Raw pointer coercion

active/0000-raw-pointer-coercion.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
+
+`*mut T` and `&mut T` should coerce to `*const T`.
+
+# Motivation
+
+Precedence:
+
+- `&mut T` coerces to `&T`
+- `&T` coerces to `*const T`
+- `&mut T` coerces to `*mut T`
+
+Converting from `*mut` to `*const` is a safe operation which happens regularly in FFI code:
+
+```c
+T *new(void);
+void non_mutating_operation(T const *);
+void mutating_operation(T *);
+```
+
+```rust
+struct TWrapper {
+    raw: *mut T,
+}
+
+impl TWrapper {
+    fn non_mutating_operation(&self) {
+        unsafe { non_mutating_operation(self.raw as *const T); }
+    }
+
+    fn mutating_operation(&mut self) {
+        unsafe { mutating_operation(self.raw); }
+    }
+}
+```
+
+# Drawbacks
+
+None right now.
+
+# Detailed design
+
+Add these rules:
+
+- `*mut T` coerces to `*const T`
+- `&mut T` coerces to `*const T`
+
+So that the following code compiles:
+
+```rust
+let a = &mut 1u;
+let b: *mut uint = a;
+
+let x: *const uint = a;
+let y: *const uint = b;
+```
+
+# Alternatives
+
+Leave it the way it is.
+
+# Unresolved questions
+
+None right now.