Rust unstable book: basic desc and example for conservative_impl_trait.

Author: frewsxcv

src/doc/unstable-book/src/conservative-impl-trait.md

@@ -6,5 +6,61 @@ The tracking issue for this feature is: [#34511]
 
 ------------------------
 
+The `conservative_impl_trait` feature allows a conservative form of abstract
+return types.
 
+Abstract return types allow a function to hide a concrete return type behind a
+trait interface similar to trait objects, while still generating the same
+statically dispatched code as with concrete types.
 
+## Examples
+
+```rust
+#![feature(conservative_impl_trait)]
+
+fn even_iter() -> impl Iterator<Item=u32> {
+    (0..).map(|n| n * 2)
+}
+
+fn main() {
+    let first_four_even_numbers = even_iter().take(4).collect::<Vec<_>>();
+    assert_eq!(first_four_even_numbers, vec![0, 2, 4, 6]);
+}
+```
+
+## Background
+
+In today's Rust, you can write function signatures like:
+
+````rust,ignore
+fn consume_iter_static<I: Iterator<u8>>(iter: I) { }
+
+fn consume_iter_dynamic(iter: Box<Iterator<u8>>) { }
+````
+
+In both cases, the function does not depend on the exact type of the argument.
+The type held is "abstract", and is assumed only to satisfy a trait bound.
+
+* In the `_static` version using generics, each use of the function is
+  specialized to a concrete, statically-known type, giving static dispatch,
+  inline layout, and other performance wins.
+* In the `_dynamic` version using trait objects, the concrete argument type is
+  only known at runtime using a vtable.
+
+On the other hand, while you can write:
+
+````rust,ignore
+fn produce_iter_dynamic() -> Box<Iterator<u8>> { }
+````
+
+...but you _cannot_ write something like:
+
+````rust,ignore
+fn produce_iter_static() -> Iterator<u8> { }
+````
+
+That is, in today's Rust, abstract return types can only be written using trait
+objects, which can be a significant performance penalty. This RFC proposes
+"unboxed abstract types" as a way of achieving signatures like
+`produce_iter_static`. Like generics, unboxed abstract types guarantee static
+dispatch and inline data layout.