RFC: movable array iterators

text/0000-array_into_iter_move.md

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+- Feature Name: array_into_iter_move
+- Start Date: 2017-10-21
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Implement into_iter on array types (not reference, so it moves).
+
+# Motivation
+[motivation]: #motivation
+
+Arrays are particularly useful in `flat_map`, but currently a Vec is required as a movable
+iterator because arrays only have slice iterator semantics. Obviously this is not optimal.
+
+Now with `ManuallyDrop` implemented, it should be possible to implement moving into_iter on arrays.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+You can convert an array into an iterator with move, just like Vec:
+
+```rust
+for x in [1, 2] {
+    // x is {integer} instead of &{integer}
+}
+```
+
+```rust
+let v: Vec<_> = (0..5).flat_map(|x| [x, x*2]).collect();
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Proposed implementation:
+
+- a type `array::IntoIter` is created. The `array::into_iter` method is implemented.
+- Inside the method:
+  * Wrap the array with `ManuallyDrop`.
+  * Create a `IntoIter` struct with contents moved.
+- Inside the iterator:
+  * Keep track of valid range (index) and move (`ptr::read`) items out as `next()` is called.
+  * Don't forget to drop the items if the iterator itself is dropped in middle. This should be done with `drop_in_place`.
+
+We should add a lint for redundant Vec in clippy to promote the use of this.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This is not 100% backwards compatible as it changes the signature of into_iter (only if directly
+called like below). A crater run is required.
+
+```rust
+[1, 2].into_iter();
+// This was originally yielding references, but now values.
+```
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+TBD
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- What should the type signature look like? Possibly utilizing const generics.