Feature-gate the #[unsafe_no_drop_flag] attribute.

See RFC 320, "Non-zeroing dynamic drops."

Fix #22173

[breaking-change]

src/test/auxiliary/issue-10028.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(unsafe_no_drop_flag)]
+
 #[unsafe_no_drop_flag]
 pub struct ZeroLengthThingWithDestructor;
 impl Drop for ZeroLengthThingWithDestructor {

src/test/run-pass/attr-no-drop-flag-size.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(unsafe_destructor)]
+#![feature(unsafe_no_drop_flag)]
 
 use std::mem::size_of;
 

src/test/run-pass/issue-10734.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(unsafe_no_drop_flag)]
+
 static mut drop_count: uint = 0;
 
 #[unsafe_no_drop_flag]

src/test/run-pass/zero-size-type-destructors.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(unsafe_no_drop_flag)]
+
 static mut destructions : int = 3;
 
 pub fn foo() {