RFC - Add an owning "borrowed" pointer type &move

text/0000-move-pointer.md

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+- Feature Name: `move_pointer`
+- Start Date: 2016-05-12
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Introduce a new pointer type `&move` that logically owns the pointed-to data, but does not control the backing memory. Also introduces the DerefMove trait to provide access to such a pointer.
+
+# Motivation
+[motivation]: #motivation
+
+This provides an elegant solution to passing DSTs by value, allowing `Box<FnOnce>` to work. It also will allow other usecases where trait methods should take self by "value".
+
+# Detailed design
+[design]: #detailed-design
+
+- Add a new pointer type `&move T`
+ - This pointer will get the same lifetime rules as `&` and `&mut` (such that it cannot outlive the allocation)
+ - but, it will allow the value to become invalid before the allocation does.
+ - Deref coercions as per RFC #241 apply.
+ - Variables of type `&move T` will only allow mutating `T` if they themselves are `mut`
+- Add a new unary operation `&move <value>`. With a special case such that `&move |x| x` parses as a move closure, requiring parentheses to parse as an owned pointer to a closure.
+ - This precedence can be implemented by ignoring the `move` when parsing unary operators if it is followed by a `|` or `||` token.
+- Add a new operator trait `DerefMove` to allow smart pointer types to return owned pointers to their contained data. 
+ - A type that implements `DerefMove` cannot implement `Drop` (as `DerefMove` provides equivalent functinality)
+
+```rust
+trait DerefMove: DerefMut
+{
+    /// Return an owned pointer to inner data
+    fn deref_move(&mut self) -> &move Self::Target;
+    /// Equivalent to `Drop::drop` except that the destructor for `Self::Target` is not called
+    fn deallocate(&mut self);
+}
+```
+
+When an owned pointer is dropped (without having been moved out of), the destructor for the contained data is called (unlike `&mut` pointers, which are just borrows). The backing memory for this pointer is not freed until a point after the `&move` is dropped (likely either at the end of the statement, or at the end of the owning block).
+
+For example, the following code moves out of a `Box<T>` into a `&move T` and passes it to a function
+```rust
+fn takes_move(val: &move SomeStruct) {
+    // ...
+}
+fn main() {
+    let val = Box::new( SomeStruct::new() );
+    takes_move( &move val );
+    println!("Hello");
+}
+```
+This becomes the following operations
+```rust
+fn main() {
+    let val = Box::new( SomeStruct::new() );
+    takes_move( DerefMove::deref_move(&mut val) );
+    DerefMove::deallocate(&mut val);
+    println!("Hello");
+}
+```
+
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- Adding a new pointer type to the language is a large change
+
+# Alternatives
+[alternatives]: #alternatives
+
+- Previous discussions have used `&own` as the pointer type
+ - This name is far closer to the actual nature of the pointer.
+ - But, since `own` is not a reserved word, such a change would be breaking.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Potential interactions of what happens when a `&move` is stored.
+ - If a `&move` is stored in the current scope, when is the original storage freed?
+ - If a `&move` isn't stored, is the storage freed right then, or when it would have otherwise gone out of scope?
+
+- `IndexMove` trait to handle moving out of collection types in a similar way to `DerefMove`
+- Should (can?) the `box` destructuring pattern be implemented using `DerefMove`?
+ - This would be a larger RFC specifying the `box` pattern using the `Deref` family of traits
+
+
+# Appendix: Implementations of `DerefMove`
+```rust
+impl<T: ?Sized> DerefMove for Box<T>
+{
+    fn deref_move(&mut self) -> &move Self::Target {
+        unsafe {
+            &move *(self.0)
+        }
+    }
+    fn deallocate(&mut self) {
+        unsafe {
+            heap::deallocate(self.0, mem::size_of_val(&*self.0), mem::align_of_val(&*self.0));
+        }
+    }
+}
+```
+