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//! See Rustc Dev Guide chapters on [trait-resolution] and [trait-specialization] for more info on
//! how this works.
//!
//! [trait-resolution]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
//! [trait-specialization]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html
use crate::infer::outlives::env::OutlivesEnvironment;
use crate::infer::InferOk;
use crate::solve::inspect;
use crate::solve::inspect::{InspectGoal, ProofTreeInferCtxtExt, ProofTreeVisitor};
use crate::traits::engine::TraitEngineExt;
use crate::traits::outlives_bounds::InferCtxtExt as _;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::select::{IntercrateAmbiguityCause, TreatInductiveCycleAs};
use crate::traits::structural_normalize::StructurallyNormalizeExt;
use crate::traits::util::impl_subject_and_oblig;
use crate::traits::NormalizeExt;
use crate::traits::SkipLeakCheck;
use crate::traits::{
self, Obligation, ObligationCause, ObligationCtxt, PredicateObligation, PredicateObligations,
SelectionContext,
};
use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::Diagnostic;
use rustc_hir::def_id::{DefId, CRATE_DEF_ID, LOCAL_CRATE};
use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, TyCtxtInferExt};
use rustc_infer::traits::{util, TraitEngine};
use rustc_middle::traits::query::NoSolution;
use rustc_middle::traits::solve::{Certainty, Goal};
use rustc_middle::traits::specialization_graph::OverlapMode;
use rustc_middle::traits::DefiningAnchor;
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
use rustc_session::lint::builtin::COINDUCTIVE_OVERLAP_IN_COHERENCE;
use rustc_span::symbol::sym;
use rustc_span::DUMMY_SP;
use std::fmt::Debug;
use std::iter;
use std::ops::ControlFlow;
/// Whether we do the orphan check relative to this crate or
/// to some remote crate.
#[derive(Copy, Clone, Debug)]
enum InCrate {
Local,
Remote,
}
#[derive(Debug, Copy, Clone)]
pub enum Conflict {
Upstream,
Downstream,
}
pub struct OverlapResult<'tcx> {
pub impl_header: ty::ImplHeader<'tcx>,
pub intercrate_ambiguity_causes: FxIndexSet<IntercrateAmbiguityCause>,
/// `true` if the overlap might've been permitted before the shift
/// to universes.
pub involves_placeholder: bool,
}
pub fn add_placeholder_note(err: &mut Diagnostic) {
err.note(
"this behavior recently changed as a result of a bug fix; \
see rust-lang/rust#56105 for details",
);
}
#[derive(Debug, Clone, Copy)]
enum TrackAmbiguityCauses {
Yes,
No,
}
impl TrackAmbiguityCauses {
fn is_yes(self) -> bool {
match self {
TrackAmbiguityCauses::Yes => true,
TrackAmbiguityCauses::No => false,
}
}
}
/// If there are types that satisfy both impls, returns `Some`
/// with a suitably-freshened `ImplHeader` with those types
/// substituted. Otherwise, returns `None`.
#[instrument(skip(tcx, skip_leak_check), level = "debug")]
pub fn overlapping_impls(
tcx: TyCtxt<'_>,
impl1_def_id: DefId,
impl2_def_id: DefId,
skip_leak_check: SkipLeakCheck,
overlap_mode: OverlapMode,
) -> Option<OverlapResult<'_>> {
// Before doing expensive operations like entering an inference context, do
// a quick check via fast_reject to tell if the impl headers could possibly
// unify.
let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsCandidateKey };
let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
let may_overlap = match (impl1_ref, impl2_ref) {
(Some(a), Some(b)) => drcx.args_refs_may_unify(a.skip_binder().args, b.skip_binder().args),
(None, None) => {
let self_ty1 = tcx.type_of(impl1_def_id).skip_binder();
let self_ty2 = tcx.type_of(impl2_def_id).skip_binder();
drcx.types_may_unify(self_ty1, self_ty2)
}
_ => bug!("unexpected impls: {impl1_def_id:?} {impl2_def_id:?}"),
};
if !may_overlap {
// Some types involved are definitely different, so the impls couldn't possibly overlap.
debug!("overlapping_impls: fast_reject early-exit");
return None;
}
let _overlap_with_bad_diagnostics = overlap(
tcx,
TrackAmbiguityCauses::No,
skip_leak_check,
impl1_def_id,
impl2_def_id,
overlap_mode,
)?;
// In the case where we detect an error, run the check again, but
// this time tracking intercrate ambiguity causes for better
// diagnostics. (These take time and can lead to false errors.)
let overlap = overlap(
tcx,
TrackAmbiguityCauses::Yes,
skip_leak_check,
impl1_def_id,
impl2_def_id,
overlap_mode,
)
.unwrap();
Some(overlap)
}
fn with_fresh_ty_vars<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
param_env: ty::ParamEnv<'tcx>,
impl_def_id: DefId,
) -> ty::ImplHeader<'tcx> {
let tcx = selcx.tcx();
let impl_args = selcx.infcx.fresh_args_for_item(DUMMY_SP, impl_def_id);
let header = ty::ImplHeader {
impl_def_id,
self_ty: tcx.type_of(impl_def_id).instantiate(tcx, impl_args),
trait_ref: tcx.impl_trait_ref(impl_def_id).map(|i| i.instantiate(tcx, impl_args)),
predicates: tcx
.predicates_of(impl_def_id)
.instantiate(tcx, impl_args)
.iter()
.map(|(c, _)| c.as_predicate())
.collect(),
};
let InferOk { value: mut header, obligations } =
selcx.infcx.at(&ObligationCause::dummy(), param_env).normalize(header);
header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
header
}
/// Can both impl `a` and impl `b` be satisfied by a common type (including
/// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
#[instrument(level = "debug", skip(tcx))]
fn overlap<'tcx>(
tcx: TyCtxt<'tcx>,
track_ambiguity_causes: TrackAmbiguityCauses,
skip_leak_check: SkipLeakCheck,
impl1_def_id: DefId,
impl2_def_id: DefId,
overlap_mode: OverlapMode,
) -> Option<OverlapResult<'tcx>> {
if overlap_mode.use_negative_impl() {
if impl_intersection_has_negative_obligation(tcx, impl1_def_id, impl2_def_id)
|| impl_intersection_has_negative_obligation(tcx, impl2_def_id, impl1_def_id)
{
return None;
}
}
let infcx = tcx
.infer_ctxt()
.with_opaque_type_inference(DefiningAnchor::Bubble)
.skip_leak_check(skip_leak_check.is_yes())
.intercrate(true)
.with_next_trait_solver(tcx.next_trait_solver_in_coherence())
.build();
let selcx = &mut SelectionContext::new(&infcx);
if track_ambiguity_causes.is_yes() {
selcx.enable_tracking_intercrate_ambiguity_causes();
}
// For the purposes of this check, we don't bring any placeholder
// types into scope; instead, we replace the generic types with
// fresh type variables, and hence we do our evaluations in an
// empty environment.
let param_env = ty::ParamEnv::empty();
let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
// Equate the headers to find their intersection (the general type, with infer vars,
// that may apply both impls).
let mut obligations = equate_impl_headers(selcx.infcx, &impl1_header, &impl2_header)?;
debug!("overlap: unification check succeeded");
obligations.extend(
[&impl1_header.predicates, &impl2_header.predicates].into_iter().flatten().map(
|&predicate| Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, predicate),
),
);
if overlap_mode.use_implicit_negative() {
for mode in [TreatInductiveCycleAs::Ambig, TreatInductiveCycleAs::Recur] {
if let Some(failing_obligation) = selcx.with_treat_inductive_cycle_as(mode, |selcx| {
impl_intersection_has_impossible_obligation(selcx, &obligations)
}) {
if matches!(mode, TreatInductiveCycleAs::Recur) {
let first_local_impl = impl1_header
.impl_def_id
.as_local()
.or(impl2_header.impl_def_id.as_local())
.expect("expected one of the impls to be local");
infcx.tcx.struct_span_lint_hir(
COINDUCTIVE_OVERLAP_IN_COHERENCE,
infcx.tcx.local_def_id_to_hir_id(first_local_impl),
infcx.tcx.def_span(first_local_impl),
format!(
"implementations {} will conflict in the future",
match impl1_header.trait_ref {
Some(trait_ref) => {
let trait_ref = infcx.resolve_vars_if_possible(trait_ref);
format!(
"of `{}` for `{}`",
trait_ref.print_only_trait_path(),
trait_ref.self_ty()
)
}
None => format!(
"for `{}`",
infcx.resolve_vars_if_possible(impl1_header.self_ty)
),
},
),
|lint| {
lint.note(
"impls that are not considered to overlap may be considered to \
overlap in the future",
)
.span_label(
infcx.tcx.def_span(impl1_header.impl_def_id),
"the first impl is here",
)
.span_label(
infcx.tcx.def_span(impl2_header.impl_def_id),
"the second impl is here",
);
lint.note(format!(
"`{}` may be considered to hold in future releases, \
causing the impls to overlap",
infcx.resolve_vars_if_possible(failing_obligation.predicate)
));
lint
},
);
}
return None;
}
}
}
// We toggle the `leak_check` by using `skip_leak_check` when constructing the
// inference context, so this may be a noop.
if infcx.leak_check(ty::UniverseIndex::ROOT, None).is_err() {
debug!("overlap: leak check failed");
return None;
}
let intercrate_ambiguity_causes = if !overlap_mode.use_implicit_negative() {
Default::default()
} else if infcx.next_trait_solver() {
compute_intercrate_ambiguity_causes(&infcx, &obligations)
} else {
selcx.take_intercrate_ambiguity_causes()
};
debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
let involves_placeholder = infcx
.inner
.borrow_mut()
.unwrap_region_constraints()
.data()
.constraints
.iter()
.any(|c| c.0.involves_placeholders());
let impl_header = selcx.infcx.resolve_vars_if_possible(impl1_header);
Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
}
#[instrument(level = "debug", skip(infcx), ret)]
fn equate_impl_headers<'tcx>(
infcx: &InferCtxt<'tcx>,
impl1: &ty::ImplHeader<'tcx>,
impl2: &ty::ImplHeader<'tcx>,
) -> Option<PredicateObligations<'tcx>> {
let result = match (impl1.trait_ref, impl2.trait_ref) {
(Some(impl1_ref), Some(impl2_ref)) => infcx
.at(&ObligationCause::dummy(), ty::ParamEnv::empty())
.eq(DefineOpaqueTypes::Yes, impl1_ref, impl2_ref),
(None, None) => infcx.at(&ObligationCause::dummy(), ty::ParamEnv::empty()).eq(
DefineOpaqueTypes::Yes,
impl1.self_ty,
impl2.self_ty,
),
_ => bug!("mk_eq_impl_headers given mismatched impl kinds"),
};
result.map(|infer_ok| infer_ok.obligations).ok()
}
/// Check if both impls can be satisfied by a common type by considering whether
/// any of either impl's obligations is not known to hold.
///
/// For example, given these two impls:
/// `impl From<MyLocalType> for Box<dyn Error>` (in my crate)
/// `impl<E> From<E> for Box<dyn Error> where E: Error` (in libstd)
///
/// After replacing both impl headers with inference vars (which happens before
/// this function is called), we get:
/// `Box<dyn Error>: From<MyLocalType>`
/// `Box<dyn Error>: From<?E>`
///
/// This gives us `?E = MyLocalType`. We then certainly know that `MyLocalType: Error`
/// never holds in intercrate mode since a local impl does not exist, and a
/// downstream impl cannot be added -- therefore can consider the intersection
/// of the two impls above to be empty.
///
/// Importantly, this works even if there isn't a `impl !Error for MyLocalType`.
fn impl_intersection_has_impossible_obligation<'a, 'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligations: &'a [PredicateObligation<'tcx>],
) -> Option<&'a PredicateObligation<'tcx>> {
let infcx = selcx.infcx;
obligations.iter().find(|obligation| {
if infcx.next_trait_solver() {
infcx.evaluate_obligation(obligation).map_or(false, |result| !result.may_apply())
} else {
// We use `evaluate_root_obligation` to correctly track intercrate
// ambiguity clauses. We cannot use this in the new solver.
selcx.evaluate_root_obligation(obligation).map_or(
false, // Overflow has occurred, and treat the obligation as possibly holding.
|result| !result.may_apply(),
)
}
})
}
/// Check if both impls can be satisfied by a common type by considering whether
/// any of first impl's obligations is known not to hold *via a negative predicate*.
///
/// For example, given these two impls:
/// `struct MyCustomBox<T: ?Sized>(Box<T>);`
/// `impl From<&str> for MyCustomBox<dyn Error>` (in my crate)
/// `impl<E> From<E> for MyCustomBox<dyn Error> where E: Error` (in my crate)
///
/// After replacing the second impl's header with inference vars, we get:
/// `MyCustomBox<dyn Error>: From<&str>`
/// `MyCustomBox<dyn Error>: From<?E>`
///
/// This gives us `?E = &str`. We then try to prove the first impl's predicates
/// after negating, giving us `&str: !Error`. This is a negative impl provided by
/// libstd, and therefore we can guarantee for certain that libstd will never add
/// a positive impl for `&str: Error` (without it being a breaking change).
fn impl_intersection_has_negative_obligation(
tcx: TyCtxt<'_>,
impl1_def_id: DefId,
impl2_def_id: DefId,
) -> bool {
debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
// Create an infcx, taking the predicates of impl1 as assumptions:
let infcx = tcx.infer_ctxt().build();
// create a parameter environment corresponding to a (placeholder) instantiation of impl1
let impl_env = tcx.param_env(impl1_def_id);
let subject1 = match traits::fully_normalize(
&infcx,
ObligationCause::dummy(),
impl_env,
tcx.impl_subject(impl1_def_id).instantiate_identity(),
) {
Ok(s) => s,
Err(err) => {
tcx.sess.delay_span_bug(
tcx.def_span(impl1_def_id),
format!("failed to fully normalize {impl1_def_id:?}: {err:?}"),
);
return false;
}
};
// Attempt to prove that impl2 applies, given all of the above.
let selcx = &mut SelectionContext::new(&infcx);
let impl2_args = infcx.fresh_args_for_item(DUMMY_SP, impl2_def_id);
let (subject2, normalization_obligations) =
impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_args, |_, _| {
ObligationCause::dummy()
});
// do the impls unify? If not, then it's not currently possible to prove any
// obligations about their intersection.
let Ok(InferOk { obligations: equate_obligations, .. }) =
infcx.at(&ObligationCause::dummy(), impl_env).eq(DefineOpaqueTypes::No, subject1, subject2)
else {
debug!("explicit_disjoint: {:?} does not unify with {:?}", subject1, subject2);
return false;
};
for obligation in normalization_obligations.into_iter().chain(equate_obligations) {
if negative_impl_exists(&infcx, &obligation, impl1_def_id) {
debug!("overlap: obligation unsatisfiable {:?}", obligation);
return true;
}
}
false
}
/// Try to prove that a negative impl exist for the obligation or its supertraits.
///
/// If such a negative impl exists, then the obligation definitely must not hold
/// due to coherence, even if it's not necessarily "knowable" in this crate. Any
/// valid impl downstream would not be able to exist due to the overlapping
/// negative impl.
#[instrument(level = "debug", skip(infcx))]
fn negative_impl_exists<'tcx>(
infcx: &InferCtxt<'tcx>,
o: &PredicateObligation<'tcx>,
body_def_id: DefId,
) -> bool {
// Try to prove a negative obligation exists for super predicates
for pred in util::elaborate(infcx.tcx, iter::once(o.predicate)) {
if prove_negated_obligation(infcx.fork(), &o.with(infcx.tcx, pred), body_def_id) {
return true;
}
}
false
}
#[instrument(level = "debug", skip(infcx))]
fn prove_negated_obligation<'tcx>(
infcx: InferCtxt<'tcx>,
o: &PredicateObligation<'tcx>,
body_def_id: DefId,
) -> bool {
let tcx = infcx.tcx;
let Some(o) = o.flip_polarity(tcx) else {
return false;
};
let param_env = o.param_env;
let ocx = ObligationCtxt::new(&infcx);
ocx.register_obligation(o);
let errors = ocx.select_all_or_error();
if !errors.is_empty() {
return false;
}
let body_def_id = body_def_id.as_local().unwrap_or(CRATE_DEF_ID);
let ocx = ObligationCtxt::new(&infcx);
let Ok(wf_tys) = ocx.assumed_wf_types(param_env, body_def_id) else {
return false;
};
let outlives_env = OutlivesEnvironment::with_bounds(
param_env,
infcx.implied_bounds_tys(param_env, body_def_id, wf_tys),
);
infcx.resolve_regions(&outlives_env).is_empty()
}
/// Returns whether all impls which would apply to the `trait_ref`
/// e.g. `Ty: Trait<Arg>` are already known in the local crate.
///
/// This both checks whether any downstream or sibling crates could
/// implement it and whether an upstream crate can add this impl
/// without breaking backwards compatibility.
#[instrument(level = "debug", skip(tcx, lazily_normalize_ty), ret)]
pub fn trait_ref_is_knowable<'tcx, E: Debug>(
tcx: TyCtxt<'tcx>,
trait_ref: ty::TraitRef<'tcx>,
mut lazily_normalize_ty: impl FnMut(Ty<'tcx>) -> Result<Ty<'tcx>, E>,
) -> Result<Result<(), Conflict>, E> {
if Some(trait_ref.def_id) == tcx.lang_items().fn_ptr_trait() {
// The only types implementing `FnPtr` are function pointers,
// so if there's no impl of `FnPtr` in the current crate,
// then such an impl will never be added in the future.
return Ok(Ok(()));
}
if orphan_check_trait_ref(trait_ref, InCrate::Remote, &mut lazily_normalize_ty)?.is_ok() {
// A downstream or cousin crate is allowed to implement some
// substitution of this trait-ref.
return Ok(Err(Conflict::Downstream));
}
if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
// This is a local or fundamental trait, so future-compatibility
// is no concern. We know that downstream/cousin crates are not
// allowed to implement a substitution of this trait ref, which
// means impls could only come from dependencies of this crate,
// which we already know about.
return Ok(Ok(()));
}
// This is a remote non-fundamental trait, so if another crate
// can be the "final owner" of a substitution of this trait-ref,
// they are allowed to implement it future-compatibly.
//
// However, if we are a final owner, then nobody else can be,
// and if we are an intermediate owner, then we don't care
// about future-compatibility, which means that we're OK if
// we are an owner.
if orphan_check_trait_ref(trait_ref, InCrate::Local, &mut lazily_normalize_ty)?.is_ok() {
Ok(Ok(()))
} else {
Ok(Err(Conflict::Upstream))
}
}
pub fn trait_ref_is_local_or_fundamental<'tcx>(
tcx: TyCtxt<'tcx>,
trait_ref: ty::TraitRef<'tcx>,
) -> bool {
trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
}
#[derive(Debug)]
pub enum OrphanCheckErr<'tcx> {
NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
}
/// Checks the coherence orphan rules. `impl_def_id` should be the
/// `DefId` of a trait impl. To pass, either the trait must be local, or else
/// two conditions must be satisfied:
///
/// 1. All type parameters in `Self` must be "covered" by some local type constructor.
/// 2. Some local type must appear in `Self`.
#[instrument(level = "debug", skip(tcx), ret)]
pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
// We only except this routine to be invoked on implementations
// of a trait, not inherent implementations.
let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().instantiate_identity();
debug!(?trait_ref);
// If the *trait* is local to the crate, ok.
if trait_ref.def_id.is_local() {
debug!("trait {:?} is local to current crate", trait_ref.def_id);
return Ok(());
}
orphan_check_trait_ref::<!>(trait_ref, InCrate::Local, |ty| Ok(ty)).unwrap()
}
/// Checks whether a trait-ref is potentially implementable by a crate.
///
/// The current rule is that a trait-ref orphan checks in a crate C:
///
/// 1. Order the parameters in the trait-ref in subst order - Self first,
/// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
/// 2. Of these type parameters, there is at least one type parameter
/// in which, walking the type as a tree, you can reach a type local
/// to C where all types in-between are fundamental types. Call the
/// first such parameter the "local key parameter".
/// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
/// going through `Box`, which is fundamental.
/// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
/// the same reason.
/// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
/// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
/// the local type and the type parameter.
/// 3. Before this local type, no generic type parameter of the impl must
/// be reachable through fundamental types.
/// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
/// - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
/// reachable through the fundamental type `Box`.
/// 4. Every type in the local key parameter not known in C, going
/// through the parameter's type tree, must appear only as a subtree of
/// a type local to C, with only fundamental types between the type
/// local to C and the local key parameter.
/// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
/// is bad, because the only local type with `T` as a subtree is
/// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
/// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
/// the second occurrence of `T` is not a subtree of *any* local type.
/// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
/// `LocalType<Vec<T>>`, which is local and has no types between it and
/// the type parameter.
///
/// The orphan rules actually serve several different purposes:
///
/// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
/// every type local to one crate is unknown in the other) can't implement
/// the same trait-ref. This follows because it can be seen that no such
/// type can orphan-check in 2 such crates.
///
/// To check that a local impl follows the orphan rules, we check it in
/// InCrate::Local mode, using type parameters for the "generic" types.
///
/// 2. They ground negative reasoning for coherence. If a user wants to
/// write both a conditional blanket impl and a specific impl, we need to
/// make sure they do not overlap. For example, if we write
/// ```ignore (illustrative)
/// impl<T> IntoIterator for Vec<T>
/// impl<T: Iterator> IntoIterator for T
/// ```
/// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
/// We can observe that this holds in the current crate, but we need to make
/// sure this will also hold in all unknown crates (both "independent" crates,
/// which we need for link-safety, and also child crates, because we don't want
/// child crates to get error for impl conflicts in a *dependency*).
///
/// For that, we only allow negative reasoning if, for every assignment to the
/// inference variables, every unknown crate would get an orphan error if they
/// try to implement this trait-ref. To check for this, we use InCrate::Remote
/// mode. That is sound because we already know all the impls from known crates.
///
/// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
/// add "non-blanket" impls without breaking negative reasoning in dependent
/// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
///
/// For that, we only a allow crate to perform negative reasoning on
/// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
///
/// Because we never perform negative reasoning generically (coherence does
/// not involve type parameters), this can be interpreted as doing the full
/// orphan check (using InCrate::Local mode), substituting non-local known
/// types for all inference variables.
///
/// This allows for crates to future-compatibly add impls as long as they
/// can't apply to types with a key parameter in a child crate - applying
/// the rules, this basically means that every type parameter in the impl
/// must appear behind a non-fundamental type (because this is not a
/// type-system requirement, crate owners might also go for "semantic
/// future-compatibility" involving things such as sealed traits, but
/// the above requirement is sufficient, and is necessary in "open world"
/// cases).
///
/// Note that this function is never called for types that have both type
/// parameters and inference variables.
#[instrument(level = "trace", skip(lazily_normalize_ty), ret)]
fn orphan_check_trait_ref<'tcx, E: Debug>(
trait_ref: ty::TraitRef<'tcx>,
in_crate: InCrate,
lazily_normalize_ty: impl FnMut(Ty<'tcx>) -> Result<Ty<'tcx>, E>,
) -> Result<Result<(), OrphanCheckErr<'tcx>>, E> {
if trait_ref.has_infer() && trait_ref.has_param() {
bug!(
"can't orphan check a trait ref with both params and inference variables {:?}",
trait_ref
);
}
let mut checker = OrphanChecker::new(in_crate, lazily_normalize_ty);
Ok(match trait_ref.visit_with(&mut checker) {
ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
ControlFlow::Break(OrphanCheckEarlyExit::NormalizationFailure(err)) => return Err(err),
ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(ty)) => {
// Does there exist some local type after the `ParamTy`.
checker.search_first_local_ty = true;
if let Some(OrphanCheckEarlyExit::LocalTy(local_ty)) =
trait_ref.visit_with(&mut checker).break_value()
{
Err(OrphanCheckErr::UncoveredTy(ty, Some(local_ty)))
} else {
Err(OrphanCheckErr::UncoveredTy(ty, None))
}
}
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(_)) => Ok(()),
})
}
struct OrphanChecker<'tcx, F> {
in_crate: InCrate,
in_self_ty: bool,
lazily_normalize_ty: F,
/// Ignore orphan check failures and exclusively search for the first
/// local type.
search_first_local_ty: bool,
non_local_tys: Vec<(Ty<'tcx>, bool)>,
}
impl<'tcx, F, E> OrphanChecker<'tcx, F>
where
F: FnOnce(Ty<'tcx>) -> Result<Ty<'tcx>, E>,
{
fn new(in_crate: InCrate, lazily_normalize_ty: F) -> Self {
OrphanChecker {
in_crate,
in_self_ty: true,
lazily_normalize_ty,
search_first_local_ty: false,
non_local_tys: Vec::new(),
}
}
fn found_non_local_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx, E>> {
self.non_local_tys.push((t, self.in_self_ty));
ControlFlow::Continue(())
}
fn found_param_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx, E>> {
if self.search_first_local_ty {
ControlFlow::Continue(())
} else {
ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(t))
}
}
fn def_id_is_local(&mut self, def_id: DefId) -> bool {
match self.in_crate {
InCrate::Local => def_id.is_local(),
InCrate::Remote => false,
}
}
}
enum OrphanCheckEarlyExit<'tcx, E> {
NormalizationFailure(E),
ParamTy(Ty<'tcx>),
LocalTy(Ty<'tcx>),
}
impl<'tcx, F, E> TypeVisitor<TyCtxt<'tcx>> for OrphanChecker<'tcx, F>
where
F: FnMut(Ty<'tcx>) -> Result<Ty<'tcx>, E>,
{
type BreakTy = OrphanCheckEarlyExit<'tcx, E>;
fn visit_region(&mut self, _r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
ControlFlow::Continue(())
}
fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
// Need to lazily normalize here in with `-Ztrait-solver=next-coherence`.
let ty = match (self.lazily_normalize_ty)(ty) {
Ok(ty) => ty,
Err(err) => return ControlFlow::Break(OrphanCheckEarlyExit::NormalizationFailure(err)),
};
let result = match *ty.kind() {
ty::Bool
| ty::Char
| ty::Int(..)
| ty::Uint(..)
| ty::Float(..)
| ty::Str
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Array(..)
| ty::Slice(..)
| ty::RawPtr(..)
| ty::Never
| ty::Tuple(..)
| ty::Alias(ty::Projection | ty::Inherent | ty::Weak, ..) => {
self.found_non_local_ty(ty)
}
ty::Param(..) => self.found_param_ty(ty),
ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match self.in_crate {
InCrate::Local => self.found_non_local_ty(ty),
// The inference variable might be unified with a local
// type in that remote crate.
InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
},
// For fundamental types, we just look inside of them.
ty::Ref(_, ty, _) => ty.visit_with(self),
ty::Adt(def, args) => {
if self.def_id_is_local(def.did()) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else if def.is_fundamental() {
args.visit_with(self)
} else {
self.found_non_local_ty(ty)
}
}
ty::Foreign(def_id) => {
if self.def_id_is_local(def_id) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else {
self.found_non_local_ty(ty)
}
}
ty::Dynamic(tt, ..) => {
let principal = tt.principal().map(|p| p.def_id());
if principal.is_some_and(|p| self.def_id_is_local(p)) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else {
self.found_non_local_ty(ty)
}
}
ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
ty::Closure(did, ..) | ty::Generator(did, ..) => {
if self.def_id_is_local(did) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else {
self.found_non_local_ty(ty)
}
}
// This should only be created when checking whether we have to check whether some
// auto trait impl applies. There will never be multiple impls, so we can just
// act as if it were a local type here.
ty::GeneratorWitness(..) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
ty::Alias(ty::Opaque, ..) => {
// This merits some explanation.
// Normally, opaque types are not involved when performing
// coherence checking, since it is illegal to directly
// implement a trait on an opaque type. However, we might
// end up looking at an opaque type during coherence checking
// if an opaque type gets used within another type (e.g. as
// the type of a field) when checking for auto trait or `Sized`
// impls. This requires us to decide whether or not an opaque
// type should be considered 'local' or not.
//
// We choose to treat all opaque types as non-local, even
// those that appear within the same crate. This seems
// somewhat surprising at first, but makes sense when
// you consider that opaque types are supposed to hide
// the underlying type *within the same crate*. When an
// opaque type is used from outside the module
// where it is declared, it should be impossible to observe
// anything about it other than the traits that it implements.
//
// The alternative would be to look at the underlying type
// to determine whether or not the opaque type itself should
// be considered local. However, this could make it a breaking change
// to switch the underlying ('defining') type from a local type
// to a remote type. This would violate the rule that opaque
// types should be completely opaque apart from the traits
// that they implement, so we don't use this behavior.
self.found_non_local_ty(ty)
}
};
// A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
// the first type we visit is always the self type.
self.in_self_ty = false;
result
}
/// All possible values for a constant parameter already exist
/// in the crate defining the trait, so they are always non-local[^1].
///
/// Because there's no way to have an impl where the first local
/// generic argument is a constant, we also don't have to fail
/// the orphan check when encountering a parameter or a generic constant.
///
/// This means that we can completely ignore constants during the orphan check.
///
/// See `tests/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
///
/// [^1]: This might not hold for function pointers or trait objects in the future.
/// As these should be quite rare as const arguments and especially rare as impl
/// parameters, allowing uncovered const parameters in impls seems more useful
/// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
ControlFlow::Continue(())
}
}
/// Compute the `intercrate_ambiguity_causes` for the new solver using
/// "proof trees".
///
/// This is a bit scuffed but seems to be good enough, at least
/// when looking at UI tests. Given that it is only used to improve
/// diagnostics this is good enough. We can always improve it once there
/// are test cases where it is currently not enough.
fn compute_intercrate_ambiguity_causes<'tcx>(
infcx: &InferCtxt<'tcx>,
obligations: &[PredicateObligation<'tcx>],
) -> FxIndexSet<IntercrateAmbiguityCause> {
let mut causes: FxIndexSet<IntercrateAmbiguityCause> = Default::default();
for obligation in obligations {
search_ambiguity_causes(infcx, obligation.clone().into(), &mut causes);
}
causes
}
struct AmbiguityCausesVisitor<'a> {
causes: &'a mut FxIndexSet<IntercrateAmbiguityCause>,
}
impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a> {
type BreakTy = !;
fn visit_goal(&mut self, goal: &InspectGoal<'_, 'tcx>) -> ControlFlow<Self::BreakTy> {
let infcx = goal.infcx();
for cand in goal.candidates() {
cand.visit_nested(self)?;
}
// When searching for intercrate ambiguity causes, we only need to look
// at ambiguous goals, as for others the coherence unknowable candidate
// was irrelevant.
match goal.result() {
Ok(Certainty::Maybe(_)) => {}
Ok(Certainty::Yes) | Err(NoSolution) => return ControlFlow::Continue(()),
}
let Goal { param_env, predicate } = goal.goal();
// For bound predicates we simply call `infcx.replace_bound_vars_with_placeholders`
// and then prove the resulting predicate as a nested goal.
let trait_ref = match predicate.kind().no_bound_vars() {
Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(tr))) => tr.trait_ref,
Some(ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj))) => {
proj.projection_ty.trait_ref(infcx.tcx)
}
_ => return ControlFlow::Continue(()),
};
let mut ambiguity_cause = None;
for cand in goal.candidates() {
// FIXME: boiiii, using string comparisions here sure is scuffed.
if let inspect::ProbeKind::MiscCandidate { name: "coherence unknowable", result: _ } =
cand.kind()
{
let lazily_normalize_ty = |ty: Ty<'tcx>| {
let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(infcx);
if matches!(ty.kind(), ty::Alias(..)) {
// FIXME(-Ztrait-solver=next-coherence): we currently don't
// normalize opaque types here, resulting in diverging behavior
// for TAITs.
match infcx
.at(&ObligationCause::dummy(), param_env)
.structurally_normalize(ty, &mut *fulfill_cx)
{
Ok(ty) => Ok(ty),
Err(_errs) => Err(()),
}
} else {
Ok(ty)
}
};
infcx.probe(|_| {
match trait_ref_is_knowable(infcx.tcx, trait_ref, lazily_normalize_ty) {
Err(()) => {}
Ok(Ok(())) => warn!("expected an unknowable trait ref: {trait_ref:?}"),
Ok(Err(conflict)) => {
if !trait_ref.references_error() {
let self_ty = trait_ref.self_ty();
let (trait_desc, self_desc) = with_no_trimmed_paths!({
let trait_desc = trait_ref.print_only_trait_path().to_string();
let self_desc = self_ty
.has_concrete_skeleton()
.then(|| self_ty.to_string());
(trait_desc, self_desc)
});
ambiguity_cause = Some(match conflict {
Conflict::Upstream => {
IntercrateAmbiguityCause::UpstreamCrateUpdate {
trait_desc,
self_desc,
}
}
Conflict::Downstream => {
IntercrateAmbiguityCause::DownstreamCrate {
trait_desc,
self_desc,
}
}
});
}
}
}
})
} else {
match cand.result() {
// We only add an ambiguity cause if the goal would otherwise
// result in an error.
//
// FIXME: While this matches the behavior of the
// old solver, it is not the only way in which the unknowable
// candidates *weaken* coherence, they can also force otherwise
// sucessful normalization to be ambiguous.
Ok(Certainty::Maybe(_) | Certainty::Yes) => {
ambiguity_cause = None;
break;
}
Err(NoSolution) => continue,
}
}
}
if let Some(ambiguity_cause) = ambiguity_cause {
self.causes.insert(ambiguity_cause);
}
ControlFlow::Continue(())
}
}
fn search_ambiguity_causes<'tcx>(
infcx: &InferCtxt<'tcx>,
goal: Goal<'tcx, ty::Predicate<'tcx>>,
causes: &mut FxIndexSet<IntercrateAmbiguityCause>,
) {
infcx.visit_proof_tree(goal, &mut AmbiguityCausesVisitor { causes });
}