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use smallvec::smallvec;
use crate::infer::outlives::components::{push_outlives_components, Component};
use crate::traits::{self, Obligation, PredicateObligation};
use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
use rustc_span::symbol::Ident;
use rustc_span::Span;
pub fn anonymize_predicate<'tcx>(
tcx: TyCtxt<'tcx>,
pred: ty::Predicate<'tcx>,
) -> ty::Predicate<'tcx> {
let new = tcx.anonymize_bound_vars(pred.kind());
tcx.reuse_or_mk_predicate(pred, new)
}
pub struct PredicateSet<'tcx> {
tcx: TyCtxt<'tcx>,
set: FxHashSet<ty::Predicate<'tcx>>,
}
impl<'tcx> PredicateSet<'tcx> {
pub fn new(tcx: TyCtxt<'tcx>) -> Self {
Self { tcx, set: Default::default() }
}
/// Adds a predicate to the set.
///
/// Returns whether the predicate was newly inserted. That is:
/// - If the set did not previously contain this predicate, `true` is returned.
/// - If the set already contained this predicate, `false` is returned,
/// and the set is not modified: original predicate is not replaced,
/// and the predicate passed as argument is dropped.
pub fn insert(&mut self, pred: ty::Predicate<'tcx>) -> bool {
// We have to be careful here because we want
//
// for<'a> Foo<&'a i32>
//
// and
//
// for<'b> Foo<&'b i32>
//
// to be considered equivalent. So normalize all late-bound
// regions before we throw things into the underlying set.
self.set.insert(anonymize_predicate(self.tcx, pred))
}
}
impl<'tcx> Extend<ty::Predicate<'tcx>> for PredicateSet<'tcx> {
fn extend<I: IntoIterator<Item = ty::Predicate<'tcx>>>(&mut self, iter: I) {
for pred in iter {
self.insert(pred);
}
}
fn extend_one(&mut self, pred: ty::Predicate<'tcx>) {
self.insert(pred);
}
fn extend_reserve(&mut self, additional: usize) {
Extend::<ty::Predicate<'tcx>>::extend_reserve(&mut self.set, additional);
}
}
///////////////////////////////////////////////////////////////////////////
// `Elaboration` iterator
///////////////////////////////////////////////////////////////////////////
/// "Elaboration" is the process of identifying all the predicates that
/// are implied by a source predicate. Currently, this basically means
/// walking the "supertraits" and other similar assumptions. For example,
/// if we know that `T: Ord`, the elaborator would deduce that `T: PartialOrd`
/// holds as well. Similarly, if we have `trait Foo: 'static`, and we know that
/// `T: Foo`, then we know that `T: 'static`.
pub struct Elaborator<'tcx, O> {
stack: Vec<O>,
visited: PredicateSet<'tcx>,
only_self: bool,
}
/// Describes how to elaborate an obligation into a sub-obligation.
///
/// For [`Obligation`], a sub-obligation is combined with the current obligation's
/// param-env and cause code. For [`ty::Predicate`], none of this is needed, since
/// there is no param-env or cause code to copy over.
pub trait Elaboratable<'tcx> {
fn predicate(&self) -> ty::Predicate<'tcx>;
// Makes a new `Self` but with a different clause that comes from elaboration.
fn child(&self, clause: ty::Clause<'tcx>) -> Self;
// Makes a new `Self` but with a different clause and a different cause
// code (if `Self` has one, such as [`PredicateObligation`]).
fn child_with_derived_cause(
&self,
clause: ty::Clause<'tcx>,
span: Span,
parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
index: usize,
) -> Self;
}
impl<'tcx> Elaboratable<'tcx> for PredicateObligation<'tcx> {
fn predicate(&self) -> ty::Predicate<'tcx> {
self.predicate
}
fn child(&self, clause: ty::Clause<'tcx>) -> Self {
Obligation {
cause: self.cause.clone(),
param_env: self.param_env,
recursion_depth: 0,
predicate: clause.as_predicate(),
}
}
fn child_with_derived_cause(
&self,
clause: ty::Clause<'tcx>,
span: Span,
parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
index: usize,
) -> Self {
let cause = self.cause.clone().derived_cause(parent_trait_pred, |derived| {
traits::ImplDerivedObligation(Box::new(traits::ImplDerivedObligationCause {
derived,
impl_or_alias_def_id: parent_trait_pred.def_id(),
impl_def_predicate_index: Some(index),
span,
}))
});
Obligation {
cause,
param_env: self.param_env,
recursion_depth: 0,
predicate: clause.as_predicate(),
}
}
}
impl<'tcx> Elaboratable<'tcx> for ty::Predicate<'tcx> {
fn predicate(&self) -> ty::Predicate<'tcx> {
*self
}
fn child(&self, clause: ty::Clause<'tcx>) -> Self {
clause.as_predicate()
}
fn child_with_derived_cause(
&self,
clause: ty::Clause<'tcx>,
_span: Span,
_parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
_index: usize,
) -> Self {
clause.as_predicate()
}
}
impl<'tcx> Elaboratable<'tcx> for (ty::Predicate<'tcx>, Span) {
fn predicate(&self) -> ty::Predicate<'tcx> {
self.0
}
fn child(&self, clause: ty::Clause<'tcx>) -> Self {
(clause.as_predicate(), self.1)
}
fn child_with_derived_cause(
&self,
clause: ty::Clause<'tcx>,
_span: Span,
_parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
_index: usize,
) -> Self {
(clause.as_predicate(), self.1)
}
}
impl<'tcx> Elaboratable<'tcx> for (ty::Clause<'tcx>, Span) {
fn predicate(&self) -> ty::Predicate<'tcx> {
self.0.as_predicate()
}
fn child(&self, clause: ty::Clause<'tcx>) -> Self {
(clause, self.1)
}
fn child_with_derived_cause(
&self,
clause: ty::Clause<'tcx>,
_span: Span,
_parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
_index: usize,
) -> Self {
(clause, self.1)
}
}
impl<'tcx> Elaboratable<'tcx> for ty::Clause<'tcx> {
fn predicate(&self) -> ty::Predicate<'tcx> {
self.as_predicate()
}
fn child(&self, clause: ty::Clause<'tcx>) -> Self {
clause
}
fn child_with_derived_cause(
&self,
clause: ty::Clause<'tcx>,
_span: Span,
_parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
_index: usize,
) -> Self {
clause
}
}
pub fn elaborate<'tcx, O: Elaboratable<'tcx>>(
tcx: TyCtxt<'tcx>,
obligations: impl IntoIterator<Item = O>,
) -> Elaborator<'tcx, O> {
let mut elaborator =
Elaborator { stack: Vec::new(), visited: PredicateSet::new(tcx), only_self: false };
elaborator.extend_deduped(obligations);
elaborator
}
impl<'tcx, O: Elaboratable<'tcx>> Elaborator<'tcx, O> {
fn extend_deduped(&mut self, obligations: impl IntoIterator<Item = O>) {
// Only keep those bounds that we haven't already seen.
// This is necessary to prevent infinite recursion in some
// cases. One common case is when people define
// `trait Sized: Sized { }` rather than `trait Sized { }`.
// let visited = &mut self.visited;
self.stack.extend(obligations.into_iter().filter(|o| self.visited.insert(o.predicate())));
}
/// Filter to only the supertraits of trait predicates, i.e. only the predicates
/// that have `Self` as their self type, instead of all implied predicates.
pub fn filter_only_self(mut self) -> Self {
self.only_self = true;
self
}
fn elaborate(&mut self, elaboratable: &O) {
let tcx = self.visited.tcx;
let bound_predicate = elaboratable.predicate().kind();
match bound_predicate.skip_binder() {
ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
// Negative trait bounds do not imply any supertrait bounds
if data.polarity == ty::ImplPolarity::Negative {
return;
}
// Get predicates implied by the trait, or only super predicates if we only care about self predicates.
let predicates = if self.only_self {
tcx.super_predicates_of(data.def_id())
} else {
tcx.implied_predicates_of(data.def_id())
};
let obligations =
predicates.predicates.iter().enumerate().map(|(index, &(clause, span))| {
elaboratable.child_with_derived_cause(
clause.subst_supertrait(tcx, &bound_predicate.rebind(data.trait_ref)),
span,
bound_predicate.rebind(data),
index,
)
});
debug!(?data, ?obligations, "super_predicates");
self.extend_deduped(obligations);
}
ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..)) => {
// Currently, we do not elaborate WF predicates,
// although we easily could.
}
ty::PredicateKind::ObjectSafe(..) => {
// Currently, we do not elaborate object-safe
// predicates.
}
ty::PredicateKind::Subtype(..) => {
// Currently, we do not "elaborate" predicates like `X <: Y`,
// though conceivably we might.
}
ty::PredicateKind::Coerce(..) => {
// Currently, we do not "elaborate" predicates like `X -> Y`,
// though conceivably we might.
}
ty::PredicateKind::Clause(ty::ClauseKind::Projection(..)) => {
// Nothing to elaborate in a projection predicate.
}
ty::PredicateKind::ClosureKind(..) => {
// Nothing to elaborate when waiting for a closure's kind to be inferred.
}
ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..)) => {
// Currently, we do not elaborate const-evaluatable
// predicates.
}
ty::PredicateKind::ConstEquate(..) => {
// Currently, we do not elaborate const-equate
// predicates.
}
ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(..)) => {
// Nothing to elaborate from `'a: 'b`.
}
ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(
ty_max,
r_min,
))) => {
// We know that `T: 'a` for some type `T`. We can
// often elaborate this. For example, if we know that
// `[U]: 'a`, that implies that `U: 'a`. Similarly, if
// we know `&'a U: 'b`, then we know that `'a: 'b` and
// `U: 'b`.
//
// We can basically ignore bound regions here. So for
// example `for<'c> Foo<'a,'c>: 'b` can be elaborated to
// `'a: 'b`.
// Ignore `for<'a> T: 'a` -- we might in the future
// consider this as evidence that `T: 'static`, but
// I'm a bit wary of such constructions and so for now
// I want to be conservative. --nmatsakis
if r_min.is_late_bound() {
return;
}
let mut components = smallvec![];
push_outlives_components(tcx, ty_max, &mut components);
self.extend_deduped(
components
.into_iter()
.filter_map(|component| match component {
Component::Region(r) => {
if r.is_late_bound() {
None
} else {
Some(ty::ClauseKind::RegionOutlives(ty::OutlivesPredicate(
r, r_min,
)))
}
}
Component::Param(p) => {
let ty = Ty::new_param(tcx, p.index, p.name);
Some(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ty, r_min)))
}
Component::UnresolvedInferenceVariable(_) => None,
Component::Alias(alias_ty) => {
// We might end up here if we have `Foo<<Bar as Baz>::Assoc>: 'a`.
// With this, we can deduce that `<Bar as Baz>::Assoc: 'a`.
Some(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(
alias_ty.to_ty(tcx),
r_min,
)))
}
Component::EscapingAlias(_) => {
// We might be able to do more here, but we don't
// want to deal with escaping vars right now.
None
}
})
.map(|clause| {
elaboratable.child(bound_predicate.rebind(clause).to_predicate(tcx))
}),
);
}
ty::PredicateKind::Ambiguous => {}
ty::PredicateKind::AliasRelate(..) => {
// No
}
ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..)) => {
// Nothing to elaborate
}
}
}
}
impl<'tcx, O: Elaboratable<'tcx>> Iterator for Elaborator<'tcx, O> {
type Item = O;
fn size_hint(&self) -> (usize, Option<usize>) {
(self.stack.len(), None)
}
fn next(&mut self) -> Option<Self::Item> {
// Extract next item from top-most stack frame, if any.
if let Some(obligation) = self.stack.pop() {
self.elaborate(&obligation);
Some(obligation)
} else {
None
}
}
}
///////////////////////////////////////////////////////////////////////////
// Supertrait iterator
///////////////////////////////////////////////////////////////////////////
pub fn supertraits<'tcx>(
tcx: TyCtxt<'tcx>,
trait_ref: ty::PolyTraitRef<'tcx>,
) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> {
elaborate(tcx, [trait_ref.to_predicate(tcx)]).filter_only_self().filter_to_traits()
}
pub fn transitive_bounds<'tcx>(
tcx: TyCtxt<'tcx>,
trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> {
elaborate(tcx, trait_refs.map(|trait_ref| trait_ref.to_predicate(tcx)))
.filter_only_self()
.filter_to_traits()
}
/// A specialized variant of `elaborate` that only elaborates trait references that may
/// define the given associated item with the name `assoc_name`. It uses the
/// `super_predicates_that_define_assoc_item` query to avoid enumerating super-predicates that
/// aren't related to `assoc_item`. This is used when resolving types like `Self::Item` or
/// `T::Item` and helps to avoid cycle errors (see e.g. #35237).
pub fn transitive_bounds_that_define_assoc_item<'tcx>(
tcx: TyCtxt<'tcx>,
bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
assoc_name: Ident,
) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> {
let mut stack: Vec<_> = bounds.collect();
let mut visited = FxIndexSet::default();
std::iter::from_fn(move || {
while let Some(trait_ref) = stack.pop() {
let anon_trait_ref = tcx.anonymize_bound_vars(trait_ref);
if visited.insert(anon_trait_ref) {
let super_predicates =
tcx.super_predicates_that_define_assoc_item((trait_ref.def_id(), assoc_name));
for (super_predicate, _) in super_predicates.predicates {
let subst_predicate = super_predicate.subst_supertrait(tcx, &trait_ref);
if let Some(binder) = subst_predicate.as_trait_clause() {
stack.push(binder.map_bound(|t| t.trait_ref));
}
}
return Some(trait_ref);
}
}
return None;
})
}
///////////////////////////////////////////////////////////////////////////
// Other
///////////////////////////////////////////////////////////////////////////
impl<'tcx> Elaborator<'tcx, ty::Predicate<'tcx>> {
fn filter_to_traits(self) -> FilterToTraits<Self> {
FilterToTraits { base_iterator: self }
}
}
/// A filter around an iterator of predicates that makes it yield up
/// just trait references.
pub struct FilterToTraits<I> {
base_iterator: I,
}
impl<'tcx, I: Iterator<Item = ty::Predicate<'tcx>>> Iterator for FilterToTraits<I> {
type Item = ty::PolyTraitRef<'tcx>;
fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
while let Some(pred) = self.base_iterator.next() {
if let Some(data) = pred.to_opt_poly_trait_pred() {
return Some(data.map_bound(|t| t.trait_ref));
}
}
None
}
fn size_hint(&self) -> (usize, Option<usize>) {
let (_, upper) = self.base_iterator.size_hint();
(0, upper)
}
}