Document deprecation of anon params

src/SUMMARY.md

@@ -32,6 +32,7 @@
       - [`dyn Trait` for trait objects](rust-2018/trait-system/dyn-trait-for-trait-objects.md)
       - [More container types support trait objects](rust-2018/trait-system/more-container-types-support-trait-objects.md)
       - [Associated constants](rust-2018/trait-system/associated-constants.md)
+      - [No more anonymous parameters](rust-2018/trait-system/no-anon-params.md)
     - [Slice patterns](rust-2018/slice-patterns.md)
     - [Ownership and lifetimes](rust-2018/ownership-and-lifetimes/index.md)
       - [Default `match` bindings](rust-2018/ownership-and-lifetimes/default-match-bindings.md)

src/rust-2018/trait-system/no-anon-params.md

@@ -0,0 +1,20 @@
+# No more anonymous trait parameters
+
+In accordance with RFC [#1685](https://github.com/rust-lang/rfcs/pull/1685),
+parameters in trait method declarations are no longer allowed to be anonymous.
+
+For example, in the 2015 edition, this was allowed:
+```rust
+trait Foo {
+    fn foo(&self, u8);
+}
+```
+
+In the 2018 edition, all parameters must be given an argument name  (even if it's just
+`_`):
+
+```rust
+trait Foo {
+    fn foo(&self, baz: u8);
+}
+```