Rollup merge of #130914 - compiler-errors:insignificant-dtor, r=Amanieu

Mark some more types as having insignificant dtor

These were caught by https://github.com/rust-lang/rust/pull/129864#issuecomment-2376658407, which is implementing a lint for some changes in drop order for temporaries in tail expressions.

Specifically, the destructors of `CString` and the bitpacked repr for `std::io::Error` are insignificant insofar as they don't have side-effects on things like locking or synchronization; they just free memory.

See some discussion on #89144 for what makes a drop impl "significant"

library/alloc/src/boxed.rs

@@ -228,6 +228,7 @@ mod thin;
 #[lang = "owned_box"]
 #[fundamental]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
 // The declaration of the `Box` struct must be kept in sync with the
 // compiler or ICEs will happen.
 pub struct Box<

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/alloc/src/sync.rs

@@ -237,6 +237,7 @@ macro_rules! acquire {
 /// [rc_examples]: crate::rc#examples
 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
 pub struct Arc<
     T: ?Sized,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,

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.