diff --git a/src/liballoc/tests/cow_str.rs b/src/liballoc/tests/cow_str.rs
index eb6adb159b0..6f357eda9b8 100644
--- a/src/liballoc/tests/cow_str.rs
+++ b/src/liballoc/tests/cow_str.rs
@@ -7,9 +7,9 @@ fn check_cow_add_cow() {
     let borrowed2 = Cow::Borrowed("World!");
     let borrow_empty = Cow::Borrowed("");
 
-    let owned1: Cow<str> = Cow::Owned(String::from("Hi, "));
-    let owned2: Cow<str> = Cow::Owned(String::from("Rustaceans!"));
-    let owned_empty: Cow<str> = Cow::Owned(String::new());
+    let owned1: Cow<'_, str> = Cow::Owned(String::from("Hi, "));
+    let owned2: Cow<'_, str> = Cow::Owned(String::from("Rustaceans!"));
+    let owned_empty: Cow<'_, str> = Cow::Owned(String::new());
 
     assert_eq!("Hello, World!", borrowed1.clone() + borrowed2.clone());
     assert_eq!("Hello, Rustaceans!", borrowed1.clone() + owned2.clone());
@@ -36,8 +36,8 @@ fn check_cow_add_str() {
     let borrowed = Cow::Borrowed("Hello, ");
     let borrow_empty = Cow::Borrowed("");
 
-    let owned: Cow<str> = Cow::Owned(String::from("Hi, "));
-    let owned_empty: Cow<str> = Cow::Owned(String::new());
+    let owned: Cow<'_, str> = Cow::Owned(String::from("Hi, "));
+    let owned_empty: Cow<'_, str> = Cow::Owned(String::new());
 
     assert_eq!("Hello, World!", borrowed.clone() + "World!");
 
@@ -60,9 +60,9 @@ fn check_cow_add_assign_cow() {
     let borrowed2 = Cow::Borrowed("World!");
     let borrow_empty = Cow::Borrowed("");
 
-    let mut owned1: Cow<str> = Cow::Owned(String::from("Hi, "));
-    let owned2: Cow<str> = Cow::Owned(String::from("Rustaceans!"));
-    let owned_empty: Cow<str> = Cow::Owned(String::new());
+    let mut owned1: Cow<'_, str> = Cow::Owned(String::from("Hi, "));
+    let owned2: Cow<'_, str> = Cow::Owned(String::from("Rustaceans!"));
+    let owned_empty: Cow<'_, str> = Cow::Owned(String::new());
 
     let mut s = borrowed1.clone();
     s += borrow_empty.clone();
@@ -101,8 +101,8 @@ fn check_cow_add_assign_str() {
     let mut borrowed = Cow::Borrowed("Hello, ");
     let borrow_empty = Cow::Borrowed("");
 
-    let mut owned: Cow<str> = Cow::Owned(String::from("Hi, "));
-    let owned_empty: Cow<str> = Cow::Owned(String::new());
+    let mut owned: Cow<'_, str> = Cow::Owned(String::from("Hi, "));
+    let owned_empty: Cow<'_, str> = Cow::Owned(String::new());
 
     let mut s = borrowed.clone();
     s += "";
@@ -132,10 +132,10 @@ fn check_cow_add_assign_str() {
 
 #[test]
 fn check_cow_clone_from() {
-    let mut c1: Cow<str> = Cow::Owned(String::with_capacity(25));
+    let mut c1: Cow<'_, str> = Cow::Owned(String::with_capacity(25));
     let s: String = "hi".to_string();
     assert!(s.capacity() < 25);
-    let c2: Cow<str> = Cow::Owned(s);
+    let c2: Cow<'_, str> = Cow::Owned(s);
     c1.clone_from(&c2);
     assert!(c1.into_owned().capacity() >= 25);
 }
diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index 90921b6af9f..b736750c576 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -7,6 +7,7 @@
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(vecdeque_rotate)]
+#![deny(rust_2018_idioms)]
 
 use std::hash::{Hash, Hasher};
 use std::collections::hash_map::DefaultHasher;
diff --git a/src/liballoc/tests/string.rs b/src/liballoc/tests/string.rs
index 7e75b8c4f28..765210e5aa6 100644
--- a/src/liballoc/tests/string.rs
+++ b/src/liballoc/tests/string.rs
@@ -54,11 +54,11 @@ fn test_from_utf8() {
 #[test]
 fn test_from_utf8_lossy() {
     let xs = b"hello";
-    let ys: Cow<str> = "hello".into_cow();
+    let ys: Cow<'_, str> = "hello".into_cow();
     assert_eq!(String::from_utf8_lossy(xs), ys);
 
     let xs = "ศไทย中华Việt Nam".as_bytes();
-    let ys: Cow<str> = "ศไทย中华Việt Nam".into_cow();
+    let ys: Cow<'_, str> = "ศไทย中华Việt Nam".into_cow();
     assert_eq!(String::from_utf8_lossy(xs), ys);
 
     let xs = b"Hello\xC2 There\xFF Goodbye";