RFC: Add DerefMove

active/0000-deref-move.md

@@ -0,0 +1,76 @@
+- Start Date: 2014-07-23
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+Summary
+=======
+
+Add a new lang-item-defined trait, `DerefMove`, which allows moves out of custom
+pointer types.
+
+Motivation
+==========
+
+There has been some effort to remove special casing from `Box`—[RFC PR
+130](https://github.com/rust-lang/rfcs/pull/130) and RFC PRs
+[139](https://github.com/rust-lang/rfcs/pull/139) and
+[112](https://github.com/rust-lang/rfcs/pull/112) are examples of this trend.
+However, there is still one more problem with `Box` that would have to be fixed
+for `Box` to be properly special-case-free (bar the construction syntax and
+patterns, which is mostly acceptable because of the plan to support `box`
+for other pointer types): `Box` pointers can be moved out of, even though
+none of the `Deref*` traits support moves.
+
+Detailed design
+===============
+
+Add a new trait, `DerefMove`, with a corresponding lang item, `deref_move`, and
+define it in `core::ops`:
+
+```rust
+#[lang="deref_move"]
+pub trait DerefMove<Result> {
+    /// The method called to move out of a dereference
+    fn deref_move(self) -> Result;
+}
+```
+
+The `deref_move` method would be called in any of the following situations:
+
+* Something is being dereferenced, and the only `Deref*` trait it implements is
+  `DerefMove`;
+* Something is being dereferenced, and while it *does* implement `Deref` and/or
+  `DerefMut`, it also implements `DerefMove` and a value is being moved out of
+  the dereference.
+
+This applies to implicit derefences as well.
+
+Remove all special treatment of `Box` by the borrow checker. Instead, `Box`
+implements `DerefMove` in the standard library roughly as follows:
+
+```rust
+impl<T> DerefMove<T> for Box<T> {
+    fn deref_move(self) -> T {
+        let Box(ptr) = self;
+        std::ptr::read(ptr)
+    }
+}
+```
+
+Drawbacks
+=========
+
+Adds yet another `Deref*` trait and another lang item, adding complexity to the
+language.
+
+Alternatives
+============
+
+* Do nothing, and blame `Box`’s special-casing on the fact that it is a lang item
+  anyway.
+* Add a `DerefSet` trait as well, for assignments of the form `*ptr = val`.
+
+Unresolved questions
+====================
+
+None.