rustc_hir_analysis/hir_ty_lowering/

dyn_compatibility.rs

use rustc_data_structures::fx::{FxHashSet, 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_lint_defs::builtin::UNUSED_ASSOCIATED_TYPE_BOUNDS;
use rustc_middle::ty::elaborate::ClauseWithSupertraitSpan;
use rustc_middle::ty::{
    self, BottomUpFolder, DynKind, ExistentialPredicateStableCmpExt as _, Ty, TyCtxt, TypeFoldable,
    TypeVisitableExt, Upcast,
};
use rustc_span::{ErrorGuaranteed, Span};
use rustc_trait_selection::error_reporting::traits::report_dyn_incompatibility;
use rustc_trait_selection::traits::{self, hir_ty_lowering_dyn_compatibility_violations};
use smallvec::{SmallVec, smallvec};
use tracing::{debug, instrument};

use super::HirTyLowerer;
use crate::hir_ty_lowering::{
    GenericArgCountMismatch, GenericArgCountResult, PredicateFilter, RegionInferReason,
};

impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
    /// Lower a trait object type from the HIR to our internal notion of a type.
    #[instrument(level = "debug", skip_all, ret)]
    pub(super) fn lower_trait_object_ty(
        &self,
        span: Span,
        hir_id: hir::HirId,
        hir_bounds: &[hir::PolyTraitRef<'tcx>],
        lifetime: &hir::Lifetime,
        representation: DynKind,
    ) -> Ty<'tcx> {
        let tcx = self.tcx();
        let dummy_self = tcx.types.trait_object_dummy_self;

        let mut user_written_bounds = Vec::new();
        let mut potential_assoc_types = Vec::new();
        for trait_bound in hir_bounds.iter() {
            if let hir::BoundPolarity::Maybe(_) = trait_bound.modifiers.polarity {
                continue;
            }
            if let GenericArgCountResult {
                correct:
                    Err(GenericArgCountMismatch { invalid_args: cur_potential_assoc_types, .. }),
                ..
            } = self.lower_poly_trait_ref(
                &trait_bound.trait_ref,
                trait_bound.span,
                hir::BoundConstness::Never,
                hir::BoundPolarity::Positive,
                dummy_self,
                &mut user_written_bounds,
                PredicateFilter::SelfOnly,
            ) {
                potential_assoc_types.extend(cur_potential_assoc_types);
            }
        }

        let (elaborated_trait_bounds, elaborated_projection_bounds) =
            traits::expand_trait_aliases(tcx, user_written_bounds.iter().copied());
        let (regular_traits, mut auto_traits): (Vec<_>, Vec<_>) = elaborated_trait_bounds
            .into_iter()
            .partition(|(trait_ref, _)| !tcx.trait_is_auto(trait_ref.def_id()));

        // We  don't support empty trait objects.
        if regular_traits.is_empty() && auto_traits.is_empty() {
            let guar = self.report_trait_object_with_no_traits_error(
                span,
                user_written_bounds.iter().copied(),
            );
            return Ty::new_error(tcx, guar);
        }
        // We don't support >1 principal
        if regular_traits.len() > 1 {
            let guar = self.report_trait_object_addition_traits_error(&regular_traits);
            return Ty::new_error(tcx, guar);
        }
        // Don't create a dyn trait if we have errors in the principal.
        if let Err(guar) = regular_traits.error_reported() {
            return Ty::new_error(tcx, guar);
        }

        // Check that there are no gross dyn-compatibility violations;
        // most importantly, that the supertraits don't contain `Self`,
        // to avoid ICEs.
        for (clause, span) in user_written_bounds {
            if let Some(trait_pred) = clause.as_trait_clause() {
                let violations =
                    hir_ty_lowering_dyn_compatibility_violations(tcx, trait_pred.def_id());
                if !violations.is_empty() {
                    let reported = report_dyn_incompatibility(
                        tcx,
                        span,
                        Some(hir_id),
                        trait_pred.def_id(),
                        &violations,
                    )
                    .emit();
                    return Ty::new_error(tcx, reported);
                }
            }
        }

        // Map the projection bounds onto a key that makes it easy to remove redundant
        // bounds that are constrained by supertraits of the principal def id.
        //
        // Also make sure we detect conflicting bounds from expanding a trait alias and
        // also specifying it manually, like:
        // ```
        // type Alias = Trait<Assoc = i32>;
        // let _: &dyn Alias<Assoc = u32> = /* ... */;
        // ```
        let mut projection_bounds = FxIndexMap::default();
        for (proj, proj_span) in elaborated_projection_bounds {
            let key = (
                proj.skip_binder().projection_term.def_id,
                tcx.anonymize_bound_vars(
                    proj.map_bound(|proj| proj.projection_term.trait_ref(tcx)),
                ),
            );
            if let Some((old_proj, old_proj_span)) =
                projection_bounds.insert(key, (proj, proj_span))
                && tcx.anonymize_bound_vars(proj) != tcx.anonymize_bound_vars(old_proj)
            {
                let item = tcx.item_name(proj.item_def_id());
                self.dcx()
                    .struct_span_err(
                        span,
                        format!(
                            "conflicting associated type bounds for `{item}` when \
                            expanding trait alias"
                        ),
                    )
                    .with_span_label(
                        old_proj_span,
                        format!("`{item}` is specified to be `{}` here", old_proj.term()),
                    )
                    .with_span_label(
                        proj_span,
                        format!("`{item}` is specified to be `{}` here", proj.term()),
                    )
                    .emit();
            }
        }

        let principal_trait = regular_traits.into_iter().next();

        // A stable ordering of associated types from the principal trait and all its
        // supertraits. We use this to ensure that different substitutions of a trait
        // don't result in `dyn Trait` types with different projections lists, which
        // can be unsound: <https://github.com/rust-lang/rust/pull/136458>.
        // We achieve a stable ordering by walking over the unsubstituted principal
        // trait ref.
        let mut ordered_associated_types = vec![];

        if let Some((principal_trait, ref spans)) = principal_trait {
            let principal_trait = principal_trait.map_bound(|trait_pred| {
                assert_eq!(trait_pred.polarity, ty::PredicatePolarity::Positive);
                trait_pred.trait_ref
            });

            for ClauseWithSupertraitSpan { clause, supertrait_span } in traits::elaborate(
                tcx,
                [ClauseWithSupertraitSpan::new(
                    ty::TraitRef::identity(tcx, principal_trait.def_id()).upcast(tcx),
                    *spans.last().unwrap(),
                )],
            )
            .filter_only_self()
            {
                let clause = clause.instantiate_supertrait(tcx, principal_trait);
                debug!("observing object predicate `{clause:?}`");

                let bound_predicate = clause.kind();
                match bound_predicate.skip_binder() {
                    ty::ClauseKind::Trait(pred) => {
                        // FIXME(negative_bounds): Handle this correctly...
                        let trait_ref =
                            tcx.anonymize_bound_vars(bound_predicate.rebind(pred.trait_ref));
                        ordered_associated_types.extend(
                            tcx.associated_items(pred.trait_ref.def_id)
                                .in_definition_order()
                                // We only care about associated types.
                                .filter(|item| item.kind == ty::AssocKind::Type)
                                // No RPITITs -- they're not dyn-compatible for now.
                                .filter(|item| !item.is_impl_trait_in_trait())
                                .map(|item| (item.def_id, trait_ref)),
                        );
                    }
                    ty::ClauseKind::Projection(pred) => {
                        let pred = bound_predicate.rebind(pred);
                        // A `Self` within the original bound will be instantiated with a
                        // `trait_object_dummy_self`, so check for that.
                        let references_self = match pred.skip_binder().term.unpack() {
                            ty::TermKind::Ty(ty) => ty.walk().any(|arg| arg == dummy_self.into()),
                            // FIXME(associated_const_equality): We should walk the const instead of not doing anything
                            ty::TermKind::Const(_) => false,
                        };

                        // If the projection output contains `Self`, force the user to
                        // elaborate it explicitly to avoid a lot of complexity.
                        //
                        // The "classically useful" case is the following:
                        // ```
                        //     trait MyTrait: FnMut() -> <Self as MyTrait>::MyOutput {
                        //         type MyOutput;
                        //     }
                        // ```
                        //
                        // Here, the user could theoretically write `dyn MyTrait<MyOutput = X>`,
                        // but actually supporting that would "expand" to an infinitely-long type
                        // `fix $ τ → dyn MyTrait<MyOutput = X, Output = <τ as MyTrait>::MyOutput`.
                        //
                        // Instead, we force the user to write
                        // `dyn MyTrait<MyOutput = X, Output = X>`, which is uglier but works. See
                        // the discussion in #56288 for alternatives.
                        if !references_self {
                            let key = (
                                pred.skip_binder().projection_term.def_id,
                                tcx.anonymize_bound_vars(
                                    pred.map_bound(|proj| proj.projection_term.trait_ref(tcx)),
                                ),
                            );
                            if !projection_bounds.contains_key(&key) {
                                projection_bounds.insert(key, (pred, supertrait_span));
                            }
                        }

                        self.check_elaborated_projection_mentions_input_lifetimes(
                            pred,
                            *spans.first().unwrap(),
                            supertrait_span,
                        );
                    }
                    _ => (),
                }
            }
        }

        // `dyn Trait<Assoc = Foo>` desugars to (not Rust syntax) `dyn Trait where
        // <Self as Trait>::Assoc = Foo`. So every `Projection` clause is an
        // `Assoc = Foo` bound. `needed_associated_types` contains all associated
        // types that we expect to be provided by the user, so the following loop
        // removes all the associated types that have a corresponding `Projection`
        // clause, either from expanding trait aliases or written by the user.
        for &(projection_bound, span) in projection_bounds.values() {
            let def_id = projection_bound.item_def_id();
            if tcx.generics_require_sized_self(def_id) {
                tcx.emit_node_span_lint(
                    UNUSED_ASSOCIATED_TYPE_BOUNDS,
                    hir_id,
                    span,
                    crate::errors::UnusedAssociatedTypeBounds { span },
                );
            }
        }

        // We compute the list of projection bounds taking the ordered associated types,
        // and check if there was an entry in the collected `projection_bounds`. Those
        // are computed by first taking the user-written associated types, then elaborating
        // the principal trait ref, and only using those if there was no user-written.
        // See note below about how we handle missing associated types with `Self: Sized`,
        // which are not required to be provided, but are still used if they are provided.
        let mut missing_assoc_types = FxIndexSet::default();
        let projection_bounds: Vec<_> = ordered_associated_types
            .into_iter()
            .filter_map(|key| {
                if let Some(assoc) = projection_bounds.get(&key) {
                    Some(*assoc)
                } else {
                    // If the associated type has a `where Self: Sized` bound, then
                    // we do not need to provide the associated type. This results in
                    // a `dyn Trait` type that has a different number of projection
                    // bounds, which may lead to type mismatches.
                    if !tcx.generics_require_sized_self(key.0) {
                        missing_assoc_types.insert(key);
                    }
                    None
                }
            })
            .collect();

        if let Err(guar) = self.check_for_required_assoc_tys(
            principal_trait.as_ref().map_or(smallvec![], |(_, spans)| spans.clone()),
            missing_assoc_types,
            potential_assoc_types,
            hir_bounds,
        ) {
            return Ty::new_error(tcx, guar);
        }

        // De-duplicate auto traits so that, e.g., `dyn Trait + Send + Send` is the same as
        // `dyn Trait + Send`.
        // We remove duplicates by inserting into a `FxHashSet` to avoid re-ordering
        // the bounds
        let mut duplicates = FxHashSet::default();
        auto_traits.retain(|(trait_pred, _)| duplicates.insert(trait_pred.def_id()));

        debug!(?principal_trait);
        debug!(?auto_traits);

        // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
        let principal_trait_ref = principal_trait.map(|(trait_pred, spans)| {
            trait_pred.map_bound(|trait_pred| {
                let trait_ref = trait_pred.trait_ref;
                assert_eq!(trait_pred.polarity, ty::PredicatePolarity::Positive);
                assert_eq!(trait_ref.self_ty(), dummy_self);

                let span = *spans.first().unwrap();

                // Verify that `dummy_self` did not leak inside default type parameters. This
                // could not be done at path creation, since we need to see through trait aliases.
                let mut missing_type_params = vec![];
                let generics = tcx.generics_of(trait_ref.def_id);
                let args: Vec<_> = trait_ref
                    .args
                    .iter()
                    .enumerate()
                    // Skip `Self`
                    .skip(1)
                    .map(|(index, arg)| {
                        if arg.walk().any(|arg| arg == dummy_self.into()) {
                            let param = &generics.own_params[index];
                            missing_type_params.push(param.name);
                            Ty::new_misc_error(tcx).into()
                        } else {
                            arg
                        }
                    })
                    .collect();

                let empty_generic_args = hir_bounds.iter().any(|hir_bound| {
                    hir_bound.trait_ref.path.res == Res::Def(DefKind::Trait, trait_ref.def_id)
                        && hir_bound.span.contains(span)
                });
                self.complain_about_missing_type_params(
                    missing_type_params,
                    trait_ref.def_id,
                    span,
                    empty_generic_args,
                );

                ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::new(
                    tcx,
                    trait_ref.def_id,
                    args,
                ))
            })
        });

        let existential_projections = projection_bounds.into_iter().map(|(bound, _)| {
            bound.map_bound(|mut b| {
                assert_eq!(b.projection_term.self_ty(), dummy_self);

                // Like for trait refs, verify that `dummy_self` did not leak inside default type
                // parameters.
                let references_self = b.projection_term.args.iter().skip(1).any(|arg| {
                    if arg.walk().any(|arg| arg == dummy_self.into()) {
                        return true;
                    }
                    false
                });
                if references_self {
                    let guar = tcx
                        .dcx()
                        .span_delayed_bug(span, "trait object projection bounds reference `Self`");
                    b.projection_term = replace_dummy_self_with_error(tcx, b.projection_term, guar);
                }

                ty::ExistentialPredicate::Projection(ty::ExistentialProjection::erase_self_ty(
                    tcx, b,
                ))
            })
        });

        let mut auto_trait_predicates: Vec<_> = auto_traits
            .into_iter()
            .map(|(trait_pred, _)| {
                assert_eq!(trait_pred.polarity(), ty::PredicatePolarity::Positive);
                assert_eq!(trait_pred.self_ty().skip_binder(), dummy_self);

                ty::Binder::dummy(ty::ExistentialPredicate::AutoTrait(trait_pred.def_id()))
            })
            .collect();
        auto_trait_predicates.dedup();

        // N.b. principal, projections, auto traits
        // FIXME: This is actually wrong with multiple principals in regards to symbol mangling
        let mut v = principal_trait_ref
            .into_iter()
            .chain(existential_projections)
            .chain(auto_trait_predicates)
            .collect::<SmallVec<[_; 8]>>();
        v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
        let existential_predicates = tcx.mk_poly_existential_predicates(&v);

        // Use explicitly-specified region bound, unless the bound is missing.
        let region_bound = if !lifetime.is_elided() {
            self.lower_lifetime(lifetime, RegionInferReason::ExplicitObjectLifetime)
        } else {
            self.compute_object_lifetime_bound(span, existential_predicates).unwrap_or_else(|| {
                // Curiously, we prefer object lifetime default for `+ '_`...
                if tcx.named_bound_var(lifetime.hir_id).is_some() {
                    self.lower_lifetime(lifetime, RegionInferReason::ExplicitObjectLifetime)
                } else {
                    let reason =
                        if let hir::LifetimeName::ImplicitObjectLifetimeDefault = lifetime.res {
                            if let hir::Node::Ty(hir::Ty {
                                kind: hir::TyKind::Ref(parent_lifetime, _),
                                ..
                            }) = tcx.parent_hir_node(hir_id)
                                && tcx.named_bound_var(parent_lifetime.hir_id).is_none()
                            {
                                // Parent lifetime must have failed to resolve. Don't emit a redundant error.
                                RegionInferReason::ExplicitObjectLifetime
                            } else {
                                RegionInferReason::ObjectLifetimeDefault
                            }
                        } else {
                            RegionInferReason::ExplicitObjectLifetime
                        };
                    self.re_infer(span, reason)
                }
            })
        };
        debug!(?region_bound);

        Ty::new_dynamic(tcx, existential_predicates, region_bound, representation)
    }

    /// Check that elaborating the principal of a trait ref doesn't lead to projections
    /// that are unconstrained. This can happen because an otherwise unconstrained
    /// *type variable* can be substituted with a type that has late-bound regions. See
    /// `elaborated-predicates-unconstrained-late-bound.rs` for a test.
    fn check_elaborated_projection_mentions_input_lifetimes(
        &self,
        pred: ty::PolyProjectionPredicate<'tcx>,
        span: Span,
        supertrait_span: Span,
    ) {
        let tcx = self.tcx();

        // 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_term =
            tcx.collect_constrained_late_bound_regions(pred.map_bound(|pred| pred.projection_term));
        let late_bound_in_term =
            tcx.collect_referenced_late_bound_regions(pred.map_bound(|pred| pred.term));
        debug!(?late_bound_in_projection_term);
        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(associated_const_equality): This error should be impossible to trigger
        //                                  with associated const equality constraints.
        self.validate_late_bound_regions(
            late_bound_in_projection_term,
            late_bound_in_term,
            |br_name| {
                let item_name = tcx.item_name(pred.item_def_id());
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0582,
                    "binding for associated type `{}` references {}, \
                             which does not appear in the trait input types",
                    item_name,
                    br_name
                )
                .with_span_label(supertrait_span, "due to this supertrait")
            },
        );
    }
}

fn replace_dummy_self_with_error<'tcx, T: TypeFoldable<TyCtxt<'tcx>>>(
    tcx: TyCtxt<'tcx>,
    t: T,
    guar: ErrorGuaranteed,
) -> T {
    t.fold_with(&mut BottomUpFolder {
        tcx,
        ty_op: |ty| {
            if ty == tcx.types.trait_object_dummy_self { Ty::new_error(tcx, guar) } else { ty }
        },
        lt_op: |lt| lt,
        ct_op: |ct| ct,
    })
}