Set all documentation examples to 2018 edition
diff --git a/src/lib.rs b/src/lib.rs
index ff43d3e..b4590dd 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -64,7 +64,7 @@
//! proc-macro = true
//! ```
//!
-//! ```rust
+//! ```edition2018
//! # extern crate quote;
//! #
//! extern crate proc_macro;
@@ -97,7 +97,7 @@
//!
//! [`heapsize`]: https://github.com/dtolnay/syn/tree/master/examples/heapsize
//!
-//! ```rust
+//! ```edition2018
//! pub trait HeapSize {
//! /// Total number of bytes of heap memory owned by `self`.
//! fn heap_size_of_children(&self) -> usize;
@@ -107,7 +107,7 @@
//! The custom derive allows users to write `#[derive(HeapSize)]` on data
//! structures in their program.
//!
-//! ```rust
+//! ```edition2018
//! # const IGNORE_TOKENS: &str = stringify! {
//! #[derive(HeapSize)]
//! # };
@@ -125,7 +125,7 @@
//! compiler's error messages are displayed in user code. Consider the error the
//! user sees if one of their field types does not implement `HeapSize`.
//!
-//! ```rust
+//! ```edition2018
//! # const IGNORE_TOKENS: &str = stringify! {
//! #[derive(HeapSize)]
//! # };
@@ -159,7 +159,7 @@
//! The example reimplements the popular `lazy_static` crate from crates.io as a
//! procedural macro.
//!
-//! ```
+//! ```edition2018
//! # macro_rules! lazy_static {
//! # ($($tt:tt)*) => {}
//! # }
@@ -429,7 +429,7 @@
///
/// [`Visit`]: trait.Visit.html
///
- /// ```rust
+ /// ```edition2018
/// # use syn::{Attribute, BinOp, Expr, ExprBinary};
/// #
/// pub trait Visit<'ast> {
@@ -465,7 +465,7 @@
///
/// [`VisitMut`]: trait.VisitMut.html
///
- /// ```rust
+ /// ```edition2018
/// # use syn::{Attribute, BinOp, Expr, ExprBinary};
/// #
/// pub trait VisitMut {
@@ -501,7 +501,7 @@
///
/// [`Fold`]: trait.Fold.html
///
- /// ```rust
+ /// ```edition2018
/// # use syn::{Attribute, BinOp, Expr, ExprBinary};
/// #
/// pub trait Fold {
@@ -584,7 +584,7 @@
///
/// # Examples
///
-/// ```rust
+/// ```edition2018
/// # extern crate quote;
/// #
/// extern crate proc_macro;
@@ -646,7 +646,7 @@
///
/// # Examples
///
-/// ```rust
+/// ```edition2018
/// use syn::{Expr, Result};
///
/// fn run() -> Result<()> {
@@ -680,7 +680,7 @@
///
/// # Examples
///
-/// ```rust,no_run
+/// ```edition2018,no_run
/// use std::error::Error;
/// use std::fs::File;
/// use std::io::Read;