Put the RFC behind a feature gate result_ffi_guarantees

compiler/rustc_feature/src/unstable.rs

@@ -585,6 +585,9 @@ declare_features! (
     (incomplete, repr128, "1.16.0", Some(56071)),
     /// Allows `repr(simd)` and importing the various simd intrinsics.
     (unstable, repr_simd, "1.4.0", Some(27731)),
+    /// Allows enums like Result<T, E> to be used across FFI, if T's niche value can
+    /// be used to describe E or vise-versa.
+    (unstable, result_ffi_guarantees, "CURRENT_RUSTC_VERSION", Some(110503)),
     /// Allows bounding the return type of AFIT/RPITIT.
     (incomplete, return_type_notation, "1.70.0", Some(109417)),
     /// Allows `extern "rust-cold"`.

compiler/rustc_lint/src/types.rs

@@ -1142,6 +1142,10 @@ pub(crate) fn repr_nullable_ptr<'tcx>(
             [var_one, var_two] => match (&var_one.fields.raw[..], &var_two.fields.raw[..]) {
                 ([], [field]) | ([field], []) => field.ty(tcx, args),
                 ([field1], [field2]) => {
+                    if !tcx.features().result_ffi_guarantees {
+                        return None;
+                    }
+
                     let ty1 = field1.ty(tcx, args);
                     let ty2 = field2.ty(tcx, args);
 

compiler/rustc_span/src/symbol.rs

@@ -1509,6 +1509,7 @@ symbols! {
         require,
         residual,
         result,
+        result_ffi_guarantees,
         resume,
         return_position_impl_trait_in_trait,
         return_type_notation,

src/doc/unstable-book/src/language-features/result-ffi-guarantees.md

@@ -0,0 +1,14 @@
+# `result_ffi_guarantees`
+
+The tracking issue for this feature is: [#110503]
+
+[#110503]: https://github.com/rust-lang/rust/issues/110503
+
+------------------------
+
+This feature adds the possibility of using `Result<T, E>` in FFI if T's niche
+value can be used to describe E or vise-versa.
+
+See [RFC 3391] for more information.
+
+[RFC 3391]: https://github.com/rust-lang/rfcs/blob/master/text/3391-result_ffi_guarantees.md

tests/ui/feature-gates/feature-gate-result_ffi_guarantees.rs

@@ -0,0 +1,99 @@
+#![allow(dead_code)]
+#![deny(improper_ctypes)]
+#![feature(ptr_internals)]
+
+use std::num;
+
+enum Z {}
+
+#[repr(transparent)]
+struct TransparentStruct<T>(T, std::marker::PhantomData<Z>);
+
+#[repr(transparent)]
+enum TransparentEnum<T> {
+    Variant(T, std::marker::PhantomData<Z>),
+}
+
+struct NoField;
+
+extern "C" {
+    fn result_ref_t(x: Result<&'static u8, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_fn_t(x: Result<extern "C" fn(), ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonnull_t(x: Result<std::ptr::NonNull<u8>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_unique_t(x: Result<std::ptr::Unique<u8>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u8_t(x: Result<num::NonZero<u8>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u16_t(x: Result<num::NonZero<u16>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u32_t(x: Result<num::NonZero<u32>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u64_t(x: Result<num::NonZero<u64>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_usize_t(x: Result<num::NonZero<usize>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i8_t(x: Result<num::NonZero<i8>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i16_t(x: Result<num::NonZero<i16>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i32_t(x: Result<num::NonZero<i32>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i64_t(x: Result<num::NonZero<i64>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_isize_t(x: Result<num::NonZero<isize>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_transparent_struct_t(x: Result<TransparentStruct<num::NonZero<u8>>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_transparent_enum_t(x: Result<TransparentEnum<num::NonZero<u8>>, ()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_phantom_t(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_1zst_exhaustive_no_variant_t(x: Result<num::NonZero<u8>, Z>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_1zst_exhaustive_no_field_t(x: Result<num::NonZero<u8>, NoField>);
+    //~^ ERROR `extern` block uses type `Result
+
+    fn result_ref_e(x: Result<(), &'static u8>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_fn_e(x: Result<(), extern "C" fn()>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonnull_e(x: Result<(), std::ptr::NonNull<u8>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_unique_e(x: Result<(), std::ptr::Unique<u8>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u8_e(x: Result<(), num::NonZero<u8>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u16_e(x: Result<(), num::NonZero<u16>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u32_e(x: Result<(), num::NonZero<u32>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_u64_e(x: Result<(), num::NonZero<u64>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_usize_e(x: Result<(), num::NonZero<usize>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i8_e(x: Result<(), num::NonZero<i8>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i16_e(x: Result<(), num::NonZero<i16>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i32_e(x: Result<(), num::NonZero<i32>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_i64_e(x: Result<(), num::NonZero<i64>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_nonzero_isize_e(x: Result<(), num::NonZero<isize>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_transparent_struct_e(x: Result<(), TransparentStruct<num::NonZero<u8>>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_transparent_enum_e(x: Result<(), TransparentEnum<num::NonZero<u8>>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_phantom_e(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_1zst_exhaustive_no_variant_e(x: Result<Z, num::NonZero<u8>>);
+    //~^ ERROR `extern` block uses type `Result
+    fn result_1zst_exhaustive_no_field_e(x: Result<NoField, num::NonZero<u8>>);
+    //~^ ERROR `extern` block uses type `Result
+}
+
+pub fn main() {}