Introduce Serializer::collect_seq

This function serializes the given iterator as a sequence. Its iter parameter
has type I: IntoIterator, <I as IntoIterator>::Item: Serialize, which means it
will work both for iterators passed by value, therefore consuming them, and as
the value for a #[serde(serialize_with)] attribute, where it will be called as
Serializer::collect_seq(&self.field, serializer), relying on the common practice
of implementing IntoIterator for &C where C is a data type representing some
kind of collection.

serde/src/ser/mod.rs

@@ -102,6 +102,7 @@ use error;
 use core::cell::RefCell;
 
 use core::fmt::Display;
+use core::iter::IntoIterator;
 
 mod impls;
 mod impossible;
@@ -242,7 +243,7 @@ pub trait Serialize {
 /// 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 {
+pub trait Serializer: Sized {
     /// 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
@@ -607,6 +608,23 @@ pub trait Serializer {
         variant: &'static str,
         len: usize,
     ) -> Result<Self::SerializeStructVariant, Self::Error>;
+
+    /// Collect an iterator as a sequence.
+    ///
+    /// The default implementation serializes each item yielded by the iterator
+    /// using `Self::SerializeSeq`. Implementors should not need to override
+    /// this method.
+    fn collect_seq<I>(self, iter: I) -> Result<Self::Ok, Self::Error>
+        where I: IntoIterator,
+              <I as IntoIterator>::Item: Serialize,
+    {
+        let iter = iter.into_iter();
+        let mut serializer = try!(self.serialize_seq(iterator_len_hint(&iter)));
+        for item in iter {
+            try!(serializer.serialize_element(&item));
+        }
+        serializer.end()
+    }
 }
 
 /// Returned from `Serializer::serialize_seq` and
@@ -826,3 +844,10 @@ pub fn iterator<I>(iter: I) -> Iterator<I>
         data: RefCell::new(Some(iter)),
     }
 }
+
+fn iterator_len_hint<I: Iterator>(iter: &I) -> Option<usize> {
+    match iter.size_hint() {
+        (lo, Some(hi)) if lo == hi => Some(lo),
+        _ => None,
+    }
+}