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"
diff --git a/src/main.rs b/src/main.rs
index 1b5f252..5b08585 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,3 @@
-extern crate num;
 use num::Complex;
 
 /// Try to determine if `c` is in the Mandelbrot set, using at most `limit`
@@ -127,8 +126,6 @@ fn render(pixels: &mut [u8],
     }
 }
 
-extern crate image;
-
 use image::ColorType;
 use image::png::PNGEncoder;
 use std::fs::File;