Fix description of lifetime elision in impl

Fixes #84

Author: steveklabnik

src/rust-2018/ownership-and-lifetimes/lifetime-elision-in-impl.md

@@ -2,19 +2,74 @@
 
 ![Minimum Rust version: nightly](https://img.shields.io/badge/Minimum%20Rust%20Version-nightly-red.svg)
 
-When writing an `impl`, you can mention lifetimes without them being bound in
-the argument list.
+When writing `impl` blocks, you can now elide lifetime annotations in some
+situations.
 
-In Rust 2015:
+Consider a trait like `MyIterator`:
 
 ```rust,ignore
-impl<'a> Iterator for MyIter<'a> { ... }
-impl<'a, 'b> SomeTrait<'a> for SomeType<'a, 'b> { ... }
+trait MyIterator {
+    type Item;
+    fn next(&mut self) -> Option<Self::Item>;
+}
 ```
 
-In Rust 2018:
+In Rust 2015, if we wanted to implement this iterator for mutable references
+to `Iterators`, we'd need to write this:
 
 ```rust,ignore
-impl Iterator for MyIter<'iter> { ... }
-impl SomeTrait<'tcx> for SomeType<'tcx, 'gcx> { ... }
-```
+impl<'a, I: MyIterator> MyIterator for &'a mut I {
+    type Item = I::Item;
+    fn next(&mut self) -> Option<Self::Item> {
+        (*self).next()
+    }
+}
+```
+
+Note all of the `'a` annotations. In Rust 2018, we can write this:
+
+```rust,ignore
+impl<I: MyIterator> MyIterator for &mut I {
+    type Item = I::Item;
+    fn next(&mut self) -> Option<Self::Item> {
+        (*self).next()
+    }
+}
+```
+
+Similarly, lifetime annotations can appear due to a struct that contains
+references:
+
+```rust,ignore
+struct SetOnDrop<'a, T> {
+    borrow: &'a mut T,
+    value: Option<T>,
+}
+```
+
+In Rust 2015, to implement `Drop` on this struct, we'd write:
+
+```rust,ignore
+impl<'a, T> Drop for SetOnDrop<'a, T> {
+    fn drop(&mut self) {
+        if let Some(x) = self.value.take() {
+            *self.borrow = x;
+        }
+    }
+}
+```
+
+But in Rust 2018, we can combine elision with [the anonymous lifetime] and
+write this instead.
+
+```rust,ignore
+impl<T> Drop for SetOnDrop<'_, T> {
+    fn drop(&mut self) {
+        if let Some(x) = self.value.take() {
+            *self.borrow = x;
+        }
+    }
+}
+```
+
+[the anonymous lifetime]: the-anonymous-lifetime.html