Add test for struct fields

tests/compile-fail/used_underscore_binding.rs

@@ -12,6 +12,17 @@ fn in_macro(_foo: u32) {
     println!("{}", _foo); //~ ERROR used binding which is prefixed with an underscore
 }
 
+// Struct for testing use of fields prefixed with an underscore
+struct StructFieldTest {
+    _underscore_field: u32,
+}
+
+/// Test that we lint the use of a struct field which is prefixed with an underscore
+fn in_struct_field() {
+    let mut s = StructFieldTest { _underscore_field: 0 };
+    s._underscore_field += 1; //~ Error used binding which is prefixed with an underscore
+}
+
 /// Test that we do not lint if the underscore is not a prefix
 fn non_prefix_underscore(some_foo: u32) -> u32 {
     some_foo + 1
@@ -22,7 +33,6 @@ fn unused_underscore_simple(_foo: u32) -> u32 {
     1
 }
 
-#[deny(unused_variables)]
 /// Test that we do not lint if we do not use the binding (complex case). This checks for
 /// compatibility with the built-in `unused_variables` lint.
 fn unused_underscore_complex(mut _foo: u32) -> u32 {
@@ -61,6 +71,7 @@ fn main() {
     // tests of unused_underscore lint
     let _ = prefix_underscore(foo);
     in_macro(foo);
+    in_struct_field();
     // possible false positives
     let _ = non_prefix_underscore(foo);
     let _ = unused_underscore_simple(foo);