rustc_hir_analysis/hir_ty_lowering/

bounds.rs

use std::ops::ControlFlow;

use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
use rustc_errors::codes::*;
use rustc_errors::struct_span_code_err;
use rustc_hir as hir;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::def_id::DefId;
use rustc_hir::{PolyTraitRef, find_attr};
use rustc_middle::bug;
use rustc_middle::ty::{
    self as ty, IsSuggestable, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
    TypeVisitor, Upcast,
};
use rustc_span::{DesugaringKind, ErrorGuaranteed, Ident, Span, kw};
use rustc_trait_selection::traits;
use tracing::{debug, instrument};

use crate::diagnostics;
use crate::hir_ty_lowering::{
    AssocItemQSelf, GenericsArgsErrExtend, HirTyLowerer, ImpliedBoundsContext,
    OverlappingAsssocItemConstraints, PredicateFilter, RegionInferReason,
};

#[derive(Debug, Default)]
struct CollectedBound {
    /// `Trait`
    positive: Option<Span>,
    /// `?Trait`
    maybe: Option<Span>,
    /// `!Trait`
    negative: Option<Span>,
}

impl CollectedBound {
    /// Returns `true` if any of `Trait`, `?Trait` or `!Trait` were encountered.
    fn any(&self) -> bool {
        self.positive.is_some() || self.maybe.is_some() || self.negative.is_some()
    }
}

#[derive(Debug)]
struct CollectedSizednessBounds {
    // Collected `Sized` bounds
    sized: CollectedBound,
    // Collected `MetaSized` bounds
    meta_sized: CollectedBound,
    // Collected `PointeeSized` bounds
    pointee_sized: CollectedBound,
}

impl CollectedSizednessBounds {
    /// Returns `true` if any of `Trait`, `?Trait` or `!Trait` were encountered for `Sized`,
    /// `MetaSized` or `PointeeSized`.
    fn any(&self) -> bool {
        self.sized.any() || self.meta_sized.any() || self.pointee_sized.any()
    }
}

fn search_bounds_for<'tcx>(
    hir_bounds: &'tcx [hir::GenericBound<'tcx>],
    where_bounds: &'tcx [hir::WherePredicate<'tcx>],
    context: ImpliedBoundsContext,
    mut f: impl FnMut(&'tcx PolyTraitRef<'tcx>),
) {
    let mut search_bounds = |hir_bounds: &'tcx [hir::GenericBound<'tcx>]| {
        for hir_bound in hir_bounds {
            let hir::GenericBound::Trait(ptr) = hir_bound else {
                continue;
            };

            f(ptr)
        }
    };

    search_bounds(hir_bounds);
    if let ImpliedBoundsContext::TyParam(self_ty) = context {
        for clause in where_bounds {
            if let hir::WherePredicateKind::BoundPredicate(pred) = clause.kind
                && pred.is_param_bound(self_ty.to_def_id())
            {
                search_bounds(pred.bounds);
            }
        }
    }
}

fn collect_bounds<'a, 'tcx>(
    hir_bounds: &'a [hir::GenericBound<'tcx>],
    where_bounds: &'tcx [hir::WherePredicate<'tcx>],
    context: ImpliedBoundsContext,
    target_did: DefId,
) -> CollectedBound {
    let mut collect_into = CollectedBound::default();
    search_bounds_for(hir_bounds, where_bounds, context, |ptr| {
        if !matches!(ptr.trait_ref.path.res, Res::Def(DefKind::Trait, did) if did == target_did) {
            return;
        }

        match ptr.modifiers.polarity {
            hir::BoundPolarity::Maybe(_) => collect_into.maybe = Some(ptr.span),
            hir::BoundPolarity::Negative(_) => collect_into.negative = Some(ptr.span),
            hir::BoundPolarity::Positive => collect_into.positive = Some(ptr.span),
        }
    });
    collect_into
}

fn collect_sizedness_bounds<'tcx>(
    tcx: TyCtxt<'tcx>,
    hir_bounds: &'tcx [hir::GenericBound<'tcx>],
    where_bounds: &'tcx [hir::WherePredicate<'tcx>],
    context: ImpliedBoundsContext,
    span: Span,
) -> CollectedSizednessBounds {
    let sized_did = tcx.require_lang_item(hir::LangItem::Sized, span);
    let sized = collect_bounds(hir_bounds, where_bounds, context, sized_did);

    let meta_sized_did = tcx.require_lang_item(hir::LangItem::MetaSized, span);
    let meta_sized = collect_bounds(hir_bounds, where_bounds, context, meta_sized_did);

    let pointee_sized_did = tcx.require_lang_item(hir::LangItem::PointeeSized, span);
    let pointee_sized = collect_bounds(hir_bounds, where_bounds, context, pointee_sized_did);

    CollectedSizednessBounds { sized, meta_sized, pointee_sized }
}

/// Add a trait bound for `did`.
fn add_trait_bound<'tcx>(
    tcx: TyCtxt<'tcx>,
    bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
    self_ty: Ty<'tcx>,
    did: DefId,
    span: Span,
) {
    let trait_ref = ty::TraitRef::new(tcx, did, [self_ty]);
    // Preferable to put sizedness obligations first, since we report better errors for `Sized`
    // ambiguity.
    bounds.insert(0, (trait_ref.upcast(tcx), span));
}

impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
    /// Adds sizedness bounds to a trait, trait alias, parameter, opaque type or associated type.
    ///
    /// - On parameters, opaque type and associated types, add default `Sized` bound if no explicit
    ///   sizedness bounds are present.
    /// - On traits and trait aliases, add default `MetaSized` supertrait if no explicit sizedness
    ///   bounds are present.
    /// - On parameters, opaque type, associated types and trait aliases, add a `MetaSized` bound if
    ///   a `?Sized` bound is present.
    pub(crate) fn add_implicit_sizedness_bounds(
        &self,
        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
        self_ty: Ty<'tcx>,
        hir_bounds: &'tcx [hir::GenericBound<'tcx>],
        where_bounds: &'tcx [hir::WherePredicate<'tcx>],
        context: ImpliedBoundsContext,
        span: Span,
    ) {
        let tcx = self.tcx();

        // Skip adding any default bounds if `#![rustc_no_implicit_bounds]`
        if find_attr!(tcx, crate, RustcNoImplicitBounds) {
            return;
        }

        let meta_sized_did = tcx.require_lang_item(hir::LangItem::MetaSized, span);
        let pointee_sized_did = tcx.require_lang_item(hir::LangItem::PointeeSized, span);

        // If adding sizedness bounds to a trait, then there are some relevant early exits
        match context {
            ImpliedBoundsContext::TraitDef(trait_did) => {
                let trait_did = trait_did.to_def_id();
                // Never add a default supertrait to `PointeeSized`.
                if trait_did == pointee_sized_did {
                    return;
                }
                // Don't add default sizedness supertraits to auto traits because it isn't possible to
                // relax an automatically added supertrait on the defn itself.
                if tcx.trait_is_auto(trait_did) {
                    return;
                }
            }
            ImpliedBoundsContext::TyParam(..)
            | ImpliedBoundsContext::AssociatedType(..)
            | ImpliedBoundsContext::TraitObject
            | ImpliedBoundsContext::TraitAscription
            | ImpliedBoundsContext::ImplTrait => {}
        }
        let collected = collect_sizedness_bounds(tcx, hir_bounds, where_bounds, context, span);
        if let Some(span) = collected.sized.maybe.or(collected.sized.negative)
            && collected.sized.positive.is_none()
            && !collected.meta_sized.any()
            && !collected.pointee_sized.any()
        {
            // `?Sized` is equivalent to `MetaSized` (but only add the bound if there aren't any
            // other explicit ones) - this can happen for trait aliases as well as bounds.
            add_trait_bound(tcx, bounds, self_ty, meta_sized_did, span);
        } else if !collected.any() {
            let span = match context {
                ImpliedBoundsContext::TraitDef(def)
                | ImpliedBoundsContext::TyParam(def)
                | ImpliedBoundsContext::AssociatedType(def) => {
                    self.tcx().with_stable_hashing_context(|hcx| {
                        span.mark_with_reason(
                            None,
                            DesugaringKind::DefaultBound { def: def.into() },
                            span.edition(),
                            hcx,
                        )
                    })
                }
                _ => span,
            };
            match context {
                ImpliedBoundsContext::TraitDef(..) => {
                    // If there are no explicit sizedness bounds on a trait then add a default
                    // `MetaSized` supertrait.
                    add_trait_bound(tcx, bounds, self_ty, meta_sized_did, span);
                }
                ImpliedBoundsContext::TyParam(..)
                | ImpliedBoundsContext::AssociatedType(..)
                | ImpliedBoundsContext::TraitObject
                | ImpliedBoundsContext::TraitAscription
                | ImpliedBoundsContext::ImplTrait => {
                    // If there are no explicit sizedness bounds on a parameter then add a default
                    // `Sized` bound.
                    let sized_did = tcx.require_lang_item(hir::LangItem::Sized, span);
                    add_trait_bound(tcx, bounds, self_ty, sized_did, span);
                }
            }
        }
    }

    pub(crate) fn add_default_traits(
        &self,
        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
        self_ty: Ty<'tcx>,
        hir_bounds: &[hir::GenericBound<'tcx>],
        where_bounds: &'tcx [hir::WherePredicate<'tcx>],
        context: ImpliedBoundsContext,
        span: Span,
    ) {
        self.tcx().default_traits().iter().for_each(|default_trait| {
            self.add_default_trait(
                *default_trait,
                bounds,
                self_ty,
                hir_bounds,
                where_bounds,
                context,
                span,
            );
        });
    }

    /// Add a `experimental_default_bounds` bound to the `bounds` if appropriate.
    ///
    /// Doesn't add the bound if the HIR bounds contain any of `Trait`, `?Trait` or `!Trait`.
    pub(crate) fn add_default_trait(
        &self,
        trait_: hir::LangItem,
        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
        self_ty: Ty<'tcx>,
        hir_bounds: &[hir::GenericBound<'tcx>],
        where_bounds: &'tcx [hir::WherePredicate<'tcx>],
        context: ImpliedBoundsContext,
        span: Span,
    ) {
        let tcx = self.tcx();

        // Supertraits for auto trait are unsound according to the unstable book:
        // https://doc.rust-lang.org/beta/unstable-book/language-features/auto-traits.html#supertraits
        if let ImpliedBoundsContext::TraitDef(trait_did) = context
            && self.tcx().trait_is_auto(trait_did.into())
        {
            return;
        }

        if let Some(trait_did) = tcx.lang_items().get(trait_)
            && self.should_add_default_traits(trait_did, hir_bounds, where_bounds, context)
        {
            add_trait_bound(tcx, bounds, self_ty, trait_did, span);
        }
    }

    /// Returns `true` if default trait bound should be added.
    fn should_add_default_traits<'a>(
        &self,
        trait_def_id: DefId,
        hir_bounds: &'a [hir::GenericBound<'tcx>],
        where_bounds: &'tcx [hir::WherePredicate<'tcx>],
        context: ImpliedBoundsContext,
    ) -> bool {
        let collected = collect_bounds(hir_bounds, where_bounds, context, trait_def_id);
        !find_attr!(self.tcx(), crate, RustcNoImplicitBounds) && !collected.any()
    }

    pub(crate) fn require_bound_to_relax_default_trait(
        &self,
        trait_ref: hir::TraitRef<'_>,
        span: Span,
    ) {
        let tcx = self.tcx();

        if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res
            && (tcx.is_lang_item(def_id, hir::LangItem::Sized) || tcx.is_default_trait(def_id))
        {
            return;
        }

        self.dcx().span_err(
            span,
            if tcx.sess.opts.unstable_opts.experimental_default_bounds
                || tcx.features().more_maybe_bounds()
            {
                "bound modifier `?` can only be applied to default traits"
            } else {
                "bound modifier `?` can only be applied to `Sized`"
            },
        );
    }

    /// Lower HIR bounds into `bounds` given the self type `param_ty` and the overarching late-bound vars if any.
    ///
    /// ### Examples
    ///
    /// ```ignore (illustrative)
    /// fn foo<T>() where for<'a> T: Trait<'a> + Copy {}
    /// //                ^^^^^^^ ^  ^^^^^^^^^^^^^^^^ `hir_bounds`, in HIR form
    /// //                |       |
    /// //                |       `param_ty`, in ty form
    /// //                `bound_vars`, in ty form
    ///
    /// fn bar<T>() where T: for<'a> Trait<'a> + Copy {} // no overarching `bound_vars` here!
    /// //                ^  ^^^^^^^^^^^^^^^^^^^^^^^^ `hir_bounds`, in HIR form
    /// //                |
    /// //                `param_ty`, in ty form
    /// ```
    ///
    /// ### A Note on Binders
    ///
    /// There is an implied binder around `param_ty` and `hir_bounds`.
    /// See `lower_poly_trait_ref` for more details.
    #[instrument(level = "debug", skip(self, hir_bounds, bounds))]
    pub(crate) fn lower_bounds<'hir, I: IntoIterator<Item = &'hir hir::GenericBound<'tcx>>>(
        &self,
        param_ty: Ty<'tcx>,
        hir_bounds: I,
        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
        bound_vars: &'tcx ty::List<ty::BoundVariableKind<'tcx>>,
        predicate_filter: PredicateFilter,
        overlapping_assoc_constraints: OverlappingAsssocItemConstraints,
    ) where
        'tcx: 'hir,
    {
        for hir_bound in hir_bounds {
            // In order to avoid cycles, when we're lowering `SelfTraitThatDefines`,
            // we skip over any traits that don't define the given associated type.
            if let PredicateFilter::SelfTraitThatDefines(assoc_ident) = predicate_filter {
                if let Some(trait_ref) = hir_bound.trait_ref()
                    && let Some(trait_did) = trait_ref.trait_def_id()
                    && self.tcx().trait_may_define_assoc_item(trait_did, assoc_ident)
                {
                    // Okay
                } else {
                    continue;
                }
            }

            match hir_bound {
                hir::GenericBound::Trait(poly_trait_ref) => {
                    let _ = self.lower_poly_trait_ref(
                        poly_trait_ref,
                        param_ty,
                        bounds,
                        predicate_filter,
                        overlapping_assoc_constraints,
                    );
                }
                hir::GenericBound::Outlives(lifetime) => {
                    // `ConstIfConst` is only interested in `[const]` bounds.
                    if matches!(
                        predicate_filter,
                        PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst
                    ) {
                        continue;
                    }

                    let region = self.lower_lifetime(lifetime, RegionInferReason::OutlivesBound);
                    let bound = ty::Binder::bind_with_vars(
                        ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(param_ty, region)),
                        bound_vars,
                    );
                    bounds.push((bound.upcast(self.tcx()), lifetime.ident.span));
                }
                hir::GenericBound::Use(..) => {
                    // We don't actually lower `use` into the type layer.
                }
            }
        }
    }

    /// Lower an associated item constraint from the HIR into `bounds`.
    ///
    /// ### A Note on Binders
    ///
    /// Given something like `T: for<'a> Iterator<Item = &'a u32>`,
    /// the `trait_ref` here will be `for<'a> T: Iterator`.
    /// The `constraint` data however is from *inside* the binder
    /// (e.g., `&'a u32`) and hence may reference bound regions.
    #[instrument(level = "debug", skip(self, bounds, duplicates, path_span))]
    pub(super) fn lower_assoc_item_constraint(
        &self,
        hir_ref_id: hir::HirId,
        trait_ref: ty::PolyTraitRef<'tcx>,
        constraint: &hir::AssocItemConstraint<'tcx>,
        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
        duplicates: Option<&mut FxIndexMap<DefId, Span>>,
        path_span: Span,
        predicate_filter: PredicateFilter,
    ) -> Result<(), ErrorGuaranteed> {
        let tcx = self.tcx();

        let assoc_tag = if constraint.gen_args.parenthesized
            == hir::GenericArgsParentheses::ReturnTypeNotation
        {
            ty::AssocTag::Fn
        } else if let hir::AssocItemConstraintKind::Equality { term: hir::Term::Const(_) } =
            constraint.kind
        {
            ty::AssocTag::Const
        } else {
            ty::AssocTag::Type
        };

        // Given something like `U: Trait<T = X>`, we want to produce a predicate like
        // `<U as Trait>::T = X`.
        // This is somewhat subtle in the event that `T` is defined in a supertrait of `Trait`,
        // because in that case we need to upcast. I.e., we want to produce
        // `<B as SuperTrait<i32>>::T == X` for `B: SubTrait<T = X>` where
        //
        //     trait SubTrait: SuperTrait<i32> {}
        //     trait SuperTrait<A> { type T; }
        let candidate = if self.probe_trait_that_defines_assoc_item(
            trait_ref.def_id(),
            assoc_tag,
            constraint.ident,
        ) {
            // Simple case: The assoc item is defined in the current trait.
            trait_ref
        } else {
            // Otherwise, we have to walk through the supertraits to find
            // one that does define it.
            self.probe_single_bound_for_assoc_item(
                || traits::supertraits(tcx, trait_ref),
                AssocItemQSelf::Trait(trait_ref.def_id()),
                assoc_tag,
                constraint.ident,
                path_span,
                Some(constraint),
            )?
        };

        let assoc_item = self
            .probe_assoc_item(
                constraint.ident,
                assoc_tag,
                hir_ref_id,
                constraint.span,
                candidate.def_id(),
            )
            .expect("failed to find associated item");

        if let Some(duplicates) = duplicates {
            duplicates
                .entry(assoc_item.def_id)
                .and_modify(|prev_span| {
                    self.dcx().emit_err(diagnostics::ValueOfAssociatedStructAlreadySpecified {
                        span: constraint.span,
                        prev_span: *prev_span,
                        item_name: constraint.ident,
                        def_path: tcx.def_path_str(assoc_item.container_id(tcx)),
                    });
                })
                .or_insert(constraint.span);
        }

        let projection_term = if let ty::AssocTag::Fn = assoc_tag {
            let bound_vars = tcx.late_bound_vars(constraint.hir_id);
            ty::Binder::bind_with_vars(
                self.lower_return_type_notation_ty(candidate, assoc_item.def_id, path_span)?.into(),
                bound_vars,
            )
        } else {
            // Create the generic arguments for the associated type or constant by joining the
            // parent arguments (the arguments of the trait) and the own arguments (the ones of
            // the associated item itself) and construct an alias type using them.
            candidate.map_bound(|trait_ref| {
                let item_segment = hir::PathSegment {
                    ident: constraint.ident,
                    hir_id: constraint.hir_id,
                    res: Res::Err,
                    args: Some(constraint.gen_args),
                    infer_args: false,
                };

                let alias_args = self.lower_generic_args_of_assoc_item(
                    path_span,
                    assoc_item.def_id,
                    &item_segment,
                    trait_ref.args,
                );
                debug!(?alias_args);

                ty::AliasTerm::new_from_def_id(tcx, assoc_item.def_id, alias_args)
            })
        };

        match constraint.kind {
            hir::AssocItemConstraintKind::Equality { .. } if let ty::AssocTag::Fn = assoc_tag => {
                return Err(self.dcx().emit_err(
                    crate::diagnostics::ReturnTypeNotationEqualityBound { span: constraint.span },
                ));
            }
            // Lower an equality constraint like `Item = u32` as found in HIR bound `T: Iterator<Item = u32>`
            // to a projection predicate: `<T as Iterator>::Item = u32`.
            hir::AssocItemConstraintKind::Equality { term } => {
                let term = match term {
                    hir::Term::Ty(ty) => self.lower_ty(ty).into(),
                    hir::Term::Const(ct) => {
                        let ty = projection_term
                            .map_bound(|alias| alias.expect_ct().type_of(tcx).skip_norm_wip());
                        let ty = check_assoc_const_binding_type(
                            self,
                            constraint.ident,
                            ty,
                            constraint.hir_id,
                        );

                        self.lower_const_arg(ct, ty).into()
                    }
                };

                // Find any late-bound regions declared in `ty` that are not
                // declared in the trait-ref or assoc_item. These are not well-formed.
                //
                // Example:
                //
                //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
                //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
                let late_bound_in_projection_ty =
                    tcx.collect_constrained_late_bound_regions(projection_term);
                let late_bound_in_term =
                    tcx.collect_referenced_late_bound_regions(trait_ref.rebind(term));
                debug!(?late_bound_in_projection_ty);
                debug!(?late_bound_in_term);

                // FIXME: point at the type params that don't have appropriate lifetimes:
                // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
                //                         ----  ----     ^^^^^^^
                // NOTE(mgca): This error should be impossible to trigger with assoc const bindings.
                self.validate_late_bound_regions(
                    late_bound_in_projection_ty,
                    late_bound_in_term,
                    |br_name| {
                        struct_span_code_err!(
                            self.dcx(),
                            constraint.span,
                            E0582,
                            "binding for associated type `{}` references {}, \
                             which does not appear in the trait input types",
                            constraint.ident,
                            br_name
                        )
                    },
                );

                match predicate_filter {
                    PredicateFilter::All
                    | PredicateFilter::SelfOnly
                    | PredicateFilter::SelfAndAssociatedTypeBounds => {
                        let bound = projection_term.map_bound(|projection_term| {
                            ty::ClauseKind::Projection(ty::ProjectionPredicate {
                                projection_term,
                                term,
                            })
                        });

                        if let ty::AssocTag::Const = assoc_tag
                            && !self.tcx().is_type_const(assoc_item.def_id)
                        {
                            if tcx.features().min_generic_const_args() {
                                let mut err = self.dcx().struct_span_err(
                                    constraint.span,
                                    "use of trait associated const not defined as `type const`",
                                );
                                err.note("the declaration in the trait must begin with `type const` not just `const` alone");
                                return Err(err.emit());
                            } else {
                                let err = self.dcx().span_delayed_bug(
                                    constraint.span,
                                    "use of trait associated const defined as `type const`",
                                );
                                return Err(err);
                            }
                        } else {
                            bounds.push((bound.upcast(tcx), constraint.span));
                        }
                    }
                    // SelfTraitThatDefines is only interested in trait predicates.
                    PredicateFilter::SelfTraitThatDefines(_) => {}
                    // `ConstIfConst` is only interested in `[const]` bounds.
                    PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {}
                }
            }
            // Lower a constraint like `Item: Debug` as found in HIR bound `T: Iterator<Item: Debug>`
            // to a bound involving a projection: `<T as Iterator>::Item: Debug`.
            hir::AssocItemConstraintKind::Bound { bounds: hir_bounds } => {
                match predicate_filter {
                    PredicateFilter::All
                    | PredicateFilter::SelfAndAssociatedTypeBounds
                    | PredicateFilter::ConstIfConst => {
                        let projection_ty = projection_term
                            .map_bound(|projection_term| projection_term.expect_ty());
                        // Calling `skip_binder` is okay, because `lower_bounds` expects the `param_ty`
                        // parameter to have a skipped binder.
                        let param_ty = Ty::new_alias(tcx, projection_ty.skip_binder());
                        self.lower_bounds(
                            param_ty,
                            hir_bounds,
                            bounds,
                            projection_ty.bound_vars(),
                            predicate_filter,
                            OverlappingAsssocItemConstraints::Allowed,
                        );
                    }
                    PredicateFilter::SelfOnly
                    | PredicateFilter::SelfTraitThatDefines(_)
                    | PredicateFilter::SelfConstIfConst => {}
                }
            }
        }
        Ok(())
    }

    /// Lower a type, possibly specially handling the type if it's a return type notation
    /// which we otherwise deny in other positions.
    pub fn lower_ty_maybe_return_type_notation(&self, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
        let hir::TyKind::Path(qpath) = hir_ty.kind else {
            return self.lower_ty(hir_ty);
        };

        let tcx = self.tcx();
        match qpath {
            hir::QPath::Resolved(opt_self_ty, path)
                if let [mod_segments @ .., trait_segment, item_segment] = &path.segments[..]
                    && item_segment.args.is_some_and(|args| {
                        matches!(
                            args.parenthesized,
                            hir::GenericArgsParentheses::ReturnTypeNotation
                        )
                    }) =>
            {
                // We don't allow generics on the module segments.
                let _ =
                    self.prohibit_generic_args(mod_segments.iter(), GenericsArgsErrExtend::None);

                let item_def_id = match path.res {
                    Res::Def(DefKind::AssocFn, item_def_id) => item_def_id,
                    Res::Err => {
                        return Ty::new_error_with_message(
                            tcx,
                            hir_ty.span,
                            "failed to resolve RTN",
                        );
                    }
                    _ => bug!("only expected method resolution for fully qualified RTN"),
                };
                let trait_def_id = tcx.parent(item_def_id);

                // Good error for `where Trait::method(..): Send`.
                let Some(self_ty) = opt_self_ty else {
                    let guar = self.report_missing_self_ty_for_resolved_path(
                        trait_def_id,
                        hir_ty.span,
                        item_segment,
                        ty::AssocTag::Type,
                    );
                    return Ty::new_error(tcx, guar);
                };
                let self_ty = self.lower_ty(self_ty);

                let trait_ref = self.lower_mono_trait_ref(
                    hir_ty.span,
                    trait_def_id,
                    self_ty,
                    trait_segment,
                    false,
                );

                // SUBTLE: As noted at the end of `try_append_return_type_notation_params`
                // in `resolve_bound_vars`, we stash the explicit bound vars of the where
                // clause onto the item segment of the RTN type. This allows us to know
                // how many bound vars are *not* coming from the signature of the function
                // from lowering RTN itself.
                //
                // For example, in `where for<'a> <T as Trait<'a>>::method(..): Other`,
                // the `late_bound_vars` of the where clause predicate (i.e. this HIR ty's
                // parent) will include `'a` AND all the early- and late-bound vars of the
                // method. But when lowering the RTN type, we just want the list of vars
                // we used to resolve the trait ref. We explicitly stored those back onto
                // the item segment, since there's no other good place to put them.
                let candidate =
                    ty::Binder::bind_with_vars(trait_ref, tcx.late_bound_vars(item_segment.hir_id));

                match self.lower_return_type_notation_ty(candidate, item_def_id, hir_ty.span) {
                    Ok(ty) => Ty::new_alias(tcx, ty),
                    Err(guar) => Ty::new_error(tcx, guar),
                }
            }
            hir::QPath::TypeRelative(hir_self_ty, segment)
                if segment.args.is_some_and(|args| {
                    matches!(args.parenthesized, hir::GenericArgsParentheses::ReturnTypeNotation)
                }) =>
            {
                let self_ty = self.lower_ty(hir_self_ty);
                let (item_def_id, bound) = match self.resolve_type_relative_path(
                    self_ty,
                    hir_self_ty,
                    ty::AssocTag::Fn,
                    segment,
                    hir_ty.hir_id,
                    hir_ty.span,
                    None,
                ) {
                    Ok(result) => result,
                    Err(guar) => return Ty::new_error(tcx, guar),
                };

                // Don't let `T::method` resolve to some `for<'a> <T as Tr<'a>>::method`,
                // which may happen via a higher-ranked where clause or supertrait.
                // This is the same restrictions as associated types; even though we could
                // support it, it just makes things a lot more difficult to support in
                // `resolve_bound_vars`, since we'd need to introduce those as elided
                // bound vars on the where clause too.
                if bound.has_bound_vars() {
                    return Ty::new_error(
                        tcx,
                        self.dcx().emit_err(
                            diagnostics::AssociatedItemTraitUninferredGenericParams {
                                span: hir_ty.span,
                                inferred_sugg: Some(hir_ty.span.with_hi(segment.ident.span.lo())),
                                bound: format!(
                                    "{}::",
                                    tcx.anonymize_bound_vars(bound).skip_binder()
                                ),
                                mpart_sugg: None,
                                what: tcx.def_descr(item_def_id),
                            },
                        ),
                    );
                }

                match self.lower_return_type_notation_ty(bound, item_def_id, hir_ty.span) {
                    Ok(ty) => Ty::new_alias(tcx, ty),
                    Err(guar) => Ty::new_error(tcx, guar),
                }
            }
            _ => self.lower_ty(hir_ty),
        }
    }

    /// Do the common parts of lowering an RTN type. This involves extending the
    /// candidate binder to include all of the early- and late-bound vars that are
    /// defined on the function itself, and constructing a projection to the RPITIT
    /// return type of that function.
    fn lower_return_type_notation_ty(
        &self,
        candidate: ty::PolyTraitRef<'tcx>,
        item_def_id: DefId,
        path_span: Span,
    ) -> Result<ty::AliasTy<'tcx>, ErrorGuaranteed> {
        let tcx = self.tcx();
        let mut emitted_bad_param_err = None;
        // If we have an method return type bound, then we need to instantiate
        // the method's early bound params with suitable late-bound params.
        let mut num_bound_vars = candidate.bound_vars().len();
        let args = candidate.skip_binder().args.extend_to(tcx, item_def_id, |param, _| {
            let arg = match param.kind {
                ty::GenericParamDefKind::Lifetime => ty::Region::new_bound(
                    tcx,
                    ty::INNERMOST,
                    ty::BoundRegion {
                        var: ty::BoundVar::from_usize(num_bound_vars),
                        kind: ty::BoundRegionKind::Named(param.def_id),
                    },
                )
                .into(),
                ty::GenericParamDefKind::Type { .. } => {
                    let guar = *emitted_bad_param_err.get_or_insert_with(|| {
                        self.dcx().emit_err(
                            crate::diagnostics::ReturnTypeNotationIllegalParam::Type {
                                span: path_span,
                                param_span: tcx.def_span(param.def_id),
                            },
                        )
                    });
                    Ty::new_error(tcx, guar).into()
                }
                ty::GenericParamDefKind::Const { .. } => {
                    let guar = *emitted_bad_param_err.get_or_insert_with(|| {
                        self.dcx().emit_err(
                            crate::diagnostics::ReturnTypeNotationIllegalParam::Const {
                                span: path_span,
                                param_span: tcx.def_span(param.def_id),
                            },
                        )
                    });
                    ty::Const::new_error(tcx, guar).into()
                }
            };
            num_bound_vars += 1;
            arg
        });

        // Next, we need to check that the return-type notation is being used on
        // an RPITIT (return-position impl trait in trait) or AFIT (async fn in trait).
        let output = tcx.fn_sig(item_def_id).skip_binder().output();
        let output = if let ty::Alias(alias_ty) = *output.skip_binder().kind()
            && let ty::AliasTy { kind: ty::Projection { def_id: projection_def_id }, .. } = alias_ty
            && tcx.is_impl_trait_in_trait(projection_def_id)
        {
            alias_ty
        } else {
            return Err(self.dcx().emit_err(crate::diagnostics::ReturnTypeNotationOnNonRpitit {
                span: path_span,
                ty: tcx.liberate_late_bound_regions(item_def_id, output),
                fn_span: tcx.hir_span_if_local(item_def_id),
                note: (),
            }));
        };

        // Finally, move the fn return type's bound vars over to account for the early bound
        // params (and trait ref's late bound params). This logic is very similar to
        // `rustc_middle::ty::predicate::Clause::instantiate_supertrait`
        // and it's no coincidence why.
        let shifted_output = tcx.shift_bound_var_indices(num_bound_vars, output);
        Ok(ty::EarlyBinder::bind(shifted_output).instantiate(tcx, args).skip_norm_wip())
    }
}

/// Detect and reject early-bound & escaping late-bound generic params in the type of assoc const bindings.
///
/// FIXME(const_generics): This is a temporary and semi-artificial restriction until the
/// arrival of *generic const generics*[^1].
///
/// It might actually be possible that we can already support early-bound generic params
/// in such types if we just lifted some more checks in other places, too, for example
/// inside `HirTyLowerer::lower_anon_const`. However, even if that were the case, we should
/// probably gate this behind another feature flag.
///
/// [^1]: <https://github.com/rust-lang/project-const-generics/issues/28>.
pub(crate) fn check_assoc_const_binding_type<'tcx>(
    cx: &dyn HirTyLowerer<'tcx>,
    assoc_const: Ident,
    ty: ty::Binder<'tcx, Ty<'tcx>>,
    hir_id: hir::HirId,
) -> Ty<'tcx> {
    // We can't perform the checks for early-bound params during name resolution unlike E0770
    // because this information depends on *type* resolution.
    // We can't perform these checks in `resolve_bound_vars` either for the same reason.
    // Consider the trait ref `for<'a> Trait<'a, C = { &0 }>`. We need to know the fully
    // resolved type of `Trait::C` in order to know if it references `'a` or not.

    let ty = ty.skip_binder();
    if !ty.has_param() && !ty.has_escaping_bound_vars() {
        return ty;
    }

    let mut collector = GenericParamAndBoundVarCollector {
        cx,
        params: Default::default(),
        vars: Default::default(),
        depth: ty::INNERMOST,
    };
    let mut guar = ty.visit_with(&mut collector).break_value();

    let tcx = cx.tcx();
    let ty_note = ty
        .make_suggestable(tcx, false, None)
        .map(|ty| crate::diagnostics::TyOfAssocConstBindingNote { assoc_const, ty });

    let enclosing_item_owner_id = tcx
        .hir_parent_owner_iter(hir_id)
        .find_map(|(owner_id, parent)| parent.generics().map(|_| owner_id))
        .unwrap();
    let generics = tcx.generics_of(enclosing_item_owner_id);
    for index in collector.params {
        let param = generics.param_at(index as _, tcx);
        let is_self_param = param.name == kw::SelfUpper;
        guar.get_or_insert(cx.dcx().emit_err(crate::diagnostics::ParamInTyOfAssocConstBinding {
            span: assoc_const.span,
            assoc_const,
            param_name: param.name,
            param_def_kind: tcx.def_descr(param.def_id),
            param_category: if is_self_param {
                "self"
            } else if param.kind.is_synthetic() {
                "synthetic"
            } else {
                "normal"
            },
            param_defined_here_label:
                (!is_self_param).then(|| tcx.def_ident_span(param.def_id).unwrap()),
            ty_note,
        }));
    }
    for var_def_id in collector.vars {
        guar.get_or_insert(cx.dcx().emit_err(
            crate::diagnostics::EscapingBoundVarInTyOfAssocConstBinding {
                span: assoc_const.span,
                assoc_const,
                var_name: cx.tcx().item_name(var_def_id),
                var_def_kind: tcx.def_descr(var_def_id),
                var_defined_here_label: tcx.def_ident_span(var_def_id).unwrap(),
                ty_note,
            },
        ));
    }

    let guar = guar.unwrap_or_else(|| bug!("failed to find gen params or bound vars in ty"));
    Ty::new_error(tcx, guar)
}

struct GenericParamAndBoundVarCollector<'a, 'tcx> {
    cx: &'a dyn HirTyLowerer<'tcx>,
    params: FxIndexSet<u32>,
    vars: FxIndexSet<DefId>,
    depth: ty::DebruijnIndex,
}

impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'_, 'tcx> {
    type Result = ControlFlow<ErrorGuaranteed>;

    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
        &mut self,
        binder: &ty::Binder<'tcx, T>,
    ) -> Self::Result {
        self.depth.shift_in(1);
        let result = binder.super_visit_with(self);
        self.depth.shift_out(1);
        result
    }

    fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
        match ty.kind() {
            ty::Param(param) => {
                self.params.insert(param.index);
            }
            ty::Bound(ty::BoundVarIndexKind::Bound(db), bt) if *db >= self.depth => {
                self.vars.insert(match bt.kind {
                    ty::BoundTyKind::Param(def_id) => def_id,
                    ty::BoundTyKind::Anon => {
                        let reported = self
                            .cx
                            .dcx()
                            .delayed_bug(format!("unexpected anon bound ty: {:?}", bt.var));
                        return ControlFlow::Break(reported);
                    }
                });
            }
            _ if ty.has_param() || ty.has_bound_vars() => return ty.super_visit_with(self),
            _ => {}
        }
        ControlFlow::Continue(())
    }

    fn visit_region(&mut self, re: ty::Region<'tcx>) -> Self::Result {
        match re.kind() {
            ty::ReEarlyParam(param) => {
                self.params.insert(param.index);
            }
            ty::ReBound(ty::BoundVarIndexKind::Bound(db), br) if db >= self.depth => {
                self.vars.insert(match br.kind {
                    ty::BoundRegionKind::Named(def_id) => def_id,
                    ty::BoundRegionKind::Anon | ty::BoundRegionKind::ClosureEnv => {
                        let guar = self
                            .cx
                            .dcx()
                            .delayed_bug(format!("unexpected bound region kind: {:?}", br.kind));
                        return ControlFlow::Break(guar);
                    }
                    ty::BoundRegionKind::NamedForPrinting(_) => {
                        bug!("only used for pretty printing")
                    }
                });
            }
            _ => {}
        }
        ControlFlow::Continue(())
    }

    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
        match ct.kind() {
            ty::ConstKind::Param(param) => {
                self.params.insert(param.index);
            }
            ty::ConstKind::Bound(ty::BoundVarIndexKind::Bound(db), _) if db >= self.depth => {
                let guar = self.cx.dcx().delayed_bug("unexpected escaping late-bound const var");
                return ControlFlow::Break(guar);
            }
            _ if ct.has_param() || ct.has_bound_vars() => return ct.super_visit_with(self),
            _ => {}
        }
        ControlFlow::Continue(())
    }
}