Introduce Option::<&T>::borrowed

text/0000-option_borrowed.md

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+- Feature Name: `option_borrowed`
+- Start Date: 2016-11-14
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+The standard library provides the `Option::<&T>::cloned` method for `T: Clone`
+as the standard way to convert from `Option<&T>` to `Option<T>`. This RFC
+proposes the addition of `Option::<&T>::borrowed` for `T: Borrow<U>` to convert
+from `Option<&T>` to `Option<&U>`.
+
+# Motivation
+[motivation]: #motivation
+
+How to convert from `Option<String>` to `Option<&str>` is sometimes asked on
+Stack Overflow [1] [2]. This use case is also common in Servo, along other
+cases that would also be covered by such a method.
+
+[1] http://stackoverflow.com/q/31233938
+[2] http://stackoverflow.com/q/34974732
+
+# Detailed design
+[design]: #detailed-design
+
+This implementation will be added to the `core::option` module:
+
+```rust
+use core::borrow::Borrow;
+
+impl<'a, T> Option<&'a T> {
+    fn borrowed<U>(self) -> Option<&'a U> where T: Borrow<U> {
+        self.map(T::borrow)
+    }
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+None.
+
+# Alternatives
+[alternatives]: #alternatives
+
+We could instead add a `dereferenced` method using the Deref trait, but that's
+a mouthful and is less flexible than using `Borrow`.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.