Add unsafe member attribute

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+- Start Date: 2015-01-14
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Adds the possibility to mark an member as unsafe to access.
+
+# Motivation
+
+Struct are commonly used to encapsulate possibly unsafe code by providing a safe interface. A private member variable may be used to store a state on which the `unsafe` code relies in order to provide a safe interface. However, it is possible for any other function in the same module to change this state without requiring an `unsafe` block or being marked as being unsafe. Thus the whole module and not only the unsafe parts require extensive auditing to ensure safety. An example is `Vec` which requires its `length` to be set correctly.
+
+# Detailed design
+
+This RFC proposes to introduce the member attribute `unsafe` (like the previous `priv` attribute) which can be used to mark a member as unsafe to change, indicating that special care is required, when changing them. The presence of this attribute requires an `unsafe`-block or function for every `mut` access to this member. `unsafe` seems to be the most sensible choice for the attribute name since it is already present in the language. Thus it can intuitively related to the other uses.
+
+The implementation should be straight-forward since member attributes once were part of the language.
+
+## Example
+
+    struct Vec {
+        unsafe length: usize
+    }
+
+    fn change_length(v: &mut Vec) {
+        unsafe {  v.length = 0 }
+    }
+
+# Drawbacks
+
+Introduces complexity to the language.
+
+# Alternatives
+
+The alternative would be to require an unsafe block for every `mut` access to any member `unsafe` blocks or functions depend on. This could be implementing by automatically tracing such dependencies in the AST.
+
+If this RFC is not accepted we would stay at the status quo: Modules which contain structures that encapsulate possibly unsafe behavior require overall special attention.
+
+# Unresolved questions
+
+None.