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use std::marker::PhantomData;
use smallvec::smallvec;
use crate::data_structures::HashSet;
use crate::inherent::*;
use crate::outlives::{push_outlives_components, Component};
use crate::{self as ty, Interner, Upcast as _};
/// "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<I: Interner, O> {
cx: I,
stack: Vec<O>,
visited: HashSet<ty::Binder<I, ty::PredicateKind<I>>>,
mode: Filter,
}
enum Filter {
All,
OnlySelf,
}
/// Describes how to elaborate an obligation into a sub-obligation.
pub trait Elaboratable<I: Interner> {
fn predicate(&self) -> I::Predicate;
// Makes a new `Self` but with a different clause that comes from elaboration.
fn child(&self, clause: I::Clause) -> 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: I::Clause,
span: I::Span,
parent_trait_pred: ty::Binder<I, ty::TraitPredicate<I>>,
index: usize,
) -> Self;
}
pub fn elaborate<I: Interner, O: Elaboratable<I>>(
cx: I,
obligations: impl IntoIterator<Item = O>,
) -> Elaborator<I, O> {
let mut elaborator =
Elaborator { cx, stack: Vec::new(), visited: HashSet::default(), mode: Filter::All };
elaborator.extend_deduped(obligations);
elaborator
}
impl<I: Interner, O: Elaboratable<I>> Elaborator<I, 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 { }`.
self.stack.extend(
obligations.into_iter().filter(|o| {
self.visited.insert(self.cx.anonymize_bound_vars(o.predicate().kind()))
}),
);
}
/// 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.mode = Filter::OnlySelf;
self
}
fn elaborate(&mut self, elaboratable: &O) {
let cx = self.cx;
// We only elaborate clauses.
let Some(clause) = elaboratable.predicate().as_clause() else {
return;
};
let bound_clause = clause.kind();
match bound_clause.skip_binder() {
ty::ClauseKind::Trait(data) => {
// Negative trait bounds do not imply any supertrait bounds
if data.polarity != ty::PredicatePolarity::Positive {
return;
}
let map_to_child_clause =
|(index, (clause, span)): (usize, (I::Clause, I::Span))| {
elaboratable.child_with_derived_cause(
clause.instantiate_supertrait(cx, bound_clause.rebind(data.trait_ref)),
span,
bound_clause.rebind(data),
index,
)
};
// Get predicates implied by the trait, or only super predicates if we only care about self predicates.
match self.mode {
Filter::All => self.extend_deduped(
cx.explicit_implied_predicates_of(data.def_id())
.iter_identity()
.enumerate()
.map(map_to_child_clause),
),
Filter::OnlySelf => self.extend_deduped(
cx.explicit_super_predicates_of(data.def_id())
.iter_identity()
.enumerate()
.map(map_to_child_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_bound() {
return;
}
let mut components = smallvec![];
push_outlives_components(cx, ty_max, &mut components);
self.extend_deduped(
components
.into_iter()
.filter_map(|component| elaborate_component_to_clause(cx, component, r_min))
.map(|clause| elaboratable.child(bound_clause.rebind(clause).upcast(cx))),
);
}
ty::ClauseKind::RegionOutlives(..) => {
// Nothing to elaborate from `'a: 'b`.
}
ty::ClauseKind::WellFormed(..) => {
// Currently, we do not elaborate WF predicates,
// although we easily could.
}
ty::ClauseKind::Projection(..) => {
// Nothing to elaborate in a projection predicate.
}
ty::ClauseKind::ConstEvaluatable(..) => {
// Currently, we do not elaborate const-evaluatable
// predicates.
}
ty::ClauseKind::ConstArgHasType(..) => {
// Nothing to elaborate
}
}
}
}
fn elaborate_component_to_clause<I: Interner>(
cx: I,
component: Component<I>,
outlives_region: I::Region,
) -> Option<ty::ClauseKind<I>> {
match component {
Component::Region(r) => {
if r.is_bound() {
None
} else {
Some(ty::ClauseKind::RegionOutlives(ty::OutlivesPredicate(r, outlives_region)))
}
}
Component::Param(p) => {
let ty = Ty::new_param(cx, p);
Some(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ty, outlives_region)))
}
Component::Placeholder(p) => {
let ty = Ty::new_placeholder(cx, p);
Some(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ty, outlives_region)))
}
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(cx),
outlives_region,
)))
}
Component::EscapingAlias(_) => {
// We might be able to do more here, but we don't
// want to deal with escaping vars right now.
None
}
}
}
impl<I: Interner, O: Elaboratable<I>> Iterator for Elaborator<I, 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
///////////////////////////////////////////////////////////////////////////
/// Computes the def-ids of the transitive supertraits of `trait_def_id`. This (intentionally)
/// does not compute the full elaborated super-predicates but just the set of def-ids. It is used
/// to identify which traits may define a given associated type to help avoid cycle errors,
/// and to make size estimates for vtable layout computation.
pub fn supertrait_def_ids<I: Interner>(
cx: I,
trait_def_id: I::DefId,
) -> impl Iterator<Item = I::DefId> {
let mut set: HashSet<I::DefId> = HashSet::default();
let mut stack = vec![trait_def_id];
set.insert(trait_def_id);
std::iter::from_fn(move || {
let trait_def_id = stack.pop()?;
for (predicate, _) in cx.explicit_super_predicates_of(trait_def_id).iter_identity() {
if let ty::ClauseKind::Trait(data) = predicate.kind().skip_binder() {
if set.insert(data.def_id()) {
stack.push(data.def_id());
}
}
}
Some(trait_def_id)
})
}
pub fn supertraits<I: Interner>(
cx: I,
trait_ref: ty::Binder<I, ty::TraitRef<I>>,
) -> FilterToTraits<I, Elaborator<I, I::Clause>> {
elaborate(cx, [trait_ref.upcast(cx)]).filter_only_self().filter_to_traits()
}
impl<I: Interner> Elaborator<I, I::Clause> {
fn filter_to_traits(self) -> FilterToTraits<I, Self> {
FilterToTraits { _cx: PhantomData, base_iterator: self }
}
}
/// A filter around an iterator of predicates that makes it yield up
/// just trait references.
pub struct FilterToTraits<I: Interner, It: Iterator<Item = I::Clause>> {
_cx: PhantomData<I>,
base_iterator: It,
}
impl<I: Interner, It: Iterator<Item = I::Clause>> Iterator for FilterToTraits<I, It> {
type Item = ty::Binder<I, ty::TraitRef<I>>;
fn next(&mut self) -> Option<ty::Binder<I, ty::TraitRef<I>>> {
while let Some(pred) = self.base_iterator.next() {
if let Some(data) = pred.as_trait_clause() {
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)
}
}