value_sink)?;
+        Ok(())
     }
 
-    fn end(self) -> Result<Self::Ok, Error> {
+    fn end(self) -> Result<Self::Ok> {
         Ok(self.urlencoder)
     }
 }
@@ -521,23 +504,11 @@ where
         &mut self,
         key: &'static str,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         self.inner.serialize_field(key, value)
     }
 
-    fn end(self) -> Result<Self::Ok, Error> {
+    fn end(self) -> Result<Self::Ok> {
         self.inner.end()
     }
 }
-
-impl Error {
-    fn top_level() -> Self {
-        let msg = "top-level serializer supports only maps and structs";
-        Error::Custom(msg.into())
-    }
-
-    fn no_key() -> Self {
-        let msg = "tried to serialize a value before serializing key";
-        Error::Custom(msg.into())
-    }
-}

src/ser/pair.rs

@@ -1,34 +1,67 @@
-use serde::ser;
 use std::borrow::Cow;
 use std::mem;
-use url::form_urlencoded::Serializer as UrlEncodedSerializer;
-use url::form_urlencoded::Target as UrlEncodedTarget;
 
-use crate::ser::{Error, part::PartSerializer, key::KeySink, value::ValueSink};
+use serde::ser;
+
+use url::form_urlencoded::{
+    Serializer as UrlEncodedSerializer, Target as UrlEncodedTarget,
+};
+
+use crate::{
+    error::{Error, Result},
+    ser::{key::KeySink, part::PartSerializer, value::ValueSink},
+};
+
+macro_rules! serialize_pair {
+    ($($ty:ty => $name:ident,)*) => {
+        $(
+            fn $name(self, value: $ty) -> Result<()> {
+                let key = if let Some(key) = self.key {
+                    key.clone()
+                } else {
+                    return Err(Error::no_key());
+                };
+                let value_sink = ValueSink::new(self.urlencoder, &key);
+                let value_serializer = PartSerializer::new(value_sink);
+                value_serializer.$name(value)
+            }
+        )*
+    };
+}
+
 pub struct PairSerializer<'input, 'target, Target: 'target + UrlEncodedTarget> {
     urlencoder: &'target mut UrlEncodedSerializer<'input, Target>,
     state: PairState,
+    key: Option<&'target Cow<'target, str>>,
+    count: &'target mut usize,
 }
 
 impl<'input, 'target, Target> PairSerializer<'input, 'target, Target>
 where
     Target: 'target + UrlEncodedTarget,
 {
-    pub fn new(urlencoder: &'target mut UrlEncodedSerializer<'input, Target>) -> Self {
+    pub fn new(
+        urlencoder: &'target mut UrlEncodedSerializer<'input, Target>,
+        key: Option<&'target Cow<'target, str>>,
+        count: &'target mut usize,
+    ) -> Self {
         PairSerializer {
-            urlencoder: urlencoder,
+            urlencoder,
             state: PairState::WaitingForKey,
+            key,
+            count,
         }
     }
 }
 
-impl<'input, 'target, Target> ser::Serializer for PairSerializer<'input, 'target, Target>
+impl<'input, 'target, Target> ser::Serializer
+    for PairSerializer<'input, 'target, Target>
 where
     Target: 'target + UrlEncodedTarget,
 {
     type Ok = ();
     type Error = Error;
-    type SerializeSeq = ser::Impossible<(), Error>;
+    type SerializeSeq = Self;
     type SerializeTuple = Self;
     type SerializeTupleStruct = ser::Impossible<(), Error>;
     type SerializeTupleVariant = ser::Impossible<(), Error>;
@@ -36,67 +69,31 @@ where
     type SerializeStruct = ser::Impossible<(), Error>;
     type SerializeStructVariant = ser::Impossible<(), Error>;
 
-    fn serialize_bool(self, _v: bool) -> Result<(), Error> {
+    serialize_pair! {
+        bool => serialize_bool,
+        u8  => serialize_u8,
+        u16 => serialize_u16,
+        u32 => serialize_u32,
+        u64 => serialize_u64,
+        i8  => serialize_i8,
+        i16 => serialize_i16,
+        i32 => serialize_i32,
+        i64 => serialize_i64,
+        f32 => serialize_f32,
+        f64 => serialize_f64,
+        char => serialize_char,
+        &str => serialize_str,
+    }
+
+    fn serialize_bytes(self, _value: &[u8]) -> Result<()> {
         Err(Error::unsupported_pair())
     }
 
-    fn serialize_i8(self, _v: i8) -> Result<(), Error> {
+    fn serialize_unit(self) -> Result<()> {
         Err(Error::unsupported_pair())
     }
 
-    fn serialize_i16(self, _v: i16) -> Result<(), Error> {
+    fn serialize_unit_struct(self, _name: &'static str) -> Result<()> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_i32(self, _v: i32) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_i64(self, _v: i64) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_u8(self, _v: u8) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_u16(self, _v: u16) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_u32(self, _v: u32) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_u64(self, _v: u64) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_f32(self, _v: f32) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_f64(self, _v: f64) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_char(self, _v: char) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_str(self, _value: &str) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_bytes(self, _value: &[u8]) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_unit(self) -> Result<(), Error> {
-        Err(Error::unsupported_pair())
-    }
-
-    fn serialize_unit_struct(self, _name: &'static str) -> Result<(), Error> {
         Err(Error::unsupported_pair())
     }
 
@@ -104,16 +101,25 @@ where
         self,
         _name: &'static str,
         _variant_index: u32,
-        _variant: &'static str,
+        variant: &'static str,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
-        Err(Error::unsupported_pair())
+        let key = if let Some(key) = self.key {
+            key.clone()
+        } else {
+            let key = Cow::Owned(self.count.to_string());
+            *self.count += 1;
+            key
+        };
+        let value_sink = ValueSink::new(self.urlencoder, &key);
+        let value_serializer = PartSerializer::new(value_sink);
+        value_serializer.serialize_str(variant)
     }
 
     fn serialize_newtype_struct<T: ?Sized + ser::Serialize>(
         self,
         _name: &'static str,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         value.serialize(self)
     }
 
@@ -123,29 +129,26 @@ where
         _variant_index: u32,
         _variant: &'static str,
         _value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         Err(Error::unsupported_pair())
     }
 
-    fn serialize_none(self) -> Result<(), Error> {
+    fn serialize_none(self) -> Result<()> {
         Ok(())
     }
 
     fn serialize_some<T: ?Sized + ser::Serialize>(
         self,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
         value.serialize(self)
     }
 
-    fn serialize_seq(
+    fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq> {
-        self,
+        Ok(self)
-        _len: Option<usize>,
-    ) -> Result<Self::SerializeSeq, Error> {
-        Err(Error::unsupported_pair())
     }
 
-    fn serialize_tuple(self, len: usize) -> Result<Self, Error> {
+    fn serialize_tuple(self, len: usize) -> Result<Self> {
         if len == 2 {
             Ok(self)
         } else {
@@ -157,7 +160,7 @@ where
         self,
         _name: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeTupleStruct, Error> {
+    ) -> Result<Self::SerializeTupleStruct> {
         Err(Error::unsupported_pair())
     }
 
@@ -167,14 +170,11 @@ where
         _variant_index: u32,
         _variant: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeTupleVariant, Error> {
+    ) -> Result<Self::SerializeTupleVariant> {
         Err(Error::unsupported_pair())
     }
 
-    fn serialize_map(
+    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap> {
-        self,
-        _len: Option<usize>,
-    ) -> Result<Self::SerializeMap, Error> {
         Err(Error::unsupported_pair())
     }
 
@@ -182,7 +182,7 @@ where
         self,
         _name: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeStruct, Error> {
+    ) -> Result<Self::SerializeStruct> {
         Err(Error::unsupported_pair())
     }
 
@@ -192,12 +192,13 @@ where
         _variant_index: u32,
         _variant: &'static str,
         _len: usize,
-    ) -> Result<Self::SerializeStructVariant, Error> {
+    ) -> Result<Self::SerializeStructVariant> {
         Err(Error::unsupported_pair())
     }
 }
 
-impl<'input, 'target, Target> ser::SerializeTuple for PairSerializer<'input, 'target, Target>
+impl<'input, 'target, Target> ser::SerializeSeq
+    for PairSerializer<'input, 'target, Target>
 where
     Target: 'target + UrlEncodedTarget,
 {
@@ -207,7 +208,31 @@ where
     fn serialize_element<T: ?Sized + ser::Serialize>(
         &mut self,
         value: &T,
-    ) -> Result<(), Error> {
+    ) -> Result<()> {
+        value.serialize(PairSerializer::new(
+            self.urlencoder,
+            self.key,
+            &mut self.count,
+        ))
+    }
+
+    fn end(self) -> Result<Self::Ok> {
+        Ok(())
+    }
+}
+
+impl<'input, 'target, Target> ser::SerializeTuple
+    for PairSerializer<'input, 'target, Target>
+where
+    Target: 'target + UrlEncodedTarget,
+{
+    type Ok = ();
+    type Error = Error;
+
+    fn serialize_element<T: ?Sized + ser::Serialize>(
+        &mut self,
+        value: &T,
+    ) -> Result<()> {
         match mem::replace(&mut self.state, PairState::Done) {
             PairState::WaitingForKey => {
                 let key_sink = KeySink::new(|key| Ok(key.into()));
@@ -234,7 +259,7 @@ where
         }
     }
 
-    fn end(self) -> Result<(), Error> {
+    fn end(self) -> Result<()> {
         if let PairState::Done = self.state {
             Ok(())
         } else {
@@ -248,17 +273,3 @@ enum PairState {
     WaitingForValue { key: Cow<'static, str> },
     Done,
 }
-
-impl Error {
-    fn done() -> Self {
-        Error::Custom("this pair has already been serialized".into())
-    }
-
-    fn not_done() -> Self {
-        Error::Custom("this pair has not yet been serialized".into())
-    }
-
-    fn unsupported_pair() -> Self {
-        Error::Custom("unsupported pair".into())
-    }
-}

src/ser/part.rs

@@ -4,7 +4,7 @@ use dtoa;
 use itoa;
 use serde::ser;
 
-use crate::ser::Error;
+use crate::error::Error;
 pub struct PartSerializer<S> {
     sink: S,
 }

src/ser/value.rs

@@ -1,9 +1,14 @@
 use std::str;
 
 use serde::Serialize;
-use url::form_urlencoded::{Serializer as UrlEncodedSerializer, Target as UrlEncodedTarget};
+use url::form_urlencoded::{
+    Serializer as UrlEncodedSerializer, Target as UrlEncodedTarget,
+};
 
-use crate::ser::{part::{PartSerializer, Sink}, Error};
+use crate::{
+    error::Error,
+    ser::part::{PartSerializer, Sink},
+};
 
 pub struct ValueSink<'input, 'key, 'target, Target>
 where
@@ -21,14 +26,12 @@ where
         urlencoder: &'target mut UrlEncodedSerializer<'input, Target>,
         key: &'key str,
     ) -> Self {
-        ValueSink {
+        ValueSink { urlencoder, key }
-            urlencoder: urlencoder,
-            key: key,
-        }
     }
 }
 
-impl<'input, 'key, 'target, Target> Sink for ValueSink<'input, 'key, 'target, Target>
+impl<'input, 'key, 'target, Target> Sink
+    for ValueSink<'input, 'key, 'target, Target>
 where
     Target: 'target + UrlEncodedTarget,
 {

tests/test_deserialize.rs

@@ -86,3 +86,114 @@ fn deserialize_unit_enum() {
 fn deserialize_unit_type() {
     assert_eq!(serde_urlencoded::from_str(""), Ok(()));
 }
+
+#[derive(Debug, Deserialize, PartialEq)]
+struct Wrapper<T> {
+    item: T,
+}
+
+#[derive(Debug, PartialEq, Deserialize)]
+struct NewStruct {
+    list: Vec<String>,
+}
+
+#[derive(Debug, PartialEq, Deserialize)]
+struct Struct {
+    list: Vec<Option<String>>,
+}
+
+#[derive(Debug, PartialEq, Deserialize)]
+struct NumList {
+    list: Vec<u8>,
+}
+
+#[derive(Debug, PartialEq, Deserialize)]
+struct ListStruct {
+    list: Vec<NewType<usize>>,
+}
+
+#[derive(Debug, PartialEq, Deserialize)]
+struct MapStruct {
+    a: usize,
+    b: String,
+    c: Option<u8>,
+}
+
+#[test]
+fn deserialize_mapstruct() {
+    let de = MapStruct {
+        a: 10,
+        b: "Hello".into(),
+        c: None,
+    };
+    assert_eq!(
+        de,
+        serde_urlencoded::from_str::<MapStruct>("a=10&b=Hello").unwrap()
+    );
+}
+
+#[test]
+fn deserialize_newstruct() {
+    let de = NewStruct {
+        list: vec!["hello".into(), "world".into()],
+    };
+    assert_eq!(
+        de,
+        serde_urlencoded::from_str::<NewStruct>("list=hello&list=world")
+            .unwrap()
+    );
+}
+
+#[test]
+fn deserialize_numlist() {
+    let de = NumList {
+        list: vec![1, 2, 3, 4],
+    };
+    assert_eq!(
+        de,
+        serde_urlencoded::from_str::<NumList>("list=1&list=2&list=3&list=4")
+            .unwrap()
+    );
+}
+
+#[test]
+fn deserialize_vec_bool() {
+    assert_eq!(
+        Wrapper {
+            item: vec![true, false, false]
+        },
+        serde_urlencoded::from_str::<Wrapper<_>>(
+            "item=true&item=false&item=false"
+        )
+        .unwrap()
+    );
+}
+
+#[test]
+fn deserialize_vec_string() {
+    assert_eq!(
+        Wrapper {
+            item: vec![
+                "hello".to_string(),
+                "matrix".to_string(),
+                "hello".to_string()
+            ],
+        },
+        serde_urlencoded::from_str::<Wrapper<_>>(
+            "item=hello&item=matrix&item=hello"
+        )
+        .unwrap()
+    );
+}
+
+#[test]
+fn deserialize_struct_unit_enum() {
+    let result = Wrapper {
+        item: vec![X::A, X::B, X::C],
+    };
+
+    assert_eq!(
+        serde_urlencoded::from_str("item=A&item=B&item=C"),
+        Ok(result)
+    );
+}

tests/test_serialize.rs

@@ -3,17 +3,6 @@ use serde::Serialize;
 #[derive(Serialize)]
 struct NewType<T>(T);
 
-#[derive(Serialize)]
-struct NewStruct {
-    list: Vec<String>,
-}
-
-#[test]
-fn serialize_newstruct() {
-    let s = NewStruct { list: vec![ "hello".into() ] };
-    println!("{:?}", serde_urlencoded::to_string(s));
-}
-
 #[test]
 fn serialize_newtype_i32() {
     let params = &[("field", Some(NewType(11)))];
@@ -95,3 +84,111 @@ fn serialize_unit_struct() {
 fn serialize_unit_type() {
     assert_eq!(serde_urlencoded::to_string(()), Ok("".to_owned()));
 }
+
+#[derive(Serialize)]
+struct Wrapper<T> {
+    item: T,
+}
+
+#[derive(Serialize)]
+struct NewStruct {
+    list: Vec<String>,
+}
+
+#[derive(Serialize)]
+struct Struct {
+    list: Vec<Option<String>>,
+}
+
+#[derive(Serialize)]
+struct ListStruct {
+    list: Vec<NewType<usize>>,
+}
+
+#[test]
+fn serialize_newstruct() {
+    let s = NewStruct {
+        list: vec!["hello".into(), "world".into()],
+    };
+    assert_eq!(
+        "list=hello&list=world".to_string(),
+        serde_urlencoded::to_string(s).unwrap()
+    );
+}
+
+#[test]
+fn serialize_vec_bool() {
+    let params = Wrapper {
+        item: vec![true, false, false],
+    };
+    assert_eq!(
+        serde_urlencoded::to_string(params).unwrap(),
+        "item=true&item=false&item=false".to_owned()
+    );
+}
+
+#[test]
+fn serialize_vec_num() {
+    let params = Wrapper {
+        item: vec![0, 1, 2],
+    };
+    assert_eq!(
+        serde_urlencoded::to_string(params).unwrap(),
+        "item=0&item=1&item=2".to_owned()
+    );
+}
+
+#[test]
+fn serialize_vec_str() {
+    let params = Wrapper {
+        item: vec!["hello", "world", "hello"],
+    };
+    assert_eq!(
+        serde_urlencoded::to_string(params).unwrap(),
+        "item=hello&item=world&item=hello".to_owned()
+    );
+}
+
+#[test]
+fn serialize_struct_opt() {
+    let s = Struct {
+        list: vec![Some("hello".into()), Some("world".into())],
+    };
+    assert_eq!(
+        "list=hello&list=world".to_string(),
+        serde_urlencoded::to_string(s).unwrap()
+    );
+}
+
+#[test]
+fn serialize_struct_newtype() {
+    let s = ListStruct {
+        list: vec![NewType(0), NewType(1)],
+    };
+    assert_eq!(
+        "list=0&list=1".to_string(),
+        serde_urlencoded::to_string(s).unwrap()
+    );
+}
+
+#[test]
+fn serialize_struct_unit_enum() {
+    let params = Wrapper {
+        item: vec![X::A, X::B, X::C],
+    };
+    assert_eq!(
+        serde_urlencoded::to_string(params),
+        Ok("item=A&item=B&item=C".to_owned())
+    );
+}
+
+#[test]
+fn serialize_map() {
+    let mut s = std::collections::BTreeMap::new();
+    s.insert("hello", "world");
+    s.insert("seri", "alize");
+    s.insert("matrix", "ruma");
+
+    let encoded = serde_urlencoded::to_string(s).unwrap();
+    assert_eq!("hello=world&matrix=ruma&seri=alize", encoded);
+}