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//! Orphan checker: every impl either implements a trait defined in this
//! crate or pertains to a type defined in this crate.
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::{struct_span_err, DelayDm};
use rustc_errors::{Diagnostic, ErrorGuaranteed};
use rustc_hir as hir;
use rustc_middle::ty::util::CheckRegions;
use rustc_middle::ty::GenericArgs;
use rustc_middle::ty::{
self, AliasKind, ImplPolarity, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
TypeVisitor,
};
use rustc_session::lint;
use rustc_span::def_id::{DefId, LocalDefId};
use rustc_span::Span;
use rustc_trait_selection::traits;
use std::ops::ControlFlow;
#[instrument(skip(tcx), level = "debug")]
pub(crate) fn orphan_check_impl(
tcx: TyCtxt<'_>,
impl_def_id: LocalDefId,
) -> Result<(), ErrorGuaranteed> {
let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().instantiate_identity();
trait_ref.error_reported()?;
let ret = do_orphan_check_impl(tcx, trait_ref, impl_def_id);
if tcx.trait_is_auto(trait_ref.def_id) {
lint_auto_trait_impl(tcx, trait_ref, impl_def_id);
}
ret
}
fn do_orphan_check_impl<'tcx>(
tcx: TyCtxt<'tcx>,
trait_ref: ty::TraitRef<'tcx>,
def_id: LocalDefId,
) -> Result<(), ErrorGuaranteed> {
let trait_def_id = trait_ref.def_id;
match traits::orphan_check(tcx, def_id.to_def_id()) {
Ok(()) => {}
Err(err) => {
let item = tcx.hir().expect_item(def_id);
let hir::ItemKind::Impl(impl_) = item.kind else {
bug!("{:?} is not an impl: {:?}", def_id, item);
};
let tr = impl_.of_trait.as_ref().unwrap();
let sp = tcx.def_span(def_id);
emit_orphan_check_error(
tcx,
sp,
item.span,
tr.path.span,
trait_ref,
impl_.self_ty.span,
&impl_.generics,
err,
)?
}
}
// In addition to the above rules, we restrict impls of auto traits
// so that they can only be implemented on nominal types, such as structs,
// enums or foreign types. To see why this restriction exists, consider the
// following example (#22978). Imagine that crate A defines an auto trait
// `Foo` and a fn that operates on pairs of types:
//
// ```
// // Crate A
// auto trait Foo { }
// fn two_foos<A:Foo,B:Foo>(..) {
// one_foo::<(A,B)>(..)
// }
// fn one_foo<T:Foo>(..) { .. }
// ```
//
// This type-checks fine; in particular the fn
// `two_foos` is able to conclude that `(A,B):Foo`
// because `A:Foo` and `B:Foo`.
//
// Now imagine that crate B comes along and does the following:
//
// ```
// struct A { }
// struct B { }
// impl Foo for A { }
// impl Foo for B { }
// impl !Foo for (A, B) { }
// ```
//
// This final impl is legal according to the orphan
// rules, but it invalidates the reasoning from
// `two_foos` above.
debug!(
"trait_ref={:?} trait_def_id={:?} trait_is_auto={}",
trait_ref,
trait_def_id,
tcx.trait_is_auto(trait_def_id)
);
if tcx.trait_is_auto(trait_def_id) {
let self_ty = trait_ref.self_ty();
// If the impl is in the same crate as the auto-trait, almost anything
// goes.
//
// impl MyAuto for Rc<Something> {} // okay
// impl<T> !MyAuto for *const T {} // okay
// impl<T> MyAuto for T {} // okay
//
// But there is one important exception: implementing for a trait object
// is not allowed.
//
// impl MyAuto for dyn Trait {} // NOT OKAY
// impl<T: ?Sized> MyAuto for T {} // NOT OKAY
//
// With this restriction, it's guaranteed that an auto-trait is
// implemented for a trait object if and only if the auto-trait is one
// of the trait object's trait bounds (or a supertrait of a bound). In
// other words `dyn Trait + AutoTrait` always implements AutoTrait,
// while `dyn Trait` never implements AutoTrait.
//
// This is necessary in order for autotrait bounds on methods of trait
// objects to be sound.
//
// auto trait AutoTrait {}
//
// trait ObjectSafeTrait {
// fn f(&self) where Self: AutoTrait;
// }
//
// We can allow f to be called on `dyn ObjectSafeTrait + AutoTrait`.
//
// If we didn't deny `impl AutoTrait for dyn Trait`, it would be unsound
// for the ObjectSafeTrait shown above to be object safe because someone
// could take some type implementing ObjectSafeTrait but not AutoTrait,
// unsize it to `dyn ObjectSafeTrait`, and call .f() which has no
// concrete implementation (issue #50781).
enum LocalImpl {
Allow,
Disallow { problematic_kind: &'static str },
}
// If the auto-trait is from a dependency, it must only be getting
// implemented for a nominal type, and specifically one local to the
// current crate.
//
// impl<T> Sync for MyStruct<T> {} // okay
//
// impl Sync for Rc<MyStruct> {} // NOT OKAY
enum NonlocalImpl {
Allow,
DisallowBecauseNonlocal,
DisallowOther,
}
// Exhaustive match considering that this logic is essential for
// soundness.
let (local_impl, nonlocal_impl) = match self_ty.kind() {
// struct Struct<T>;
// impl AutoTrait for Struct<Foo> {}
ty::Adt(self_def, _) => (
LocalImpl::Allow,
if self_def.did().is_local() {
NonlocalImpl::Allow
} else {
NonlocalImpl::DisallowBecauseNonlocal
},
),
// extern { type OpaqueType; }
// impl AutoTrait for OpaqueType {}
ty::Foreign(did) => (
LocalImpl::Allow,
if did.is_local() {
NonlocalImpl::Allow
} else {
NonlocalImpl::DisallowBecauseNonlocal
},
),
// impl AutoTrait for dyn Trait {}
ty::Dynamic(..) => (
LocalImpl::Disallow { problematic_kind: "trait object" },
NonlocalImpl::DisallowOther,
),
// impl<T> AutoTrait for T {}
// impl<T: ?Sized> AutoTrait for T {}
ty::Param(..) => (
if self_ty.is_sized(tcx, tcx.param_env(def_id)) {
LocalImpl::Allow
} else {
LocalImpl::Disallow { problematic_kind: "generic type" }
},
NonlocalImpl::DisallowOther,
),
ty::Alias(kind, _) => {
let problematic_kind = match kind {
// trait Id { type This: ?Sized; }
// impl<T: ?Sized> Id for T {
// type This = T;
// }
// impl<T: ?Sized> AutoTrait for <T as Id>::This {}
AliasKind::Projection => "associated type",
// type Foo = (impl Sized, bool)
// impl AutoTrait for Foo {}
AliasKind::Weak => "type alias",
// type Opaque = impl Trait;
// impl AutoTrait for Opaque {}
AliasKind::Opaque => "opaque type",
// ```
// struct S<T>(T);
// impl<T: ?Sized> S<T> {
// type This = T;
// }
// impl<T: ?Sized> AutoTrait for S<T>::This {}
// ```
// FIXME(inherent_associated_types): The example code above currently leads to a cycle
AliasKind::Inherent => "associated type",
};
(LocalImpl::Disallow { problematic_kind }, NonlocalImpl::DisallowOther)
}
ty::Bool
| ty::Char
| ty::Int(..)
| ty::Uint(..)
| ty::Float(..)
| ty::Str
| ty::Array(..)
| ty::Slice(..)
| ty::RawPtr(..)
| ty::Ref(..)
| ty::FnDef(..)
| ty::FnPtr(..)
| ty::Never
| ty::Tuple(..) => (LocalImpl::Allow, NonlocalImpl::DisallowOther),
ty::Closure(..)
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Bound(..)
| ty::Placeholder(..)
| ty::Infer(..) => {
let sp = tcx.def_span(def_id);
span_bug!(sp, "weird self type for autotrait impl")
}
ty::Error(..) => (LocalImpl::Allow, NonlocalImpl::Allow),
};
if trait_def_id.is_local() {
match local_impl {
LocalImpl::Allow => {}
LocalImpl::Disallow { problematic_kind } => {
let msg = format!(
"traits with a default impl, like `{trait}`, \
cannot be implemented for {problematic_kind} `{self_ty}`",
trait = tcx.def_path_str(trait_def_id),
);
let label = format!(
"a trait object implements `{trait}` if and only if `{trait}` \
is one of the trait object's trait bounds",
trait = tcx.def_path_str(trait_def_id),
);
let sp = tcx.def_span(def_id);
let reported =
struct_span_err!(tcx.sess, sp, E0321, "{}", msg).note(label).emit();
return Err(reported);
}
}
} else {
if let Some((msg, label)) = match nonlocal_impl {
NonlocalImpl::Allow => None,
NonlocalImpl::DisallowBecauseNonlocal => Some((
format!(
"cross-crate traits with a default impl, like `{}`, \
can only be implemented for a struct/enum type \
defined in the current crate",
tcx.def_path_str(trait_def_id)
),
"can't implement cross-crate trait for type in another crate",
)),
NonlocalImpl::DisallowOther => Some((
format!(
"cross-crate traits with a default impl, like `{}`, can \
only be implemented for a struct/enum type, not `{}`",
tcx.def_path_str(trait_def_id),
self_ty
),
"can't implement cross-crate trait with a default impl for \
non-struct/enum type",
)),
} {
let sp = tcx.def_span(def_id);
let reported =
struct_span_err!(tcx.sess, sp, E0321, "{}", msg).span_label(sp, label).emit();
return Err(reported);
}
}
}
Ok(())
}
fn emit_orphan_check_error<'tcx>(
tcx: TyCtxt<'tcx>,
sp: Span,
full_impl_span: Span,
trait_span: Span,
trait_ref: ty::TraitRef<'tcx>,
self_ty_span: Span,
generics: &hir::Generics<'tcx>,
err: traits::OrphanCheckErr<'tcx>,
) -> Result<!, ErrorGuaranteed> {
let self_ty = trait_ref.self_ty();
Err(match err {
traits::OrphanCheckErr::NonLocalInputType(tys) => {
let msg = match self_ty.kind() {
ty::Adt(..) => "can be implemented for types defined outside of the crate",
_ if self_ty.is_primitive() => "can be implemented for primitive types",
_ => "can be implemented for arbitrary types",
};
let mut err = struct_span_err!(
tcx.sess,
sp,
E0117,
"only traits defined in the current crate {msg}"
);
err.span_label(sp, "impl doesn't use only types from inside the current crate");
for &(mut ty, is_target_ty) in &tys {
ty = tcx.erase_regions(ty);
ty = match ty.kind() {
// Remove the type arguments from the output, as they are not relevant.
// You can think of this as the reverse of `resolve_vars_if_possible`.
// That way if we had `Vec<MyType>`, we will properly attribute the
// problem to `Vec<T>` and avoid confusing the user if they were to see
// `MyType` in the error.
ty::Adt(def, _) => Ty::new_adt(tcx, *def, ty::List::empty()),
_ => ty,
};
let msg = |ty: &str, postfix: &str| {
format!("{ty} is not defined in the current crate{postfix}")
};
let this = |name: &str| {
if !trait_ref.def_id.is_local() && !is_target_ty {
msg("this", " because this is a foreign trait")
} else {
msg("this", &format!(" because {name} are always foreign"))
}
};
let msg = match &ty.kind() {
ty::Slice(_) => this("slices"),
ty::Array(..) => this("arrays"),
ty::Tuple(..) => this("tuples"),
ty::Alias(ty::Opaque, ..) => {
"type alias impl trait is treated as if it were foreign, \
because its hidden type could be from a foreign crate"
.to_string()
}
ty::RawPtr(ptr_ty) => {
emit_newtype_suggestion_for_raw_ptr(
full_impl_span,
self_ty,
self_ty_span,
ptr_ty,
&mut err,
);
msg(&format!("`{ty}`"), " because raw pointers are always foreign")
}
_ => msg(&format!("`{ty}`"), ""),
};
if is_target_ty {
// Point at `D<A>` in `impl<A, B> for C<B> in D<A>`
err.span_label(self_ty_span, msg);
} else {
// Point at `C<B>` in `impl<A, B> for C<B> in D<A>`
err.span_label(trait_span, msg);
}
}
err.note("define and implement a trait or new type instead");
err.emit()
}
traits::OrphanCheckErr::UncoveredTy(param_ty, local_type) => {
let mut sp = sp;
for param in generics.params {
if param.name.ident().to_string() == param_ty.to_string() {
sp = param.span;
}
}
match local_type {
Some(local_type) => struct_span_err!(
tcx.sess,
sp,
E0210,
"type parameter `{}` must be covered by another type \
when it appears before the first local type (`{}`)",
param_ty,
local_type
)
.span_label(
sp,
format!(
"type parameter `{param_ty}` must be covered by another type \
when it appears before the first local type (`{local_type}`)"
),
)
.note(
"implementing a foreign trait is only possible if at \
least one of the types for which it is implemented is local, \
and no uncovered type parameters appear before that first \
local type",
)
.note(
"in this case, 'before' refers to the following order: \
`impl<..> ForeignTrait<T1, ..., Tn> for T0`, \
where `T0` is the first and `Tn` is the last",
)
.emit(),
None => struct_span_err!(
tcx.sess,
sp,
E0210,
"type parameter `{}` must be used as the type parameter for some \
local type (e.g., `MyStruct<{}>`)",
param_ty,
param_ty
)
.span_label(
sp,
format!(
"type parameter `{param_ty}` must be used as the type parameter for some \
local type",
),
)
.note(
"implementing a foreign trait is only possible if at \
least one of the types for which it is implemented is local",
)
.note(
"only traits defined in the current crate can be \
implemented for a type parameter",
)
.emit(),
}
}
})
}
fn emit_newtype_suggestion_for_raw_ptr(
full_impl_span: Span,
self_ty: Ty<'_>,
self_ty_span: Span,
ptr_ty: &ty::TypeAndMut<'_>,
diag: &mut Diagnostic,
) {
if !self_ty.has_param() {
let mut_key = ptr_ty.mutbl.prefix_str();
let msg_sugg = "consider introducing a new wrapper type".to_owned();
let sugg = vec![
(
full_impl_span.shrink_to_lo(),
format!("struct WrapperType(*{}{});\n\n", mut_key, ptr_ty.ty),
),
(self_ty_span, "WrapperType".to_owned()),
];
diag.multipart_suggestion(msg_sugg, sugg, rustc_errors::Applicability::MaybeIncorrect);
}
}
/// Lint impls of auto traits if they are likely to have
/// unsound or surprising effects on auto impls.
fn lint_auto_trait_impl<'tcx>(
tcx: TyCtxt<'tcx>,
trait_ref: ty::TraitRef<'tcx>,
impl_def_id: LocalDefId,
) {
assert_eq!(trait_ref.args.len(), 1);
let self_ty = trait_ref.self_ty();
let (self_type_did, args) = match self_ty.kind() {
ty::Adt(def, args) => (def.did(), args),
_ => {
// FIXME: should also lint for stuff like `&i32` but
// considering that auto traits are unstable, that
// isn't too important for now as this only affects
// crates using `nightly`, and std.
return;
}
};
// Impls which completely cover a given root type are fine as they
// disable auto impls entirely. So only lint if the args
// are not a permutation of the identity args.
let Err(arg) = tcx.uses_unique_generic_params(args, CheckRegions::No) else {
// ok
return;
};
// Ideally:
//
// - compute the requirements for the auto impl candidate
// - check whether these are implied by the non covering impls
// - if not, emit the lint
//
// What we do here is a bit simpler:
//
// - badly check if an auto impl candidate definitely does not apply
// for the given simplified type
// - if so, do not lint
if fast_reject_auto_impl(tcx, trait_ref.def_id, self_ty) {
// ok
return;
}
tcx.struct_span_lint_hir(
lint::builtin::SUSPICIOUS_AUTO_TRAIT_IMPLS,
tcx.hir().local_def_id_to_hir_id(impl_def_id),
tcx.def_span(impl_def_id),
DelayDm(|| {
format!(
"cross-crate traits with a default impl, like `{}`, \
should not be specialized",
tcx.def_path_str(trait_ref.def_id),
)
}),
|lint| {
let item_span = tcx.def_span(self_type_did);
let self_descr = tcx.def_descr(self_type_did);
match arg {
ty::util::NotUniqueParam::DuplicateParam(arg) => {
lint.note(format!("`{arg}` is mentioned multiple times"));
}
ty::util::NotUniqueParam::NotParam(arg) => {
lint.note(format!("`{arg}` is not a generic parameter"));
}
}
lint.span_note(
item_span,
format!(
"try using the same sequence of generic parameters as the {self_descr} definition",
),
)
},
);
}
fn fast_reject_auto_impl<'tcx>(tcx: TyCtxt<'tcx>, trait_def_id: DefId, self_ty: Ty<'tcx>) -> bool {
struct DisableAutoTraitVisitor<'tcx> {
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
self_ty_root: Ty<'tcx>,
seen: FxHashSet<DefId>,
}
impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for DisableAutoTraitVisitor<'tcx> {
type BreakTy = ();
fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
let tcx = self.tcx;
if ty != self.self_ty_root {
for impl_def_id in tcx.non_blanket_impls_for_ty(self.trait_def_id, ty) {
match tcx.impl_polarity(impl_def_id) {
ImplPolarity::Negative => return ControlFlow::Break(()),
ImplPolarity::Reservation => {}
// FIXME(@lcnr): That's probably not good enough, idk
//
// We might just want to take the rustdoc code and somehow avoid
// explicit impls for `Self`.
ImplPolarity::Positive => return ControlFlow::Continue(()),
}
}
}
match ty.kind() {
ty::Adt(def, args) if def.is_phantom_data() => args.visit_with(self),
ty::Adt(def, args) => {
// @lcnr: This is the only place where cycles can happen. We avoid this
// by only visiting each `DefId` once.
//
// This will be is incorrect in subtle cases, but I don't care :)
if self.seen.insert(def.did()) {
for ty in def.all_fields().map(|field| field.ty(tcx, args)) {
ty.visit_with(self)?;
}
}
ControlFlow::Continue(())
}
_ => ty.super_visit_with(self),
}
}
}
let self_ty_root = match self_ty.kind() {
ty::Adt(def, _) => Ty::new_adt(tcx, *def, GenericArgs::identity_for_item(tcx, def.did())),
_ => unimplemented!("unexpected self ty {:?}", self_ty),
};
self_ty_root
.visit_with(&mut DisableAutoTraitVisitor {
tcx,
self_ty_root,
trait_def_id,
seen: FxHashSet::default(),
})
.is_break()
}