Update to 2021 edition

Author: cuviper

Cargo.toml

@@ -11,7 +11,7 @@ repository = "https://github.com/rust-num/num-derive"
 version = "0.3.3"
 readme = "README.md"
 exclude = ["/bors.toml", "/ci/*", "/.github/*"]
-edition = "2018"
+edition = "2021"
 rust-version = "1.56.0"
 
 [dependencies]
@@ -28,13 +28,21 @@ name = "num_derive"
 proc-macro = true
 test = false
 
-# Most of the tests are left implicily detected, compiled for Rust 2018,
-# but let's try a few of them with the old 2015 edition too.
+# Most of the tests are left implicily detected, compiled for Rust 2021,
+# but let's try a few of them with the older editions too.
 
 [[test]]
 name = "newtype-2015"
 edition = "2015"
 
+[[test]]
+name = "newtype-2018"
+edition = "2018"
+
 [[test]]
 name = "trivial-2015"
 edition = "2015"
+
+[[test]]
+name = "trivial-2018"
+edition = "2018"

tests/newtype-2018.rs

@@ -0,0 +1,2 @@
+// Same source, just compiled for 2018 edition
+include!("newtype.rs");

tests/trivial-2018.rs

@@ -0,0 +1,2 @@
+// Same source, just compiled for 2018 edition
+include!("trivial.rs");