hir_ty/

dyn_compatibility.rs

//! Compute the dyn-compatibility of a trait

use std::ops::ControlFlow;

use hir_def::{
    AssocItemId, ConstId, FunctionId, GenericDefId, HasModule, TraitId, TypeAliasId,
    TypeOrConstParamId, TypeParamId,
    hir::generics::{GenericParams, LocalTypeOrConstParamId},
    signatures::{FunctionSignature, TraitFlags, TraitSignature},
    unstable_features::UnstableFeatures,
};
use rustc_hash::FxHashSet;
use rustc_type_ir::{
    AliasTyKind, ClauseKind, PredicatePolarity, TypeSuperVisitable as _, TypeVisitable as _,
    Upcast, elaborate, inherent::IntoKind,
};
use smallvec::SmallVec;

use crate::{
    ImplTraitId,
    db::{HirDatabase, InternedOpaqueTyId},
    lower::{GenericPredicates, associated_ty_item_bounds},
    next_solver::{
        AliasTy, Binder, Clause, Clauses, DbInterner, EarlyBinder, GenericArgs, ParamEnv, ParamTy,
        SolverDefId, TraitPredicate, TraitRef, Ty, TypingMode, Unnormalized,
        infer::{
            DbInternerInferExt,
            traits::{Obligation, ObligationCause},
        },
        mk_param,
    },
};

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum DynCompatibilityViolation {
    SizedSelf,
    SelfReferential,
    Method(FunctionId, MethodViolationCode),
    AssocConst(ConstId),
    GAT(TypeAliasId),
    // This doesn't exist in rustc, but added for better visualization
    HasNonCompatibleSuperTrait(TraitId),
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum MethodViolationCode {
    StaticMethod,
    ReferencesSelfInput,
    ReferencesSelfOutput,
    ReferencesImplTraitInTrait,
    AsyncFn,
    WhereClauseReferencesSelf,
    Generic,
    UndispatchableReceiver,
}

pub fn dyn_compatibility(
    db: &dyn HirDatabase,
    trait_: TraitId,
) -> Option<DynCompatibilityViolation> {
    let interner = DbInterner::new_no_crate(db);
    for super_trait in elaborate::supertrait_def_ids(interner, trait_.into()) {
        if let Some(v) = db.dyn_compatibility_of_trait(super_trait.0) {
            return if super_trait.0 == trait_ {
                Some(v)
            } else {
                Some(DynCompatibilityViolation::HasNonCompatibleSuperTrait(super_trait.0))
            };
        }
    }

    None
}

pub fn dyn_compatibility_with_callback<F>(
    db: &dyn HirDatabase,
    trait_: TraitId,
    cb: &mut F,
) -> ControlFlow<()>
where
    F: FnMut(DynCompatibilityViolation) -> ControlFlow<()>,
{
    let interner = DbInterner::new_no_crate(db);
    for super_trait in elaborate::supertrait_def_ids(interner, trait_.into()).skip(1) {
        if db.dyn_compatibility_of_trait(super_trait.0).is_some() {
            cb(DynCompatibilityViolation::HasNonCompatibleSuperTrait(trait_))?;
        }
    }

    dyn_compatibility_of_trait_with_callback(db, trait_, cb)
}

pub fn dyn_compatibility_of_trait_with_callback<F>(
    db: &dyn HirDatabase,
    trait_: TraitId,
    cb: &mut F,
) -> ControlFlow<()>
where
    F: FnMut(DynCompatibilityViolation) -> ControlFlow<()>,
{
    // Check whether this has a `Sized` bound
    if generics_require_sized_self(db, trait_.into()) {
        cb(DynCompatibilityViolation::SizedSelf)?;
    }

    // Check if there exist bounds that referencing self
    if predicates_reference_self(db, trait_) {
        cb(DynCompatibilityViolation::SelfReferential)?;
    }
    if bounds_reference_self(db, trait_) {
        cb(DynCompatibilityViolation::SelfReferential)?;
    }

    // rustc checks for non-lifetime binders here, but we don't support HRTB yet

    let trait_data = trait_.trait_items(db);
    let mut features = None;
    for (_, assoc_item) in &trait_data.items {
        dyn_compatibility_violation_for_assoc_item(db, &mut features, trait_, *assoc_item, cb)?;
    }

    ControlFlow::Continue(())
}

#[salsa::tracked]
pub fn dyn_compatibility_of_trait_query(
    db: &dyn HirDatabase,
    trait_: TraitId,
) -> Option<DynCompatibilityViolation> {
    let mut res = None;
    _ = dyn_compatibility_of_trait_with_callback(db, trait_, &mut |osv| {
        res = Some(osv);
        ControlFlow::Break(())
    });

    res
}

pub fn generics_require_sized_self(db: &dyn HirDatabase, def: GenericDefId) -> bool {
    let krate = def.module(db).krate(db);
    let interner = DbInterner::new_with(db, krate);
    let Some(sized) = interner.lang_items().Sized else {
        return false;
    };

    let predicates = GenericPredicates::query_explicit(db, def);
    // FIXME: We should use `explicit_predicates_of` here, which hasn't been implemented to
    // rust-analyzer yet
    // https://github.com/rust-lang/rust/blob/ddaf12390d3ffb7d5ba74491a48f3cd528e5d777/compiler/rustc_hir_analysis/src/collect/predicates_of.rs#L490
    elaborate::elaborate(interner, predicates.iter_identity().map(Unnormalized::skip_norm_wip)).any(
        |pred| match pred.kind().skip_binder() {
            ClauseKind::Trait(trait_pred) => {
                if sized == trait_pred.def_id().0
                    && let rustc_type_ir::TyKind::Param(param_ty) =
                        trait_pred.trait_ref.self_ty().kind()
                    && param_ty.index == 0
                {
                    true
                } else {
                    false
                }
            }
            _ => false,
        },
    )
}

// rustc gathers all the spans that references `Self` for error rendering,
// but we don't have good way to render such locations.
// So, just return single boolean value for existence of such `Self` reference
fn predicates_reference_self(db: &dyn HirDatabase, trait_: TraitId) -> bool {
    GenericPredicates::query_explicit(db, trait_.into()).iter_identity().any(|pred| {
        predicate_references_self(db, trait_, pred.skip_norm_wip(), AllowSelfProjection::No)
    })
}

// Same as the above, `predicates_reference_self`
fn bounds_reference_self(db: &dyn HirDatabase, trait_: TraitId) -> bool {
    let trait_data = trait_.trait_items(db);
    trait_data
        .items
        .iter()
        .filter_map(|(_, it)| match *it {
            AssocItemId::TypeAliasId(id) => Some(associated_ty_item_bounds(db, id)),
            _ => None,
        })
        .any(|bounds| {
            bounds.skip_binder().iter().any(|pred| match pred.skip_binder() {
                rustc_type_ir::ExistentialPredicate::Trait(it) => it.args.iter().any(|arg| {
                    contains_illegal_self_type_reference(db, trait_, &arg, AllowSelfProjection::Yes)
                }),
                rustc_type_ir::ExistentialPredicate::Projection(it) => it.args.iter().any(|arg| {
                    contains_illegal_self_type_reference(db, trait_, &arg, AllowSelfProjection::Yes)
                }),
                rustc_type_ir::ExistentialPredicate::AutoTrait(_) => false,
            })
        })
}

#[derive(Clone, Copy)]
enum AllowSelfProjection {
    Yes,
    No,
}

fn predicate_references_self<'db>(
    db: &'db dyn HirDatabase,
    trait_: TraitId,
    predicate: Clause<'db>,
    allow_self_projection: AllowSelfProjection,
) -> bool {
    match predicate.kind().skip_binder() {
        ClauseKind::Trait(trait_pred) => trait_pred.trait_ref.args.iter().skip(1).any(|arg| {
            contains_illegal_self_type_reference(db, trait_, &arg, allow_self_projection)
        }),
        ClauseKind::Projection(proj_pred) => {
            proj_pred.projection_term.args.iter().skip(1).any(|arg| {
                contains_illegal_self_type_reference(db, trait_, &arg, allow_self_projection)
            })
        }
        _ => false,
    }
}

fn contains_illegal_self_type_reference<'db, T: rustc_type_ir::TypeVisitable<DbInterner<'db>>>(
    db: &'db dyn HirDatabase,
    trait_: TraitId,
    t: &T,
    allow_self_projection: AllowSelfProjection,
) -> bool {
    struct IllegalSelfTypeVisitor<'db> {
        db: &'db dyn HirDatabase,
        trait_: TraitId,
        super_traits: Option<SmallVec<[TraitId; 4]>>,
        allow_self_projection: AllowSelfProjection,
    }
    impl<'db> rustc_type_ir::TypeVisitor<DbInterner<'db>> for IllegalSelfTypeVisitor<'db> {
        type Result = ControlFlow<()>;

        fn visit_ty(
            &mut self,
            ty: <DbInterner<'db> as rustc_type_ir::Interner>::Ty,
        ) -> Self::Result {
            let interner = DbInterner::new_no_crate(self.db);
            match ty.kind() {
                rustc_type_ir::TyKind::Param(param) if param.index == 0 => ControlFlow::Break(()),
                rustc_type_ir::TyKind::Param(_) => ControlFlow::Continue(()),
                rustc_type_ir::TyKind::Alias(
                    proj @ AliasTy { kind: AliasTyKind::Projection { .. }, .. },
                ) => match self.allow_self_projection {
                    AllowSelfProjection::Yes => {
                        let trait_ = proj.trait_def_id(interner).0;
                        if self.super_traits.is_none() {
                            self.super_traits = Some(
                                elaborate::supertrait_def_ids(interner, self.trait_.into())
                                    .map(|super_trait| super_trait.0)
                                    .collect(),
                            )
                        }
                        if self.super_traits.as_ref().is_some_and(|s| s.contains(&trait_)) {
                            ControlFlow::Continue(())
                        } else {
                            ty.super_visit_with(self)
                        }
                    }
                    AllowSelfProjection::No => ty.super_visit_with(self),
                },
                _ => ty.super_visit_with(self),
            }
        }
    }

    let mut visitor =
        IllegalSelfTypeVisitor { db, trait_, super_traits: None, allow_self_projection };
    t.visit_with(&mut visitor).is_break()
}

fn dyn_compatibility_violation_for_assoc_item<'db, F>(
    db: &'db dyn HirDatabase,
    features: &mut Option<&'db UnstableFeatures>,
    trait_: TraitId,
    item: AssocItemId,
    cb: &mut F,
) -> ControlFlow<()>
where
    F: FnMut(DynCompatibilityViolation) -> ControlFlow<()>,
{
    // Any item that has a `Self : Sized` requisite is otherwise
    // exempt from the regulations.
    if generics_require_sized_self(db, item.into()) {
        return ControlFlow::Continue(());
    }

    match item {
        AssocItemId::ConstId(it) => cb(DynCompatibilityViolation::AssocConst(it)),
        AssocItemId::FunctionId(it) => {
            virtual_call_violations_for_method(db, trait_, it, &mut |mvc| {
                cb(DynCompatibilityViolation::Method(it, mvc))
            })
        }
        AssocItemId::TypeAliasId(it) => {
            if features
                .get_or_insert_with(|| UnstableFeatures::query(db, trait_.krate(db)))
                .generic_associated_type_extended
            {
                ControlFlow::Continue(())
            } else {
                let generic_params = GenericParams::of(db, item.into());
                if !generic_params.is_empty() {
                    cb(DynCompatibilityViolation::GAT(it))
                } else {
                    ControlFlow::Continue(())
                }
            }
        }
    }
}

fn virtual_call_violations_for_method<F>(
    db: &dyn HirDatabase,
    trait_: TraitId,
    func: FunctionId,
    cb: &mut F,
) -> ControlFlow<()>
where
    F: FnMut(MethodViolationCode) -> ControlFlow<()>,
{
    let func_data = FunctionSignature::of(db, func);
    if !func_data.has_self_param() {
        cb(MethodViolationCode::StaticMethod)?;
    }

    if func_data.is_async() {
        cb(MethodViolationCode::AsyncFn)?;
    }

    let sig = db.callable_item_signature(func.into());
    if sig.skip_binder().inputs().iter().skip(1).any(|ty| {
        contains_illegal_self_type_reference(db, trait_, ty.skip_binder(), AllowSelfProjection::Yes)
    }) {
        cb(MethodViolationCode::ReferencesSelfInput)?;
    }

    if contains_illegal_self_type_reference(
        db,
        trait_,
        &sig.skip_binder().output(),
        AllowSelfProjection::Yes,
    ) {
        cb(MethodViolationCode::ReferencesSelfOutput)?;
    }

    if !func_data.is_async()
        && let Some(mvc) = contains_illegal_impl_trait_in_trait(db, &sig)
    {
        cb(mvc)?;
    }

    let generic_params = GenericParams::of(db, func.into());
    if generic_params.len_type_or_consts() > 0 {
        cb(MethodViolationCode::Generic)?;
    }

    if func_data.has_self_param() && !receiver_is_dispatchable(db, trait_, func, &sig) {
        cb(MethodViolationCode::UndispatchableReceiver)?;
    }

    let predicates = GenericPredicates::query_own_explicit(db, func.into());
    for pred in predicates.iter_identity() {
        let pred = pred.kind().skip_binder();

        if matches!(pred, ClauseKind::TypeOutlives(_)) {
            continue;
        }

        // Allow `impl AutoTrait` predicates
        if let ClauseKind::Trait(TraitPredicate {
            trait_ref: pred_trait_ref,
            polarity: PredicatePolarity::Positive,
        }) = pred
            && let trait_data = TraitSignature::of(db, pred_trait_ref.def_id.0)
            && trait_data.flags.contains(TraitFlags::AUTO)
            && let rustc_type_ir::TyKind::Param(ParamTy { index: 0, .. }) =
                pred_trait_ref.self_ty().kind()
        {
            continue;
        }

        if contains_illegal_self_type_reference(db, trait_, &pred, AllowSelfProjection::Yes) {
            cb(MethodViolationCode::WhereClauseReferencesSelf)?;
            break;
        }
    }

    ControlFlow::Continue(())
}

fn receiver_is_dispatchable<'db>(
    db: &'db dyn HirDatabase,
    trait_: TraitId,
    func: FunctionId,
    sig: &EarlyBinder<'db, Binder<'db, rustc_type_ir::FnSig<DbInterner<'db>>>>,
) -> bool {
    let sig = sig.instantiate_identity().skip_norm_wip();

    let module = trait_.module(db);
    let interner = DbInterner::new_with(db, module.krate(db));
    let self_param_id = TypeParamId::from_unchecked(TypeOrConstParamId {
        parent: trait_.into(),
        local_id: LocalTypeOrConstParamId::from_raw(la_arena::RawIdx::from_u32(0)),
    });
    let self_param_ty =
        Ty::new(interner, rustc_type_ir::TyKind::Param(ParamTy { index: 0, id: self_param_id }));

    // `self: Self` can't be dispatched on, but this is already considered dyn-compatible
    // See rustc's comment on https://github.com/rust-lang/rust/blob/3f121b9461cce02a703a0e7e450568849dfaa074/compiler/rustc_trait_selection/src/traits/object_safety.rs#L433-L437
    if sig.inputs().iter().next().is_some_and(|p| *p.skip_binder() == self_param_ty) {
        return true;
    }

    let receiver_ty = interner.liberate_late_bound_regions(func.into(), sig.input(0));

    let lang_items = interner.lang_items();
    let traits = (lang_items.Unsize, lang_items.DispatchFromDyn);
    let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
        return false;
    };

    let meta_sized_did = lang_items.MetaSized;

    // TODO: This is for supporting dyn compatibility for toolchains doesn't contain `MetaSized`
    // trait. Uncomment and short circuit here once `MINIMUM_SUPPORTED_TOOLCHAIN_VERSION`
    // become > 1.88.0
    //
    // let Some(meta_sized_did) = meta_sized_did else {
    //     return false;
    // };

    // Type `U`
    // FIXME: That seems problematic to fake a generic param like that?
    let unsized_self_ty = Ty::new_param(interner, self_param_id, u32::MAX);
    // `Receiver[Self => U]`
    let unsized_receiver_ty = receiver_for_self_ty(interner, func, receiver_ty, unsized_self_ty);

    let param_env = {
        let generic_predicates = GenericPredicates::query_all(db, func.into());

        // Self: Unsize<U>
        let unsize_predicate =
            TraitRef::new(interner, unsize_did.into(), [self_param_ty, unsized_self_ty]);

        // U: Trait<Arg1, ..., ArgN>
        let args = GenericArgs::for_item(interner, trait_.into(), |index, kind, _, _| {
            if index == 0 { unsized_self_ty.into() } else { mk_param(interner, index, kind) }
        });
        let trait_predicate = TraitRef::new_from_args(interner, trait_.into(), args);

        let meta_sized_predicate = meta_sized_did
            .map(|did| TraitRef::new(interner, did.into(), [unsized_self_ty]).upcast(interner));

        ParamEnv {
            clauses: Clauses::new_from_iter(
                interner,
                generic_predicates
                    .iter_identity()
                    .map(Unnormalized::skip_norm_wip)
                    .chain([unsize_predicate.upcast(interner), trait_predicate.upcast(interner)])
                    .chain(meta_sized_predicate),
            ),
        }
    };

    // Receiver: DispatchFromDyn<Receiver[Self => U]>
    let predicate =
        TraitRef::new(interner, dispatch_from_dyn_did.into(), [receiver_ty, unsized_receiver_ty]);
    let obligation = Obligation::new(interner, ObligationCause::dummy(), param_env, predicate);

    let infcx = interner.infer_ctxt().build(TypingMode::non_body_analysis());
    // the receiver is dispatchable iff the obligation holds
    infcx.predicate_must_hold_modulo_regions(&obligation)
}

fn receiver_for_self_ty<'db>(
    interner: DbInterner<'db>,
    func: FunctionId,
    receiver_ty: Ty<'db>,
    self_ty: Ty<'db>,
) -> Ty<'db> {
    let args =
        GenericArgs::for_item(interner, SolverDefId::FunctionId(func), |index, kind, _, _| {
            if index == 0 { self_ty.into() } else { mk_param(interner, index, kind) }
        });

    EarlyBinder::bind(receiver_ty).instantiate(interner, args).skip_norm_wip()
}

fn contains_illegal_impl_trait_in_trait<'db>(
    db: &'db dyn HirDatabase,
    sig: &EarlyBinder<'db, Binder<'db, rustc_type_ir::FnSig<DbInterner<'db>>>>,
) -> Option<MethodViolationCode> {
    struct OpaqueTypeCollector<'db>(FxHashSet<InternedOpaqueTyId<'db>>);

    impl<'db> rustc_type_ir::TypeVisitor<DbInterner<'db>> for OpaqueTypeCollector<'db> {
        type Result = ControlFlow<()>;

        fn visit_ty(
            &mut self,
            ty: <DbInterner<'db> as rustc_type_ir::Interner>::Ty,
        ) -> Self::Result {
            if let rustc_type_ir::TyKind::Alias(AliasTy {
                kind: AliasTyKind::Opaque { def_id },
                ..
            }) = ty.kind()
            {
                self.0.insert(def_id.0);
            }
            ty.super_visit_with(self)
        }
    }

    let ret = sig.skip_binder().output();
    let mut visitor = OpaqueTypeCollector(FxHashSet::default());
    _ = ret.visit_with(&mut visitor);

    // Since we haven't implemented RPITIT in proper way like rustc yet,
    // just check whether `ret` contains RPIT for now
    for opaque_ty in visitor.0 {
        let impl_trait_id = opaque_ty.loc(db);
        if matches!(impl_trait_id, ImplTraitId::ReturnTypeImplTrait(..)) {
            return Some(MethodViolationCode::ReferencesImplTraitInTrait);
        }
    }

    None
}

#[cfg(test)]
mod tests;