Enable portable-atomic's require-cas feature

Provide a better error message if the end user forgets to use cfg

Cargo.toml

@@ -14,7 +14,8 @@ rust-version = "1.38"
 
 [dependencies]
 lock_api_crate = { package = "lock_api", version = "0.4", optional = true }
-portable-atomic = { version = "1", optional = true, default-features = false }
+# Enable require-cas feature to provide a better error message if the end user forgets to use cfg.
+portable-atomic = { version = "1", optional = true, default-features = false, features = ["require-cas"], git = "https://github.com/taiki-e/portable-atomic.git", branch = "require-cas" }
 
 [features]
 default = ["lock_api", "mutex", "spin_mutex", "rwlock", "once", "lazy", "barrier"]