diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 9641d336c3f3c..eec0d0eb5298a 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/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"`.
diff --git a/compiler/rustc_lint/src/types.rs b/compiler/rustc_lint/src/types.rs
index 09fdceb156cf6..b3fd03d3f78bf 100644
--- a/compiler/rustc_lint/src/types.rs
+++ b/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);
 
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 8abf42e2c1392..ff9b2a413c952 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/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,
diff --git a/src/doc/unstable-book/src/language-features/result-ffi-guarantees.md b/src/doc/unstable-book/src/language-features/result-ffi-guarantees.md
new file mode 100644
index 0000000000000..dc9c196524ed4
--- /dev/null
+++ b/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
diff --git a/tests/ui/feature-gates/feature-gate-result_ffi_guarantees.rs b/tests/ui/feature-gates/feature-gate-result_ffi_guarantees.rs
new file mode 100644
index 0000000000000..dda317aecc3c3
--- /dev/null
+++ b/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() {}
diff --git a/tests/ui/feature-gates/feature-gate-result_ffi_guarantees.stderr b/tests/ui/feature-gates/feature-gate-result_ffi_guarantees.stderr
new file mode 100644
index 0000000000000..94416eb99c878
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-result_ffi_guarantees.stderr
@@ -0,0 +1,349 @@
+error: `extern` block uses type `Result<&u8, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:20:24
+   |
+LL |     fn result_ref_t(x: Result<&'static u8, ()>);
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+note: the lint level is defined here
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:2:9
+   |
+LL | #![deny(improper_ctypes)]
+   |         ^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `Result<extern "C" fn(), ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:22:23
+   |
+LL |     fn result_fn_t(x: Result<extern "C" fn(), ()>);
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonNull<u8>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:24:28
+   |
+LL |     fn result_nonnull_t(x: Result<std::ptr::NonNull<u8>, ()>);
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<Unique<u8>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:26:27
+   |
+LL |     fn result_unique_t(x: Result<std::ptr::Unique<u8>, ()>);
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u8>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:28:31
+   |
+LL |     fn result_nonzero_u8_t(x: Result<num::NonZero<u8>, ()>);
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u16>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:30:32
+   |
+LL |     fn result_nonzero_u16_t(x: Result<num::NonZero<u16>, ()>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u32>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:32:32
+   |
+LL |     fn result_nonzero_u32_t(x: Result<num::NonZero<u32>, ()>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u64>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:34:32
+   |
+LL |     fn result_nonzero_u64_t(x: Result<num::NonZero<u64>, ()>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<usize>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:36:34
+   |
+LL |     fn result_nonzero_usize_t(x: Result<num::NonZero<usize>, ()>);
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<i8>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:38:31
+   |
+LL |     fn result_nonzero_i8_t(x: Result<num::NonZero<i8>, ()>);
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<i16>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:40:32
+   |
+LL |     fn result_nonzero_i16_t(x: Result<num::NonZero<i16>, ()>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<i32>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:42:32
+   |
+LL |     fn result_nonzero_i32_t(x: Result<num::NonZero<i32>, ()>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<i64>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:44:32
+   |
+LL |     fn result_nonzero_i64_t(x: Result<num::NonZero<i64>, ()>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<isize>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:46:34
+   |
+LL |     fn result_nonzero_isize_t(x: Result<num::NonZero<isize>, ()>);
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<TransparentStruct<NonZero<u8>>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:48:39
+   |
+LL |     fn result_transparent_struct_t(x: Result<TransparentStruct<num::NonZero<u8>>, ()>);
+   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<TransparentEnum<NonZero<u8>>, ()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:50:37
+   |
+LL |     fn result_transparent_enum_t(x: Result<TransparentEnum<num::NonZero<u8>>, ()>);
+   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u8>, PhantomData<()>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:52:28
+   |
+LL |     fn result_phantom_t(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u8>, Z>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:54:47
+   |
+LL |     fn result_1zst_exhaustive_no_variant_t(x: Result<num::NonZero<u8>, Z>);
+   |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u8>, NoField>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:56:45
+   |
+LL |     fn result_1zst_exhaustive_no_field_t(x: Result<num::NonZero<u8>, NoField>);
+   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), &u8>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:59:24
+   |
+LL |     fn result_ref_e(x: Result<(), &'static u8>);
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), extern "C" fn()>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:61:23
+   |
+LL |     fn result_fn_e(x: Result<(), extern "C" fn()>);
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonNull<u8>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:63:28
+   |
+LL |     fn result_nonnull_e(x: Result<(), std::ptr::NonNull<u8>>);
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), Unique<u8>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:65:27
+   |
+LL |     fn result_unique_e(x: Result<(), std::ptr::Unique<u8>>);
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<u8>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:67:31
+   |
+LL |     fn result_nonzero_u8_e(x: Result<(), num::NonZero<u8>>);
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<u16>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:69:32
+   |
+LL |     fn result_nonzero_u16_e(x: Result<(), num::NonZero<u16>>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<u32>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:71:32
+   |
+LL |     fn result_nonzero_u32_e(x: Result<(), num::NonZero<u32>>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<u64>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:73:32
+   |
+LL |     fn result_nonzero_u64_e(x: Result<(), num::NonZero<u64>>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<usize>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:75:34
+   |
+LL |     fn result_nonzero_usize_e(x: Result<(), num::NonZero<usize>>);
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<i8>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:77:31
+   |
+LL |     fn result_nonzero_i8_e(x: Result<(), num::NonZero<i8>>);
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<i16>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:79:32
+   |
+LL |     fn result_nonzero_i16_e(x: Result<(), num::NonZero<i16>>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<i32>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:81:32
+   |
+LL |     fn result_nonzero_i32_e(x: Result<(), num::NonZero<i32>>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<i64>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:83:32
+   |
+LL |     fn result_nonzero_i64_e(x: Result<(), num::NonZero<i64>>);
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), NonZero<isize>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:85:34
+   |
+LL |     fn result_nonzero_isize_e(x: Result<(), num::NonZero<isize>>);
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), TransparentStruct<NonZero<u8>>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:87:39
+   |
+LL |     fn result_transparent_struct_e(x: Result<(), TransparentStruct<num::NonZero<u8>>>);
+   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<(), TransparentEnum<NonZero<u8>>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:89:37
+   |
+LL |     fn result_transparent_enum_e(x: Result<(), TransparentEnum<num::NonZero<u8>>>);
+   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NonZero<u8>, PhantomData<()>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:91:28
+   |
+LL |     fn result_phantom_e(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<Z, NonZero<u8>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:93:47
+   |
+LL |     fn result_1zst_exhaustive_no_variant_e(x: Result<Z, num::NonZero<u8>>);
+   |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Result<NoField, NonZero<u8>>`, which is not FFI-safe
+  --> $DIR/feature-gate-result_ffi_guarantees.rs:95:45
+   |
+LL |     fn result_1zst_exhaustive_no_field_e(x: Result<NoField, num::NonZero<u8>>);
+   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: aborting due to 38 previous errors
+
diff --git a/tests/ui/lint/lint-ctypes-enum.rs b/tests/ui/lint/lint-ctypes-enum.rs
index 19af1de95760b..cb8e9e8067513 100644
--- a/tests/ui/lint/lint-ctypes-enum.rs
+++ b/tests/ui/lint/lint-ctypes-enum.rs
@@ -2,6 +2,7 @@
 #![deny(improper_ctypes)]
 #![feature(ptr_internals)]
 #![feature(transparent_unions)]
+#![feature(result_ffi_guarantees)]
 
 use std::num;
 
diff --git a/tests/ui/lint/lint-ctypes-enum.stderr b/tests/ui/lint/lint-ctypes-enum.stderr
index 8d7b28bfb7ddf..bba5b09b69cc9 100644
--- a/tests/ui/lint/lint-ctypes-enum.stderr
+++ b/tests/ui/lint/lint-ctypes-enum.stderr
@@ -1,5 +1,5 @@
 error: `extern` block uses type `U`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:68:14
+  --> $DIR/lint-ctypes-enum.rs:69:14
    |
 LL |     fn uf(x: U);
    |              ^ not FFI-safe
@@ -7,7 +7,7 @@ LL |     fn uf(x: U);
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
 note: the type is defined here
-  --> $DIR/lint-ctypes-enum.rs:9:1
+  --> $DIR/lint-ctypes-enum.rs:10:1
    |
 LL | enum U {
    | ^^^^^^
@@ -18,7 +18,7 @@ LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `B`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:69:14
+  --> $DIR/lint-ctypes-enum.rs:70:14
    |
 LL |     fn bf(x: B);
    |              ^ not FFI-safe
@@ -26,13 +26,13 @@ LL |     fn bf(x: B);
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
 note: the type is defined here
-  --> $DIR/lint-ctypes-enum.rs:12:1
+  --> $DIR/lint-ctypes-enum.rs:13:1
    |
 LL | enum B {
    | ^^^^^^
 
 error: `extern` block uses type `T`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:70:14
+  --> $DIR/lint-ctypes-enum.rs:71:14
    |
 LL |     fn tf(x: T);
    |              ^ not FFI-safe
@@ -40,13 +40,13 @@ LL |     fn tf(x: T);
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
 note: the type is defined here
-  --> $DIR/lint-ctypes-enum.rs:16:1
+  --> $DIR/lint-ctypes-enum.rs:17:1
    |
 LL | enum T {
    | ^^^^^^
 
 error: `extern` block uses type `u128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:82:31
+  --> $DIR/lint-ctypes-enum.rs:83:31
    |
 LL |     fn option_nonzero_u128(x: Option<num::NonZero<u128>>);
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -54,7 +54,7 @@ LL |     fn option_nonzero_u128(x: Option<num::NonZero<u128>>);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `i128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:89:31
+  --> $DIR/lint-ctypes-enum.rs:90:31
    |
 LL |     fn option_nonzero_i128(x: Option<num::NonZero<i128>>);
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -62,7 +62,7 @@ LL |     fn option_nonzero_i128(x: Option<num::NonZero<i128>>);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `Option<TransparentUnion<NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:94:36
+  --> $DIR/lint-ctypes-enum.rs:95:36
    |
 LL |     fn option_transparent_union(x: Option<TransparentUnion<num::NonZero<u8>>>);
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -71,7 +71,7 @@ LL |     fn option_transparent_union(x: Option<TransparentUnion<num::NonZero<u8>
    = note: enum has no representation hint
 
 error: `extern` block uses type `Option<Rust<NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:96:28
+  --> $DIR/lint-ctypes-enum.rs:97:28
    |
 LL |     fn option_repr_rust(x: Option<Rust<num::NonZero<u8>>>);
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -80,7 +80,7 @@ LL |     fn option_repr_rust(x: Option<Rust<num::NonZero<u8>>>);
    = note: enum has no representation hint
 
 error: `extern` block uses type `u128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:106:33
+  --> $DIR/lint-ctypes-enum.rs:107:33
    |
 LL |     fn result_nonzero_u128_t(x: Result<num::NonZero<u128>, ()>);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -88,7 +88,7 @@ LL |     fn result_nonzero_u128_t(x: Result<num::NonZero<u128>, ()>);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `i128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:113:33
+  --> $DIR/lint-ctypes-enum.rs:114:33
    |
 LL |     fn result_nonzero_i128_t(x: Result<num::NonZero<i128>, ()>);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -96,7 +96,7 @@ LL |     fn result_nonzero_i128_t(x: Result<num::NonZero<i128>, ()>);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `Result<TransparentUnion<NonZero<u8>>, ()>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:118:38
+  --> $DIR/lint-ctypes-enum.rs:119:38
    |
 LL |     fn result_transparent_union_t(x: Result<TransparentUnion<num::NonZero<u8>>, ()>);
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -105,7 +105,7 @@ LL |     fn result_transparent_union_t(x: Result<TransparentUnion<num::NonZero<u
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<Rust<NonZero<u8>>, ()>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:120:30
+  --> $DIR/lint-ctypes-enum.rs:121:30
    |
 LL |     fn result_repr_rust_t(x: Result<Rust<num::NonZero<u8>>, ()>);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -114,7 +114,7 @@ LL |     fn result_repr_rust_t(x: Result<Rust<num::NonZero<u8>>, ()>);
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonZero<u8>, U>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:124:51
+  --> $DIR/lint-ctypes-enum.rs:125:51
    |
 LL |     fn result_1zst_exhaustive_single_variant_t(x: Result<num::NonZero<u8>, U>);
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -123,7 +123,7 @@ LL |     fn result_1zst_exhaustive_single_variant_t(x: Result<num::NonZero<u8>,
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonZero<u8>, B>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:126:53
+  --> $DIR/lint-ctypes-enum.rs:127:53
    |
 LL |     fn result_1zst_exhaustive_multiple_variant_t(x: Result<num::NonZero<u8>, B>);
    |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -132,7 +132,7 @@ LL |     fn result_1zst_exhaustive_multiple_variant_t(x: Result<num::NonZero<u8>
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonZero<u8>, NonExhaustive>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:128:51
+  --> $DIR/lint-ctypes-enum.rs:129:51
    |
 LL |     fn result_1zst_non_exhaustive_no_variant_t(x: Result<num::NonZero<u8>, NonExhaustive>);
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -141,7 +141,7 @@ LL |     fn result_1zst_non_exhaustive_no_variant_t(x: Result<num::NonZero<u8>,
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonZero<u8>, Field>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:131:49
+  --> $DIR/lint-ctypes-enum.rs:132:49
    |
 LL |     fn result_1zst_exhaustive_single_field_t(x: Result<num::NonZero<u8>, Field>);
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -150,7 +150,7 @@ LL |     fn result_1zst_exhaustive_single_field_t(x: Result<num::NonZero<u8>, Fi
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<Result<(), NonZero<u8>>, ()>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:133:30
+  --> $DIR/lint-ctypes-enum.rs:134:30
    |
 LL |     fn result_cascading_t(x: Result<Result<(), num::NonZero<u8>>, ()>);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -159,7 +159,7 @@ LL |     fn result_cascading_t(x: Result<Result<(), num::NonZero<u8>>, ()>);
    = note: enum has no representation hint
 
 error: `extern` block uses type `u128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:144:33
+  --> $DIR/lint-ctypes-enum.rs:145:33
    |
 LL |     fn result_nonzero_u128_e(x: Result<(), num::NonZero<u128>>);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -167,7 +167,7 @@ LL |     fn result_nonzero_u128_e(x: Result<(), num::NonZero<u128>>);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `i128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:151:33
+  --> $DIR/lint-ctypes-enum.rs:152:33
    |
 LL |     fn result_nonzero_i128_e(x: Result<(), num::NonZero<i128>>);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -175,7 +175,7 @@ LL |     fn result_nonzero_i128_e(x: Result<(), num::NonZero<i128>>);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `Result<(), TransparentUnion<NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:156:38
+  --> $DIR/lint-ctypes-enum.rs:157:38
    |
 LL |     fn result_transparent_union_e(x: Result<(), TransparentUnion<num::NonZero<u8>>>);
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -184,7 +184,7 @@ LL |     fn result_transparent_union_e(x: Result<(), TransparentUnion<num::NonZe
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<(), Rust<NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:158:30
+  --> $DIR/lint-ctypes-enum.rs:159:30
    |
 LL |     fn result_repr_rust_e(x: Result<(), Rust<num::NonZero<u8>>>);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -193,7 +193,7 @@ LL |     fn result_repr_rust_e(x: Result<(), Rust<num::NonZero<u8>>>);
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<U, NonZero<u8>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:162:51
+  --> $DIR/lint-ctypes-enum.rs:163:51
    |
 LL |     fn result_1zst_exhaustive_single_variant_e(x: Result<U, num::NonZero<u8>>);
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -202,7 +202,7 @@ LL |     fn result_1zst_exhaustive_single_variant_e(x: Result<U, num::NonZero<u8
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<B, NonZero<u8>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:164:53
+  --> $DIR/lint-ctypes-enum.rs:165:53
    |
 LL |     fn result_1zst_exhaustive_multiple_variant_e(x: Result<B, num::NonZero<u8>>);
    |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -211,7 +211,7 @@ LL |     fn result_1zst_exhaustive_multiple_variant_e(x: Result<B, num::NonZero<
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonExhaustive, NonZero<u8>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:166:51
+  --> $DIR/lint-ctypes-enum.rs:167:51
    |
 LL |     fn result_1zst_non_exhaustive_no_variant_e(x: Result<NonExhaustive, num::NonZero<u8>>);
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -220,7 +220,7 @@ LL |     fn result_1zst_non_exhaustive_no_variant_e(x: Result<NonExhaustive, num
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<Field, NonZero<u8>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:169:49
+  --> $DIR/lint-ctypes-enum.rs:170:49
    |
 LL |     fn result_1zst_exhaustive_single_field_e(x: Result<Field, num::NonZero<u8>>);
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -229,7 +229,7 @@ LL |     fn result_1zst_exhaustive_single_field_e(x: Result<Field, num::NonZero<
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<(), Result<(), NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:171:30
+  --> $DIR/lint-ctypes-enum.rs:172:30
    |
 LL |     fn result_cascading_e(x: Result<(), Result<(), num::NonZero<u8>>>);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe