Edition notes about ?

src/SUMMARY.md

@@ -53,6 +53,7 @@
     - [Macros](rust-2018/macros/index.md)
       - [Custom Derive](rust-2018/macros/custom-derive.md)
       - [Macro changes](rust-2018/macros/macro-changes.md)
+      - [At most one repetition](rust-2018/macros/at-most-once.md)
     - [The compiler](rust-2018/the-compiler/index.md)
       - [Improved error messages](rust-2018/the-compiler/improved-error-messages.md)
       - [Incremental Compilation for faster compiles](rust-2018/the-compiler/incremental-compilation-for-faster-compiles.md)

src/rust-2018/macros/at-most-once.md

@@ -0,0 +1,38 @@
+# At most one repetition
+
+In Rust 2018, we have made a couple of changes to the macros-by-example syntax.
+
+1. We have added a new Kleene operator `?` which means "at most one"
+   repetition. This operator does not accept a separator token.
+2. We have disallowed using `?` as a separator to remove ambiguity with `?`.
+
+For example, consider the following Rust 2015 code:
+
+```rust2018
+macro_rules! foo {
+    ($a:ident, $b:expr) => {
+        println!("{}", $a);
+        println!("{}", $b);
+    }
+    ($a:ident) => {
+        println!("{}", $a);
+    }
+}
+```
+
+Macro `foo` can be called with 1 or 2 arguments; the second one is optional,
+but you need a whole other matcher to represent this possibility. This is
+annoying if your matchers are long. In Rust 2018, one can simply write the
+following:
+
+```rust2018
+macro_rules! foo {
+    ($a:ident $(, $b:expr)?) => {
+        println!("{}", $a);
+
+        $(
+            println!("{}", $b);
+         )?
+    }
+}
+```