Auto merge of rust-lang#131078 - tgross35:rollup-66to2u9, r=tgross35

Rollup of 5 pull requests

Successful merges:

 - rust-lang#129638 (Hook up std::net to wasi-libc on wasm32-wasip2 target)
 - rust-lang#130877 (rustc_target: Add RISC-V atomic-related features)
 - rust-lang#130914 (Mark some more types as having insignificant dtor)
 - rust-lang#130961 (Enable `f16` tests on x86 Apple platforms)
 - rust-lang#130966 (make ptr metadata functions callable from stable const fn)

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -264,6 +264,10 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFea
         ("riscv32" | "riscv64", "unaligned-scalar-mem") if get_version().0 == 18 => {
             Some(LLVMFeature::new("fast-unaligned-access"))
         }
+        // Filter out features that are not supported by the current LLVM version
+        ("riscv32" | "riscv64", "zaamo") if get_version().0 < 19 => None,
+        ("riscv32" | "riscv64", "zabha") if get_version().0 < 19 => None,
+        ("riscv32" | "riscv64", "zalrsc") if get_version().0 < 19 => None,
         // Enable the evex512 target feature if an avx512 target feature is enabled.
         ("x86", s) if s.starts_with("avx512") => {
             Some(LLVMFeature::with_dependency(s, TargetFeatureFoldStrength::EnableOnly("evex512")))

compiler/rustc_target/src/target_features.rs

@@ -373,7 +373,7 @@ const MIPS_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
 
 const RISCV_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     // tidy-alphabetical-start
-    ("a", Stable, &[]),
+    ("a", Stable, &["zaamo", "zalrsc"]),
     ("c", Stable, &[]),
     ("d", Unstable(sym::riscv_target_feature), &["f"]),
     ("e", Unstable(sym::riscv_target_feature), &[]),
@@ -382,6 +382,9 @@ const RISCV_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("relax", Unstable(sym::riscv_target_feature), &[]),
     ("unaligned-scalar-mem", Unstable(sym::riscv_target_feature), &[]),
     ("v", Unstable(sym::riscv_target_feature), &[]),
+    ("zaamo", Unstable(sym::riscv_target_feature), &[]),
+    ("zabha", Unstable(sym::riscv_target_feature), &["zaamo"]),
+    ("zalrsc", Unstable(sym::riscv_target_feature), &[]),
     ("zba", Stable, &[]),
     ("zbb", Stable, &[]),
     ("zbc", Stable, &[]),

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/core/src/intrinsics.rs

@@ -3165,7 +3165,7 @@ pub const fn type_id<T: ?Sized + 'static>() -> u128 {
 /// change the possible layouts of pointers.
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
 pub const fn aggregate_raw_ptr<P: AggregateRawPtr<D, Metadata = M>, D, M>(_data: D, _meta: M) -> P {
@@ -3190,7 +3190,7 @@ impl<P: ?Sized, T: ptr::Thin> AggregateRawPtr<*mut T> for *mut P {
 /// This is used to implement functions like `ptr::metadata`.
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
 pub const fn ptr_metadata<P: ptr::Pointee<Metadata = M> + ?Sized, M>(_ptr: *const P) -> M {

library/core/src/ptr/const_ptr.rs

@@ -92,7 +92,7 @@ impl<T: ?Sized> *const T {
     /// }
     /// ```
     #[unstable(feature = "set_ptr_value", issue = "75091")]
-    #[rustc_const_unstable(feature = "set_ptr_value", issue = "75091")]
+    #[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[inline]
     pub const fn with_metadata_of<U>(self, meta: *const U) -> *const U
@@ -412,7 +412,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset(self, count: isize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `offset`.
@@ -495,7 +494,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_offset(self, count: isize) -> Self {
         self.cast::<u8>().wrapping_offset(count).with_metadata_of(self)
     }
@@ -645,7 +643,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize {
         // SAFETY: the caller must uphold the safety contract for `offset_from`.
@@ -873,7 +870,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_add(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `add`.
@@ -956,7 +952,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_sub(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `sub`.
@@ -1039,7 +1034,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_add(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_add(count).with_metadata_of(self)
     }
@@ -1120,7 +1114,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_sub(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_sub(count).with_metadata_of(self)
     }
@@ -1554,7 +1547,6 @@ impl<T> *const [T] {
     #[inline]
     #[stable(feature = "slice_ptr_len", since = "1.79.0")]
     #[rustc_const_stable(feature = "const_slice_ptr_len", since = "1.79.0")]
-    #[rustc_allow_const_fn_unstable(ptr_metadata)]
     pub const fn len(self) -> usize {
         metadata(self)
     }

library/core/src/ptr/metadata.rs

@@ -92,7 +92,7 @@ pub trait Thin = Pointee<Metadata = ()>;
 ///
 /// assert_eq!(std::ptr::metadata("foo"), 3_usize);
 /// ```
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[inline]
 pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
     ptr_metadata(ptr)
@@ -106,7 +106,7 @@ pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
 ///
 /// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
 #[unstable(feature = "ptr_metadata", issue = "81513")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[inline]
 pub const fn from_raw_parts<T: ?Sized>(
     data_pointer: *const impl Thin,
@@ -120,7 +120,7 @@ pub const fn from_raw_parts<T: ?Sized>(
 ///
 /// See the documentation of [`from_raw_parts`] for more details.
 #[unstable(feature = "ptr_metadata", issue = "81513")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[inline]
 pub const fn from_raw_parts_mut<T: ?Sized>(
     data_pointer: *mut impl Thin,

library/core/src/ptr/mod.rs

@@ -599,7 +599,6 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_promotable]
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
-#[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_null"]
 pub const fn null<T: ?Sized + Thin>() -> *const T {
     from_raw_parts(without_provenance::<()>(0), ())
@@ -625,7 +624,6 @@ pub const fn null<T: ?Sized + Thin>() -> *const T {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_promotable]
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
-#[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_null_mut"]
 pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {
     from_raw_parts_mut(without_provenance_mut::<()>(0), ())
@@ -949,7 +947,6 @@ pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
 #[inline]
 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
 #[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
-#[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_slice_from_raw_parts"]
 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
     from_raw_parts(data, len)

library/core/src/ptr/mut_ptr.rs

@@ -74,7 +74,7 @@ impl<T: ?Sized> *mut T {
     /// }
     /// ```
     #[unstable(feature = "set_ptr_value", issue = "75091")]
-    #[rustc_const_unstable(feature = "set_ptr_value", issue = "75091")]
+    #[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[inline]
     pub const fn with_metadata_of<U>(self, meta: *const U) -> *mut U
@@ -412,7 +412,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset(self, count: isize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `offset`.
@@ -492,7 +491,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_offset(self, count: isize) -> Self {
         self.cast::<u8>().wrapping_offset(count).with_metadata_of(self)
     }
@@ -808,7 +806,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize {
         // SAFETY: the caller must uphold the safety contract for `offset_from`.
@@ -954,7 +951,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_add(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `add`.
@@ -1037,7 +1033,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_sub(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `sub`.
@@ -1118,7 +1113,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_add(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_add(count).with_metadata_of(self)
     }
@@ -1197,7 +1191,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_sub(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_sub(count).with_metadata_of(self)
     }
@@ -1804,7 +1797,6 @@ impl<T> *mut [T] {
     #[inline(always)]
     #[stable(feature = "slice_ptr_len", since = "1.79.0")]
     #[rustc_const_stable(feature = "const_slice_ptr_len", since = "1.79.0")]
-    #[rustc_allow_const_fn_unstable(ptr_metadata)]
     pub const fn len(self) -> usize {
         metadata(self)
     }

library/core/src/ptr/non_null.rs

@@ -567,7 +567,6 @@ impl<T: ?Sized> NonNull<T> {
     #[must_use]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
     #[rustc_const_stable(feature = "non_null_convenience", since = "1.80.0")]
     pub const unsafe fn byte_add(self, count: usize) -> Self {
@@ -651,7 +650,6 @@ impl<T: ?Sized> NonNull<T> {
     #[must_use]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
     #[rustc_const_stable(feature = "non_null_convenience", since = "1.80.0")]
     pub const unsafe fn byte_sub(self, count: usize) -> Self {

library/core/src/slice/mod.rs

@@ -111,7 +111,6 @@ impl<T> [T] {
     #[lang = "slice_len_fn"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_slice_len", since = "1.39.0")]
-    #[rustc_allow_const_fn_unstable(ptr_metadata)]
     #[inline]
     #[must_use]
     pub const fn len(&self) -> usize {

library/std/build.rs

@@ -102,9 +102,6 @@ fn main() {
         ("arm64ec", _) => false,
         // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
         ("x86_64", "windows") => false,
-        // Apple has a special ABI for `f16` that we do not yet support
-        // FIXME(builtins): fixed by <https://github.com/rust-lang/compiler-builtins/pull/675>
-        ("x86" | "x86_64", _) if target_vendor == "apple" => false,
         // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
         ("csky", _) => false,
         ("hexagon", _) => false,

library/std/src/collections/hash/map/tests.rs

@@ -1018,6 +1018,7 @@ mod test_extract_if {
     }
 
     #[test]
+    #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
     fn drop_panic_leak() {
         static PREDS: AtomicUsize = AtomicUsize::new(0);
         static DROPS: AtomicUsize = AtomicUsize::new(0);
@@ -1047,6 +1048,7 @@ mod test_extract_if {
     }
 
     #[test]
+    #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
     fn pred_panic_leak() {
         static PREDS: AtomicUsize = AtomicUsize::new(0);
         static DROPS: AtomicUsize = AtomicUsize::new(0);
@@ -1076,6 +1078,7 @@ mod test_extract_if {
 
     // Same as above, but attempt to use the iterator again after the panic in the predicate
     #[test]
+    #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
     fn pred_panic_reuse() {
         static PREDS: AtomicUsize = AtomicUsize::new(0);
         static DROPS: AtomicUsize = AtomicUsize::new(0);

library/std/src/collections/hash/set/tests.rs

@@ -429,6 +429,7 @@ fn test_extract_if() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn test_extract_if_drop_panic_leak() {
     static PREDS: AtomicU32 = AtomicU32::new(0);
     static DROPS: AtomicU32 = AtomicU32::new(0);
@@ -459,6 +460,7 @@ fn test_extract_if_drop_panic_leak() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn test_extract_if_pred_panic_leak() {
     static PREDS: AtomicU32 = AtomicU32::new(0);
     static DROPS: AtomicU32 = AtomicU32::new(0);

library/std/src/env/tests.rs

@@ -1,7 +1,7 @@
 use super::*;
 
 #[test]
-#[cfg_attr(any(target_os = "emscripten", target_env = "sgx"), ignore)]
+#[cfg_attr(any(target_os = "emscripten", target_os = "wasi", target_env = "sgx"), ignore)]
 fn test_self_exe_path() {
     let path = current_exe();
     assert!(path.is_ok());

library/std/src/fs.rs

@@ -8,7 +8,15 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 #![deny(unsafe_op_in_unsafe_fn)]
 
-#[cfg(all(test, not(any(target_os = "emscripten", target_env = "sgx", target_os = "xous"))))]
+#[cfg(all(
+    test,
+    not(any(
+        target_os = "emscripten",
+        target_os = "wasi",
+        target_env = "sgx",
+        target_os = "xous"
+    ))
+))]
 mod tests;
 
 use crate::ffi::OsString;

library/std/src/io/buffered/tests.rs

@@ -164,6 +164,7 @@ fn test_buffered_reader_stream_position() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn test_buffered_reader_stream_position_panic() {
     let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
     let mut reader = BufReader::with_capacity(4, io::Cursor::new(inner));
@@ -487,7 +488,7 @@ fn dont_panic_in_drop_on_panicked_flush() {
 }
 
 #[test]
-#[cfg_attr(target_os = "emscripten", ignore)]
+#[cfg_attr(any(target_os = "emscripten", target_os = "wasi"), ignore)] // no threads
 fn panic_in_write_doesnt_flush_in_drop() {
     static WRITES: AtomicUsize = AtomicUsize::new(0);
 

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.