Deny warnings in test modules

tests/compile-test.rs

@@ -1,5 +1,7 @@
 #![feature(test)] // compiletest_rs requires this attribute
 #![feature(once_cell)]
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2018_idioms, unused_lifetimes)]
 
 use compiletest_rs as compiletest;
 use compiletest_rs::common::Mode as TestMode;
@@ -31,11 +33,17 @@ static TEST_DEPENDENCIES: &[&str] = &[
 
 // Test dependencies may need an `extern crate` here to ensure that they show up
 // in the depinfo file (otherwise cargo thinks they are unused)
+#[allow(unused_extern_crates)]
 extern crate clippy_utils;
+#[allow(unused_extern_crates)]
 extern crate derive_new;
+#[allow(unused_extern_crates)]
 extern crate if_chain;
+#[allow(unused_extern_crates)]
 extern crate itertools;
+#[allow(unused_extern_crates)]
 extern crate quote;
+#[allow(unused_extern_crates)]
 extern crate syn;
 
 fn host_lib() -> PathBuf {

tests/dogfood.rs

@@ -6,6 +6,8 @@
 // Dogfood cannot run on Windows
 #![cfg(not(windows))]
 #![feature(once_cell)]
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2018_idioms, unused_lifetimes)]
 
 use std::lazy::SyncLazy;
 use std::path::PathBuf;

tests/fmt.rs

@@ -1,3 +1,6 @@
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2018_idioms, unused_lifetimes)]
+
 use std::path::PathBuf;
 use std::process::Command;
 

tests/integration.rs

@@ -1,4 +1,6 @@
 #![cfg(feature = "integration")]
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2018_idioms, unused_lifetimes)]
 
 use std::env;
 use std::ffi::OsStr;

tests/lint_message_convention.rs

@@ -1,3 +1,6 @@
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2018_idioms, unused_lifetimes)]
+
 use std::ffi::OsStr;
 use std::path::PathBuf;
 

tests/missing-test-files.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2018_idioms, unused_lifetimes)]
 #![allow(clippy::assertions_on_constants)]
 
 use std::fs::{self, DirEntry};

tests/versioncheck.rs

@@ -1,4 +1,7 @@
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2018_idioms, unused_lifetimes)]
 #![allow(clippy::single_match_else)]
+
 use rustc_tools_util::VersionInfo;
 
 #[test]