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committedMay 5, 2024
Auto merge of #122253 - MasterAwesome:master, r=davidtwco
Support Result<T, E> across FFI when niche optimization can be used Allow allow enums like `Result<T, E>` to be used across FFI if the T/E can be niche optimized and the non-niche-optimized type is FFI safe. Implementation of rust-lang/rfcs#3391 Tracking issue: #110503 Additional ABI and codegen tests were added in #115372
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‎compiler/rustc_feature/src/unstable.rs

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -581,6 +581,9 @@ declare_features! (
581581
(incomplete, repr128, "1.16.0", Some(56071)),
582582
/// Allows `repr(simd)` and importing the various simd intrinsics.
583583
(unstable, repr_simd, "1.4.0", Some(27731)),
584+
/// Allows enums like Result<T, E> to be used across FFI, if T's niche value can
585+
/// be used to describe E or vise-versa.
586+
(unstable, result_ffi_guarantees, "CURRENT_RUSTC_VERSION", Some(110503)),
584587
/// Allows bounding the return type of AFIT/RPITIT.
585588
(incomplete, return_type_notation, "1.70.0", Some(109417)),
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/// Allows `extern "rust-cold"`.

‎compiler/rustc_lint/src/types.rs

Lines changed: 56 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -1101,6 +1101,32 @@ fn get_nullable_type<'tcx>(
11011101
})
11021102
}
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1104+
/// A type is niche-optimization candidate iff:
1105+
/// - Is a zero-sized type with alignment 1 (a “1-ZST”).
1106+
/// - Has no fields.
1107+
/// - Does not have the `#[non_exhaustive]` attribute.
1108+
fn is_niche_optimization_candidate<'tcx>(
1109+
tcx: TyCtxt<'tcx>,
1110+
param_env: ty::ParamEnv<'tcx>,
1111+
ty: Ty<'tcx>,
1112+
) -> bool {
1113+
if tcx.layout_of(param_env.and(ty)).is_ok_and(|layout| !layout.is_1zst()) {
1114+
return false;
1115+
}
1116+
1117+
match ty.kind() {
1118+
ty::Adt(ty_def, _) => {
1119+
let non_exhaustive = ty_def.is_variant_list_non_exhaustive();
1120+
let empty = (ty_def.is_struct() && ty_def.all_fields().next().is_none())
1121+
|| (ty_def.is_enum() && ty_def.variants().is_empty());
1122+
1123+
!non_exhaustive && empty
1124+
}
1125+
ty::Tuple(tys) => tys.is_empty(),
1126+
_ => false,
1127+
}
1128+
}
1129+
11041130
/// Check if this enum can be safely exported based on the "nullable pointer optimization". If it
11051131
/// can, return the type that `ty` can be safely converted to, otherwise return `None`.
11061132
/// Currently restricted to function pointers, boxes, references, `core::num::NonZero`,
@@ -1117,6 +1143,22 @@ pub(crate) fn repr_nullable_ptr<'tcx>(
11171143
let field_ty = match &ty_def.variants().raw[..] {
11181144
[var_one, var_two] => match (&var_one.fields.raw[..], &var_two.fields.raw[..]) {
11191145
([], [field]) | ([field], []) => field.ty(tcx, args),
1146+
([field1], [field2]) => {
1147+
if !tcx.features().result_ffi_guarantees {
1148+
return None;
1149+
}
1150+
1151+
let ty1 = field1.ty(tcx, args);
1152+
let ty2 = field2.ty(tcx, args);
1153+
1154+
if is_niche_optimization_candidate(tcx, param_env, ty1) {
1155+
ty2
1156+
} else if is_niche_optimization_candidate(tcx, param_env, ty2) {
1157+
ty1
1158+
} else {
1159+
return None;
1160+
}
1161+
}
11201162
_ => return None,
11211163
},
11221164
_ => return None,
@@ -1202,7 +1244,6 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
12021244
args: GenericArgsRef<'tcx>,
12031245
) -> FfiResult<'tcx> {
12041246
use FfiResult::*;
1205-
12061247
let transparent_with_all_zst_fields = if def.repr().transparent() {
12071248
if let Some(field) = transparent_newtype_field(self.cx.tcx, variant) {
12081249
// Transparent newtypes have at most one non-ZST field which needs to be checked..
@@ -1329,27 +1370,29 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
13291370
return FfiSafe;
13301371
}
13311372

1373+
if def.is_variant_list_non_exhaustive() && !def.did().is_local() {
1374+
return FfiUnsafe {
1375+
ty,
1376+
reason: fluent::lint_improper_ctypes_non_exhaustive,
1377+
help: None,
1378+
};
1379+
}
1380+
13321381
// Check for a repr() attribute to specify the size of the
13331382
// discriminant.
13341383
if !def.repr().c() && !def.repr().transparent() && def.repr().int.is_none()
13351384
{
1336-
// Special-case types like `Option<extern fn()>`.
1337-
if repr_nullable_ptr(self.cx.tcx, self.cx.param_env, ty, self.mode)
1338-
.is_none()
1385+
// Special-case types like `Option<extern fn()>` and `Result<extern fn(), ()>`
1386+
if let Some(ty) =
1387+
repr_nullable_ptr(self.cx.tcx, self.cx.param_env, ty, self.mode)
13391388
{
1340-
return FfiUnsafe {
1341-
ty,
1342-
reason: fluent::lint_improper_ctypes_enum_repr_reason,
1343-
help: Some(fluent::lint_improper_ctypes_enum_repr_help),
1344-
};
1389+
return self.check_type_for_ffi(cache, ty);
13451390
}
1346-
}
13471391

1348-
if def.is_variant_list_non_exhaustive() && !def.did().is_local() {
13491392
return FfiUnsafe {
13501393
ty,
1351-
reason: fluent::lint_improper_ctypes_non_exhaustive,
1352-
help: None,
1394+
reason: fluent::lint_improper_ctypes_enum_repr_reason,
1395+
help: Some(fluent::lint_improper_ctypes_enum_repr_help),
13531396
};
13541397
}
13551398

‎compiler/rustc_span/src/symbol.rs

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Original file line numberDiff line numberDiff line change
@@ -1511,6 +1511,7 @@ symbols! {
15111511
require,
15121512
residual,
15131513
result,
1514+
result_ffi_guarantees,
15141515
resume,
15151516
return_position_impl_trait_in_trait,
15161517
return_type_notation,

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

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,14 @@
1+
# `result_ffi_guarantees`
2+
3+
The tracking issue for this feature is: [#110503]
4+
5+
[#110503]: https://github.com/rust-lang/rust/issues/110503
6+
7+
------------------------
8+
9+
This feature adds the possibility of using `Result<T, E>` in FFI if T's niche
10+
value can be used to describe E or vise-versa.
11+
12+
See [RFC 3391] for more information.
13+
14+
[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

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@@ -0,0 +1,99 @@
1+
#![allow(dead_code)]
2+
#![deny(improper_ctypes)]
3+
#![feature(ptr_internals)]
4+
5+
use std::num;
6+
7+
enum Z {}
8+
9+
#[repr(transparent)]
10+
struct TransparentStruct<T>(T, std::marker::PhantomData<Z>);
11+
12+
#[repr(transparent)]
13+
enum TransparentEnum<T> {
14+
Variant(T, std::marker::PhantomData<Z>),
15+
}
16+
17+
struct NoField;
18+
19+
extern "C" {
20+
fn result_ref_t(x: Result<&'static u8, ()>);
21+
//~^ ERROR `extern` block uses type `Result
22+
fn result_fn_t(x: Result<extern "C" fn(), ()>);
23+
//~^ ERROR `extern` block uses type `Result
24+
fn result_nonnull_t(x: Result<std::ptr::NonNull<u8>, ()>);
25+
//~^ ERROR `extern` block uses type `Result
26+
fn result_unique_t(x: Result<std::ptr::Unique<u8>, ()>);
27+
//~^ ERROR `extern` block uses type `Result
28+
fn result_nonzero_u8_t(x: Result<num::NonZero<u8>, ()>);
29+
//~^ ERROR `extern` block uses type `Result
30+
fn result_nonzero_u16_t(x: Result<num::NonZero<u16>, ()>);
31+
//~^ ERROR `extern` block uses type `Result
32+
fn result_nonzero_u32_t(x: Result<num::NonZero<u32>, ()>);
33+
//~^ ERROR `extern` block uses type `Result
34+
fn result_nonzero_u64_t(x: Result<num::NonZero<u64>, ()>);
35+
//~^ ERROR `extern` block uses type `Result
36+
fn result_nonzero_usize_t(x: Result<num::NonZero<usize>, ()>);
37+
//~^ ERROR `extern` block uses type `Result
38+
fn result_nonzero_i8_t(x: Result<num::NonZero<i8>, ()>);
39+
//~^ ERROR `extern` block uses type `Result
40+
fn result_nonzero_i16_t(x: Result<num::NonZero<i16>, ()>);
41+
//~^ ERROR `extern` block uses type `Result
42+
fn result_nonzero_i32_t(x: Result<num::NonZero<i32>, ()>);
43+
//~^ ERROR `extern` block uses type `Result
44+
fn result_nonzero_i64_t(x: Result<num::NonZero<i64>, ()>);
45+
//~^ ERROR `extern` block uses type `Result
46+
fn result_nonzero_isize_t(x: Result<num::NonZero<isize>, ()>);
47+
//~^ ERROR `extern` block uses type `Result
48+
fn result_transparent_struct_t(x: Result<TransparentStruct<num::NonZero<u8>>, ()>);
49+
//~^ ERROR `extern` block uses type `Result
50+
fn result_transparent_enum_t(x: Result<TransparentEnum<num::NonZero<u8>>, ()>);
51+
//~^ ERROR `extern` block uses type `Result
52+
fn result_phantom_t(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
53+
//~^ ERROR `extern` block uses type `Result
54+
fn result_1zst_exhaustive_no_variant_t(x: Result<num::NonZero<u8>, Z>);
55+
//~^ ERROR `extern` block uses type `Result
56+
fn result_1zst_exhaustive_no_field_t(x: Result<num::NonZero<u8>, NoField>);
57+
//~^ ERROR `extern` block uses type `Result
58+
59+
fn result_ref_e(x: Result<(), &'static u8>);
60+
//~^ ERROR `extern` block uses type `Result
61+
fn result_fn_e(x: Result<(), extern "C" fn()>);
62+
//~^ ERROR `extern` block uses type `Result
63+
fn result_nonnull_e(x: Result<(), std::ptr::NonNull<u8>>);
64+
//~^ ERROR `extern` block uses type `Result
65+
fn result_unique_e(x: Result<(), std::ptr::Unique<u8>>);
66+
//~^ ERROR `extern` block uses type `Result
67+
fn result_nonzero_u8_e(x: Result<(), num::NonZero<u8>>);
68+
//~^ ERROR `extern` block uses type `Result
69+
fn result_nonzero_u16_e(x: Result<(), num::NonZero<u16>>);
70+
//~^ ERROR `extern` block uses type `Result
71+
fn result_nonzero_u32_e(x: Result<(), num::NonZero<u32>>);
72+
//~^ ERROR `extern` block uses type `Result
73+
fn result_nonzero_u64_e(x: Result<(), num::NonZero<u64>>);
74+
//~^ ERROR `extern` block uses type `Result
75+
fn result_nonzero_usize_e(x: Result<(), num::NonZero<usize>>);
76+
//~^ ERROR `extern` block uses type `Result
77+
fn result_nonzero_i8_e(x: Result<(), num::NonZero<i8>>);
78+
//~^ ERROR `extern` block uses type `Result
79+
fn result_nonzero_i16_e(x: Result<(), num::NonZero<i16>>);
80+
//~^ ERROR `extern` block uses type `Result
81+
fn result_nonzero_i32_e(x: Result<(), num::NonZero<i32>>);
82+
//~^ ERROR `extern` block uses type `Result
83+
fn result_nonzero_i64_e(x: Result<(), num::NonZero<i64>>);
84+
//~^ ERROR `extern` block uses type `Result
85+
fn result_nonzero_isize_e(x: Result<(), num::NonZero<isize>>);
86+
//~^ ERROR `extern` block uses type `Result
87+
fn result_transparent_struct_e(x: Result<(), TransparentStruct<num::NonZero<u8>>>);
88+
//~^ ERROR `extern` block uses type `Result
89+
fn result_transparent_enum_e(x: Result<(), TransparentEnum<num::NonZero<u8>>>);
90+
//~^ ERROR `extern` block uses type `Result
91+
fn result_phantom_e(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
92+
//~^ ERROR `extern` block uses type `Result
93+
fn result_1zst_exhaustive_no_variant_e(x: Result<Z, num::NonZero<u8>>);
94+
//~^ ERROR `extern` block uses type `Result
95+
fn result_1zst_exhaustive_no_field_e(x: Result<NoField, num::NonZero<u8>>);
96+
//~^ ERROR `extern` block uses type `Result
97+
}
98+
99+
pub fn main() {}

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