Implement FromIterator for the unit type

text/0000-unit-from-iterator.md

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+- Feature Name: unit_from_iterator
+- Start Date: 2015-05-20
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Implement `std::iter::FromIterator<()>` for the empty tuple type `()`, also known as “unit”.
+
+# Motivation
+
+The `Result` type implements `FromIterator` in a way that creates a result out of an iterable of results, extracting the first error if any, and otherwise returning `Ok` with the contents of all values:
+
+```rust
+println!("{:?}", Result::<Vec<i32>, MyError>::from_iter(vec![Ok(4), Ok(7), Ok(-3903)])); // Ok([4, 7, -3903])
+println!("{:?}", Result::<Vec<i32>, MyError>::from_iter(vec![Ok(5), Err(MyError::DivisionByZero), Ok(842), Err(MyError::Overflow)])); // Err(DivisionByZero)
+```
+
+Implementing this RFC would allow this pattern to be used with iterables whose item type is of the form `Result<(), T>`.
+
+For example, suppose we have a function which moves all values from a `mpsc::Receiver` into a `mpsc::Sender`. Currently, this could be written as follows:
+
+```rust
+fn forward_values<T>(src: Receiver<T>, dst: Sender<T>) -> Result<(), SendError<T>> {
+    src.iter().map(|val| dst.send(val)).fold(Ok(()), Result::and))
+}
+```
+
+This has the flaw of exhausting the receiver even after an error is encountered. With the proposed trait implementation, it could be refactored into the following:
+
+```rust
+fn forward_values<T>(src: Receiver<T>, dst: Sender<T>) -> Result<(), SendError<T>> {
+    src.iter().map(|val| dst.send(val)).collect()
+}
+```
+
+This version of the function immediately returns when the first error is encountered.
+
+# Detailed design
+
+Implement the trait `std::iter::FromIterator<()>` for the primitive type `()`.
+
+The implementation is very short:
+
+```rust
+impl FromIterator<()> for () {
+    fn from_iter<T>(_: T) -> () where T: IntoIterator<Item = ()> {
+        ()
+    }
+}
+```
+
+# Drawbacks
+
+The only known drawback is that the use-cases for this functionality seem to be quite limited and as such may not warrant an addition to the standard library. However, the `()` type has only one possible value, so if more use-cases requiring an implementation of this trait for `()` are found, the proposed implementation must already be correct.
+
+# Alternatives
+
+*   Do nothing. The same short-circuiting behavior shown in the example can be achieved by using a for loop and `try!`:
+
+    ```rust
+    fn forward_values<T>(src: Receiver<T>, dst: Sender<T>) -> Result<(), SendError<T>> {
+        for val in src {
+            try!(dst.send(val));
+        }
+        Ok(())
+    }
+    ```
+*   Add a special-cased `FromIterator` implementation for `Result`:
+
+    ```rust
+    Impl<E> FromIterator<Result<(), E>> for Result<(), E> {
+        fn from_iter<T>(iterable: T) -> Result<(), E> where T: Iterator<Item = Result<(), E>> {
+            for val in iterable {
+                try!(val);
+            }
+            Ok(())
+        }
+    }
+    ```
+
+# Unresolved questions
+
+*   Should the iterator be exhausted rather than discarded? An exhausting implementation:
+
+    ```rust
+    impl FromIterator<()> for () {
+        fn from_iter<T>(iter: T) -> () where T: IntoIterator<Item = ()> {
+            for _ in iter {}
+        }
+    }
+    ```
+
+    This would decrease performance in the `Result` example, but open up other possibilities for using this where for loops are used today.
+
+    *   If so, should the implementation be expanded to exhaust iterables of any item type?
+
+        ```rust
+        impl<X> FromIterator<X> for () {
+             fn from_iter<T>(iter: T) -> () where T: IntoIterator<Item = X> {
+                  for _ in iter {}
+             }
+        }
+        ```
+
+        This would allow `.collect::<()>()` to be used as a general-purpose “exhaust and discard the entire iterable” function.