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diff --git a/tests/ui/abi/compatibility.rs b/tests/ui/abi/compatibility.rs
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+// check-pass
+// revisions: host
+// revisions: arm
+//[arm] compile-flags: --target arm-unknown-linux-gnueabi
+//[arm] needs-llvm-components: arm
+// revisions: aarch64
+//[aarch64] compile-flags: --target aarch64-unknown-linux-gnu
+//[aarch64] needs-llvm-components: aarch64
+// revisions: s390x
+//[s390x] compile-flags: --target s390x-unknown-linux-gnu
+//[s390x] needs-llvm-components: systemz
+// revisions: mips
+//[mips] compile-flags: --target mips-unknown-linux-gnu
+//[mips] needs-llvm-components: mips
+// revisions: mips64
+//[mips64] compile-flags: --target mips64-unknown-linux-gnuabi64
+//[mips64] needs-llvm-components: mips
+// revisions: sparc
+//[sparc] compile-flags: --target sparc-unknown-linux-gnu
+//[sparc] needs-llvm-components: sparc
+// revisions: sparc64
+//[sparc64] compile-flags: --target sparc64-unknown-linux-gnu
+//[sparc64] needs-llvm-components: sparc
+// revisions: powerpc64
+//[powerpc64] compile-flags: --target powerpc64-unknown-linux-gnu
+//[powerpc64] needs-llvm-components: powerpc
+// revisions: riscv
+//[riscv] compile-flags: --target riscv64gc-unknown-linux-gnu
+//[riscv] needs-llvm-components: riscv
+// revisions: loongarch64
+//[loongarch64] compile-flags: --target loongarch64-unknown-linux-gnu
+//[loongarch64] needs-llvm-components: loongarch
+// revisions: wasm
+//[wasm] compile-flags: --target wasm32-unknown-unknown
+//[wasm] needs-llvm-components: webassembly
+// revisions: wasi
+//[wasi] compile-flags: --target wasm32-wasi
+//[wasi] needs-llvm-components: webassembly
+// revisions: nvptx64
+//[nvptx64] compile-flags: --target nvptx64-nvidia-cuda
+//[nvptx64] needs-llvm-components: nvptx
+#![feature(rustc_attrs, unsized_fn_params, transparent_unions)]
+#![cfg_attr(not(host), feature(no_core, lang_items), no_std, no_core)]
+#![allow(unused, improper_ctypes_definitions, internal_features)]
+
+// FIXME: some targets are broken in various ways.
+// Hence there are `cfg` throughout this test to disable parts of it on those targets.
+// sparc64: https://github.com/rust-lang/rust/issues/115336
+// mips64: https://github.com/rust-lang/rust/issues/115404
+
+#[cfg(host)]
+use std::{
+    any::Any, marker::PhantomData, mem::ManuallyDrop, num::NonZeroI32, ptr::NonNull, rc::Rc,
+    sync::Arc,
+};
+
+/// To work cross-target this test must be no_core.
+/// This little prelude supplies what we need.
+#[cfg(not(host))]
+mod prelude {
+    #[lang = "sized"]
+    pub trait Sized {}
+
+    #[lang = "receiver"]
+    pub trait Receiver {}
+    impl<T: ?Sized> Receiver for &T {}
+    impl<T: ?Sized> Receiver for &mut T {}
+
+    #[lang = "copy"]
+    pub trait Copy: Sized {}
+    impl Copy for i32 {}
+    impl Copy for f32 {}
+    impl<T: ?Sized> Copy for &T {}
+    impl<T: ?Sized> Copy for *const T {}
+    impl<T: ?Sized> Copy for *mut T {}
+
+    #[lang = "clone"]
+    pub trait Clone: Sized {
+        fn clone(&self) -> Self;
+    }
+
+    #[lang = "phantom_data"]
+    pub struct PhantomData<T: ?Sized>;
+    impl<T: ?Sized> Copy for PhantomData<T> {}
+
+    #[lang = "unsafe_cell"]
+    #[repr(transparent)]
+    pub struct UnsafeCell<T: ?Sized> {
+        value: T,
+    }
+
+    pub trait Any: 'static {}
+
+    pub enum Option<T> {
+        None,
+        Some(T),
+    }
+    impl<T: Copy> Copy for Option<T> {}
+
+    pub enum Result<T, E> {
+        Ok(T),
+        Err(E),
+    }
+    impl<T: Copy, E: Copy> Copy for Result<T, E> {}
+
+    #[lang = "manually_drop"]
+    #[repr(transparent)]
+    pub struct ManuallyDrop<T: ?Sized> {
+        value: T,
+    }
+    impl<T: Copy + ?Sized> Copy for ManuallyDrop<T> {}
+
+    #[repr(transparent)]
+    #[rustc_layout_scalar_valid_range_start(1)]
+    #[rustc_nonnull_optimization_guaranteed]
+    pub struct NonNull<T: ?Sized> {
+        pointer: *const T,
+    }
+    impl<T: ?Sized> Copy for NonNull<T> {}
+
+    #[repr(transparent)]
+    #[rustc_layout_scalar_valid_range_start(1)]
+    #[rustc_nonnull_optimization_guaranteed]
+    pub struct NonZeroI32(i32);
+
+    // This just stands in for a non-trivial type.
+    pub struct Vec<T> {
+        ptr: NonNull<T>,
+        cap: usize,
+        len: usize,
+    }
+
+    pub struct Unique<T: ?Sized> {
+        pub pointer: NonNull<T>,
+        pub _marker: PhantomData<T>,
+    }
+
+    pub struct Global;
+
+    #[lang = "owned_box"]
+    pub struct Box<T: ?Sized, A = Global>(Unique<T>, A);
+
+    #[repr(C)]
+    struct RcBox<T: ?Sized> {
+        strong: UnsafeCell<usize>,
+        weak: UnsafeCell<usize>,
+        value: T,
+    }
+    pub struct Rc<T: ?Sized, A = Global> {
+        ptr: NonNull<RcBox<T>>,
+        phantom: PhantomData<RcBox<T>>,
+        alloc: A,
+    }
+
+    #[repr(C, align(8))]
+    struct AtomicUsize(usize);
+    #[repr(C)]
+    struct ArcInner<T: ?Sized> {
+        strong: AtomicUsize,
+        weak: AtomicUsize,
+        data: T,
+    }
+    pub struct Arc<T: ?Sized, A = Global> {
+        ptr: NonNull<ArcInner<T>>,
+        phantom: PhantomData<ArcInner<T>>,
+        alloc: A,
+    }
+}
+#[cfg(not(host))]
+use prelude::*;
+
+macro_rules! assert_abi_compatible {
+    ($name:ident, $t1:ty, $t2:ty) => {
+        mod $name {
+            use super::*;
+            // Declaring a `type` doesn't even check well-formedness, so we also declare a function.
+            fn check_wf(_x: $t1, _y: $t2) {}
+            // Test argument and return value, `Rust` and `C` ABIs.
+            #[rustc_abi(assert_eq)]
+            type TestRust = (fn($t1) -> $t1, fn($t2) -> $t2);
+            #[rustc_abi(assert_eq)]
+            type TestC = (extern "C" fn($t1) -> $t1, extern "C" fn($t2) -> $t2);
+        }
+    };
+}
+
+struct Zst;
+impl Copy for Zst {}
+impl Clone for Zst {
+    fn clone(&self) -> Self {
+        Zst
+    }
+}
+
+#[repr(C)]
+struct ReprC1<T: ?Sized>(T);
+#[repr(C)]
+struct ReprC2Int<T>(i32, T);
+#[repr(C)]
+struct ReprC2Float<T>(f32, T);
+#[repr(C)]
+struct ReprC4<T>(T, Vec<i32>, Zst, T);
+#[repr(C)]
+struct ReprC4Mixed<T>(T, f32, i32, T);
+#[repr(C)]
+enum ReprCEnum<T> {
+    Variant1,
+    Variant2(T),
+}
+#[repr(C)]
+union ReprCUnion<T> {
+    nothing: (),
+    something: ManuallyDrop<T>,
+}
+
+macro_rules! test_abi_compatible {
+    ($name:ident, $t1:ty, $t2:ty) => {
+        mod $name {
+            use super::*;
+            assert_abi_compatible!(plain, $t1, $t2);
+            // We also do some tests with differences in fields of `repr(C)` types.
+            assert_abi_compatible!(repr_c_1, ReprC1<$t1>, ReprC1<$t2>);
+            assert_abi_compatible!(repr_c_2_int, ReprC2Int<$t1>, ReprC2Int<$t2>);
+            assert_abi_compatible!(repr_c_2_float, ReprC2Float<$t1>, ReprC2Float<$t2>);
+            assert_abi_compatible!(repr_c_4, ReprC4<$t1>, ReprC4<$t2>);
+            assert_abi_compatible!(repr_c_4mixed, ReprC4Mixed<$t1>, ReprC4Mixed<$t2>);
+            assert_abi_compatible!(repr_c_enum, ReprCEnum<$t1>, ReprCEnum<$t2>);
+            assert_abi_compatible!(repr_c_union, ReprCUnion<$t1>, ReprCUnion<$t2>);
+        }
+    };
+}
+
+// Compatibility of pointers is probably de-facto guaranteed,
+// but that does not seem to be documented.
+test_abi_compatible!(ptr_mut, *const i32, *mut i32);
+test_abi_compatible!(ptr_pointee, *const i32, *const Vec<i32>);
+test_abi_compatible!(ref_mut, &i32, &mut i32);
+test_abi_compatible!(ref_ptr, &i32, *const i32);
+test_abi_compatible!(box_ptr, Box<i32>, *const i32);
+test_abi_compatible!(nonnull_ptr, NonNull<i32>, *const i32);
+test_abi_compatible!(fn_fn, fn(), fn(i32) -> i32);
+
+// Some further guarantees we will likely (have to) make.
+test_abi_compatible!(zst_unit, Zst, ());
+#[cfg(not(any(target_arch = "sparc64")))]
+test_abi_compatible!(zst_array, Zst, [u8; 0]);
+test_abi_compatible!(nonzero_int, NonZeroI32, i32);
+
+// `DispatchFromDyn` relies on ABI compatibility.
+// This is interesting since these types are not `repr(transparent)`.
+test_abi_compatible!(rc, Rc<i32>, *mut i32);
+test_abi_compatible!(arc, Arc<i32>, *mut i32);
+
+// `repr(transparent)` compatibility.
+#[repr(transparent)]
+struct Wrapper1<T: ?Sized>(T);
+#[repr(transparent)]
+struct Wrapper2<T: ?Sized>((), Zst, T);
+#[repr(transparent)]
+struct Wrapper3<T>(T, [u8; 0], PhantomData<u64>);
+#[repr(transparent)]
+union WrapperUnion<T> {
+    nothing: (),
+    something: ManuallyDrop<T>,
+}
+
+macro_rules! test_transparent {
+    ($name:ident, $t:ty) => {
+        mod $name {
+            use super::*;
+            test_abi_compatible!(wrap1, $t, Wrapper1<$t>);
+            test_abi_compatible!(wrap2, $t, Wrapper2<$t>);
+            test_abi_compatible!(wrap3, $t, Wrapper3<$t>);
+            test_abi_compatible!(wrap4, $t, WrapperUnion<$t>);
+        }
+    };
+}
+
+test_transparent!(simple, i32);
+test_transparent!(reference, &'static i32);
+test_transparent!(zst, Zst);
+test_transparent!(unit, ());
+test_transparent!(enum_, Option<i32>);
+test_transparent!(enum_niched, Option<&'static i32>);
+#[cfg(not(any(target_arch = "mips64", target_arch = "sparc64")))]
+mod tuples {
+    use super::*;
+    // mixing in some floats since they often get special treatment
+    test_transparent!(pair, (i32, f32));
+    // chosen to fit into 64bit
+    test_transparent!(triple, (i8, i16, f32));
+    // Pure-float types that are not ScalarPair seem to be tricky.
+    test_transparent!(triple_f32, (f32, f32, f32));
+    test_transparent!(triple_f64, (f64, f64, f64));
+    // and also something that's larger than 2 pointers
+    test_transparent!(tuple, (i32, f32, i64, f64));
+}
+// Some targets have special rules for arrays.
+#[cfg(not(any(target_arch = "mips64", target_arch = "sparc64")))]
+mod arrays {
+    use super::*;
+    test_transparent!(empty_array, [u32; 0]);
+    test_transparent!(empty_1zst_array, [u8; 0]);
+    test_transparent!(small_array, [i32; 2]); // chosen to fit into 64bit
+    test_transparent!(large_array, [i32; 16]);
+}
+
+// Some tests with unsized types (not all wrappers are compatible with that).
+macro_rules! test_transparent_unsized {
+    ($name:ident, $t:ty) => {
+        mod $name {
+            use super::*;
+            assert_abi_compatible!(wrap1, $t, Wrapper1<$t>);
+            assert_abi_compatible!(wrap1_reprc, ReprC1<$t>, ReprC1<Wrapper1<$t>>);
+            assert_abi_compatible!(wrap2, $t, Wrapper2<$t>);
+            assert_abi_compatible!(wrap2_reprc, ReprC1<$t>, ReprC1<Wrapper2<$t>>);
+        }
+    };
+}
+
+#[cfg(not(any(target_arch = "mips64", target_arch = "sparc64")))]
+mod unsized_ {
+    use super::*;
+    test_transparent_unsized!(str_, str);
+    test_transparent_unsized!(slice, [u8]);
+    test_transparent_unsized!(dyn_trait, dyn Any);
+}
+
+// RFC 3391 <https://rust-lang.github.io/rfcs/3391-result_ffi_guarantees.html>.
+macro_rules! test_nonnull {
+    ($name:ident, $t:ty) => {
+        mod $name {
+            use super::*;
+            test_abi_compatible!(option, Option<$t>, $t);
+            test_abi_compatible!(result_err_unit, Result<$t, ()>, $t);
+            test_abi_compatible!(result_ok_unit, Result<(), $t>, $t);
+            test_abi_compatible!(result_err_zst, Result<$t, Zst>, $t);
+            test_abi_compatible!(result_ok_zst, Result<Zst, $t>, $t);
+            test_abi_compatible!(result_err_arr, Result<$t, [i8; 0]>, $t);
+            test_abi_compatible!(result_ok_arr, Result<[i8; 0], $t>, $t);
+        }
+    }
+}
+
+test_nonnull!(ref_, &i32);
+test_nonnull!(mut_, &mut i32);
+test_nonnull!(ref_unsized, &[i32]);
+test_nonnull!(mut_unsized, &mut [i32]);
+test_nonnull!(fn_, fn());
+test_nonnull!(nonnull, NonNull<i32>);
+test_nonnull!(nonnull_unsized, NonNull<dyn Any>);
+test_nonnull!(non_zero, NonZeroI32);
+
+fn main() {}