RFC to remove some special treatment of Box by borrow checker

active/0000-box-not-special.md

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+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Remove special treatment of `Box<T>` from the borrow checker.
+
+# Motivation
+
+Currently the `Box<T>` type is special-cased and converted to the old
+`~T` internally. This is mostly invisible to the user, but it shows up
+in some places that give special treatment to `Box<T>`. This RFC is
+specifically concerned with the fact that the borrow checker has
+greater precision when derefencing `Box<T>` vs other smart pointers
+that rely on the `Deref` traits. Unlike the other kinds of special
+treatment, we do not currently have a plan for how to extend this
+behavior to all smart pointer types, and hence we would like to remove
+it.
+
+Here is an example that illustrates the extra precision afforded to
+`Box<T>` vs other types that implement the `Deref` traits. The
+following program, written using the `Box` type, compiles
+successfully:
+
+    struct Pair {
+        a: uint,
+        b: uint
+    }
+
+    fn example1(mut smaht: Box<Pair>) {
+        let a = &mut smaht.a;
+        let b = &mut smaht.b;
+        ...
+    }
+
+This program compiles because the type checker can see that
+`(*smaht).a` and `(*smaht).b` are always distinct paths. In contrast,
+if I use a smart pointer, I get compilation errors:
+
+    fn example2(cell: RefCell<Pair>) {
+        let mut smaht: RefMut<Pair> = cell.borrow_mut();
+        let a = &mut smaht.a;
+
+        // Error: cannot borrow `smaht` as mutable more than once at a time
+        let b = &mut smaht.b;
+    }
+
+To see why this, consider the desugaring:
+
+    fn example2(smaht: RefCell<Pair>) {
+        let mut smaht = smaht.borrow_mut();
+
+        let tmp1: &mut Pair = smaht.deref_mut(); // borrows `smaht`
+        let a = &mut tmp1.a;
+
+        let tmp2: &mut Pair = smaht.deref_mut(); // borrows `smaht` again!
+        let b = &mut tmp2.b;
+    }
+
+It is a violation of the Rust type system to invoke `deref_mut` when
+the reference to `a` is valid and usable, since `deref_mut` requires
+`&mut self`, which in turn implies no alias to `self` or anything
+owned by `self`.
+
+This desugaring suggests how the problem can be worked around in user
+code. The idea is to pull the result of the deref into a new temporary:
+
+    fn example3(smaht: RefCell<Pair>) {
+        let mut smaht: RefMut<Pair> = smaht.borrow_mut();
+        let temp: &mut Pair = &mut *smaht;
+        let a = &mut temp.a;
+        let b = &mut temp.b;
+    }
+
+# Detailed design
+
+Removing this treatment from the borrow checker basically means
+changing the construction of loan paths for unique pointers.
+
+I don't actually know how best to implement this in the borrow
+checker, particularly concerning the desire to keep the ability to
+move out of boxes and use them in patterns. This requires some
+investigation. The easiest and best way may be to "do it right" and is
+probably to handle derefs of `Box<T>` in a similar way to how
+overloaded derefs are handled, but somewhat differently to account for
+the possibility of moving out of them. Some investigation is needed.
+
+# Drawbacks
+
+The borrow checker rules are that much more restrictive.
+
+# Alternatives
+
+We have ruled out inconsistent behavior between `Box` and other smart
+pointer types. We considered a number of ways to extend the current
+treatment of box to other smart pointer types:
+
+1. *Require* compiler to introduce deref temporaries automatically
+   where possible. This is plausible as a future extension but
+   requires some thought to work through all cases. It may be
+   surprising. Note that this would be a required optimization because
+   if the optimization is not performed it affects what programs can
+   successfully type check. (Naturally it is also observable.)
+
+2. Some sort of unsafe deref trait that acknolwedges possibliity of
+   other pointers into the referent. Unappealing because the problem
+   is not that bad as to require unsafety.
+
+3. Determining conditions (perhaps based on parametricity?) where it
+   is provably safe to call deref. It is dubious and unknown if such
+   conditions exist or what that even means. Rust also does not really
+   enjoy parametricity properties due to presence of reflection and
+   unsafe code.
+
+# Unresolved questions
+
+Best implementation strategy.