Mark some more types as having insignificant dtor

library/alloc/src/ffi/c_str.rs

@@ -696,6 +696,7 @@ impl CString {
 // memory-unsafe code from working by accident. Inline
 // to prevent LLVM from optimizing it away in debug builds.
 #[stable(feature = "cstring_drop", since = "1.13.0")]
+#[rustc_insignificant_dtor]
 impl Drop for CString {
     #[inline]
     fn drop(&mut self) {

library/std/src/io/error/repr_bitpacked.rs

@@ -124,6 +124,7 @@ const TAG_SIMPLE: usize = 0b11;
 /// is_unwind_safe::<std::io::Error>();
 /// ```
 #[repr(transparent)]
+#[rustc_insignificant_dtor]
 pub(super) struct Repr(NonNull<()>, PhantomData<ErrorData<Box<Custom>>>);
 
 // All the types `Repr` stores internally are Send + Sync, and so is it.