Merge pull request rust-lang#112 from pcwalton/remove-cross-borrowing

Accepted RFC rust-lang#32: Remove cross-borrowing

active/0032-remove-cross-borrowing.md

@@ -0,0 +1,31 @@
+- Start Date: 2014-06-09
+- RFC PR #: 112
+- Rust Issue #: #10504
+
+# Summary
+
+Remove the coercion from `Box<T>` to `&mut T` from the language.
+
+# Motivation
+
+Currently, the coercion between `Box<T>` to `&mut T` can be a hazard because it can lead to surprising mutation where it was not expected.
+
+# Detailed design
+
+The coercion between `Box<T>` and `&mut T` should be removed.
+
+Note that methods that take `&mut self` can still be called on values of type `Box<T>` without any special referencing or dereferencing. That is because the semantics of auto-deref and auto-ref conspire to make it work: the types unify after one autoderef followed by one autoref.
+
+# Drawbacks
+
+Borrowing from `Box<T>` to `&mut T` may be convenient.
+
+# Alternatives
+
+An alternative is to remove `&T` coercions as well, but this was decided against as they are convenient.
+
+The impact of not doing this is that the coercion will remain.
+
+# Unresolved questions
+
+None.