Document closure to fn coercion feature

src/type-coercions.md

@@ -1,8 +1,10 @@
 # Type coercions
 
-Coercions are defined in [RFC 401]. A coercion is implicit and has no syntax.
+Coercions are defined in [RFC 401]. [RFC 1558] then expanded on that.
+A coercion is implicit and has no syntax.
 
 [RFC 401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
+[RFC 1558]: https://github.com/rust-lang/rfcs/blob/master/text/1558-closure-to-fn-coercion.md
 
 ## Coercion sites
 
@@ -143,3 +145,5 @@ Coercion is allowed between the following types:
     In the future, coerce_inner will be recursively extended to tuples and
     structs. In addition, coercions from sub-traits to super-traits will be
     added. See [RFC 401] for more details.
+
+* Non capturing closures to `fn` pointers

src/types.md

@@ -286,9 +286,22 @@ more of the closure traits:
 * `Fn`
   : The closure can be called multiple times through a shared reference.
     A closure called as `Fn` can neither move out from nor mutate values
-    from its environment. `Fn` inherits from `FnMut`, which itself
-    inherits from `FnOnce`.
+    from its environment, but read-only access to such values is allowed.
+    `Fn` inherits from `FnMut`, which itself inherits from `FnOnce`.
 
+Closures that don't use anything from their environment ("non capturing closures")
+can be coerced to function pointers (`fn`) with the matching signature.
+To adopt the example from the section above:
+
+```rust
+let add = |x, y| x + y;
+
+let mut x = add(5,7);
+
+type Binop = fn(i32, i32) -> i32;
+let bo: Binop = add;
+x = bo(5,7);
+```
 
 ## Trait objects