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Auto merge of #124747 - MasterAwesome:master, r=davidtwco

Support Result<T, E> across FFI when niche optimization can be used (v2)

This PR is identical to #122253, which was approved and merged but then removed from master by a force-push due to a [CI bug](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/ci.20broken.3F).

r? ghost

Original PR description:

---

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 https://github.com/rust-lang/rfcs/pull/3391
Tracking issue: https://github.com/rust-lang/rust/issues/110503

Additional ABI and codegen tests were added in https://github.com/rust-lang/rust/pull/115372
This commit is contained in:
bors 2024-05-06 00:55:49 +00:00
commit 80420a693f
8 changed files with 830 additions and 77 deletions

View file

@ -1101,6 +1101,32 @@ fn get_nullable_type<'tcx>(
})
}
/// A type is niche-optimization candidate iff:
/// - Is a zero-sized type with alignment 1 (a “1-ZST”).
/// - Has no fields.
/// - Does not have the `#[non_exhaustive]` attribute.
fn is_niche_optimization_candidate<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
ty: Ty<'tcx>,
) -> bool {
if tcx.layout_of(param_env.and(ty)).is_ok_and(|layout| !layout.is_1zst()) {
return false;
}
match ty.kind() {
ty::Adt(ty_def, _) => {
let non_exhaustive = ty_def.is_variant_list_non_exhaustive();
let empty = (ty_def.is_struct() && ty_def.all_fields().next().is_none())
|| (ty_def.is_enum() && ty_def.variants().is_empty());
!non_exhaustive && empty
}
ty::Tuple(tys) => tys.is_empty(),
_ => false,
}
}
/// Check if this enum can be safely exported based on the "nullable pointer optimization". If it
/// can, return the type that `ty` can be safely converted to, otherwise return `None`.
/// Currently restricted to function pointers, boxes, references, `core::num::NonZero`,
@ -1117,6 +1143,22 @@ pub(crate) fn repr_nullable_ptr<'tcx>(
let field_ty = match &ty_def.variants().raw[..] {
[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);
if is_niche_optimization_candidate(tcx, param_env, ty1) {
ty2
} else if is_niche_optimization_candidate(tcx, param_env, ty2) {
ty1
} else {
return None;
}
}
_ => return None,
},
_ => return None,
@ -1202,7 +1244,6 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
args: GenericArgsRef<'tcx>,
) -> FfiResult<'tcx> {
use FfiResult::*;
let transparent_with_all_zst_fields = if def.repr().transparent() {
if let Some(field) = transparent_newtype_field(self.cx.tcx, variant) {
// Transparent newtypes have at most one non-ZST field which needs to be checked..
@ -1329,22 +1370,6 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
return FfiSafe;
}
// Check for a repr() attribute to specify the size of the
// discriminant.
if !def.repr().c() && !def.repr().transparent() && def.repr().int.is_none()
{
// Special-case types like `Option<extern fn()>`.
if repr_nullable_ptr(self.cx.tcx, self.cx.param_env, ty, self.mode)
.is_none()
{
return FfiUnsafe {
ty,
reason: fluent::lint_improper_ctypes_enum_repr_reason,
help: Some(fluent::lint_improper_ctypes_enum_repr_help),
};
}
}
if def.is_variant_list_non_exhaustive() && !def.did().is_local() {
return FfiUnsafe {
ty,
@ -1353,6 +1378,24 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
};
}
// Check for a repr() attribute to specify the size of the
// discriminant.
if !def.repr().c() && !def.repr().transparent() && def.repr().int.is_none()
{
// Special-case types like `Option<extern fn()>` and `Result<extern fn(), ()>`
if let Some(ty) =
repr_nullable_ptr(self.cx.tcx, self.cx.param_env, ty, self.mode)
{
return self.check_type_for_ffi(cache, ty);
}
return FfiUnsafe {
ty,
reason: fluent::lint_improper_ctypes_enum_repr_reason,
help: Some(fluent::lint_improper_ctypes_enum_repr_help),
};
}
// Check the contained variants.
for variant in def.variants() {
let is_non_exhaustive = variant.is_field_list_non_exhaustive();