diff --git a/clippy_workspace_tests/Cargo.toml b/clippy_workspace_tests/Cargo.toml
index a282378c951..7a235b215d3 100644
--- a/clippy_workspace_tests/Cargo.toml
+++ b/clippy_workspace_tests/Cargo.toml
@@ -1,6 +1,7 @@
 [package]
 name = "clippy_workspace_tests"
 version = "0.1.0"
+edition = "2018"
 
 [workspace]
 members = ["subcrate"]
diff --git a/clippy_workspace_tests/src/main.rs b/clippy_workspace_tests/src/main.rs
index f79c691f085..0dcdf6383fd 100644
--- a/clippy_workspace_tests/src/main.rs
+++ b/clippy_workspace_tests/src/main.rs
@@ -1,2 +1,4 @@
+#![deny(rust_2018_idioms)]
+
 fn main() {
 }
diff --git a/mini-macro/Cargo.toml b/mini-macro/Cargo.toml
index 2cdba4c3236..a21e7fec6d4 100644
--- a/mini-macro/Cargo.toml
+++ b/mini-macro/Cargo.toml
@@ -11,6 +11,7 @@ authors = [
 license = "MPL-2.0"
 description = "A macro to test clippy's procedural macro checks"
 repository = "https://github.com/rust-lang/rust-clippy"
+edition = "2018"
 
 [lib]
 name = "clippy_mini_macro_test"
diff --git a/mini-macro/src/lib.rs b/mini-macro/src/lib.rs
index 0a96be71b35..ec489344a6f 100644
--- a/mini-macro/src/lib.rs
+++ b/mini-macro/src/lib.rs
@@ -1,4 +1,5 @@
 #![feature(proc_macro_quote, proc_macro_hygiene)]
+#![deny(rust_2018_idioms)]
 extern crate proc_macro;
 
 use proc_macro::{TokenStream, quote};