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//! Trait Resolution. See the [rustc dev guide] for more information on how this works.
//!
//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
pub mod auto_trait;
pub(crate) mod coherence;
pub mod const_evaluatable;
mod engine;
mod fulfill;
pub mod misc;
pub mod normalize;
mod object_safety;
pub mod outlives_bounds;
pub mod project;
pub mod query;
#[allow(hidden_glob_reexports)]
mod select;
mod specialize;
mod structural_normalize;
#[allow(hidden_glob_reexports)]
mod util;
pub mod vtable;
pub mod wf;
use std::fmt::Debug;
use std::ops::ControlFlow;
use rustc_errors::ErrorGuaranteed;
pub use rustc_infer::traits::*;
use rustc_middle::query::Providers;
use rustc_middle::span_bug;
use rustc_middle::ty::error::{ExpectedFound, TypeError};
use rustc_middle::ty::fold::TypeFoldable;
use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
use rustc_middle::ty::{
self, GenericArgs, GenericArgsRef, Ty, TyCtxt, TypeFolder, TypeSuperVisitable, Upcast,
};
use rustc_span::def_id::DefId;
use rustc_span::Span;
use tracing::{debug, instrument};
pub use self::coherence::{
add_placeholder_note, orphan_check_trait_ref, overlapping_impls, InCrate, IsFirstInputType,
OrphanCheckErr, OrphanCheckMode, OverlapResult, UncoveredTyParams,
};
pub use self::engine::{ObligationCtxt, TraitEngineExt};
pub use self::fulfill::{FulfillmentContext, OldSolverError, PendingPredicateObligation};
pub use self::normalize::NormalizeExt;
pub use self::object_safety::{
hir_ty_lowering_object_safety_violations, is_vtable_safe_method,
object_safety_violations_for_assoc_item, ObjectSafetyViolation,
};
pub use self::project::{normalize_inherent_projection, normalize_projection_ty};
pub use self::select::{
EvaluationCache, EvaluationResult, IntercrateAmbiguityCause, OverflowError, SelectionCache,
SelectionContext,
};
pub use self::specialize::specialization_graph::{
FutureCompatOverlapError, FutureCompatOverlapErrorKind,
};
pub use self::specialize::{
specialization_graph, translate_args, translate_args_with_cause, OverlapError,
};
pub use self::structural_normalize::StructurallyNormalizeExt;
pub use self::util::{
elaborate, expand_trait_aliases, impl_item_is_final, supertraits,
transitive_bounds_that_define_assoc_item, upcast_choices, with_replaced_escaping_bound_vars,
BoundVarReplacer, PlaceholderReplacer, TraitAliasExpander, TraitAliasExpansionInfo,
};
use crate::error_reporting::InferCtxtErrorExt;
use crate::infer::outlives::env::OutlivesEnvironment;
use crate::infer::{InferCtxt, TyCtxtInferExt};
use crate::regions::InferCtxtRegionExt;
use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
pub struct FulfillmentError<'tcx> {
pub obligation: PredicateObligation<'tcx>,
pub code: FulfillmentErrorCode<'tcx>,
/// Diagnostics only: the 'root' obligation which resulted in
/// the failure to process `obligation`. This is the obligation
/// that was initially passed to `register_predicate_obligation`
pub root_obligation: PredicateObligation<'tcx>,
}
impl<'tcx> FulfillmentError<'tcx> {
pub fn new(
obligation: PredicateObligation<'tcx>,
code: FulfillmentErrorCode<'tcx>,
root_obligation: PredicateObligation<'tcx>,
) -> FulfillmentError<'tcx> {
FulfillmentError { obligation, code, root_obligation }
}
pub fn is_true_error(&self) -> bool {
match self.code {
FulfillmentErrorCode::Select(_)
| FulfillmentErrorCode::Project(_)
| FulfillmentErrorCode::Subtype(_, _)
| FulfillmentErrorCode::ConstEquate(_, _) => true,
FulfillmentErrorCode::Cycle(_) | FulfillmentErrorCode::Ambiguity { overflow: _ } => {
false
}
}
}
}
impl<'tcx> Debug for FulfillmentError<'tcx> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "FulfillmentError({:?},{:?})", self.obligation, self.code)
}
}
#[derive(Clone)]
pub enum FulfillmentErrorCode<'tcx> {
/// Inherently impossible to fulfill; this trait is implemented if and only
/// if it is already implemented.
Cycle(Vec<PredicateObligation<'tcx>>),
Select(SelectionError<'tcx>),
Project(MismatchedProjectionTypes<'tcx>),
Subtype(ExpectedFound<Ty<'tcx>>, TypeError<'tcx>), // always comes from a SubtypePredicate
ConstEquate(ExpectedFound<ty::Const<'tcx>>, TypeError<'tcx>),
Ambiguity {
/// Overflow is only `Some(suggest_recursion_limit)` when using the next generation
/// trait solver `-Znext-solver`. With the old solver overflow is eagerly handled by
/// emitting a fatal error instead.
overflow: Option<bool>,
},
}
impl<'tcx> Debug for FulfillmentErrorCode<'tcx> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match *self {
FulfillmentErrorCode::Select(ref e) => write!(f, "{e:?}"),
FulfillmentErrorCode::Project(ref e) => write!(f, "{e:?}"),
FulfillmentErrorCode::Subtype(ref a, ref b) => {
write!(f, "CodeSubtypeError({a:?}, {b:?})")
}
FulfillmentErrorCode::ConstEquate(ref a, ref b) => {
write!(f, "CodeConstEquateError({a:?}, {b:?})")
}
FulfillmentErrorCode::Ambiguity { overflow: None } => write!(f, "Ambiguity"),
FulfillmentErrorCode::Ambiguity { overflow: Some(suggest_increasing_limit) } => {
write!(f, "Overflow({suggest_increasing_limit})")
}
FulfillmentErrorCode::Cycle(ref cycle) => write!(f, "Cycle({cycle:?})"),
}
}
}
/// Whether to skip the leak check, as part of a future compatibility warning step.
///
/// The "default" for skip-leak-check corresponds to the current
/// behavior (do not skip the leak check) -- not the behavior we are
/// transitioning into.
#[derive(Copy, Clone, PartialEq, Eq, Debug, Default)]
pub enum SkipLeakCheck {
Yes,
#[default]
No,
}
impl SkipLeakCheck {
fn is_yes(self) -> bool {
self == SkipLeakCheck::Yes
}
}
/// The mode that trait queries run in.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub enum TraitQueryMode {
/// Standard/un-canonicalized queries get accurate
/// spans etc. passed in and hence can do reasonable
/// error reporting on their own.
Standard,
/// Canonical queries get dummy spans and hence
/// must generally propagate errors to
/// pre-canonicalization callsites.
Canonical,
}
/// Creates predicate obligations from the generic bounds.
#[instrument(level = "debug", skip(cause, param_env))]
pub fn predicates_for_generics<'tcx>(
cause: impl Fn(usize, Span) -> ObligationCause<'tcx>,
param_env: ty::ParamEnv<'tcx>,
generic_bounds: ty::InstantiatedPredicates<'tcx>,
) -> impl Iterator<Item = PredicateObligation<'tcx>> {
generic_bounds.into_iter().enumerate().map(move |(idx, (clause, span))| Obligation {
cause: cause(idx, span),
recursion_depth: 0,
param_env,
predicate: clause.as_predicate(),
})
}
/// Determines whether the type `ty` is known to meet `bound` and
/// returns true if so. Returns false if `ty` either does not meet
/// `bound` or is not known to meet bound (note that this is
/// conservative towards *no impl*, which is the opposite of the
/// `evaluate` methods).
pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
ty: Ty<'tcx>,
def_id: DefId,
) -> bool {
let trait_ref = ty::TraitRef::new(infcx.tcx, def_id, [ty]);
pred_known_to_hold_modulo_regions(infcx, param_env, trait_ref)
}
/// FIXME(@lcnr): this function doesn't seem right and shouldn't exist?
///
/// Ping me on zulip if you want to use this method and need help with finding
/// an appropriate replacement.
#[instrument(level = "debug", skip(infcx, param_env, pred), ret)]
fn pred_known_to_hold_modulo_regions<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
pred: impl Upcast<TyCtxt<'tcx>, ty::Predicate<'tcx>>,
) -> bool {
let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, pred);
let result = infcx.evaluate_obligation_no_overflow(&obligation);
debug!(?result);
if result.must_apply_modulo_regions() {
true
} else if result.may_apply() {
// Sometimes obligations are ambiguous because the recursive evaluator
// is not smart enough, so we fall back to fulfillment when we're not certain
// that an obligation holds or not. Even still, we must make sure that
// the we do no inference in the process of checking this obligation.
let goal = infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
infcx.probe(|_| {
let ocx = ObligationCtxt::new(infcx);
ocx.register_obligation(obligation);
let errors = ocx.select_all_or_error();
match errors.as_slice() {
// Only known to hold if we did no inference.
[] => infcx.resolve_vars_if_possible(goal) == goal,
errors => {
debug!(?errors);
false
}
}
})
} else {
false
}
}
#[instrument(level = "debug", skip(tcx, elaborated_env))]
fn do_normalize_predicates<'tcx>(
tcx: TyCtxt<'tcx>,
cause: ObligationCause<'tcx>,
elaborated_env: ty::ParamEnv<'tcx>,
predicates: Vec<ty::Clause<'tcx>>,
) -> Result<Vec<ty::Clause<'tcx>>, ErrorGuaranteed> {
let span = cause.span;
// FIXME. We should really... do something with these region
// obligations. But this call just continues the older
// behavior (i.e., doesn't cause any new bugs), and it would
// take some further refactoring to actually solve them. In
// particular, we would have to handle implied bounds
// properly, and that code is currently largely confined to
// regionck (though I made some efforts to extract it
// out). -nmatsakis
//
// @arielby: In any case, these obligations are checked
// by wfcheck anyway, so I'm not sure we have to check
// them here too, and we will remove this function when
// we move over to lazy normalization *anyway*.
let infcx = tcx.infer_ctxt().ignoring_regions().build();
let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
let predicates = ocx.normalize(&cause, elaborated_env, predicates);
let errors = ocx.select_all_or_error();
if !errors.is_empty() {
let reported = infcx.err_ctxt().report_fulfillment_errors(errors);
return Err(reported);
}
debug!("do_normalize_predicates: normalized predicates = {:?}", predicates);
// We can use the `elaborated_env` here; the region code only
// cares about declarations like `'a: 'b`.
let outlives_env = OutlivesEnvironment::new(elaborated_env);
// FIXME: It's very weird that we ignore region obligations but apparently
// still need to use `resolve_regions` as we need the resolved regions in
// the normalized predicates.
let errors = infcx.resolve_regions(&outlives_env);
if !errors.is_empty() {
tcx.dcx().span_delayed_bug(
span,
format!("failed region resolution while normalizing {elaborated_env:?}: {errors:?}"),
);
}
match infcx.fully_resolve(predicates) {
Ok(predicates) => Ok(predicates),
Err(fixup_err) => {
// If we encounter a fixup error, it means that some type
// variable wound up unconstrained. I actually don't know
// if this can happen, and I certainly don't expect it to
// happen often, but if it did happen it probably
// represents a legitimate failure due to some kind of
// unconstrained variable.
//
// @lcnr: Let's still ICE here for now. I want a test case
// for that.
span_bug!(
span,
"inference variables in normalized parameter environment: {}",
fixup_err
);
}
}
}
// FIXME: this is gonna need to be removed ...
/// Normalizes the parameter environment, reporting errors if they occur.
#[instrument(level = "debug", skip(tcx))]
pub fn normalize_param_env_or_error<'tcx>(
tcx: TyCtxt<'tcx>,
unnormalized_env: ty::ParamEnv<'tcx>,
cause: ObligationCause<'tcx>,
) -> ty::ParamEnv<'tcx> {
// I'm not wild about reporting errors here; I'd prefer to
// have the errors get reported at a defined place (e.g.,
// during typeck). Instead I have all parameter
// environments, in effect, going through this function
// and hence potentially reporting errors. This ensures of
// course that we never forget to normalize (the
// alternative seemed like it would involve a lot of
// manual invocations of this fn -- and then we'd have to
// deal with the errors at each of those sites).
//
// In any case, in practice, typeck constructs all the
// parameter environments once for every fn as it goes,
// and errors will get reported then; so outside of type inference we
// can be sure that no errors should occur.
let mut predicates: Vec<_> = util::elaborate(
tcx,
unnormalized_env.caller_bounds().into_iter().map(|predicate| {
if tcx.features().generic_const_exprs {
return predicate;
}
struct ConstNormalizer<'tcx>(TyCtxt<'tcx>);
impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ConstNormalizer<'tcx> {
fn cx(&self) -> TyCtxt<'tcx> {
self.0
}
fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx> {
// FIXME(return_type_notation): track binders in this normalizer, as
// `ty::Const::normalize` can only work with properly preserved binders.
if c.has_escaping_bound_vars() {
return ty::Const::new_misc_error(self.0);
}
// While it is pretty sus to be evaluating things with an empty param env, it
// should actually be okay since without `feature(generic_const_exprs)` the only
// const arguments that have a non-empty param env are array repeat counts. These
// do not appear in the type system though.
c.normalize(self.0, ty::ParamEnv::empty())
}
}
// This whole normalization step is a hack to work around the fact that
// `normalize_param_env_or_error` is fundamentally broken from using an
// unnormalized param env with a trait solver that expects the param env
// to be normalized.
//
// When normalizing the param env we can end up evaluating obligations
// that have been normalized but can only be proven via a where clause
// which is still in its unnormalized form. example:
//
// Attempting to prove `T: Trait<<u8 as Identity>::Assoc>` in a param env
// with a `T: Trait<<u8 as Identity>::Assoc>` where clause will fail because
// we first normalize obligations before proving them so we end up proving
// `T: Trait<u8>`. Since lazy normalization is not implemented equating `u8`
// with `<u8 as Identity>::Assoc` fails outright so we incorrectly believe that
// we cannot prove `T: Trait<u8>`.
//
// The same thing is true for const generics- attempting to prove
// `T: Trait<ConstKind::Unevaluated(...)>` with the same thing as a where clauses
// will fail. After normalization we may be attempting to prove `T: Trait<4>` with
// the unnormalized where clause `T: Trait<ConstKind::Unevaluated(...)>`. In order
// for the obligation to hold `4` must be equal to `ConstKind::Unevaluated(...)`
// but as we do not have lazy norm implemented, equating the two consts fails outright.
//
// Ideally we would not normalize consts here at all but it is required for backwards
// compatibility. Eventually when lazy norm is implemented this can just be removed.
// We do not normalize types here as there is no backwards compatibility requirement
// for us to do so.
//
// FIXME(-Znext-solver): remove this hack since we have deferred projection equality
predicate.fold_with(&mut ConstNormalizer(tcx))
}),
)
.collect();
debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
let elaborated_env = ty::ParamEnv::new(tcx.mk_clauses(&predicates), unnormalized_env.reveal());
// HACK: we are trying to normalize the param-env inside *itself*. The problem is that
// normalization expects its param-env to be already normalized, which means we have
// a circularity.
//
// The way we handle this is by normalizing the param-env inside an unnormalized version
// of the param-env, which means that if the param-env contains unnormalized projections,
// we'll have some normalization failures. This is unfortunate.
//
// Lazy normalization would basically handle this by treating just the
// normalizing-a-trait-ref-requires-itself cycles as evaluation failures.
//
// Inferred outlives bounds can create a lot of `TypeOutlives` predicates for associated
// types, so to make the situation less bad, we normalize all the predicates *but*
// the `TypeOutlives` predicates first inside the unnormalized parameter environment, and
// then we normalize the `TypeOutlives` bounds inside the normalized parameter environment.
//
// This works fairly well because trait matching does not actually care about param-env
// TypeOutlives predicates - these are normally used by regionck.
let outlives_predicates: Vec<_> = predicates
.extract_if(|predicate| {
matches!(predicate.kind().skip_binder(), ty::ClauseKind::TypeOutlives(..))
})
.collect();
debug!(
"normalize_param_env_or_error: predicates=(non-outlives={:?}, outlives={:?})",
predicates, outlives_predicates
);
let Ok(non_outlives_predicates) =
do_normalize_predicates(tcx, cause.clone(), elaborated_env, predicates)
else {
// An unnormalized env is better than nothing.
debug!("normalize_param_env_or_error: errored resolving non-outlives predicates");
return elaborated_env;
};
debug!("normalize_param_env_or_error: non-outlives predicates={:?}", non_outlives_predicates);
// Not sure whether it is better to include the unnormalized TypeOutlives predicates
// here. I believe they should not matter, because we are ignoring TypeOutlives param-env
// predicates here anyway. Keeping them here anyway because it seems safer.
let outlives_env = non_outlives_predicates.iter().chain(&outlives_predicates).cloned();
let outlives_env =
ty::ParamEnv::new(tcx.mk_clauses_from_iter(outlives_env), unnormalized_env.reveal());
let Ok(outlives_predicates) =
do_normalize_predicates(tcx, cause, outlives_env, outlives_predicates)
else {
// An unnormalized env is better than nothing.
debug!("normalize_param_env_or_error: errored resolving outlives predicates");
return elaborated_env;
};
debug!("normalize_param_env_or_error: outlives predicates={:?}", outlives_predicates);
let mut predicates = non_outlives_predicates;
predicates.extend(outlives_predicates);
debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
ty::ParamEnv::new(tcx.mk_clauses(&predicates), unnormalized_env.reveal())
}
/// Normalizes the predicates and checks whether they hold in an empty environment. If this
/// returns true, then either normalize encountered an error or one of the predicates did not
/// hold. Used when creating vtables to check for unsatisfiable methods.
pub fn impossible_predicates<'tcx>(tcx: TyCtxt<'tcx>, predicates: Vec<ty::Clause<'tcx>>) -> bool {
debug!("impossible_predicates(predicates={:?})", predicates);
let infcx = tcx.infer_ctxt().build();
let param_env = ty::ParamEnv::reveal_all();
let ocx = ObligationCtxt::new(&infcx);
let predicates = ocx.normalize(&ObligationCause::dummy(), param_env, predicates);
for predicate in predicates {
let obligation = Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate);
ocx.register_obligation(obligation);
}
let errors = ocx.select_all_or_error();
let result = !errors.is_empty();
debug!("impossible_predicates = {:?}", result);
result
}
fn instantiate_and_check_impossible_predicates<'tcx>(
tcx: TyCtxt<'tcx>,
key: (DefId, GenericArgsRef<'tcx>),
) -> bool {
debug!("instantiate_and_check_impossible_predicates(key={:?})", key);
let mut predicates = tcx.predicates_of(key.0).instantiate(tcx, key.1).predicates;
// Specifically check trait fulfillment to avoid an error when trying to resolve
// associated items.
if let Some(trait_def_id) = tcx.trait_of_item(key.0) {
let trait_ref = ty::TraitRef::from_method(tcx, trait_def_id, key.1);
predicates.push(trait_ref.upcast(tcx));
}
predicates.retain(|predicate| !predicate.has_param());
let result = impossible_predicates(tcx, predicates);
debug!("instantiate_and_check_impossible_predicates(key={:?}) = {:?}", key, result);
result
}
/// Checks whether a trait's associated item is impossible to reference on a given impl.
///
/// This only considers predicates that reference the impl's generics, and not
/// those that reference the method's generics.
fn is_impossible_associated_item(
tcx: TyCtxt<'_>,
(impl_def_id, trait_item_def_id): (DefId, DefId),
) -> bool {
struct ReferencesOnlyParentGenerics<'tcx> {
tcx: TyCtxt<'tcx>,
generics: &'tcx ty::Generics,
trait_item_def_id: DefId,
}
impl<'tcx> ty::TypeVisitor<TyCtxt<'tcx>> for ReferencesOnlyParentGenerics<'tcx> {
type Result = ControlFlow<()>;
fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
// If this is a parameter from the trait item's own generics, then bail
if let ty::Param(param) = *t.kind()
&& let param_def_id = self.generics.type_param(param, self.tcx).def_id
&& self.tcx.parent(param_def_id) == self.trait_item_def_id
{
return ControlFlow::Break(());
}
t.super_visit_with(self)
}
fn visit_region(&mut self, r: ty::Region<'tcx>) -> Self::Result {
if let ty::ReEarlyParam(param) = r.kind()
&& let param_def_id = self.generics.region_param(param, self.tcx).def_id
&& self.tcx.parent(param_def_id) == self.trait_item_def_id
{
return ControlFlow::Break(());
}
ControlFlow::Continue(())
}
fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
if let ty::ConstKind::Param(param) = ct.kind()
&& let param_def_id = self.generics.const_param(param, self.tcx).def_id
&& self.tcx.parent(param_def_id) == self.trait_item_def_id
{
return ControlFlow::Break(());
}
ct.super_visit_with(self)
}
}
let generics = tcx.generics_of(trait_item_def_id);
let predicates = tcx.predicates_of(trait_item_def_id);
let impl_trait_ref = tcx
.impl_trait_ref(impl_def_id)
.expect("expected impl to correspond to trait")
.instantiate_identity();
let param_env = tcx.param_env(impl_def_id);
let mut visitor = ReferencesOnlyParentGenerics { tcx, generics, trait_item_def_id };
let predicates_for_trait = predicates.predicates.iter().filter_map(|(pred, span)| {
pred.visit_with(&mut visitor).is_continue().then(|| {
Obligation::new(
tcx,
ObligationCause::dummy_with_span(*span),
param_env,
ty::EarlyBinder::bind(*pred).instantiate(tcx, impl_trait_ref.args),
)
})
});
let infcx = tcx.infer_ctxt().ignoring_regions().build();
for obligation in predicates_for_trait {
// Ignore overflow error, to be conservative.
if let Ok(result) = infcx.evaluate_obligation(&obligation)
&& !result.may_apply()
{
return true;
}
}
false
}
pub fn provide(providers: &mut Providers) {
object_safety::provide(providers);
vtable::provide(providers);
*providers = Providers {
specialization_graph_of: specialize::specialization_graph_provider,
specializes: specialize::specializes,
specialization_enabled_in: specialize::specialization_enabled_in,
instantiate_and_check_impossible_predicates,
is_impossible_associated_item,
..*providers
};
}