diff --git a/Cargo.toml b/Cargo.toml
index afec3b2..5fec1b4 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,6 +2,7 @@
 name = "mandelbrot"
 version = "0.2.0"
 authors = ["Jim Blandy <jimb@red-bean.com>"]
+edition = "2018"
 
 [dependencies]
 num = "0.1.34"