Update tests to use #[feature(rustc_attrs)]

Author: nikomatsakis

Diff for: src/test/compile-fail/variance-trait-bounds.rs

@@ -10,6 +10,7 @@
 
 #![deny(bivariance)]
 #![allow(dead_code)]
+#![feature(rustc_attrs)]
 
 // Check that bounds on type parameters (other than `Self`) do not
 // influence variance.

Diff for: src/test/compile-fail/variance-types-bounds.rs

@@ -11,6 +11,8 @@
 // Test that we correctly infer variance for type parameters in
 // various types and traits.
 
+#![feature(rustc_attrs)]
+
 #[rustc_variance]
 struct TestImm<A, B> { //~ ERROR types=[[+, +];[];[]]
     x: A,

Diff for: src/test/compile-fail/variance-types.rs

@@ -10,6 +10,7 @@
 
 #![deny(bivariance)]
 #![allow(dead_code)]
+#![feature(rustc_attrs)]
 
 use std::cell::Cell;
 

Diff for: src/test/compile-fail/variance-use-contravariant-struct-1.rs

@@ -11,6 +11,8 @@
 // Test various uses of structs with distint variances to make sure
 // they permit lifetimes to be approximated as expected.
 
+#![feature(rustc_attrs)]
+
 struct SomeStruct<T>(fn(T));
 
 fn foo<'min,'max>(v: SomeStruct<&'max ()>)

Diff for: src/test/compile-fail/variance-use-contravariant-struct-2.rs

@@ -12,6 +12,7 @@
 // they permit lifetimes to be approximated as expected.
 
 #![allow(dead_code)]
+#![feature(rustc_attrs)]
 
 struct SomeStruct<T>(fn(T));
 

Diff for: src/test/compile-fail/variance-use-covariant-struct-2.rs

@@ -12,6 +12,7 @@
 // be shortened.
 
 #![allow(dead_code)]
+#![feature(rustc_attrs)]
 
 struct SomeStruct<T>(T);
 

Diff for: src/test/compile-fail/variance-use-invariant-struct-1.rs

@@ -11,6 +11,8 @@
 // Test various uses of structs with distint variances to make sure
 // they permit lifetimes to be approximated as expected.
 
+#![feature(rustc_attrs)]
+
 struct SomeStruct<T>(*mut T);
 
 fn foo<'min,'max>(v: SomeStruct<&'max ()>)