RFC: Add a replace method to Option

text/0000-option-replace.md

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+- Feature Name: `option-replace`
+- Start Date: 2017-01-16
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+This RFC proposes the addition of `Option::replace` to complete the `Option::take` method, it replaces the actual value in the option by `Some` with the value given in parameter, returning the old value if present, without deinitializing either one.
+
+# Motivation
+[motivation]: #motivation
+
+You can see the `Option` as a container and other containers already have this kind of method to change a value in-place like the [HashMap::replace](https://doc.rust-lang.org/std/collections/struct.HashSet.html#method.replace) method.
+
+How do you replace a value inside an `Option`, you can use `mem::replace` but it can be really unconvenient to import the `mem` module just for that. Why not adding a useful method to do that ?
+
+This is the symmetry of the already present `Option::take` method.
+
+# Detailed design
+[design]: #detailed-design
+
+This method will be added to the `core::option::Option` type implementation:
+
+```rust
+use core::mem::replace;
+
+impl<T> Option<T> {
+    // ...
+
+    pub fn replace(&mut self, value: T) -> Option<T> {
+        mem::replace(self, Some(value))
+    }
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It increases the size of the standard library by a tiny bit.
+
+# Alternatives
+[alternatives]: #alternatives
+
+- Don't use the `replace` name and use `give` instead in symmetry with the actual `take` method.
+- Use directly `mem::replace`.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.