hir_ty/next_solver/

interner.rs

//! Things related to the Interner in the next-trait-solver.

use std::{fmt, ops::ControlFlow};

use either::Either;
use intern::{Interned, InternedRef, InternedSliceRef, impl_internable};
use macros::GenericTypeVisitable;
use rustc_abi::ReprOptions;
use rustc_ast_ir::{FloatTy, IntTy, UintTy};
pub use tls_cache::clear_tls_solver_cache;
pub use tls_db::{attach_db, attach_db_allow_change, with_attached_db};

use base_db::Crate;
use hir_def::{
    AdtId, CallableDefId, EnumId, GenericDefId, GenericParamId, HasModule, ItemContainerId,
    StructId, TraitId, TypeAliasId, UnionId, VariantId,
    attrs::AttrFlags,
    expr_store::{ExpressionStore, StoreVisitor},
    hir::{ClosureKind as HirClosureKind, CoroutineKind as HirCoroutineKind, ExprId, PatId},
    lang_item::LangItems,
    signatures::{
        EnumFlags, EnumSignature, FnFlags, FunctionSignature, ImplFlags, ImplSignature,
        StructFlags, StructSignature, TraitFlags, TraitSignature, UnionSignature,
    },
};
use rustc_abi::ExternAbi;
use rustc_index::bit_set::DenseBitSet;
use rustc_type_ir::{
    AliasTy, BoundVar, CoroutineWitnessTypes, DebruijnIndex, EarlyBinder, FlagComputation, Flags,
    FnSigKind, GenericArgKind, GenericTypeVisitable, ImplPolarity, InferTy, Interner, TraitRef,
    TypeFlags, TypeVisitableExt, Variance, VisitorResult,
    error::TypeError,
    fast_reject,
    inherent::{self, Const as _, GenericsOf, IntoKind, SliceLike as _, Span as _, Ty as _},
    lang_items::{SolverAdtLangItem, SolverProjectionLangItem, SolverTraitLangItem},
    solve::{AdtDestructorKind, SizedTraitKind},
    try_visit,
};

use crate::{
    InferBodyId, Span,
    db::{HirDatabase, InternedClosure, InternedCoroutineId},
    lower::GenericPredicates,
    method_resolution::TraitImpls,
    next_solver::{
        AdtIdWrapper, AliasTermKind, AliasTyKind, AnyImplId, BoundConst, CallableIdWrapper,
        CanonicalVarKind, ClosureIdWrapper, Consts, CoroutineClosureIdWrapper, CoroutineIdWrapper,
        Ctor, FnSig, FreeConstAliasId, FreeTermAliasId, FreeTyAliasId, FxIndexMap,
        GeneralConstIdWrapper, ImplOrTraitAssocConstId, ImplOrTraitAssocTermId,
        ImplOrTraitAssocTyId, InherentAssocConstId, InherentAssocTermId, InherentAssocTyId,
        LateParamRegion, OpaqueTyIdWrapper, OpaqueTypeKey, RegionAssumptions, ScalarInt,
        SimplifiedType, SolverContext, SolverDefIds, TermId, TraitAssocConstId, TraitAssocTermId,
        TraitAssocTyId, TraitIdWrapper, TypeAliasIdWrapper, UnevaluatedConst,
        util::{ItemBounds, impl_super_outlives},
    },
};

use super::{
    Binder, BoundExistentialPredicates, BoundTy, BoundTyKind, Clause, ClauseKind, Clauses, Const,
    ErrorGuaranteed, ExprConst, ExternalConstraints, GenericArg, GenericArgs, ParamConst, ParamEnv,
    ParamTy, PredefinedOpaques, Predicate, SolverDefId, Term, Ty, TyKind, Tys, ValTree, ValueConst,
    abi::Safety,
    fold::{BoundVarReplacer, BoundVarReplacerDelegate, FnMutDelegate},
    generics::{Generics, generics},
    region::{BoundRegion, BoundRegionKind, EarlyParamRegion, Region},
    util::sizedness_constraint_for_ty,
};

macro_rules! interned_slice {
    ($storage:ident, $name:ident, $stored_name:ident, $default_types_field:ident, $ty_db:ty, $ty_static:ty $(,)?) => {
        const _: () = {
            #[allow(unused_lifetimes)]
            fn _ensure_correct_types<'db: 'static>(v: $ty_db) -> $ty_static { v }
        };

        ::intern::impl_slice_internable!(gc; $storage, (), $ty_static);

        #[derive(Clone, Copy, PartialEq, Eq, Hash)]
        pub struct $name<'db> {
            interned: ::intern::InternedSliceRef<'db, $storage>,
        }

        impl<'db> std::fmt::Debug for $name<'db> {
            fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                self.as_slice().fmt(fmt)
            }
        }

        impl<'db> $name<'db> {
            #[inline]
            pub fn empty() -> Self {
                $crate::next_solver::default_types().empty.$default_types_field
            }

            #[inline]
            pub fn new_from_slice(slice: &[$ty_db]) -> Self {
                if slice.is_empty() {
                    // Common case: avoid looking up the empty slice.
                    Self::empty()
                } else {
                    Self::new_from_slice_no_empty_check(slice)
                }
            }

            /// Same as [`Self::new_from_slice()`] but won't use the global empty slice `slice.is_empty()`, because someone
            /// needs to intern the global slice as well.
            #[inline]
            pub(crate) fn new_from_slice_no_empty_check(slice: &[$ty_db]) -> Self {
                let slice = unsafe { ::std::mem::transmute::<&[$ty_db], &[$ty_static]>(slice) };
                Self { interned: ::intern::InternedSlice::from_header_and_slice((), slice) }
            }

            #[inline]
            pub fn new_from_iter<I, T>(args: I) -> T::Output
            where
                I: IntoIterator<Item = T>,
                T: ::rustc_type_ir::CollectAndApply<$ty_db, Self>,
            {
                ::rustc_type_ir::CollectAndApply::collect_and_apply(args.into_iter(), |g| {
                    Self::new_from_slice(g)
                })
            }

            #[inline]
            pub fn as_slice(self) -> &'db [$ty_db] {
                let slice = &self.interned.get().slice;
                unsafe { ::std::mem::transmute::<&[$ty_static], &[$ty_db]>(slice) }
            }

            #[inline]
            pub fn iter(self) -> ::std::iter::Copied<::std::slice::Iter<'db, $ty_db>> {
                self.as_slice().iter().copied()
            }

            #[inline]
            pub fn len(self) -> usize {
                self.as_slice().len()
            }

            #[inline]
            pub fn is_empty(self) -> bool {
                self.as_slice().is_empty()
            }
        }

        impl<'db> IntoIterator for $name<'db> {
            type IntoIter = ::std::iter::Copied<::std::slice::Iter<'db, $ty_db>>;
            type Item = $ty_db;
            #[inline]
            fn into_iter(self) -> Self::IntoIter { self.iter() }
        }

        impl<'db> ::std::ops::Deref for $name<'db> {
            type Target = [$ty_db];

            #[inline]
            fn deref(&self) -> &Self::Target {
                (*self).as_slice()
            }
        }

        impl<'db> rustc_type_ir::inherent::SliceLike for $name<'db> {
            type Item = $ty_db;

            type IntoIter = ::std::iter::Copied<::std::slice::Iter<'db, $ty_db>>;

            #[inline]
            fn iter(self) -> Self::IntoIter {
                self.iter()
            }

            #[inline]
            fn as_slice(&self) -> &[Self::Item] {
                (*self).as_slice()
            }
        }

        impl<'db> Default for $name<'db> {
            #[inline]
            fn default() -> Self {
                $name::empty()
            }
        }


        impl<'db, V: $crate::next_solver::interner::WorldExposer>
            rustc_type_ir::GenericTypeVisitable<V> for $name<'db>
        {
            #[inline]
            fn generic_visit_with(&self, visitor: &mut V) {
                if visitor.on_interned_slice(self.interned).is_continue() {
                    self.as_slice().iter().for_each(|it| it.generic_visit_with(visitor));
                }
            }
        }

        $crate::next_solver::interner::impl_stored_interned_slice!($storage, $name, $stored_name);
    };
}
pub(crate) use interned_slice;

macro_rules! impl_stored_interned_slice {
    ( $storage:ident, $name:ident, $stored_name:ident $(,)? ) => {
        #[derive(Clone, PartialEq, Eq, Hash)]
        pub struct $stored_name {
            interned: ::intern::InternedSlice<$storage>,
        }

        impl $stored_name {
            #[inline]
            fn new(it: $name<'_>) -> Self {
                Self { interned: it.interned.to_owned() }
            }

            // FIXME: This transmute is not safe as is!
            #[inline]
            pub fn as_ref<'a, 'db>(&'a self) -> $name<'db> {
                let it = $name { interned: self.interned.as_ref() };
                unsafe { std::mem::transmute::<$name<'a>, $name<'db>>(it) }
            }
        }

        // SAFETY: It is safe to store this type in queries (but not `$name`).
        unsafe impl salsa::SalsaValue for $stored_name {}

        impl std::fmt::Debug for $stored_name {
            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                self.as_ref().fmt(f)
            }
        }

        impl $name<'_> {
            #[inline]
            pub fn store(self) -> $stored_name {
                $stored_name::new(self)
            }
        }
    };
}
pub(crate) use impl_stored_interned_slice;

macro_rules! impl_foldable_for_interned_slice {
    ($name:ident) => {
        impl<'db> ::rustc_type_ir::TypeVisitable<DbInterner<'db>> for $name<'db> {
            fn visit_with<V: rustc_type_ir::TypeVisitor<DbInterner<'db>>>(
                &self,
                visitor: &mut V,
            ) -> V::Result {
                use rustc_ast_ir::visit::VisitorResult;
                rustc_ast_ir::walk_visitable_list!(visitor, (*self).iter());
                V::Result::output()
            }
        }

        impl<'db> rustc_type_ir::TypeFoldable<DbInterner<'db>> for $name<'db> {
            fn try_fold_with<F: rustc_type_ir::FallibleTypeFolder<DbInterner<'db>>>(
                self,
                folder: &mut F,
            ) -> Result<Self, F::Error> {
                Self::new_from_iter(self.iter().map(|it| it.try_fold_with(folder)))
            }
            fn fold_with<F: rustc_type_ir::TypeFolder<DbInterner<'db>>>(
                self,
                folder: &mut F,
            ) -> Self {
                Self::new_from_iter(self.iter().map(|it| it.fold_with(folder)))
            }
        }
    };
}
pub(crate) use impl_foldable_for_interned_slice;

macro_rules! impl_foldable_for_stored_type {
    ($name:ident) => {
        impl<'db> ::rustc_type_ir::TypeVisitable<DbInterner<'db>> for $name {
            fn visit_with<V: rustc_type_ir::TypeVisitor<DbInterner<'db>>>(
                &self,
                visitor: &mut V,
            ) -> V::Result {
                self.as_ref().visit_with(visitor)
            }
        }

        impl<'db> rustc_type_ir::TypeFoldable<DbInterner<'db>> for $name {
            fn try_fold_with<F: rustc_type_ir::FallibleTypeFolder<DbInterner<'db>>>(
                self,
                folder: &mut F,
            ) -> Result<Self, F::Error> {
                Ok(self.as_ref().try_fold_with(folder)?.store())
            }
            fn fold_with<F: rustc_type_ir::TypeFolder<DbInterner<'db>>>(
                self,
                folder: &mut F,
            ) -> Self {
                self.as_ref().fold_with(folder).store()
            }
        }
    };
}
pub(crate) use impl_foldable_for_stored_type;

macro_rules! impl_stored_interned {
    ( $storage:ident, $name:ident, $stored_name:ident $(,)? ) => {
        #[derive(Clone, PartialEq, Eq, Hash, ::salsa::SalsaValue)]
        pub struct $stored_name {
            interned: ::intern::Interned<$storage>,
        }

        impl $stored_name {
            #[inline]
            fn new(it: $name<'_>) -> Self {
                Self { interned: it.interned.to_owned() }
            }

            #[inline]
            pub fn as_ref<'a, 'db>(&'a self) -> $name<'db> {
                let it = $name { interned: self.interned.as_ref() };
                unsafe { std::mem::transmute::<$name<'a>, $name<'db>>(it) }
            }
        }

        impl std::fmt::Debug for $stored_name {
            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                self.as_ref().fmt(f)
            }
        }

        impl $name<'_> {
            #[inline]
            pub fn store(self) -> $stored_name {
                $stored_name::new(self)
            }
        }
    };
}
pub(crate) use impl_stored_interned;

/// This is a visitor trait that treats any interned thing specifically. Visitables are expected to call
/// the trait's methods when encountering an interned. This is used to implement marking in GC.
pub trait WorldExposer {
    fn on_interned<T: intern::Internable>(
        &mut self,
        interned: InternedRef<'_, T>,
    ) -> ControlFlow<()>;
    fn on_interned_slice<T: intern::SliceInternable>(
        &mut self,
        interned: InternedSliceRef<'_, T>,
    ) -> ControlFlow<()>;
}

#[derive(Debug, Copy, Clone)]
pub struct DbInterner<'db> {
    pub db: &'db dyn HirDatabase,
    krate: Option<Crate>,
    lang_items: Option<&'db LangItems>,
}

// FIXME: very wrong, see https://github.com/rust-lang/rust/pull/144808
unsafe impl Send for DbInterner<'_> {}
unsafe impl Sync for DbInterner<'_> {}

impl<'db> DbInterner<'db> {
    // FIXME(next-solver): remove this method
    #[doc(hidden)]
    pub fn conjure() -> DbInterner<'db> {
        // Here we can not reinit the cache since we do that when we attach the db.
        crate::with_attached_db(|db| DbInterner {
            db: unsafe { std::mem::transmute::<&dyn HirDatabase, &'db dyn HirDatabase>(db) },
            krate: None,
            lang_items: None,
        })
    }

    /// Creates a new interner without an active crate. Good only for interning things, not for trait solving etc..
    /// As a rule of thumb, when you create an `InferCtxt`, you need to provide the crate (and the block).
    ///
    /// Elaboration is a special kind: it needs lang items (for `Sized`), therefore it needs `new_with()`.
    pub fn new_no_crate(db: &'db dyn HirDatabase) -> Self {
        // We do not reinit the cache here, since anything accessing the cache needs an InferCtxt,
        // and we panic when trying to construct an InferCtxt for an Interner without a crate.
        DbInterner { db, krate: None, lang_items: None }
    }

    pub fn new_with(db: &'db dyn HirDatabase, krate: Crate) -> DbInterner<'db> {
        tls_cache::reinit_cache(db);
        DbInterner {
            db,
            krate: Some(krate),
            // As an approximation, when we call `new_with` we're trait solving, therefore we need the lang items.
            // This is also convenient since here we have a starting crate but not in `new_no_crate`.
            lang_items: Some(hir_def::lang_item::lang_items(db, krate)),
        }
    }

    #[inline]
    pub fn db(&self) -> &'db dyn HirDatabase {
        self.db
    }

    #[inline]
    #[track_caller]
    pub fn lang_items(&self) -> &'db LangItems {
        self.lang_items.expect(
            "Must have `DbInterner::lang_items`.\n\n\
            Note: you might have called `DbInterner::new_no_crate()` \
            where you should've called `DbInterner::new_with()`",
        )
    }

    #[inline]
    pub fn default_types(&self) -> &'db crate::next_solver::DefaultAny<'db> {
        crate::next_solver::default_types()
    }

    #[inline]
    pub(crate) fn expect_crate(&self) -> Crate {
        self.krate.expect("should have a crate")
    }
}

impl<'db> inherent::Span<DbInterner<'db>> for Span {
    fn dummy() -> Self {
        Span::Dummy
    }
}

interned_slice!(
    BoundVarKindsStorage,
    BoundVarKinds,
    StoredBoundVarKinds,
    bound_var_kinds,
    BoundVariableKind<'db>,
    BoundVariableKind<'static>,
);

pub type BoundVariableKind<'db> = rustc_type_ir::BoundVariableKind<DbInterner<'db>>;

interned_slice!(
    CanonicalVarsStorage,
    CanonicalVarKinds,
    StoredCanonicalVars,
    canonical_vars,
    CanonicalVarKind<'db>,
    CanonicalVarKind<'static>
);

pub struct DepNodeIndex;

#[derive(Debug)]
pub struct Tracked<T: fmt::Debug + Clone>(T);

#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
pub struct AllocId;

interned_slice!(VariancesOfStorage, VariancesOf, StoredVariancesOf, variances, Variance, Variance);

bitflags::bitflags! {
    #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
    struct AdtFlags: u8 {
        const IS_FUNDAMENTAL = 1 << 0;
        const IS_PACKED = 1 << 1;
        const HAS_REPR = 1 << 2;
        const IS_PHANTOM_DATA = 1 << 3;
        const IS_MANUALLY_DROP = 1 << 4;
        const IS_BOX = 1 << 5;
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
enum AdtDefInner {
    Struct { id: StructId, flags: AdtFlags },
    Union { id: UnionId, flags: AdtFlags },
    Enum { id: EnumId, flags: AdtFlags },
}

#[derive(Clone, Copy, PartialEq, Eq, Hash)]
pub struct AdtDef(AdtDefInner);

const _: () = assert!(size_of::<AdtDef>() == 12);

impl AdtDef {
    pub fn new<'db>(def_id: AdtId, interner: DbInterner<'db>) -> Self {
        let db = interner.db();
        let inner = match def_id {
            AdtId::StructId(id) => {
                let data = StructSignature::of(db, id);
                let mut flags = AdtFlags::empty();
                if data.flags.contains(StructFlags::FUNDAMENTAL) {
                    flags.insert(AdtFlags::IS_FUNDAMENTAL);
                }
                if data.flags.contains(StructFlags::IS_PHANTOM_DATA) {
                    flags.insert(AdtFlags::IS_PHANTOM_DATA);
                }
                if data.flags.contains(StructFlags::IS_MANUALLY_DROP) {
                    flags.insert(AdtFlags::IS_MANUALLY_DROP);
                }
                if data.flags.contains(StructFlags::IS_BOX) {
                    flags.insert(AdtFlags::IS_BOX);
                }
                if data.flags.contains(StructFlags::HAS_REPR) {
                    flags.insert(AdtFlags::HAS_REPR);
                    if data.repr(db, id).is_some_and(|repr| repr.packed()) {
                        flags.insert(AdtFlags::IS_PACKED);
                    }
                }
                AdtDefInner::Struct { id, flags }
            }
            AdtId::UnionId(id) => {
                let data = UnionSignature::of(db, id);
                let mut flags = AdtFlags::empty();
                if data.flags.contains(StructFlags::FUNDAMENTAL) {
                    flags.insert(AdtFlags::IS_FUNDAMENTAL);
                }
                if data.flags.contains(StructFlags::HAS_REPR) {
                    flags.insert(AdtFlags::HAS_REPR);
                    if data.repr(db, id).is_some_and(|repr| repr.packed()) {
                        flags.insert(AdtFlags::IS_PACKED);
                    }
                }
                AdtDefInner::Union { id, flags }
            }
            AdtId::EnumId(id) => {
                let data = EnumSignature::of(db, id);
                let mut flags = AdtFlags::empty();
                if data.flags.contains(EnumFlags::FUNDAMENTAL) {
                    flags.insert(AdtFlags::IS_FUNDAMENTAL);
                }
                if data.flags.contains(EnumFlags::HAS_REPR) {
                    flags.insert(AdtFlags::HAS_REPR);
                    if data.repr(db, id).is_some_and(|repr| repr.packed()) {
                        flags.insert(AdtFlags::IS_PACKED);
                    }
                }
                AdtDefInner::Enum { id, flags }
            }
        };
        AdtDef(inner)
    }

    #[inline]
    pub fn def_id(self) -> AdtId {
        match self.0 {
            AdtDefInner::Struct { id, .. } => AdtId::StructId(id),
            AdtDefInner::Union { id, .. } => AdtId::UnionId(id),
            AdtDefInner::Enum { id, .. } => AdtId::EnumId(id),
        }
    }

    #[inline]
    fn flags(self) -> AdtFlags {
        match self.0 {
            AdtDefInner::Struct { flags, .. }
            | AdtDefInner::Union { flags, .. }
            | AdtDefInner::Enum { flags, .. } => flags,
        }
    }

    #[inline]
    pub fn is_struct(self) -> bool {
        matches!(self.0, AdtDefInner::Struct { .. })
    }

    #[inline]
    pub fn is_union(self) -> bool {
        matches!(self.0, AdtDefInner::Union { .. })
    }

    #[inline]
    pub fn is_enum(self) -> bool {
        matches!(self.0, AdtDefInner::Enum { .. })
    }

    #[inline]
    pub fn is_box(self) -> bool {
        matches!(self.0, AdtDefInner::Struct { flags, .. } if flags.contains(AdtFlags::IS_BOX))
    }

    #[inline]
    pub fn repr(self, db: &dyn HirDatabase) -> ReprOptions {
        if self.flags().contains(AdtFlags::HAS_REPR) {
            AttrFlags::repr_assume_has(db, self.def_id()).unwrap_or_default()
        } else {
            ReprOptions::default()
        }
    }
}

impl<'db> inherent::AdtDef<DbInterner<'db>> for AdtDef {
    fn def_id(self) -> AdtIdWrapper {
        self.def_id().into()
    }

    fn is_struct(self) -> bool {
        self.is_struct()
    }

    fn is_phantom_data(self) -> bool {
        matches!(self.0, AdtDefInner::Struct { flags, .. } if flags.contains(AdtFlags::IS_PHANTOM_DATA))
    }

    fn is_manually_drop(self) -> bool {
        matches!(self.0, AdtDefInner::Struct { flags, .. } if flags.contains(AdtFlags::IS_MANUALLY_DROP))
    }

    fn is_packed(self) -> bool {
        self.flags().contains(AdtFlags::IS_PACKED)
    }

    fn is_fundamental(self) -> bool {
        self.flags().contains(AdtFlags::IS_FUNDAMENTAL)
    }

    fn struct_tail_ty(
        self,
        interner: DbInterner<'db>,
    ) -> Option<EarlyBinder<DbInterner<'db>, Ty<'db>>> {
        let hir_def::AdtId::StructId(struct_id) = self.def_id() else {
            return None;
        };
        let id: VariantId = struct_id.into();
        let field_types = interner.db().field_types(id);

        field_types.iter().last().map(|f| f.1.ty())
    }

    fn all_field_tys(
        self,
        interner: DbInterner<'db>,
    ) -> EarlyBinder<DbInterner<'db>, impl IntoIterator<Item = Ty<'db>>> {
        let db = interner.db();
        let field_tys =
            |id: VariantId| db.field_types(id).iter().map(|(_, ty)| ty.ty().skip_binder());
        let tys = match self.def_id() {
            hir_def::AdtId::StructId(id) => Either::Left(field_tys(id.into())),
            hir_def::AdtId::UnionId(id) => Either::Left(field_tys(id.into())),
            hir_def::AdtId::EnumId(id) => Either::Right(
                id.enum_variants(db)
                    .variants
                    .values()
                    .flat_map(move |&(variant_id, _)| field_tys(variant_id.into())),
            ),
        };

        EarlyBinder::bind(tys)
    }

    fn sizedness_constraint(
        self,
        interner: DbInterner<'db>,
        sizedness: SizedTraitKind,
    ) -> Option<EarlyBinder<DbInterner<'db>, Ty<'db>>> {
        let tail_ty = self.struct_tail_ty(interner)?;
        tail_ty
            .map_bound(|tail_ty| sizedness_constraint_for_ty(interner, sizedness, tail_ty))
            .transpose()
    }

    fn destructor(self, interner: DbInterner<'db>) -> Option<AdtDestructorKind> {
        crate::drop::destructor(interner.db, self.def_id()).map(|_| AdtDestructorKind::NotConst)
    }

    fn field_representing_type_info(
        self,
        _interner: DbInterner<'db>,
        _args: GenericArgs<'db>,
    ) -> Option<rustc_type_ir::FieldInfo<DbInterner<'db>>> {
        // FIXME
        None
    }
}

impl fmt::Debug for AdtDef {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        crate::with_attached_db(|db| match self.0 {
            AdtDefInner::Struct { id, .. } => {
                let data = StructSignature::of(db, id);
                f.write_str(data.name.as_str())
            }
            AdtDefInner::Union { id, .. } => {
                let data = UnionSignature::of(db, id);
                f.write_str(data.name.as_str())
            }
            AdtDefInner::Enum { id, .. } => {
                let data = EnumSignature::of(db, id);
                f.write_str(data.name.as_str())
            }
        })
    }
}

#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub struct Features;

impl<'db> inherent::Features<DbInterner<'db>> for Features {
    fn generic_const_exprs(self) -> bool {
        false
    }

    fn coroutine_clone(self) -> bool {
        false
    }

    fn generic_const_args(self) -> bool {
        false
    }

    fn feature_bound_holds_in_crate(self, _symbol: Symbol) -> bool {
        false
    }
}

#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash, GenericTypeVisitable)]
pub struct Symbol;

impl<'db> inherent::Symbol<DbInterner<'db>> for Symbol {
    fn is_kw_underscore_lifetime(self) -> bool {
        false
    }
}

#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub struct UnsizingParams(pub(crate) DenseBitSet<u32>);

impl std::ops::Deref for UnsizingParams {
    type Target = DenseBitSet<u32>;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

pub type PatternKind<'db> = rustc_type_ir::PatternKind<DbInterner<'db>>;

#[derive(Clone, Copy, PartialEq, Eq, Hash)]
pub struct Pattern<'db> {
    interned: InternedRef<'db, PatternInterned>,
}

#[derive(PartialEq, Eq, Hash, GenericTypeVisitable)]
struct PatternInterned(PatternKind<'static>);

impl_internable!(gc; PatternInterned);

const _: () = {
    const fn is_copy<T: Copy>() {}
    is_copy::<Pattern<'static>>();
};

impl<'db> Pattern<'db> {
    pub fn new(_interner: DbInterner<'db>, kind: PatternKind<'db>) -> Self {
        let kind = unsafe { std::mem::transmute::<PatternKind<'db>, PatternKind<'static>>(kind) };
        Self { interned: Interned::new_gc(PatternInterned(kind)) }
    }

    pub fn inner(&self) -> &PatternKind<'db> {
        let inner = &self.interned.0;
        unsafe { std::mem::transmute::<&PatternKind<'static>, &PatternKind<'db>>(inner) }
    }
}

impl<'db> std::fmt::Debug for Pattern<'db> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.kind().fmt(f)
    }
}

impl<'db> Flags for Pattern<'db> {
    fn flags(&self) -> TypeFlags {
        match self.inner() {
            PatternKind::Range { start, end } => {
                FlagComputation::for_const_kind(&start.kind()).flags
                    | FlagComputation::for_const_kind(&end.kind()).flags
            }
            PatternKind::Or(pats) => {
                let mut flags = pats.as_slice()[0].flags();
                for pat in pats.as_slice()[1..].iter() {
                    flags |= pat.flags();
                }
                flags
            }
            PatternKind::NotNull => TypeFlags::empty(),
        }
    }

    fn outer_exclusive_binder(&self) -> rustc_type_ir::DebruijnIndex {
        match self.inner() {
            PatternKind::Range { start, end } => {
                start.outer_exclusive_binder().max(end.outer_exclusive_binder())
            }
            PatternKind::Or(pats) => {
                let mut idx = pats.as_slice()[0].outer_exclusive_binder();
                for pat in pats.as_slice()[1..].iter() {
                    idx = idx.max(pat.outer_exclusive_binder());
                }
                idx
            }
            PatternKind::NotNull => rustc_type_ir::INNERMOST,
        }
    }
}

impl<'db> rustc_type_ir::inherent::IntoKind for Pattern<'db> {
    type Kind = rustc_type_ir::PatternKind<DbInterner<'db>>;
    fn kind(self) -> Self::Kind {
        *self.inner()
    }
}

impl<'db> rustc_type_ir::TypeVisitable<DbInterner<'db>> for Pattern<'db> {
    fn visit_with<V: rustc_type_ir::TypeVisitor<DbInterner<'db>>>(
        &self,
        visitor: &mut V,
    ) -> V::Result {
        self.kind().visit_with(visitor)
    }
}

impl<'db, V: WorldExposer> rustc_type_ir::GenericTypeVisitable<V> for Pattern<'db> {
    fn generic_visit_with(&self, visitor: &mut V) {
        if visitor.on_interned(self.interned).is_continue() {
            self.kind().generic_visit_with(visitor);
        }
    }
}

impl<'db> rustc_type_ir::TypeFoldable<DbInterner<'db>> for Pattern<'db> {
    fn try_fold_with<F: rustc_type_ir::FallibleTypeFolder<DbInterner<'db>>>(
        self,
        folder: &mut F,
    ) -> Result<Self, F::Error> {
        Ok(Pattern::new(folder.cx(), self.kind().try_fold_with(folder)?))
    }

    fn fold_with<F: rustc_type_ir::TypeFolder<DbInterner<'db>>>(self, folder: &mut F) -> Self {
        Pattern::new(folder.cx(), self.kind().fold_with(folder))
    }
}

impl<'db> rustc_type_ir::relate::Relate<DbInterner<'db>> for Pattern<'db> {
    fn relate<R: rustc_type_ir::relate::TypeRelation<DbInterner<'db>>>(
        relation: &mut R,
        a: Self,
        b: Self,
    ) -> rustc_type_ir::relate::RelateResult<DbInterner<'db>, Self> {
        let tcx = relation.cx();
        match (a.kind(), b.kind()) {
            (
                PatternKind::Range { start: start_a, end: end_a },
                PatternKind::Range { start: start_b, end: end_b },
            ) => {
                let start = relation.relate(start_a, start_b)?;
                let end = relation.relate(end_a, end_b)?;
                Ok(Pattern::new(tcx, PatternKind::Range { start, end }))
            }
            (PatternKind::Or(a), PatternKind::Or(b)) => {
                if a.len() != b.len() {
                    return Err(TypeError::Mismatch);
                }
                let pats = PatList::new_from_iter(
                    std::iter::zip(a.iter(), b.iter()).map(|(a, b)| relation.relate(a, b)),
                )?;
                Ok(Pattern::new(tcx, PatternKind::Or(pats)))
            }
            (PatternKind::NotNull, PatternKind::NotNull) => Ok(a),
            (PatternKind::Range { .. } | PatternKind::Or(_) | PatternKind::NotNull, _) => {
                Err(TypeError::Mismatch)
            }
        }
    }
}

interned_slice!(PatListStorage, PatList, StoredPatList, pat_list, Pattern<'db>, Pattern<'static>);
impl_foldable_for_interned_slice!(PatList);

macro_rules! as_lang_item {
    (
        $solver_enum:ident, $self:ident, $def_id:expr, $id_ty:ty;

        $( $variant:ident ),* $(,)?
    ) => {{
        let lang_items = $self.lang_items();
        // Ensure exhaustiveness.
        if let Some(it) = None::<$solver_enum> {
            match it {
                $( $solver_enum::$variant => {} )*
            }
        }
        match $def_id {
            $( def_id if let Some(it) = lang_items.$variant && <$id_ty>::from(it) == def_id => Some($solver_enum::$variant), )*
            _ => None
        }
    }};
}

macro_rules! is_lang_item {
    (
        $solver_enum:ident, $self:ident, $def_id:expr, $expected_variant:ident;

        $( $variant:ident ),* $(,)?
    ) => {{
        let lang_items = $self.lang_items();
        let def_id = $def_id;
        match $expected_variant {
            $( $solver_enum::$variant => lang_items.$variant.is_some_and(|it| it == def_id), )*
        }
    }};
}

impl<'db> Interner for DbInterner<'db> {
    type DefId = SolverDefId<'db>;
    type LocalDefId = SolverDefId<'db>;
    type LocalDefIds = SolverDefIds<'db>;
    type TraitId = TraitIdWrapper;
    type ForeignId = TypeAliasIdWrapper;
    type FunctionId = CallableIdWrapper;
    type ClosureId = ClosureIdWrapper<'db>;
    type CoroutineClosureId = CoroutineClosureIdWrapper<'db>;
    type CoroutineId = CoroutineIdWrapper<'db>;
    type AdtId = AdtIdWrapper;
    type ImplId = AnyImplId;
    type UnevaluatedConstId = GeneralConstIdWrapper<'db>;
    type TraitAssocTyId = TraitAssocTyId;
    type TraitAssocConstId = TraitAssocConstId;
    type TraitAssocTermId = TraitAssocTermId;
    type OpaqueTyId = OpaqueTyIdWrapper<'db>;
    type LocalOpaqueTyId = OpaqueTyIdWrapper<'db>;
    type FreeTyAliasId = FreeTyAliasId;
    type FreeConstAliasId = FreeConstAliasId;
    type FreeTermAliasId = FreeTermAliasId;
    type ImplOrTraitAssocTyId = ImplOrTraitAssocTyId;
    type ImplOrTraitAssocConstId = ImplOrTraitAssocConstId;
    type ImplOrTraitAssocTermId = ImplOrTraitAssocTermId;
    type InherentAssocTyId = InherentAssocTyId;
    type InherentAssocConstId = InherentAssocConstId;
    type InherentAssocTermId = InherentAssocTermId;
    type Span = Span;

    type GenericArgs = GenericArgs<'db>;
    type GenericArgsSlice = &'db [GenericArg<'db>];
    type GenericArg = GenericArg<'db>;

    type Term = Term<'db>;

    type BoundVarKinds = BoundVarKinds<'db>;

    type PredefinedOpaques = PredefinedOpaques<'db>;

    fn mk_predefined_opaques_in_body(
        self,
        data: &[(OpaqueTypeKey<'db>, Self::Ty)],
    ) -> Self::PredefinedOpaques {
        PredefinedOpaques::new_from_slice(data)
    }

    type CanonicalVarKinds = CanonicalVarKinds<'db>;

    fn mk_canonical_var_kinds(
        self,
        kinds: &[rustc_type_ir::CanonicalVarKind<Self>],
    ) -> Self::CanonicalVarKinds {
        CanonicalVarKinds::new_from_slice(kinds)
    }

    type ExternalConstraints = ExternalConstraints<'db>;

    fn mk_external_constraints(
        self,
        data: rustc_type_ir::solve::ExternalConstraintsData<Self>,
    ) -> Self::ExternalConstraints {
        ExternalConstraints::new(self, data)
    }

    type DepNodeIndex = DepNodeIndex;

    type Tracked<T: fmt::Debug + Clone> = Tracked<T>;

    type Ty = Ty<'db>;
    type Tys = Tys<'db>;
    type FnInputTys = &'db [Ty<'db>];
    type ParamTy = ParamTy;
    type Symbol = Symbol;

    type ErrorGuaranteed = ErrorGuaranteed;
    type BoundExistentialPredicates = BoundExistentialPredicates<'db>;
    type AllocId = AllocId;
    type Pat = Pattern<'db>;
    type PatList = PatList<'db>;
    type Safety = Safety;

    type Const = Const<'db>;
    type ParamConst = ParamConst;
    type ValueConst = ValueConst<'db>;
    type ValTree = ValTree<'db>;
    type Consts = Consts<'db>;
    type ScalarInt = ScalarInt;
    type ExprConst = ExprConst;

    type Region = Region<'db>;
    type EarlyParamRegion = EarlyParamRegion;
    type LateParamRegion = LateParamRegion<'db>;

    type RegionAssumptions = RegionAssumptions<'db>;

    type ParamEnv = ParamEnv<'db>;
    type Predicate = Predicate<'db>;
    type Clause = Clause<'db>;
    type Clauses = Clauses<'db>;

    type GenericsOf = Generics<'db>;

    type VariancesOf = VariancesOf<'db>;

    type AdtDef = AdtDef;

    type Features = Features;

    fn mk_args(self, args: &[Self::GenericArg]) -> Self::GenericArgs {
        GenericArgs::new_from_slice(args)
    }

    fn mk_args_from_iter<I, T>(self, args: I) -> T::Output
    where
        I: Iterator<Item = T>,
        T: rustc_type_ir::CollectAndApply<Self::GenericArg, Self::GenericArgs>,
    {
        GenericArgs::new_from_iter(args)
    }

    type UnsizingParams = UnsizingParams;

    fn mk_tracked<T: fmt::Debug + Clone>(
        self,
        data: T,
        _dep_node: Self::DepNodeIndex,
    ) -> Self::Tracked<T> {
        Tracked(data)
    }

    fn get_tracked<T: fmt::Debug + Clone>(self, tracked: &Self::Tracked<T>) -> T {
        tracked.0.clone()
    }

    fn with_cached_task<T>(self, task: impl FnOnce() -> T) -> (T, Self::DepNodeIndex) {
        (task(), DepNodeIndex)
    }

    fn with_global_cache<R>(
        self,
        f: impl FnOnce(&mut rustc_type_ir::search_graph::GlobalCache<Self>) -> R,
    ) -> R {
        // We make sure to reinit the cache when constructing the Interner.
        tls_cache::borrow_assume_valid(self.db, f)
    }

    fn canonical_param_env_cache_get_or_insert<R>(
        self,
        _param_env: Self::ParamEnv,
        f: impl FnOnce() -> rustc_type_ir::CanonicalParamEnvCacheEntry<Self>,
        from_entry: impl FnOnce(&rustc_type_ir::CanonicalParamEnvCacheEntry<Self>) -> R,
    ) -> R {
        from_entry(&f())
    }

    fn assert_evaluation_is_concurrent(&self) {
        // Turns out, the assumption for this function isn't perfect.
        // See https://github.com/rust-lang/trait-system-refactor-initiative/issues/234
    }

    fn expand_abstract_consts<T: rustc_type_ir::TypeFoldable<Self>>(self, _: T) -> T {
        unreachable!("only used by the old trait solver in rustc");
    }

    fn generics_of(self, def_id: Self::DefId) -> Self::GenericsOf {
        generics(self, def_id)
    }

    fn variances_of(self, def_id: Self::DefId) -> Self::VariancesOf {
        let generic_def = match def_id {
            SolverDefId::Ctor(Ctor::Enum(def_id)) | SolverDefId::EnumVariantId(def_id) => {
                def_id.loc(self.db).parent.into()
            }
            SolverDefId::InternedOpaqueTyId(_def_id) => {
                // FIXME(next-solver): track variances
                //
                // We compute them based on the only `Ty` level info in rustc,
                // move `variances_of_opaque` into `rustc_next_trait_solver` for reuse.
                return VariancesOf::new_from_iter(
                    (0..self.generics_of(def_id).count()).map(|_| Variance::Invariant),
                );
            }
            SolverDefId::Ctor(Ctor::Struct(def_id)) => def_id.into(),
            SolverDefId::AdtId(def_id) => def_id.into(),
            SolverDefId::FunctionId(def_id) => def_id.into(),
            SolverDefId::ConstId(_)
            | SolverDefId::StaticId(_)
            | SolverDefId::TraitId(_)
            | SolverDefId::TypeAliasId(_)
            | SolverDefId::ImplId(_)
            | SolverDefId::BuiltinDeriveImplId(_)
            | SolverDefId::InternedClosureId(_)
            | SolverDefId::InternedCoroutineId(_)
            | SolverDefId::InternedCoroutineClosureId(_)
            | SolverDefId::AnonConstId(_) => {
                return VariancesOf::empty();
            }
        };
        self.db.variances_of(generic_def)
    }

    fn type_of(self, def_id: Self::DefId) -> EarlyBinder<Self, Self::Ty> {
        match def_id {
            SolverDefId::TypeAliasId(id) => self.db().ty(id.into()),
            SolverDefId::AdtId(id) => self.db().ty(id.into()),
            // FIXME(next-solver): This uses the types of `query mir_borrowck` in rustc.
            //
            // We currently always use the type from HIR typeck which ignores regions. This
            // should be fine.
            SolverDefId::InternedOpaqueTyId(def_id) => {
                self.type_of_opaque_hir_typeck(def_id.into())
            }
            SolverDefId::FunctionId(id) => self.db.value_ty(id.into()).unwrap(),
            SolverDefId::Ctor(id) => {
                let id = match id {
                    Ctor::Struct(id) => id.into(),
                    Ctor::Enum(id) => id.into(),
                };
                self.db.value_ty(id).expect("`SolverDefId::Ctor` should have a function-like ctor")
            }
            _ => panic!("Unexpected def_id `{def_id:?}` provided for `type_of`"),
        }
    }

    fn adt_def(self, def_id: Self::AdtId) -> Self::AdtDef {
        AdtDef::new(def_id.0, self)
    }

    fn alias_term_kind_from_def_id(self, def_id: SolverDefId<'db>) -> AliasTermKind<'db> {
        match def_id {
            SolverDefId::InternedOpaqueTyId(def_id) => {
                AliasTermKind::OpaqueTy { def_id: def_id.into() }
            }
            SolverDefId::TypeAliasId(type_alias) => match type_alias.loc(self.db).container {
                ItemContainerId::ImplId(impl_)
                    if ImplSignature::of(self.db, impl_).target_trait.is_none() =>
                {
                    AliasTermKind::InherentTy { def_id: type_alias.into() }
                }
                ItemContainerId::TraitId(_) | ItemContainerId::ImplId(_) => {
                    AliasTermKind::ProjectionTy { def_id: type_alias.into() }
                }
                _ => AliasTermKind::FreeTy { def_id: type_alias.into() },
            },
            // rustc creates an `AnonConst` for consts, and evaluates them with CTFE (normalizing projections
            // via selection, similar to ours `find_matching_impl()`, and not with the trait solver), so mimic it.
            SolverDefId::ConstId(def_id) => {
                AliasTermKind::UnevaluatedConst { def_id: GeneralConstIdWrapper(def_id.into()) }
            }
            SolverDefId::StaticId(def_id) => {
                AliasTermKind::UnevaluatedConst { def_id: GeneralConstIdWrapper(def_id.into()) }
            }
            SolverDefId::AnonConstId(def_id) => {
                AliasTermKind::UnevaluatedConst { def_id: GeneralConstIdWrapper(def_id.into()) }
            }
            _ => unimplemented!("Unexpected alias: {:?}", def_id),
        }
    }

    fn trait_ref_and_own_args_for_alias(
        self,
        def_id: Self::TraitAssocTermId,
        args: Self::GenericArgs,
    ) -> (rustc_type_ir::TraitRef<Self>, Self::GenericArgsSlice) {
        let trait_def_id = self.projection_parent(def_id);
        let trait_ref = TraitRef::from_assoc(self, trait_def_id, args);
        (trait_ref, &args.as_slice()[trait_ref.args.len()..])
    }

    fn check_args_compatible(self, def_id: Self::DefId, args: Self::GenericArgs) -> bool {
        let generics = self.generics_of(def_id);
        generics.count() == args.len()
            && std::iter::zip(generics.iter(), args).all(|((param, _), arg)| {
                matches!(
                    (param, arg.kind()),
                    (GenericParamId::LifetimeParamId(_), GenericArgKind::Lifetime(_))
                        | (GenericParamId::TypeParamId(_), GenericArgKind::Type(_))
                        | (GenericParamId::ConstParamId(_), GenericArgKind::Const(_))
                )
            })
    }

    fn debug_assert_args_compatible(self, _def_id: Self::DefId, _args: Self::GenericArgs) {}

    fn debug_assert_existential_args_compatible(
        self,
        _def_id: Self::DefId,
        _args: Self::GenericArgs,
    ) {
    }

    fn mk_type_list_from_iter<I, T>(self, args: I) -> T::Output
    where
        I: Iterator<Item = T>,
        T: rustc_type_ir::CollectAndApply<Self::Ty, Self::Tys>,
    {
        Tys::new_from_iter(args)
    }

    fn projection_parent(self, def_id: Self::TraitAssocTermId) -> Self::TraitId {
        let container = match def_id.0 {
            TermId::TypeAliasId(def_id) => def_id.loc(self.db).container,
            TermId::ConstId(def_id) => def_id.loc(self.db).container,
        };
        let ItemContainerId::TraitId(trait_) = container else {
            panic!("a TraitAssocTermId can only come from a trait")
        };
        trait_.into()
    }

    fn impl_or_trait_assoc_term_parent(self, def_id: Self::ImplOrTraitAssocTermId) -> Self::DefId {
        let container = match def_id.0 {
            TermId::TypeAliasId(def_id) => def_id.loc(self.db).container,
            TermId::ConstId(def_id) => def_id.loc(self.db).container,
        };
        match container {
            ItemContainerId::ImplId(impl_) => impl_.into(),
            ItemContainerId::TraitId(trait_) => trait_.into(),
            ItemContainerId::ExternBlockId(_) | ItemContainerId::ModuleId(_) => {
                panic!("only impl or trait can be the parent of ImplOrTraitAssocTermId")
            }
        }
    }

    fn inherent_alias_term_parent(self, def_id: Self::InherentAssocTermId) -> Self::ImplId {
        let container = match def_id.0 {
            TermId::TypeAliasId(def_id) => def_id.loc(self.db).container,
            TermId::ConstId(def_id) => def_id.loc(self.db).container,
        };
        match container {
            ItemContainerId::ImplId(impl_) => impl_.into(),
            ItemContainerId::ExternBlockId(_)
            | ItemContainerId::ModuleId(_)
            | ItemContainerId::TraitId(_) => {
                panic!("only impl can be the parent of InherentAliasTermId")
            }
        }
    }

    fn recursion_limit(self) -> usize {
        50
    }

    fn is_type_const(self, _def_id: Self::DefId) -> bool {
        false
    }

    fn features(self) -> Features {
        Features
    }

    fn fn_sig(
        self,
        def_id: Self::FunctionId,
    ) -> EarlyBinder<Self, rustc_type_ir::Binder<Self, rustc_type_ir::FnSig<Self>>> {
        self.db().callable_item_signature(def_id.0)
    }

    fn coroutine_movability(self, def_id: Self::CoroutineId) -> rustc_ast_ir::Movability {
        match def_id.0.loc(self.db).kind {
            hir_def::hir::ClosureKind::OldCoroutine(movability) => match movability {
                hir_def::hir::Movability::Static => rustc_ast_ir::Movability::Static,
                hir_def::hir::Movability::Movable => rustc_ast_ir::Movability::Movable,
            },
            hir_def::hir::ClosureKind::Coroutine { .. } => rustc_ast_ir::Movability::Static,
            kind => panic!("unexpected kind for a coroutine: {kind:?}"),
        }
    }

    fn coroutine_for_closure(self, def_id: Self::CoroutineClosureId) -> Self::CoroutineId {
        let InternedClosure { owner, expr: coroutine_closure_expr, kind: coroutine_closure_kind } =
            def_id.0.loc(self.db);
        let coroutine_closure_kind = match coroutine_closure_kind {
            HirClosureKind::CoroutineClosure(it) => it,
            _ => {
                panic!("invalid kind closure kind {coroutine_closure_kind:?} for coroutine closure")
            }
        };
        let coroutine_expr = ExpressionStore::coroutine_for_closure(coroutine_closure_expr);
        let coroutine_kind = hir_def::hir::ClosureKind::Coroutine {
            kind: coroutine_closure_kind,
            source: hir_def::hir::CoroutineSource::Closure,
        };
        InternedCoroutineId::new(
            self.db,
            InternedClosure { owner, expr: coroutine_expr, kind: coroutine_kind },
        )
        .into()
    }

    fn generics_require_sized_self(self, def_id: Self::DefId) -> bool {
        match GenericDefId::try_from(def_id) {
            Ok(def_id) => crate::dyn_compatibility::generics_require_sized_self(self.db, def_id),
            Err(_) => false,
        }
    }

    #[tracing::instrument(skip(self))]
    fn item_bounds(
        self,
        def_id: Self::DefId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = Self::Clause>> {
        ItemBounds::of_solver_def(self.db, def_id).all_bounds()
    }

    #[tracing::instrument(skip(self))]
    fn item_self_bounds(
        self,
        def_id: Self::DefId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = Self::Clause>> {
        ItemBounds::of_solver_def(self.db, def_id).self_bounds()
    }

    fn item_non_self_bounds(
        self,
        def_id: Self::DefId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = Self::Clause>> {
        ItemBounds::of_solver_def(self.db, def_id).non_self_bounds()
    }

    fn predicates_of(
        self,
        def_id: Self::DefId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = Self::Clause>> {
        predicates_of(self.db, def_id).all_predicates()
    }

    fn own_predicates_of(
        self,
        def_id: Self::DefId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = Self::Clause>> {
        predicates_of(self.db, def_id).own_explicit_predicates()
    }

    fn explicit_super_predicates_of(
        self,
        def_id: Self::TraitId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = (Self::Clause, Self::Span)>> {
        GenericPredicates::query(self.db, def_id.0.into())
            .explicit_non_assoc_types_predicates()
            .map_bound(move |predicates| {
                predicates.filter(|p| is_clause_at_ty(p, is_ty_self)).map(|p| (p, Span::dummy()))
            })
    }

    fn explicit_implied_predicates_of(
        self,
        def_id: Self::DefId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = (Self::Clause, Self::Span)>> {
        fn is_ty_assoc_of_self(ty: Ty<'_>) -> bool {
            // FIXME: Is this correct wrt. combined kind of assoc type bounds, i.e. `where Self::Assoc: Trait<Assoc2: Trait>`
            // wrt. `Assoc2`, which we should exclude?
            if let TyKind::Alias(alias @ AliasTy { kind: AliasTyKind::Projection { .. }, .. }) =
                ty.kind()
            {
                is_ty_assoc_of_self(alias.self_ty())
            } else {
                is_ty_self(ty)
            }
        }

        let predicates = predicates_of(self.db, def_id);
        let non_assoc_types = predicates
            .explicit_non_assoc_types_predicates()
            .skip_binder()
            .filter(|p| is_clause_at_ty(p, is_ty_self));
        let assoc_types = predicates
            .explicit_assoc_types_predicates()
            .skip_binder()
            .filter(|p| is_clause_at_ty(p, is_ty_assoc_of_self));
        EarlyBinder::bind(non_assoc_types.chain(assoc_types).map(|it| (it, Span::dummy())))
    }

    fn impl_super_outlives(
        self,
        impl_id: Self::ImplId,
    ) -> EarlyBinder<Self, impl IntoIterator<Item = Self::Clause>> {
        let bounds = match impl_id {
            AnyImplId::ImplId(id) => {
                impl_super_outlives(self.db, id).get().map_bound(|it| it.as_slice())
            }
            AnyImplId::BuiltinDeriveImplId(id) => {
                crate::builtin_derive::impl_super_outlives(self, id)
            }
        };
        bounds.map_bound(|it| it.iter().copied())
    }

    #[expect(unreachable_code)]
    fn const_conditions(
        self,
        _def_id: Self::DefId,
    ) -> EarlyBinder<
        Self,
        impl IntoIterator<Item = rustc_type_ir::Binder<Self, rustc_type_ir::TraitRef<Self>>>,
    > {
        EarlyBinder::bind([unimplemented!()])
    }

    fn has_target_features(self, _def_id: Self::FunctionId) -> bool {
        false
    }

    fn require_projection_lang_item(
        self,
        lang_item: SolverProjectionLangItem,
    ) -> Self::TraitAssocTyId {
        let lang_items = self.lang_items();
        let lang_item = match lang_item {
            SolverProjectionLangItem::AsyncFnKindUpvars => lang_items.AsyncFnKindUpvars,
            SolverProjectionLangItem::AsyncFnOnceOutput => lang_items.AsyncFnOnceOutput,
            SolverProjectionLangItem::CallOnceFuture => lang_items.CallOnceFuture,
            SolverProjectionLangItem::CallRefFuture => lang_items.CallRefFuture,
            SolverProjectionLangItem::CoroutineReturn => lang_items.CoroutineReturn,
            SolverProjectionLangItem::CoroutineYield => lang_items.CoroutineYield,
            SolverProjectionLangItem::FutureOutput => lang_items.FutureOutput,
            SolverProjectionLangItem::Metadata => lang_items.Metadata,
            SolverProjectionLangItem::FieldBase => lang_items.FieldBase,
            SolverProjectionLangItem::FieldType => lang_items.FieldType,
        };
        lang_item.expect("Lang item required but not found.").into()
    }

    fn require_trait_lang_item(self, lang_item: SolverTraitLangItem) -> TraitIdWrapper {
        let lang_items = self.lang_items();
        let lang_item = match lang_item {
            SolverTraitLangItem::AsyncFn => lang_items.AsyncFn,
            SolverTraitLangItem::AsyncFnKindHelper => lang_items.AsyncFnKindHelper,
            SolverTraitLangItem::AsyncFnMut => lang_items.AsyncFnMut,
            SolverTraitLangItem::AsyncFnOnce => lang_items.AsyncFnOnce,
            SolverTraitLangItem::AsyncIterator => lang_items.AsyncIterator,
            SolverTraitLangItem::Clone => lang_items.Clone,
            SolverTraitLangItem::Copy => lang_items.Copy,
            SolverTraitLangItem::Coroutine => lang_items.Coroutine,
            SolverTraitLangItem::Destruct => lang_items.Destruct,
            SolverTraitLangItem::DiscriminantKind => lang_items.DiscriminantKind,
            SolverTraitLangItem::Drop => lang_items.Drop,
            SolverTraitLangItem::Fn => lang_items.Fn,
            SolverTraitLangItem::FnMut => lang_items.FnMut,
            SolverTraitLangItem::FnOnce => lang_items.FnOnce,
            SolverTraitLangItem::FnPtrTrait => lang_items.FnPtrTrait,
            SolverTraitLangItem::FusedIterator => lang_items.FusedIterator,
            SolverTraitLangItem::Future => lang_items.Future,
            SolverTraitLangItem::Iterator => lang_items.Iterator,
            SolverTraitLangItem::PointeeTrait => lang_items.PointeeTrait,
            SolverTraitLangItem::Sized => lang_items.Sized,
            SolverTraitLangItem::MetaSized => lang_items.MetaSized,
            SolverTraitLangItem::PointeeSized => lang_items.PointeeSized,
            SolverTraitLangItem::TransmuteTrait => lang_items.TransmuteTrait,
            SolverTraitLangItem::Tuple => lang_items.Tuple,
            SolverTraitLangItem::Unpin => lang_items.Unpin,
            SolverTraitLangItem::Unsize => lang_items.Unsize,
            SolverTraitLangItem::BikeshedGuaranteedNoDrop => lang_items.BikeshedGuaranteedNoDrop,
            SolverTraitLangItem::TrivialClone => lang_items.TrivialClone,
            SolverTraitLangItem::Field => lang_items.Field,
        };
        lang_item.expect("Lang item required but not found.").into()
    }

    fn require_adt_lang_item(self, lang_item: SolverAdtLangItem) -> AdtIdWrapper {
        let lang_items = self.lang_items();
        let lang_item = match lang_item {
            SolverAdtLangItem::Option => lang_items.Option.map(Into::into),
            SolverAdtLangItem::Poll => lang_items.Poll.map(Into::into),
            SolverAdtLangItem::DynMetadata => lang_items.DynMetadata.map(Into::into),
        };
        AdtIdWrapper(lang_item.expect("Lang item required but not found."))
    }

    fn is_projection_lang_item(
        self,
        def_id: Self::TraitAssocTyId,
        lang_item: SolverProjectionLangItem,
    ) -> bool {
        self.as_projection_lang_item(def_id)
            .map_or(false, |l| std::mem::discriminant(&l) == std::mem::discriminant(&lang_item))
    }

    fn is_trait_lang_item(self, def_id: Self::TraitId, lang_item: SolverTraitLangItem) -> bool {
        is_lang_item!(
            SolverTraitLangItem, self, def_id.0, lang_item;

            Sized,
            MetaSized,
            PointeeSized,
            Unsize,
            Copy,
            Clone,
            DiscriminantKind,
            PointeeTrait,
            FnPtrTrait,
            Drop,
            Destruct,
            TransmuteTrait,
            Fn,
            FnMut,
            FnOnce,
            Future,
            Coroutine,
            Unpin,
            Tuple,
            Iterator,
            AsyncFn,
            AsyncFnMut,
            AsyncFnOnce,
            TrivialClone,
            AsyncFnKindHelper,
            AsyncIterator,
            BikeshedGuaranteedNoDrop,
            FusedIterator,
            Field,
        )
    }

    fn is_adt_lang_item(self, def_id: Self::AdtId, lang_item: SolverAdtLangItem) -> bool {
        // FIXME: derive PartialEq on SolverTraitLangItem
        self.as_adt_lang_item(def_id)
            .map_or(false, |l| std::mem::discriminant(&l) == std::mem::discriminant(&lang_item))
    }

    fn as_projection_lang_item(
        self,
        def_id: Self::TraitAssocTyId,
    ) -> Option<SolverProjectionLangItem> {
        as_lang_item!(
            SolverProjectionLangItem, self, def_id.0, TypeAliasId;

            Metadata,
            CoroutineReturn,
            CoroutineYield,
            FutureOutput,
            CallRefFuture,
            CallOnceFuture,
            AsyncFnOnceOutput,
            AsyncFnKindUpvars,
            FieldBase,
            FieldType,
        )
    }

    fn as_trait_lang_item(self, def_id: Self::TraitId) -> Option<SolverTraitLangItem> {
        as_lang_item!(
            SolverTraitLangItem, self, def_id.0, TraitId;

            Sized,
            MetaSized,
            PointeeSized,
            Unsize,
            Copy,
            Clone,
            DiscriminantKind,
            PointeeTrait,
            FnPtrTrait,
            Drop,
            Destruct,
            TransmuteTrait,
            Fn,
            FnMut,
            FnOnce,
            Future,
            Coroutine,
            Unpin,
            Tuple,
            Iterator,
            AsyncFn,
            AsyncFnMut,
            AsyncFnOnce,
            TrivialClone,
            AsyncFnKindHelper,
            AsyncIterator,
            BikeshedGuaranteedNoDrop,
            FusedIterator,
            Field,
        )
    }

    fn as_adt_lang_item(self, def_id: Self::AdtId) -> Option<SolverAdtLangItem> {
        as_lang_item!(
            SolverAdtLangItem, self, def_id.0, AdtId;

            Option,
            Poll,
            DynMetadata,
        )
    }

    fn associated_type_def_ids(
        self,
        def_id: Self::TraitId,
    ) -> impl IntoIterator<Item = Self::DefId> {
        def_id.0.trait_items(self.db()).associated_types().map(|id| id.into())
    }

    fn for_each_relevant_impl<R: VisitorResult>(
        self,
        trait_def_id: Self::TraitId,
        self_ty: Self::Ty,
        mut f: impl FnMut(Self::ImplId) -> R,
    ) -> R {
        let krate = self.krate.expect("trait solving requires setting `DbInterner::krate`");
        let trait_block = trait_def_id.0.loc(self.db).container.block(self.db);
        let mut consider_impls_for_simplified_type = |simp: SimplifiedType<'_>| {
            let type_block = simp.def().and_then(|def_id| {
                let module = match def_id {
                    SolverDefId::AdtId(AdtId::StructId(id)) => id.module(self.db),
                    SolverDefId::AdtId(AdtId::EnumId(id)) => id.module(self.db),
                    SolverDefId::AdtId(AdtId::UnionId(id)) => id.module(self.db),
                    SolverDefId::TraitId(id) => id.module(self.db),
                    SolverDefId::TypeAliasId(id) => id.module(self.db),
                    SolverDefId::ConstId(_)
                    | SolverDefId::FunctionId(_)
                    | SolverDefId::ImplId(_)
                    | SolverDefId::BuiltinDeriveImplId(_)
                    | SolverDefId::StaticId(_)
                    | SolverDefId::InternedClosureId(_)
                    | SolverDefId::InternedCoroutineId(_)
                    | SolverDefId::InternedCoroutineClosureId(_)
                    | SolverDefId::InternedOpaqueTyId(_)
                    | SolverDefId::EnumVariantId(_)
                    | SolverDefId::AnonConstId(_)
                    | SolverDefId::Ctor(_) => return None,
                };
                module.block(self.db)
            });
            TraitImpls::for_each_crate_and_block_trait_and_type(
                self.db,
                krate,
                type_block,
                trait_block,
                &mut |impls| {
                    let (regular_impls, builtin_derive_impls) =
                        impls.for_trait_and_self_ty(trait_def_id.0, &simp);
                    for &impl_ in regular_impls {
                        try_visit!(f(impl_.into()));
                    }
                    for &impl_ in builtin_derive_impls {
                        try_visit!(f(impl_.into()));
                    }
                    R::output()
                },
            )
        };

        match self_ty.kind() {
            TyKind::Bool
            | TyKind::Char
            | TyKind::Int(_)
            | TyKind::Uint(_)
            | TyKind::Float(_)
            | TyKind::Adt(_, _)
            | TyKind::Foreign(_)
            | TyKind::Str
            | TyKind::Array(_, _)
            | TyKind::Pat(_, _)
            | TyKind::Slice(_)
            | TyKind::RawPtr(_, _)
            | TyKind::Ref(_, _, _)
            | TyKind::FnDef(_, _)
            | TyKind::FnPtr(..)
            | TyKind::Dynamic(_, _)
            | TyKind::Closure(..)
            | TyKind::CoroutineClosure(..)
            | TyKind::Coroutine(_, _)
            | TyKind::Never
            | TyKind::Tuple(_)
            | TyKind::UnsafeBinder(_) => {
                let simp =
                    fast_reject::simplify_type(self, self_ty, fast_reject::TreatParams::AsRigid)
                        .unwrap();
                try_visit!(consider_impls_for_simplified_type(simp));
            }

            // HACK: For integer and float variables we have to manually look at all impls
            // which have some integer or float as a self type.
            TyKind::Infer(InferTy::IntVar(_)) => {
                use IntTy::*;
                use UintTy::*;
                // This causes a compiler error if any new integer kinds are added.
                let (I8 | I16 | I32 | I64 | I128 | Isize): IntTy;
                let (U8 | U16 | U32 | U64 | U128 | Usize): UintTy;
                let possible_integers = [
                    // signed integers
                    SimplifiedType::Int(I8),
                    SimplifiedType::Int(I16),
                    SimplifiedType::Int(I32),
                    SimplifiedType::Int(I64),
                    SimplifiedType::Int(I128),
                    SimplifiedType::Int(Isize),
                    // unsigned integers
                    SimplifiedType::Uint(U8),
                    SimplifiedType::Uint(U16),
                    SimplifiedType::Uint(U32),
                    SimplifiedType::Uint(U64),
                    SimplifiedType::Uint(U128),
                    SimplifiedType::Uint(Usize),
                ];
                for simp in possible_integers {
                    try_visit!(consider_impls_for_simplified_type(simp));
                }
            }

            TyKind::Infer(InferTy::FloatVar(_)) => {
                // This causes a compiler error if any new float kinds are added.
                let (FloatTy::F16 | FloatTy::F32 | FloatTy::F64 | FloatTy::F128);
                let possible_floats = [
                    SimplifiedType::Float(FloatTy::F16),
                    SimplifiedType::Float(FloatTy::F32),
                    SimplifiedType::Float(FloatTy::F64),
                    SimplifiedType::Float(FloatTy::F128),
                ];

                for simp in possible_floats {
                    try_visit!(consider_impls_for_simplified_type(simp));
                }
            }

            // The only traits applying to aliases and placeholders are blanket impls.
            //
            // Impls which apply to an alias after normalization are handled by
            // `assemble_candidates_after_normalizing_self_ty`.
            TyKind::Alias(..) | TyKind::Placeholder(..) | TyKind::Error(_) => (),

            // FIXME: These should ideally not exist as a self type. It would be nice for
            // the builtin auto trait impls of coroutines to instead directly recurse
            // into the witness.
            TyKind::CoroutineWitness(..) => (),

            // These variants should not exist as a self type.
            TyKind::Infer(
                InferTy::TyVar(_)
                | InferTy::FreshTy(_)
                | InferTy::FreshIntTy(_)
                | InferTy::FreshFloatTy(_),
            )
            | TyKind::Param(_)
            | TyKind::Bound(_, _) => panic!("unexpected self type: {self_ty:?}"),
        }

        self.for_each_blanket_impl(trait_def_id, f)
    }

    fn for_each_blanket_impl<R: VisitorResult>(
        self,
        trait_def_id: Self::TraitId,
        mut f: impl FnMut(Self::ImplId) -> R,
    ) -> R {
        let Some(krate) = self.krate else {
            return R::output();
        };
        let block = trait_def_id.0.loc(self.db).container.block(self.db);

        TraitImpls::for_each_crate_and_block(self.db, krate, block, &mut |impls| {
            for &impl_ in impls.blanket_impls(trait_def_id.0) {
                try_visit!(f(impl_.into()));
            }
            R::output()
        })
    }

    fn has_item_definition(self, _def_id: Self::ImplOrTraitAssocTermId) -> bool {
        // FIXME(next-solver): should check if the associated item has a value.
        true
    }

    fn impl_is_default(self, impl_def_id: Self::ImplId) -> bool {
        match impl_def_id {
            AnyImplId::ImplId(impl_id) => ImplSignature::of(self.db, impl_id).is_default(),
            AnyImplId::BuiltinDeriveImplId(_) => false,
        }
    }

    #[tracing::instrument(skip(self), ret)]
    fn impl_trait_ref(
        self,
        impl_id: Self::ImplId,
    ) -> EarlyBinder<Self, rustc_type_ir::TraitRef<Self>> {
        match impl_id {
            AnyImplId::ImplId(impl_id) => {
                let db = self.db();
                db.impl_trait(impl_id)
                    // ImplIds for impls where the trait ref can't be resolved should never reach trait solving
                    .expect("invalid impl passed to trait solver")
            }
            AnyImplId::BuiltinDeriveImplId(impl_id) => {
                crate::builtin_derive::impl_trait(self, impl_id)
            }
        }
    }

    fn impl_polarity(self, impl_id: Self::ImplId) -> rustc_type_ir::ImplPolarity {
        let AnyImplId::ImplId(impl_id) = impl_id else {
            return ImplPolarity::Positive;
        };
        let impl_data = ImplSignature::of(self.db(), impl_id);
        if impl_data.flags.contains(ImplFlags::NEGATIVE) {
            ImplPolarity::Negative
        } else {
            ImplPolarity::Positive
        }
    }

    fn trait_is_auto(self, trait_: Self::TraitId) -> bool {
        let trait_data = TraitSignature::of(self.db(), trait_.0);
        trait_data.flags.contains(TraitFlags::AUTO)
    }

    fn trait_is_alias(self, trait_: Self::TraitId) -> bool {
        let trait_data = TraitSignature::of(self.db(), trait_.0);
        trait_data.flags.contains(TraitFlags::ALIAS)
    }

    fn trait_is_dyn_compatible(self, trait_: Self::TraitId) -> bool {
        crate::dyn_compatibility::dyn_compatibility(self.db(), trait_.0).is_none()
    }

    fn trait_is_fundamental(self, trait_: Self::TraitId) -> bool {
        let trait_data = TraitSignature::of(self.db(), trait_.0);
        trait_data.flags.contains(TraitFlags::FUNDAMENTAL)
    }

    fn is_impl_trait_in_trait(self, _def_id: Self::DefId) -> bool {
        // FIXME(next-solver)
        false
    }

    fn delay_bug(self, _msg: impl ToString) -> Self::ErrorGuaranteed {
        ErrorGuaranteed
    }

    fn is_general_coroutine(self, def_id: Self::CoroutineId) -> bool {
        matches!(def_id.0.loc(self.db).kind, HirClosureKind::OldCoroutine(_))
    }

    fn coroutine_is_async(self, def_id: Self::CoroutineId) -> bool {
        matches!(
            def_id.0.loc(self.db).kind,
            HirClosureKind::Coroutine { kind: HirCoroutineKind::Async, .. }
        )
    }

    fn coroutine_is_gen(self, def_id: Self::CoroutineId) -> bool {
        matches!(
            def_id.0.loc(self.db).kind,
            HirClosureKind::Coroutine { kind: HirCoroutineKind::Gen, .. }
        )
    }

    fn coroutine_is_async_gen(self, def_id: Self::CoroutineId) -> bool {
        matches!(
            def_id.0.loc(self.db).kind,
            HirClosureKind::Coroutine { kind: HirCoroutineKind::AsyncGen, .. }
        )
    }

    fn unsizing_params_for_adt(self, id: Self::AdtId) -> Self::UnsizingParams {
        let def = AdtDef::new(id.0, self);
        let num_params = self.generics_of(id.into()).count();

        let maybe_unsizing_param_idx = |arg: GenericArg<'db>| match arg.kind() {
            GenericArgKind::Type(ty) => match ty.kind() {
                rustc_type_ir::TyKind::Param(p) => Some(p.index),
                _ => None,
            },
            GenericArgKind::Lifetime(_) => None,
            GenericArgKind::Const(ct) => match ct.kind() {
                rustc_type_ir::ConstKind::Param(p) => Some(p.index),
                _ => None,
            },
        };

        // The last field of the structure has to exist and contain type/const parameters.
        let variant = match def.def_id() {
            AdtId::StructId(id) => VariantId::from(id),
            AdtId::UnionId(id) => id.into(),
            AdtId::EnumId(_) => panic!("expected a struct or a union"),
        };
        let fields = variant.fields(self.db());
        let mut prefix_fields = fields.fields().iter();
        let Some(tail_field) = prefix_fields.next_back() else {
            return UnsizingParams(DenseBitSet::new_empty(num_params));
        };

        let field_types = self.db().field_types(variant);
        let mut unsizing_params = DenseBitSet::new_empty(num_params);
        let ty = field_types[tail_field.0].ty();
        for arg in ty.instantiate_identity().skip_norm_wip().walk() {
            if let Some(i) = maybe_unsizing_param_idx(arg) {
                unsizing_params.insert(i);
            }
        }

        // Ensure none of the other fields mention the parameters used
        // in unsizing.
        for field in prefix_fields {
            for arg in field_types[field.0].ty().instantiate_identity().skip_norm_wip().walk() {
                if let Some(i) = maybe_unsizing_param_idx(arg) {
                    unsizing_params.remove(i);
                }
            }
        }

        UnsizingParams(unsizing_params)
    }

    fn anonymize_bound_vars<T: rustc_type_ir::TypeFoldable<Self>>(
        self,
        value: rustc_type_ir::Binder<Self, T>,
    ) -> rustc_type_ir::Binder<Self, T> {
        struct Anonymize<'a, 'db> {
            interner: DbInterner<'db>,
            map: &'a mut FxIndexMap<BoundVar, BoundVariableKind<'db>>,
        }
        impl<'db> BoundVarReplacerDelegate<'db> for Anonymize<'_, 'db> {
            fn replace_region(&mut self, br: BoundRegion<'db>) -> Region<'db> {
                let entry = self.map.entry(br.var);
                let index = entry.index();
                let var = BoundVar::from_usize(index);
                let kind = (*entry
                    .or_insert_with(|| BoundVariableKind::Region(BoundRegionKind::Anon)))
                .expect_region();
                let br = BoundRegion { var, kind };
                Region::new_bound(self.interner, DebruijnIndex::ZERO, br)
            }
            fn replace_ty(&mut self, bt: BoundTy<'db>) -> Ty<'db> {
                let entry = self.map.entry(bt.var);
                let index = entry.index();
                let var = BoundVar::from_usize(index);
                let kind = (*entry.or_insert_with(|| BoundVariableKind::Ty(BoundTyKind::Anon)))
                    .expect_ty();
                Ty::new_bound(self.interner, DebruijnIndex::ZERO, BoundTy { var, kind })
            }
            fn replace_const(&mut self, bv: BoundConst<'db>) -> Const<'db> {
                let entry = self.map.entry(bv.var);
                let index = entry.index();
                let var = BoundVar::from_usize(index);
                let () = (*entry.or_insert_with(|| BoundVariableKind::Const)).expect_const();
                Const::new_bound(self.interner, DebruijnIndex::ZERO, BoundConst::new(var))
            }
        }

        let mut map = Default::default();
        let delegate = Anonymize { interner: self, map: &mut map };
        let inner = self.replace_escaping_bound_vars_uncached(value.skip_binder(), delegate);
        let bound_vars = BoundVarKinds::new_from_iter(map.into_values());
        Binder::bind_with_vars(inner, bound_vars)
    }

    fn opaque_types_defined_by(self, def_id: Self::LocalDefId) -> Self::LocalDefIds {
        let Ok(def_id) = InferBodyId::try_from(def_id) else {
            return SolverDefIds::default();
        };
        let mut result = Vec::new();
        crate::opaques::opaque_types_defined_by(self.db, def_id, &mut result);
        SolverDefIds::new_from_slice(&result)
    }

    fn opaque_types_and_coroutines_defined_by(self, def_id: Self::LocalDefId) -> Self::LocalDefIds {
        let db = self.db;

        let Ok(def_id) = InferBodyId::try_from(def_id) else {
            return SolverDefIds::default();
        };
        let mut result = Vec::new();

        crate::opaques::opaque_types_defined_by(db, def_id, &mut result);

        // Collect coroutines.
        let (store, root_expr) = def_id.store_and_root_expr(db);
        // We can't just visit all exprs, since this may end up in unrelated anon consts.
        CoroutinesVisitor { db: self.db, owner: def_id, store, coroutines: &mut result }
            .on_expr(root_expr);

        return SolverDefIds::new_from_slice(&result);

        struct CoroutinesVisitor<'a, 'db> {
            db: &'db dyn HirDatabase,
            owner: InferBodyId<'db>,
            store: &'db ExpressionStore,
            coroutines: &'a mut Vec<SolverDefId<'db>>,
        }

        impl<'db> StoreVisitor for CoroutinesVisitor<'_, 'db> {
            fn on_expr(&mut self, expr: ExprId) {
                if let hir_def::hir::Expr::Closure {
                    closure_kind:
                        kind @ (hir_def::hir::ClosureKind::Coroutine { .. }
                        | hir_def::hir::ClosureKind::OldCoroutine(_)),
                    ..
                } = self.store[expr]
                {
                    let coroutine = InternedCoroutineId::new(
                        self.db,
                        InternedClosure { owner: self.owner, expr, kind },
                    );
                    self.coroutines.push(coroutine.into());
                }

                self.store.visit_expr_children(expr, self);
            }
            fn on_pat(&mut self, pat: PatId) {
                self.store.visit_pat_children(pat, self);
            }
            // Do not visit anon consts, they're separate bodies.
            fn on_anon_const_expr(&mut self, _expr: ExprId) {}
        }
    }

    fn alias_has_const_conditions(self, _def_id: Self::DefId) -> bool {
        // FIXME(next-solver)
        false
    }

    fn explicit_implied_const_bounds(
        self,
        _def_id: Self::DefId,
    ) -> EarlyBinder<
        Self,
        impl IntoIterator<Item = rustc_type_ir::Binder<Self, rustc_type_ir::TraitRef<Self>>>,
    > {
        // FIXME(next-solver)
        EarlyBinder::bind([])
    }

    fn fn_is_const(self, id: Self::FunctionId) -> bool {
        let id = match id.0 {
            CallableDefId::FunctionId(id) => id,
            _ => return false,
        };
        FunctionSignature::of(self.db(), id).flags.contains(FnFlags::CONST)
    }

    fn impl_is_const(self, _def_id: Self::ImplId) -> bool {
        false
    }

    fn opt_alias_variances(
        self,
        _kind: impl Into<AliasTermKind<'db>>,
    ) -> Option<Self::VariancesOf> {
        None
    }

    fn type_of_opaque_hir_typeck(
        self,
        opaque: Self::LocalOpaqueTyId,
    ) -> EarlyBinder<Self, Self::Ty> {
        let impl_trait_id = opaque.0.loc(self.db);
        // The entry is missing when this call cycles back into the still-running inference
        // of the defining body, as the cycle fallback is an empty result.
        let hidden_type = match impl_trait_id {
            crate::ImplTraitId::ReturnTypeImplTrait(func, idx) => {
                crate::opaques::rpit_hidden_types(self.db, func).get(idx)
            }
            crate::ImplTraitId::TypeAliasImplTrait(type_alias, idx) => {
                crate::opaques::tait_hidden_types(self.db, type_alias).get(idx)
            }
        };
        match hidden_type {
            Some(hidden_type) => hidden_type.get(),
            None => EarlyBinder::bind(Ty::new_error(self, ErrorGuaranteed)),
        }
    }

    fn coroutine_hidden_types(
        self,
        _def_id: Self::CoroutineId,
    ) -> EarlyBinder<Self, Binder<'db, CoroutineWitnessTypes<Self>>> {
        // FIXME: Actually implement this.
        EarlyBinder::bind(Binder::dummy(CoroutineWitnessTypes {
            types: Tys::default(),
            assumptions: RegionAssumptions::default(),
        }))
    }

    fn is_default_trait(self, def_id: Self::TraitId) -> bool {
        self.as_trait_lang_item(def_id).map_or(false, |l| matches!(l, SolverTraitLangItem::Sized))
    }

    fn trait_is_coinductive(self, trait_: Self::TraitId) -> bool {
        TraitSignature::of(self.db(), trait_.0).flags.contains(TraitFlags::COINDUCTIVE)
    }

    fn trait_is_unsafe(self, trait_: Self::TraitId) -> bool {
        TraitSignature::of(self.db(), trait_.0).flags.contains(TraitFlags::UNSAFE)
    }

    fn impl_self_is_guaranteed_unsized(self, _def_id: Self::ImplId) -> bool {
        false
    }

    fn impl_specializes(
        self,
        specializing_impl_def_id: Self::ImplId,
        parent_impl_def_id: Self::ImplId,
    ) -> bool {
        let (AnyImplId::ImplId(specializing_impl_def_id), AnyImplId::ImplId(parent_impl_def_id)) =
            (specializing_impl_def_id, parent_impl_def_id)
        else {
            // No builtin derive allow specialization currently.
            return false;
        };
        crate::specialization::specializes(self.db, specializing_impl_def_id, parent_impl_def_id)
    }

    fn next_trait_solver_globally(self) -> bool {
        true
    }

    type Probe = rustc_type_ir::solve::inspect::Probe<DbInterner<'db>>;
    fn mk_probe(self, probe: rustc_type_ir::solve::inspect::Probe<Self>) -> Self::Probe {
        probe
    }
    fn evaluate_root_goal_for_proof_tree_raw(
        self,
        canonical_goal: rustc_type_ir::solve::CanonicalInput<Self>,
    ) -> (rustc_type_ir::solve::QueryResult<Self>, Self::Probe) {
        rustc_next_trait_solver::solve::evaluate_root_goal_for_proof_tree_raw_provider::<
            SolverContext<'db>,
            Self,
        >(self, canonical_goal)
    }

    fn is_sizedness_trait(self, def_id: Self::TraitId) -> bool {
        matches!(
            self.as_trait_lang_item(def_id),
            Some(SolverTraitLangItem::Sized | SolverTraitLangItem::MetaSized)
        )
    }

    fn const_of_item(self, def_id: Self::DefId) -> rustc_type_ir::EarlyBinder<Self, Self::Const> {
        let id = match def_id {
            SolverDefId::StaticId(id) => id.into(),
            SolverDefId::ConstId(id) => id.into(),
            _ => unreachable!(),
        };
        EarlyBinder::bind(Const::new_unevaluated(
            self,
            UnevaluatedConst { def: GeneralConstIdWrapper(id), args: GenericArgs::empty() },
        ))
    }

    fn anon_const_kind(self, _def_id: Self::DefId) -> rustc_type_ir::AnonConstKind {
        // FIXME
        rustc_type_ir::AnonConstKind::GCE
    }

    fn alias_ty_kind_from_def_id(self, def_id: Self::DefId) -> AliasTyKind<'db> {
        match def_id {
            SolverDefId::TypeAliasId(type_alias) => match type_alias.loc(self.db).container {
                ItemContainerId::ExternBlockId(_) | ItemContainerId::ModuleId(_) => {
                    AliasTyKind::Free { def_id: type_alias.into() }
                }
                ItemContainerId::ImplId(_) => AliasTyKind::Inherent { def_id: type_alias.into() },
                ItemContainerId::TraitId(_) => {
                    AliasTyKind::Projection { def_id: type_alias.into() }
                }
            },
            SolverDefId::InternedOpaqueTyId(def_id) => {
                AliasTyKind::Opaque { def_id: def_id.into() }
            }
            _ => unreachable!(),
        }
    }

    fn closure_is_const(self, _def_id: Self::ClosureId) -> bool {
        // FIXME
        false
    }

    fn item_name(self, _item_index: Self::DefId) -> Self::Symbol {
        Symbol
    }
}

fn is_ty_self(ty: Ty<'_>) -> bool {
    match ty.kind() {
        TyKind::Param(param) => param.index == 0,
        _ => false,
    }
}
fn is_clause_at_ty(p: &Clause<'_>, filter: impl FnOnce(Ty<'_>) -> bool) -> bool {
    match p.kind().skip_binder() {
        // rustc has the following assertion:
        // https://github.com/rust-lang/rust/blob/52618eb338609df44978b0ca4451ab7941fd1c7a/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs#L525-L608
        ClauseKind::Trait(it) => filter(it.self_ty()),
        ClauseKind::TypeOutlives(it) => filter(it.0),
        ClauseKind::Projection(it) => filter(it.self_ty()),
        ClauseKind::HostEffect(it) => filter(it.self_ty()),
        _ => false,
    }
}

impl<'db> DbInterner<'db> {
    pub fn shift_bound_var_indices<T>(self, bound_vars: usize, value: T) -> T
    where
        T: rustc_type_ir::TypeFoldable<Self>,
    {
        let shift_bv = |bv: BoundVar| BoundVar::from_usize(bv.as_usize() + bound_vars);
        self.replace_escaping_bound_vars_uncached(
            value,
            FnMutDelegate {
                regions: &mut |r: BoundRegion<'db>| {
                    Region::new_bound(
                        self,
                        DebruijnIndex::ZERO,
                        BoundRegion { var: shift_bv(r.var), kind: r.kind },
                    )
                },
                types: &mut |t: BoundTy<'db>| {
                    Ty::new_bound(
                        self,
                        DebruijnIndex::ZERO,
                        BoundTy { var: shift_bv(t.var), kind: t.kind },
                    )
                },
                consts: &mut |c| {
                    Const::new_bound(self, DebruijnIndex::ZERO, BoundConst::new(shift_bv(c.var)))
                },
            },
        )
    }

    pub fn replace_escaping_bound_vars_uncached<T: rustc_type_ir::TypeFoldable<DbInterner<'db>>>(
        self,
        value: T,
        delegate: impl BoundVarReplacerDelegate<'db>,
    ) -> T {
        if !value.has_escaping_bound_vars() {
            value
        } else {
            let mut replacer = BoundVarReplacer::new(self, delegate);
            value.fold_with(&mut replacer)
        }
    }

    pub fn replace_bound_vars_uncached<T: rustc_type_ir::TypeFoldable<DbInterner<'db>>>(
        self,
        value: Binder<'db, T>,
        delegate: impl BoundVarReplacerDelegate<'db>,
    ) -> T {
        self.replace_escaping_bound_vars_uncached(value.skip_binder(), delegate)
    }

    // FIXME: add splat support when the experiment is complete
    pub fn mk_fn_sig<I>(
        self,
        inputs: I,
        output: Ty<'db>,
        c_variadic: bool,
        safety: Safety,
        abi: ExternAbi,
    ) -> FnSig<'db>
    where
        I: IntoIterator<Item = Ty<'db>>,
    {
        FnSig {
            inputs_and_output: Tys::new_from_iter(
                inputs.into_iter().chain(std::iter::once(output)),
            ),
            fn_sig_kind: FnSigKind::new(abi, safety, c_variadic),
        }
    }

    /// `mk_fn_sig`, but with a safe Rust ABI, and no C-variadic argument.
    pub fn mk_fn_sig_safe_rust_abi<I>(self, inputs: I, output: Ty<'db>) -> FnSig<'db>
    where
        I: IntoIterator<Item = Ty<'db>>,
    {
        self.mk_fn_sig(inputs, output, false, Safety::Safe, ExternAbi::Rust)
    }
}

fn predicates_of<'db>(
    db: &'db dyn HirDatabase,
    def_id: SolverDefId<'db>,
) -> &'db GenericPredicates {
    match def_id {
        SolverDefId::BuiltinDeriveImplId(impl_) => crate::builtin_derive::predicates(db, impl_),
        SolverDefId::AnonConstId(anon_const) => {
            let loc = anon_const.loc(db);
            if loc.allow_using_generic_params {
                GenericPredicates::query(db, loc.owner.generic_def(db))
            } else {
                GenericPredicates::empty()
            }
        }
        _ => GenericPredicates::query(db, def_id.try_into().unwrap()),
    }
}

macro_rules! TrivialTypeTraversalImpls {
    ($($ty:ty,)+) => {
        $(
            impl<'db> rustc_type_ir::TypeFoldable<DbInterner<'db>> for $ty {
                fn try_fold_with<F: rustc_type_ir::FallibleTypeFolder<DbInterner<'db>>>(
                    self,
                    _: &mut F,
                ) -> ::std::result::Result<Self, F::Error> {
                    Ok(self)
                }

                #[inline]
                fn fold_with<F: rustc_type_ir::TypeFolder<DbInterner<'db>>>(
                    self,
                    _: &mut F,
                ) -> Self {
                    self
                }
            }

            impl<'db> rustc_type_ir::TypeVisitable<DbInterner<'db>> for $ty {
                #[inline]
                fn visit_with<F: rustc_type_ir::TypeVisitor<DbInterner<'db>>>(
                    &self,
                    _: &mut F)
                    -> F::Result
                {
                    <F::Result as rustc_ast_ir::visit::VisitorResult>::output()
                }
            }

            impl<V> rustc_type_ir::GenericTypeVisitable<V> for $ty {
                #[inline]
                fn generic_visit_with(&self, _visitor: &mut V) {}
            }
        )+
    };
}

TrivialTypeTraversalImpls! {
    SolverDefId<'_>,
    TraitIdWrapper,
    TypeAliasIdWrapper,
    CallableIdWrapper,
    ClosureIdWrapper<'_>,
    CoroutineIdWrapper<'_>,
    CoroutineClosureIdWrapper<'_>,
    AdtIdWrapper,
    TraitAssocTyId,
    TraitAssocConstId,
    TraitAssocTermId,
    ImplOrTraitAssocTyId,
    ImplOrTraitAssocConstId,
    ImplOrTraitAssocTermId,
    FreeTyAliasId,
    FreeConstAliasId,
    FreeTermAliasId,
    InherentAssocTyId,
    InherentAssocConstId,
    InherentAssocTermId,
    OpaqueTyIdWrapper<'_>,
    AnyImplId,
    GeneralConstIdWrapper<'_>,
    Safety,
    Span,
    ParamConst,
    ParamTy,
    EarlyParamRegion,
    AdtDef,
    ScalarInt,
}

mod tls_db {
    use std::{cell::Cell, ptr::NonNull};

    use crate::db::HirDatabase;

    struct Attached {
        database: Cell<Option<NonNull<dyn HirDatabase>>>,
    }

    impl Attached {
        #[inline]
        fn attach<R>(&self, db: &dyn HirDatabase, op: impl FnOnce() -> R) -> R {
            struct DbGuard<'s> {
                state: Option<&'s Attached>,
            }

            impl<'s> DbGuard<'s> {
                #[inline]
                fn new(attached: &'s Attached, db: &dyn HirDatabase) -> Self {
                    match attached.database.get() {
                        Some(current_db) => {
                            let new_db = NonNull::from(db);
                            if !std::ptr::addr_eq(current_db.as_ptr(), new_db.as_ptr()) {
                                panic!(
                                    "Cannot change attached database. This is likely a bug.\n\
                                    If this is not a bug, you can use `attach_db_allow_change()`."
                                );
                            }
                            Self { state: None }
                        }
                        None => {
                            // Otherwise, set the database.
                            attached.database.set(Some(NonNull::from(db)));
                            Self { state: Some(attached) }
                        }
                    }
                }
            }

            impl Drop for DbGuard<'_> {
                #[inline]
                fn drop(&mut self) {
                    // Reset database to null if we did anything in `DbGuard::new`.
                    if let Some(attached) = self.state {
                        attached.database.set(None);
                    }
                }
            }

            let _guard = DbGuard::new(self, db);
            super::tls_cache::reinit_cache(db);
            op()
        }

        #[inline]
        fn attach_allow_change<R>(&self, db: &dyn HirDatabase, op: impl FnOnce() -> R) -> R {
            struct DbGuard<'s> {
                state: &'s Attached,
                prev: Option<NonNull<dyn HirDatabase>>,
            }

            impl<'s> DbGuard<'s> {
                #[inline]
                fn new(attached: &'s Attached, db: &dyn HirDatabase) -> Self {
                    let prev = attached.database.replace(Some(NonNull::from(db)));
                    Self { state: attached, prev }
                }
            }

            impl Drop for DbGuard<'_> {
                #[inline]
                fn drop(&mut self) {
                    self.state.database.set(self.prev);
                    if let Some(prev) = self.prev {
                        super::tls_cache::reinit_cache(unsafe { prev.as_ref() });
                    }
                }
            }

            let _guard = DbGuard::new(self, db);
            super::tls_cache::reinit_cache(db);
            op()
        }

        #[inline]
        fn with<R>(&self, op: impl FnOnce(&dyn HirDatabase) -> R) -> R {
            let db = self.database.get().expect("Tried to use attached db, but no db is attached");

            // SAFETY: The db is attached, so it must be valid.
            op(unsafe { db.as_ref() })
        }
    }

    thread_local! {
        static GLOBAL_DB: Attached = const { Attached { database: Cell::new(None) } };
    }

    #[inline]
    pub fn attach_db<R>(db: &dyn HirDatabase, op: impl FnOnce() -> R) -> R {
        GLOBAL_DB.with(|global_db| global_db.attach(db, op))
    }

    #[inline]
    pub fn attach_db_allow_change<R>(db: &dyn HirDatabase, op: impl FnOnce() -> R) -> R {
        GLOBAL_DB.with(|global_db| global_db.attach_allow_change(db, op))
    }

    #[inline]
    pub fn with_attached_db<R>(op: impl FnOnce(&dyn HirDatabase) -> R) -> R {
        GLOBAL_DB.with(
            #[inline]
            |a| a.with(op),
        )
    }
}

mod tls_cache {
    use crate::db::HirDatabase;

    use super::DbInterner;
    use base_db::Nonce;
    use rustc_type_ir::search_graph::GlobalCache;
    use salsa::Revision;
    use std::cell::RefCell;

    struct Cache {
        cache: GlobalCache<DbInterner<'static>>,
        revision: Revision,
        db_nonce: Nonce,
    }

    impl Cache {
        const fn default() -> Cache {
            Cache {
                cache: GlobalCache::new(),
                revision: Revision::max(),
                db_nonce: Nonce::invalid(),
            }
        }
    }

    thread_local! {
        static GLOBAL_CACHE: RefCell<Cache> = const { RefCell::new(Cache::default()) };
    }

    pub(super) fn reinit_cache(db: &dyn HirDatabase) {
        GLOBAL_CACHE.with_borrow_mut(|handle| {
            let (db_nonce, revision) = db.nonce_and_revision();
            if handle.revision != revision || db_nonce != handle.db_nonce {
                *handle = Cache { cache: GlobalCache::default(), revision, db_nonce };
            }
        })
    }

    #[inline]
    pub(super) fn borrow_assume_valid<'db, T>(
        db: &'db dyn HirDatabase,
        f: impl FnOnce(&mut GlobalCache<DbInterner<'db>>) -> T,
    ) -> T {
        if cfg!(debug_assertions) {
            let get_state =
                || GLOBAL_CACHE.with_borrow(|handle| (handle.db_nonce, handle.revision));
            let old_state = get_state();
            reinit_cache(db);
            let new_state = get_state();
            assert_eq!(old_state, new_state, "you assumed the cache is valid!");
        }

        GLOBAL_CACHE.with_borrow_mut(|handle| {
            // SAFETY: No idea
            f(unsafe {
                std::mem::transmute::<
                    &mut GlobalCache<DbInterner<'static>>,
                    &mut GlobalCache<DbInterner<'db>>,
                >(&mut handle.cache)
            })
        })
    }

    /// Clears the thread-local trait solver cache.
    ///
    /// Should be called before getting memory usage estimations, as the solver cache
    /// is per-revision and usually should be excluded from estimations.
    pub fn clear_tls_solver_cache() {
        GLOBAL_CACHE.with_borrow_mut(|handle| *handle = Cache::default());
    }
}

impl WorldExposer for intern::GarbageCollector {
    fn on_interned<T: intern::Internable>(
        &mut self,
        interned: InternedRef<'_, T>,
    ) -> ControlFlow<()> {
        self.mark_interned_alive(interned)
    }

    fn on_interned_slice<T: intern::SliceInternable>(
        &mut self,
        interned: InternedSliceRef<'_, T>,
    ) -> ControlFlow<()> {
        self.mark_interned_slice_alive(interned)
    }
}

/// # Safety
///
/// This cannot be called if there are some not-yet-recorded type values. Generally, if you have a mutable
/// reference to the database, and there are no other database - then you can call this safely, but you
/// also need to make sure to maintain the mutable reference while this is running.
pub unsafe fn collect_ty_garbage() {
    let mut gc = intern::GarbageCollector::default();

    gc.add_storage::<super::consts::ConstInterned>();
    gc.add_storage::<super::consts::ValTreeInterned>();
    gc.add_storage::<super::allocation::AllocationInterned>();
    gc.add_storage::<PatternInterned>();
    gc.add_storage::<super::opaques::ExternalConstraintsInterned>();
    gc.add_storage::<super::predicate::PredicateInterned>();
    gc.add_storage::<super::region::RegionInterned>();
    gc.add_storage::<super::ty::TyInterned>();

    gc.add_slice_storage::<super::consts::ConstsStorage>();
    gc.add_slice_storage::<super::predicate::ClausesStorage>();
    gc.add_slice_storage::<super::generic_arg::GenericArgsStorage>();
    gc.add_slice_storage::<BoundVarKindsStorage>();
    gc.add_slice_storage::<VariancesOfStorage>();
    gc.add_slice_storage::<CanonicalVarsStorage>();
    gc.add_slice_storage::<PatListStorage>();
    gc.add_slice_storage::<super::opaques::PredefinedOpaquesStorage>();
    gc.add_slice_storage::<super::opaques::SolverDefIdsStorage>();
    gc.add_slice_storage::<super::predicate::BoundExistentialPredicatesStorage>();
    gc.add_slice_storage::<super::region::RegionAssumptionsStorage>();
    gc.add_slice_storage::<super::ty::TysStorage>();
    gc.add_slice_storage::<crate::mir::ProjectionStorage>();

    // SAFETY:
    //  - By our precondition, there are no unrecorded types.
    //  - We implement `GcInternedVisit` and `GcInternedSliceVisit` correctly for all types.
    //  - We added all storages (FIXME: it's too easy to forget to add a new storage here).
    unsafe { gc.collect() };
}

macro_rules! impl_gc_visit {
    ( $($ty:ty),* $(,)? ) => {
        $(
            impl ::intern::GcInternedVisit for $ty {
                #[inline]
                fn visit_with(&self, gc: &mut ::intern::GarbageCollector) {
                    self.generic_visit_with(gc);
                }
            }
        )*
    };
}

impl_gc_visit!(
    super::consts::ConstInterned,
    super::consts::ValTreeInterned,
    super::allocation::AllocationInterned,
    PatternInterned,
    super::opaques::ExternalConstraintsInterned,
    super::predicate::PredicateInterned,
    super::region::RegionInterned,
    super::ty::TyInterned,
    super::predicate::ClausesCachedTypeInfo,
);

macro_rules! impl_gc_visit_slice {
    ( $($ty:ty),* $(,)? ) => {
        $(
            impl ::intern::GcInternedSliceVisit for $ty {
                #[inline]
                fn visit_header(header: &<Self as ::intern::SliceInternable>::Header, gc: &mut ::intern::GarbageCollector) {
                    header.generic_visit_with(gc);
                }

                #[inline]
                fn visit_slice(slice: &[<Self as ::intern::SliceInternable>::SliceType], gc: &mut ::intern::GarbageCollector) {
                    slice.generic_visit_with(gc);
                }
            }
        )*
    };
}

impl_gc_visit_slice!(
    super::predicate::ClausesStorage,
    super::generic_arg::GenericArgsStorage,
    BoundVarKindsStorage,
    VariancesOfStorage,
    CanonicalVarsStorage,
    PatListStorage,
    super::opaques::PredefinedOpaquesStorage,
    super::opaques::SolverDefIdsStorage,
    super::predicate::BoundExistentialPredicatesStorage,
    super::region::RegionAssumptionsStorage,
    super::ty::TysStorage,
    super::consts::ConstsStorage,
    crate::mir::ProjectionStorage,
);