RFC: Add DerefMove

active/0000-deref-move.md

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+- 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.
+
+Additionally, this lets custom pointer types like `Cell` be dereferenced. At the
+moment, `Cell` doesn’t implement `Deref` or `DerefMut`, because the contents of
+`Cell` pointers can’t be referenced for safety reasons. This would allow read
+access to the contents of `Cell` pointers (although admittedly not the ability
+to modify them).
+
+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;
+}
+```
+
+In order to determine which method to use when dereferencing with `*`, the
+following rules are used:
+
+1. If the type implements `DerefMove` but not `Deref` or `DerefMut`, all
+   dereferences are done using `deref_move`.
+2. If a reference is being taken to a dereference (e.g. `&*ptr`), call `deref`
+   or `deref_mut`, depending on the mutability of the reference.
+3. If a value is being dereferenced and implements `DerefMove` and `Deref`:
+  1. If the type is `Copy`, call `deref_move`.
+  2. Otherwise:
+    1. If the type implements `Deref<T>` where `T: Copy`, call `*deref`.
+    2. Otherwise, call `deref_move`.
+4. If a value is being dereferenced and does not implement `DerefMove` but does
+   implement `Deref`, use `deref`.
+
+This applies to implicit dereferences as well.
+
+This means that (at least with DST) it is now possible remove all special
+treatment of `Box` by the borrow checker. To do this, we would have add an
+implementation of `DerefMove` for `Box` 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)
+    }
+}
+```
+
+and also add similar implementations of `Deref` and `DerefMut` to the standard
+library. With these changes, it’s now possible to finally remove all previously
+built-in dereference functionality for `Box` from the language, because all
+dereference functionality is now provided by the standard library.
+
+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.