1 files changed, 1242 insertions, 0 deletions

diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
new file mode 100644
index 000000000..cd574e983
--- /dev/null
+++ b/crates/hir_ty/src/lower.rs
@@ -0,0 +1,1242 @@
+//! Methods for lowering the HIR to types. There are two main cases here:
+//!
+//!  - Lowering a type reference like `&usize` or `Option<foo::bar::Baz>` to a
+//!    type: The entry point for this is `Ty::from_hir`.
+//!  - Building the type for an item: This happens through the `type_for_def` query.
+//!
+//! This usually involves resolving names, collecting generic arguments etc.
+use std::{iter, sync::Arc};
+
+use arena::map::ArenaMap;
+use base_db::CrateId;
+use hir_def::{
+    adt::StructKind,
+    builtin_type::BuiltinType,
+    generics::{TypeParamProvenance, WherePredicate, WherePredicateTarget},
+    path::{GenericArg, Path, PathSegment, PathSegments},
+    resolver::{HasResolver, Resolver, TypeNs},
+    type_ref::{TypeBound, TypeRef},
+    AdtId, AssocContainerId, AssocItemId, ConstId, EnumId, EnumVariantId, FunctionId, GenericDefId,
+    HasModule, ImplId, LocalFieldId, Lookup, StaticId, StructId, TraitId, TypeAliasId, TypeParamId,
+    UnionId, VariantId,
+};
+use hir_expand::name::Name;
+use smallvec::SmallVec;
+use stdx::impl_from;
+use test_utils::mark;
+
+use crate::{
+    db::HirDatabase,
+    primitive::{FloatTy, IntTy},
+    utils::{
+        all_super_trait_refs, associated_type_by_name_including_super_traits, generics,
+        make_mut_slice, variant_data,
+    },
+    Binders, BoundVar, DebruijnIndex, FnSig, GenericPredicate, OpaqueTy, OpaqueTyId, PolyFnSig,
+    ProjectionPredicate, ProjectionTy, ReturnTypeImplTrait, ReturnTypeImplTraits, Substs,
+    TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
+};
+
+#[derive(Debug)]
+pub struct TyLoweringContext<'a> {
+    pub db: &'a dyn HirDatabase,
+    pub resolver: &'a Resolver,
+    in_binders: DebruijnIndex,
+    /// Note: Conceptually, it's thinkable that we could be in a location where
+    /// some type params should be represented as placeholders, and others
+    /// should be converted to variables. I think in practice, this isn't
+    /// possible currently, so this should be fine for now.
+    pub type_param_mode: TypeParamLoweringMode,
+    pub impl_trait_mode: ImplTraitLoweringMode,
+    impl_trait_counter: std::cell::Cell<u16>,
+    /// When turning `impl Trait` into opaque types, we have to collect the
+    /// bounds at the same time to get the IDs correct (without becoming too
+    /// complicated). I don't like using interior mutability (as for the
+    /// counter), but I've tried and failed to make the lifetimes work for
+    /// passing around a `&mut TyLoweringContext`. The core problem is that
+    /// we're grouping the mutable data (the counter and this field) together
+    /// with the immutable context (the references to the DB and resolver).
+    /// Splitting this up would be a possible fix.
+    opaque_type_data: std::cell::RefCell<Vec<ReturnTypeImplTrait>>,
+}
+
+impl<'a> TyLoweringContext<'a> {
+    pub fn new(db: &'a dyn HirDatabase, resolver: &'a Resolver) -> Self {
+        let impl_trait_counter = std::cell::Cell::new(0);
+        let impl_trait_mode = ImplTraitLoweringMode::Disallowed;
+        let type_param_mode = TypeParamLoweringMode::Placeholder;
+        let in_binders = DebruijnIndex::INNERMOST;
+        let opaque_type_data = std::cell::RefCell::new(Vec::new());
+        Self {
+            db,
+            resolver,
+            in_binders,
+            impl_trait_mode,
+            impl_trait_counter,
+            type_param_mode,
+            opaque_type_data,
+        }
+    }
+
+    pub fn with_debruijn<T>(
+        &self,
+        debruijn: DebruijnIndex,
+        f: impl FnOnce(&TyLoweringContext) -> T,
+    ) -> T {
+        let opaque_ty_data_vec = self.opaque_type_data.replace(Vec::new());
+        let new_ctx = Self {
+            in_binders: debruijn,
+            impl_trait_counter: std::cell::Cell::new(self.impl_trait_counter.get()),
+            opaque_type_data: std::cell::RefCell::new(opaque_ty_data_vec),
+            ..*self
+        };
+        let result = f(&new_ctx);
+        self.impl_trait_counter.set(new_ctx.impl_trait_counter.get());
+        self.opaque_type_data.replace(new_ctx.opaque_type_data.into_inner());
+        result
+    }
+
+    pub fn with_shifted_in<T>(
+        &self,
+        debruijn: DebruijnIndex,
+        f: impl FnOnce(&TyLoweringContext) -> T,
+    ) -> T {
+        self.with_debruijn(self.in_binders.shifted_in_from(debruijn), f)
+    }
+
+    pub fn with_impl_trait_mode(self, impl_trait_mode: ImplTraitLoweringMode) -> Self {
+        Self { impl_trait_mode, ..self }
+    }
+
+    pub fn with_type_param_mode(self, type_param_mode: TypeParamLoweringMode) -> Self {
+        Self { type_param_mode, ..self }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum ImplTraitLoweringMode {
+    /// `impl Trait` gets lowered into an opaque type that doesn't unify with
+    /// anything except itself. This is used in places where values flow 'out',
+    /// i.e. for arguments of the function we're currently checking, and return
+    /// types of functions we're calling.
+    Opaque,
+    /// `impl Trait` gets lowered into a type variable. Used for argument
+    /// position impl Trait when inside the respective function, since it allows
+    /// us to support that without Chalk.
+    Param,
+    /// `impl Trait` gets lowered into a variable that can unify with some
+    /// type. This is used in places where values flow 'in', i.e. for arguments
+    /// of functions we're calling, and the return type of the function we're
+    /// currently checking.
+    Variable,
+    /// `impl Trait` is disallowed and will be an error.
+    Disallowed,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum TypeParamLoweringMode {
+    Placeholder,
+    Variable,
+}
+
+impl Ty {
+    pub fn from_hir(ctx: &TyLoweringContext<'_>, type_ref: &TypeRef) -> Self {
+        Ty::from_hir_ext(ctx, type_ref).0
+    }
+    pub fn from_hir_ext(ctx: &TyLoweringContext<'_>, type_ref: &TypeRef) -> (Self, Option<TypeNs>) {
+        let mut res = None;
+        let ty = match type_ref {
+            TypeRef::Never => Ty::simple(TypeCtor::Never),
+            TypeRef::Tuple(inner) => {
+                let inner_tys: Arc<[Ty]> = inner.iter().map(|tr| Ty::from_hir(ctx, tr)).collect();
+                Ty::apply(
+                    TypeCtor::Tuple { cardinality: inner_tys.len() as u16 },
+                    Substs(inner_tys),
+                )
+            }
+            TypeRef::Path(path) => {
+                let (ty, res_) = Ty::from_hir_path(ctx, path);
+                res = res_;
+                ty
+            }
+            TypeRef::RawPtr(inner, mutability) => {
+                let inner_ty = Ty::from_hir(ctx, inner);
+                Ty::apply_one(TypeCtor::RawPtr(*mutability), inner_ty)
+            }
+            TypeRef::Array(inner) => {
+                let inner_ty = Ty::from_hir(ctx, inner);
+                Ty::apply_one(TypeCtor::Array, inner_ty)
+            }
+            TypeRef::Slice(inner) => {
+                let inner_ty = Ty::from_hir(ctx, inner);
+                Ty::apply_one(TypeCtor::Slice, inner_ty)
+            }
+            TypeRef::Reference(inner, mutability) => {
+                let inner_ty = Ty::from_hir(ctx, inner);
+                Ty::apply_one(TypeCtor::Ref(*mutability), inner_ty)
+            }
+            TypeRef::Placeholder => Ty::Unknown,
+            TypeRef::Fn(params, is_varargs) => {
+                let sig = Substs(params.iter().map(|tr| Ty::from_hir(ctx, tr)).collect());
+                Ty::apply(
+                    TypeCtor::FnPtr { num_args: sig.len() as u16 - 1, is_varargs: *is_varargs },
+                    sig,
+                )
+            }
+            TypeRef::DynTrait(bounds) => {
+                let self_ty = Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0));
+                let predicates = ctx.with_shifted_in(DebruijnIndex::ONE, |ctx| {
+                    bounds
+                        .iter()
+                        .flat_map(|b| GenericPredicate::from_type_bound(ctx, b, self_ty.clone()))
+                        .collect()
+                });
+                Ty::Dyn(predicates)
+            }
+            TypeRef::ImplTrait(bounds) => {
+                match ctx.impl_trait_mode {
+                    ImplTraitLoweringMode::Opaque => {
+                        let idx = ctx.impl_trait_counter.get();
+                        ctx.impl_trait_counter.set(idx + 1);
+
+                        assert!(idx as usize == ctx.opaque_type_data.borrow().len());
+                        // this dance is to make sure the data is in the right
+                        // place even if we encounter more opaque types while
+                        // lowering the bounds
+                        ctx.opaque_type_data
+                            .borrow_mut()
+                            .push(ReturnTypeImplTrait { bounds: Binders::new(1, Vec::new()) });
+                        // We don't want to lower the bounds inside the binders
+                        // we're currently in, because they don't end up inside
+                        // those binders. E.g. when we have `impl Trait<impl
+                        // OtherTrait<T>>`, the `impl OtherTrait<T>` can't refer
+                        // to the self parameter from `impl Trait`, and the
+                        // bounds aren't actually stored nested within each
+                        // other, but separately. So if the `T` refers to a type
+                        // parameter of the outer function, it's just one binder
+                        // away instead of two.
+                        let actual_opaque_type_data = ctx
+                            .with_debruijn(DebruijnIndex::INNERMOST, |ctx| {
+                                ReturnTypeImplTrait::from_hir(ctx, &bounds)
+                            });
+                        ctx.opaque_type_data.borrow_mut()[idx as usize] = actual_opaque_type_data;
+
+                        let func = match ctx.resolver.generic_def() {
+                            Some(GenericDefId::FunctionId(f)) => f,
+                            _ => panic!("opaque impl trait lowering in non-function"),
+                        };
+                        let impl_trait_id = OpaqueTyId::ReturnTypeImplTrait(func, idx);
+                        let generics = generics(ctx.db.upcast(), func.into());
+                        let parameters = Substs::bound_vars(&generics, ctx.in_binders);
+                        Ty::Opaque(OpaqueTy { opaque_ty_id: impl_trait_id, parameters })
+                    }
+                    ImplTraitLoweringMode::Param => {
+                        let idx = ctx.impl_trait_counter.get();
+                        // FIXME we're probably doing something wrong here
+                        ctx.impl_trait_counter.set(idx + count_impl_traits(type_ref) as u16);
+                        if let Some(def) = ctx.resolver.generic_def() {
+                            let generics = generics(ctx.db.upcast(), def);
+                            let param = generics
+                                .iter()
+                                .filter(|(_, data)| {
+                                    data.provenance == TypeParamProvenance::ArgumentImplTrait
+                                })
+                                .nth(idx as usize)
+                                .map_or(Ty::Unknown, |(id, _)| Ty::Placeholder(id));
+                            param
+                        } else {
+                            Ty::Unknown
+                        }
+                    }
+                    ImplTraitLoweringMode::Variable => {
+                        let idx = ctx.impl_trait_counter.get();
+                        // FIXME we're probably doing something wrong here
+                        ctx.impl_trait_counter.set(idx + count_impl_traits(type_ref) as u16);
+                        let (parent_params, self_params, list_params, _impl_trait_params) =
+                            if let Some(def) = ctx.resolver.generic_def() {
+                                let generics = generics(ctx.db.upcast(), def);
+                                generics.provenance_split()
+                            } else {
+                                (0, 0, 0, 0)
+                            };
+                        Ty::Bound(BoundVar::new(
+                            ctx.in_binders,
+                            idx as usize + parent_params + self_params + list_params,
+                        ))
+                    }
+                    ImplTraitLoweringMode::Disallowed => {
+                        // FIXME: report error
+                        Ty::Unknown
+                    }
+                }
+            }
+            TypeRef::Error => Ty::Unknown,
+        };
+        (ty, res)
+    }
+
+    /// This is only for `generic_predicates_for_param`, where we can't just
+    /// lower the self types of the predicates since that could lead to cycles.
+    /// So we just check here if the `type_ref` resolves to a generic param, and which.
+    fn from_hir_only_param(ctx: &TyLoweringContext<'_>, type_ref: &TypeRef) -> Option<TypeParamId> {
+        let path = match type_ref {
+            TypeRef::Path(path) => path,
+            _ => return None,
+        };
+        if path.type_anchor().is_some() {
+            return None;
+        }
+        if path.segments().len() > 1 {
+            return None;
+        }
+        let resolution =
+            match ctx.resolver.resolve_path_in_type_ns(ctx.db.upcast(), path.mod_path()) {
+                Some((it, None)) => it,
+                _ => return None,
+            };
+        if let TypeNs::GenericParam(param_id) = resolution {
+            Some(param_id)
+        } else {
+            None
+        }
+    }
+
+    pub(crate) fn from_type_relative_path(
+        ctx: &TyLoweringContext<'_>,
+        ty: Ty,
+        // We need the original resolution to lower `Self::AssocTy` correctly
+        res: Option<TypeNs>,
+        remaining_segments: PathSegments<'_>,
+    ) -> (Ty, Option<TypeNs>) {
+        if remaining_segments.len() == 1 {
+            // resolve unselected assoc types
+            let segment = remaining_segments.first().unwrap();
+            (Ty::select_associated_type(ctx, res, segment), None)
+        } else if remaining_segments.len() > 1 {
+            // FIXME report error (ambiguous associated type)
+            (Ty::Unknown, None)
+        } else {
+            (ty, res)
+        }
+    }
+
+    pub(crate) fn from_partly_resolved_hir_path(
+        ctx: &TyLoweringContext<'_>,
+        resolution: TypeNs,
+        resolved_segment: PathSegment<'_>,
+        remaining_segments: PathSegments<'_>,
+        infer_args: bool,
+    ) -> (Ty, Option<TypeNs>) {
+        let ty = match resolution {
+            TypeNs::TraitId(trait_) => {
+                // if this is a bare dyn Trait, we'll directly put the required ^0 for the self type in there
+                let self_ty = if remaining_segments.len() == 0 {
+                    Some(Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0)))
+                } else {
+                    None
+                };
+                let trait_ref =
+                    TraitRef::from_resolved_path(ctx, trait_, resolved_segment, self_ty);
+                let ty = if remaining_segments.len() == 1 {
+                    let segment = remaining_segments.first().unwrap();
+                    let found = associated_type_by_name_including_super_traits(
+                        ctx.db,
+                        trait_ref,
+                        &segment.name,
+                    );
+                    match found {
+                        Some((super_trait_ref, associated_ty)) => {
+                            // FIXME handle type parameters on the segment
+                            Ty::Projection(ProjectionTy {
+                                associated_ty,
+                                parameters: super_trait_ref.substs,
+                            })
+                        }
+                        None => {
+                            // FIXME: report error (associated type not found)
+                            Ty::Unknown
+                        }
+                    }
+                } else if remaining_segments.len() > 1 {
+                    // FIXME report error (ambiguous associated type)
+                    Ty::Unknown
+                } else {
+                    Ty::Dyn(Arc::new([GenericPredicate::Implemented(trait_ref)]))
+                };
+                return (ty, None);
+            }
+            TypeNs::GenericParam(param_id) => {
+                let generics = generics(
+                    ctx.db.upcast(),
+                    ctx.resolver.generic_def().expect("generics in scope"),
+                );
+                match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => Ty::Placeholder(param_id),
+                    TypeParamLoweringMode::Variable => {
+                        let idx = generics.param_idx(param_id).expect("matching generics");
+                        Ty::Bound(BoundVar::new(ctx.in_binders, idx))
+                    }
+                }
+            }
+            TypeNs::SelfType(impl_id) => {
+                let generics = generics(ctx.db.upcast(), impl_id.into());
+                let substs = match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => {
+                        Substs::type_params_for_generics(&generics)
+                    }
+                    TypeParamLoweringMode::Variable => {
+                        Substs::bound_vars(&generics, ctx.in_binders)
+                    }
+                };
+                ctx.db.impl_self_ty(impl_id).subst(&substs)
+            }
+            TypeNs::AdtSelfType(adt) => {
+                let generics = generics(ctx.db.upcast(), adt.into());
+                let substs = match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => {
+                        Substs::type_params_for_generics(&generics)
+                    }
+                    TypeParamLoweringMode::Variable => {
+                        Substs::bound_vars(&generics, ctx.in_binders)
+                    }
+                };
+                ctx.db.ty(adt.into()).subst(&substs)
+            }
+
+            TypeNs::AdtId(it) => {
+                Ty::from_hir_path_inner(ctx, resolved_segment, it.into(), infer_args)
+            }
+            TypeNs::BuiltinType(it) => {
+                Ty::from_hir_path_inner(ctx, resolved_segment, it.into(), infer_args)
+            }
+            TypeNs::TypeAliasId(it) => {
+                Ty::from_hir_path_inner(ctx, resolved_segment, it.into(), infer_args)
+            }
+            // FIXME: report error
+            TypeNs::EnumVariantId(_) => return (Ty::Unknown, None),
+        };
+
+        Ty::from_type_relative_path(ctx, ty, Some(resolution), remaining_segments)
+    }
+
+    pub(crate) fn from_hir_path(ctx: &TyLoweringContext<'_>, path: &Path) -> (Ty, Option<TypeNs>) {
+        // Resolve the path (in type namespace)
+        if let Some(type_ref) = path.type_anchor() {
+            let (ty, res) = Ty::from_hir_ext(ctx, &type_ref);
+            return Ty::from_type_relative_path(ctx, ty, res, path.segments());
+        }
+        let (resolution, remaining_index) =
+            match ctx.resolver.resolve_path_in_type_ns(ctx.db.upcast(), path.mod_path()) {
+                Some(it) => it,
+                None => return (Ty::Unknown, None),
+            };
+        let (resolved_segment, remaining_segments) = match remaining_index {
+            None => (
+                path.segments().last().expect("resolved path has at least one element"),
+                PathSegments::EMPTY,
+            ),
+            Some(i) => (path.segments().get(i - 1).unwrap(), path.segments().skip(i)),
+        };
+        Ty::from_partly_resolved_hir_path(
+            ctx,
+            resolution,
+            resolved_segment,
+            remaining_segments,
+            false,
+        )
+    }
+
+    fn select_associated_type(
+        ctx: &TyLoweringContext<'_>,
+        res: Option<TypeNs>,
+        segment: PathSegment<'_>,
+    ) -> Ty {
+        if let Some(res) = res {
+            let ty =
+                associated_type_shorthand_candidates(ctx.db, res, move |name, t, associated_ty| {
+                    if name == segment.name {
+                        let substs = match ctx.type_param_mode {
+                            TypeParamLoweringMode::Placeholder => {
+                                // if we're lowering to placeholders, we have to put
+                                // them in now
+                                let s = Substs::type_params(
+                                    ctx.db,
+                                    ctx.resolver.generic_def().expect(
+                                        "there should be generics if there's a generic param",
+                                    ),
+                                );
+                                t.substs.clone().subst_bound_vars(&s)
+                            }
+                            TypeParamLoweringMode::Variable => t.substs.clone(),
+                        };
+                        // We need to shift in the bound vars, since
+                        // associated_type_shorthand_candidates does not do that
+                        let substs = substs.shift_bound_vars(ctx.in_binders);
+                        // FIXME handle type parameters on the segment
+                        return Some(Ty::Projection(ProjectionTy {
+                            associated_ty,
+                            parameters: substs,
+                        }));
+                    }
+
+                    None
+                });
+
+            ty.unwrap_or(Ty::Unknown)
+        } else {
+            Ty::Unknown
+        }
+    }
+
+    fn from_hir_path_inner(
+        ctx: &TyLoweringContext<'_>,
+        segment: PathSegment<'_>,
+        typable: TyDefId,
+        infer_args: bool,
+    ) -> Ty {
+        let generic_def = match typable {
+            TyDefId::BuiltinType(_) => None,
+            TyDefId::AdtId(it) => Some(it.into()),
+            TyDefId::TypeAliasId(it) => Some(it.into()),
+        };
+        let substs = substs_from_path_segment(ctx, segment, generic_def, infer_args);
+        ctx.db.ty(typable).subst(&substs)
+    }
+
+    /// Collect generic arguments from a path into a `Substs`. See also
+    /// `create_substs_for_ast_path` and `def_to_ty` in rustc.
+    pub(super) fn substs_from_path(
+        ctx: &TyLoweringContext<'_>,
+        path: &Path,
+        // Note that we don't call `db.value_type(resolved)` here,
+        // `ValueTyDefId` is just a convenient way to pass generics and
+        // special-case enum variants
+        resolved: ValueTyDefId,
+        infer_args: bool,
+    ) -> Substs {
+        let last = path.segments().last().expect("path should have at least one segment");
+        let (segment, generic_def) = match resolved {
+            ValueTyDefId::FunctionId(it) => (last, Some(it.into())),
+            ValueTyDefId::StructId(it) => (last, Some(it.into())),
+            ValueTyDefId::UnionId(it) => (last, Some(it.into())),
+            ValueTyDefId::ConstId(it) => (last, Some(it.into())),
+            ValueTyDefId::StaticId(_) => (last, None),
+            ValueTyDefId::EnumVariantId(var) => {
+                // the generic args for an enum variant may be either specified
+                // on the segment referring to the enum, or on the segment
+                // referring to the variant. So `Option::<T>::None` and
+                // `Option::None::<T>` are both allowed (though the former is
+                // preferred). See also `def_ids_for_path_segments` in rustc.
+                let len = path.segments().len();
+                let penultimate = if len >= 2 { path.segments().get(len - 2) } else { None };
+                let segment = match penultimate {
+                    Some(segment) if segment.args_and_bindings.is_some() => segment,
+                    _ => last,
+                };
+                (segment, Some(var.parent.into()))
+            }
+        };
+        substs_from_path_segment(ctx, segment, generic_def, infer_args)
+    }
+}
+
+fn substs_from_path_segment(
+    ctx: &TyLoweringContext<'_>,
+    segment: PathSegment<'_>,
+    def_generic: Option<GenericDefId>,
+    infer_args: bool,
+) -> Substs {
+    let mut substs = Vec::new();
+    let def_generics = def_generic.map(|def| generics(ctx.db.upcast(), def));
+
+    let (parent_params, self_params, type_params, impl_trait_params) =
+        def_generics.map_or((0, 0, 0, 0), |g| g.provenance_split());
+    let total_len = parent_params + self_params + type_params + impl_trait_params;
+
+    substs.extend(iter::repeat(Ty::Unknown).take(parent_params));
+
+    let mut had_explicit_args = false;
+
+    if let Some(generic_args) = &segment.args_and_bindings {
+        if !generic_args.has_self_type {
+            substs.extend(iter::repeat(Ty::Unknown).take(self_params));
+        }
+        let expected_num =
+            if generic_args.has_self_type { self_params + type_params } else { type_params };
+        let skip = if generic_args.has_self_type && self_params == 0 { 1 } else { 0 };
+        // if args are provided, it should be all of them, but we can't rely on that
+        for arg in generic_args.args.iter().skip(skip).take(expected_num) {
+            match arg {
+                GenericArg::Type(type_ref) => {
+                    had_explicit_args = true;
+                    let ty = Ty::from_hir(ctx, type_ref);
+                    substs.push(ty);
+                }
+            }
+        }
+    }
+
+    // handle defaults. In expression or pattern path segments without
+    // explicitly specified type arguments, missing type arguments are inferred
+    // (i.e. defaults aren't used).
+    if !infer_args || had_explicit_args {
+        if let Some(def_generic) = def_generic {
+            let defaults = ctx.db.generic_defaults(def_generic);
+            assert_eq!(total_len, defaults.len());
+
+            for default_ty in defaults.iter().skip(substs.len()) {
+                // each default can depend on the previous parameters
+                let substs_so_far = Substs(substs.clone().into());
+                substs.push(default_ty.clone().subst(&substs_so_far));
+            }
+        }
+    }
+
+    // add placeholders for args that were not provided
+    // FIXME: emit diagnostics in contexts where this is not allowed
+    for _ in substs.len()..total_len {
+        substs.push(Ty::Unknown);
+    }
+    assert_eq!(substs.len(), total_len);
+
+    Substs(substs.into())
+}
+
+impl TraitRef {
+    fn from_path(
+        ctx: &TyLoweringContext<'_>,
+        path: &Path,
+        explicit_self_ty: Option<Ty>,
+    ) -> Option<Self> {
+        let resolved =
+            match ctx.resolver.resolve_path_in_type_ns_fully(ctx.db.upcast(), path.mod_path())? {
+                TypeNs::TraitId(tr) => tr,
+                _ => return None,
+            };
+        let segment = path.segments().last().expect("path should have at least one segment");
+        Some(TraitRef::from_resolved_path(ctx, resolved, segment, explicit_self_ty))
+    }
+
+    pub(crate) fn from_resolved_path(
+        ctx: &TyLoweringContext<'_>,
+        resolved: TraitId,
+        segment: PathSegment<'_>,
+        explicit_self_ty: Option<Ty>,
+    ) -> Self {
+        let mut substs = TraitRef::substs_from_path(ctx, segment, resolved);
+        if let Some(self_ty) = explicit_self_ty {
+            make_mut_slice(&mut substs.0)[0] = self_ty;
+        }
+        TraitRef { trait_: resolved, substs }
+    }
+
+    fn from_hir(
+        ctx: &TyLoweringContext<'_>,
+        type_ref: &TypeRef,
+        explicit_self_ty: Option<Ty>,
+    ) -> Option<Self> {
+        let path = match type_ref {
+            TypeRef::Path(path) => path,
+            _ => return None,
+        };
+        TraitRef::from_path(ctx, path, explicit_self_ty)
+    }
+
+    fn substs_from_path(
+        ctx: &TyLoweringContext<'_>,
+        segment: PathSegment<'_>,
+        resolved: TraitId,
+    ) -> Substs {
+        substs_from_path_segment(ctx, segment, Some(resolved.into()), false)
+    }
+
+    pub(crate) fn from_type_bound(
+        ctx: &TyLoweringContext<'_>,
+        bound: &TypeBound,
+        self_ty: Ty,
+    ) -> Option<TraitRef> {
+        match bound {
+            TypeBound::Path(path) => TraitRef::from_path(ctx, path, Some(self_ty)),
+            TypeBound::Error => None,
+        }
+    }
+}
+
+impl GenericPredicate {
+    pub(crate) fn from_where_predicate<'a>(
+        ctx: &'a TyLoweringContext<'a>,
+        where_predicate: &'a WherePredicate,
+    ) -> impl Iterator<Item = GenericPredicate> + 'a {
+        let self_ty = match &where_predicate.target {
+            WherePredicateTarget::TypeRef(type_ref) => Ty::from_hir(ctx, type_ref),
+            WherePredicateTarget::TypeParam(param_id) => {
+                let generic_def = ctx.resolver.generic_def().expect("generics in scope");
+                let generics = generics(ctx.db.upcast(), generic_def);
+                let param_id = hir_def::TypeParamId { parent: generic_def, local_id: *param_id };
+                match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => Ty::Placeholder(param_id),
+                    TypeParamLoweringMode::Variable => {
+                        let idx = generics.param_idx(param_id).expect("matching generics");
+                        Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, idx))
+                    }
+                }
+            }
+        };
+        GenericPredicate::from_type_bound(ctx, &where_predicate.bound, self_ty)
+    }
+
+    pub(crate) fn from_type_bound<'a>(
+        ctx: &'a TyLoweringContext<'a>,
+        bound: &'a TypeBound,
+        self_ty: Ty,
+    ) -> impl Iterator<Item = GenericPredicate> + 'a {
+        let trait_ref = TraitRef::from_type_bound(ctx, bound, self_ty);
+        iter::once(trait_ref.clone().map_or(GenericPredicate::Error, GenericPredicate::Implemented))
+            .chain(
+                trait_ref
+                    .into_iter()
+                    .flat_map(move |tr| assoc_type_bindings_from_type_bound(ctx, bound, tr)),
+            )
+    }
+}
+
+fn assoc_type_bindings_from_type_bound<'a>(
+    ctx: &'a TyLoweringContext<'a>,
+    bound: &'a TypeBound,
+    trait_ref: TraitRef,
+) -> impl Iterator<Item = GenericPredicate> + 'a {
+    let last_segment = match bound {
+        TypeBound::Path(path) => path.segments().last(),
+        TypeBound::Error => None,
+    };
+    last_segment
+        .into_iter()
+        .flat_map(|segment| segment.args_and_bindings.into_iter())
+        .flat_map(|args_and_bindings| args_and_bindings.bindings.iter())
+        .flat_map(move |binding| {
+            let found = associated_type_by_name_including_super_traits(
+                ctx.db,
+                trait_ref.clone(),
+                &binding.name,
+            );
+            let (super_trait_ref, associated_ty) = match found {
+                None => return SmallVec::<[GenericPredicate; 1]>::new(),
+                Some(t) => t,
+            };
+            let projection_ty = ProjectionTy { associated_ty, parameters: super_trait_ref.substs };
+            let mut preds = SmallVec::with_capacity(
+                binding.type_ref.as_ref().map_or(0, |_| 1) + binding.bounds.len(),
+            );
+            if let Some(type_ref) = &binding.type_ref {
+                let ty = Ty::from_hir(ctx, type_ref);
+                let projection_predicate =
+                    ProjectionPredicate { projection_ty: projection_ty.clone(), ty };
+                preds.push(GenericPredicate::Projection(projection_predicate));
+            }
+            for bound in &binding.bounds {
+                preds.extend(GenericPredicate::from_type_bound(
+                    ctx,
+                    bound,
+                    Ty::Projection(projection_ty.clone()),
+                ));
+            }
+            preds
+        })
+}
+
+impl ReturnTypeImplTrait {
+    fn from_hir(ctx: &TyLoweringContext, bounds: &[TypeBound]) -> Self {
+        mark::hit!(lower_rpit);
+        let self_ty = Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0));
+        let predicates = ctx.with_shifted_in(DebruijnIndex::ONE, |ctx| {
+            bounds
+                .iter()
+                .flat_map(|b| GenericPredicate::from_type_bound(ctx, b, self_ty.clone()))
+                .collect()
+        });
+        ReturnTypeImplTrait { bounds: Binders::new(1, predicates) }
+    }
+}
+
+fn count_impl_traits(type_ref: &TypeRef) -> usize {
+    let mut count = 0;
+    type_ref.walk(&mut |type_ref| {
+        if matches!(type_ref, TypeRef::ImplTrait(_)) {
+            count += 1;
+        }
+    });
+    count
+}
+
+/// Build the signature of a callable item (function, struct or enum variant).
+pub fn callable_item_sig(db: &dyn HirDatabase, def: CallableDefId) -> PolyFnSig {
+    match def {
+        CallableDefId::FunctionId(f) => fn_sig_for_fn(db, f),
+        CallableDefId::StructId(s) => fn_sig_for_struct_constructor(db, s),
+        CallableDefId::EnumVariantId(e) => fn_sig_for_enum_variant_constructor(db, e),
+    }
+}
+
+pub fn associated_type_shorthand_candidates<R>(
+    db: &dyn HirDatabase,
+    res: TypeNs,
+    mut cb: impl FnMut(&Name, &TraitRef, TypeAliasId) -> Option<R>,
+) -> Option<R> {
+    let traits_from_env: Vec<_> = match res {
+        TypeNs::SelfType(impl_id) => match db.impl_trait(impl_id) {
+            None => vec![],
+            Some(trait_ref) => vec![trait_ref.value],
+        },
+        TypeNs::GenericParam(param_id) => {
+            let predicates = db.generic_predicates_for_param(param_id);
+            let mut traits_: Vec<_> = predicates
+                .iter()
+                .filter_map(|pred| match &pred.value {
+                    GenericPredicate::Implemented(tr) => Some(tr.clone()),
+                    _ => None,
+                })
+                .collect();
+            // Handle `Self::Type` referring to own associated type in trait definitions
+            if let GenericDefId::TraitId(trait_id) = param_id.parent {
+                let generics = generics(db.upcast(), trait_id.into());
+                if generics.params.types[param_id.local_id].provenance
+                    == TypeParamProvenance::TraitSelf
+                {
+                    let trait_ref = TraitRef {
+                        trait_: trait_id,
+                        substs: Substs::bound_vars(&generics, DebruijnIndex::INNERMOST),
+                    };
+                    traits_.push(trait_ref);
+                }
+            }
+            traits_
+        }
+        _ => vec![],
+    };
+
+    for t in traits_from_env.into_iter().flat_map(move |t| all_super_trait_refs(db, t)) {
+        let data = db.trait_data(t.trait_);
+
+        for (name, assoc_id) in &data.items {
+            match assoc_id {
+                AssocItemId::TypeAliasId(alias) => {
+                    if let Some(result) = cb(name, &t, *alias) {
+                        return Some(result);
+                    }
+                }
+                AssocItemId::FunctionId(_) | AssocItemId::ConstId(_) => {}
+            }
+        }
+    }
+
+    None
+}
+
+/// Build the type of all specific fields of a struct or enum variant.
+pub(crate) fn field_types_query(
+    db: &dyn HirDatabase,
+    variant_id: VariantId,
+) -> Arc<ArenaMap<LocalFieldId, Binders<Ty>>> {
+    let var_data = variant_data(db.upcast(), variant_id);
+    let (resolver, def): (_, GenericDefId) = match variant_id {
+        VariantId::StructId(it) => (it.resolver(db.upcast()), it.into()),
+        VariantId::UnionId(it) => (it.resolver(db.upcast()), it.into()),
+        VariantId::EnumVariantId(it) => (it.parent.resolver(db.upcast()), it.parent.into()),
+    };
+    let generics = generics(db.upcast(), def);
+    let mut res = ArenaMap::default();
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    for (field_id, field_data) in var_data.fields().iter() {
+        res.insert(field_id, Binders::new(generics.len(), Ty::from_hir(&ctx, &field_data.type_ref)))
+    }
+    Arc::new(res)
+}
+
+/// This query exists only to be used when resolving short-hand associated types
+/// like `T::Item`.
+///
+/// See the analogous query in rustc and its comment:
+/// https://github.com/rust-lang/rust/blob/9150f844e2624eb013ec78ca08c1d416e6644026/src/librustc_typeck/astconv.rs#L46
+/// This is a query mostly to handle cycles somewhat gracefully; e.g. the
+/// following bounds are disallowed: `T: Foo<U::Item>, U: Foo<T::Item>`, but
+/// these are fine: `T: Foo<U::Item>, U: Foo<()>`.
+pub(crate) fn generic_predicates_for_param_query(
+    db: &dyn HirDatabase,
+    param_id: TypeParamId,
+) -> Arc<[Binders<GenericPredicate>]> {
+    let resolver = param_id.parent.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    let generics = generics(db.upcast(), param_id.parent);
+    resolver
+        .where_predicates_in_scope()
+        // we have to filter out all other predicates *first*, before attempting to lower them
+        .filter(|pred| match &pred.target {
+            WherePredicateTarget::TypeRef(type_ref) => {
+                Ty::from_hir_only_param(&ctx, type_ref) == Some(param_id)
+            }
+            WherePredicateTarget::TypeParam(local_id) => *local_id == param_id.local_id,
+        })
+        .flat_map(|pred| {
+            GenericPredicate::from_where_predicate(&ctx, pred)
+                .map(|p| Binders::new(generics.len(), p))
+        })
+        .collect()
+}
+
+pub(crate) fn generic_predicates_for_param_recover(
+    _db: &dyn HirDatabase,
+    _cycle: &[String],
+    _param_id: &TypeParamId,
+) -> Arc<[Binders<GenericPredicate>]> {
+    Arc::new([])
+}
+
+impl TraitEnvironment {
+    pub fn lower(db: &dyn HirDatabase, resolver: &Resolver) -> Arc<TraitEnvironment> {
+        let ctx = TyLoweringContext::new(db, &resolver)
+            .with_type_param_mode(TypeParamLoweringMode::Placeholder);
+        let mut predicates = resolver
+            .where_predicates_in_scope()
+            .flat_map(|pred| GenericPredicate::from_where_predicate(&ctx, pred))
+            .collect::<Vec<_>>();
+
+        if let Some(def) = resolver.generic_def() {
+            let container: Option<AssocContainerId> = match def {
+                // FIXME: is there a function for this?
+                GenericDefId::FunctionId(f) => Some(f.lookup(db.upcast()).container),
+                GenericDefId::AdtId(_) => None,
+                GenericDefId::TraitId(_) => None,
+                GenericDefId::TypeAliasId(t) => Some(t.lookup(db.upcast()).container),
+                GenericDefId::ImplId(_) => None,
+                GenericDefId::EnumVariantId(_) => None,
+                GenericDefId::ConstId(c) => Some(c.lookup(db.upcast()).container),
+            };
+            if let Some(AssocContainerId::TraitId(trait_id)) = container {
+                // add `Self: Trait<T1, T2, ...>` to the environment in trait
+                // function default implementations (and hypothetical code
+                // inside consts or type aliases)
+                test_utils::mark::hit!(trait_self_implements_self);
+                let substs = Substs::type_params(db, trait_id);
+                let trait_ref = TraitRef { trait_: trait_id, substs };
+                let pred = GenericPredicate::Implemented(trait_ref);
+
+                predicates.push(pred);
+            }
+        }
+
+        Arc::new(TraitEnvironment { predicates })
+    }
+}
+
+/// Resolve the where clause(s) of an item with generics.
+pub(crate) fn generic_predicates_query(
+    db: &dyn HirDatabase,
+    def: GenericDefId,
+) -> Arc<[Binders<GenericPredicate>]> {
+    let resolver = def.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    let generics = generics(db.upcast(), def);
+    resolver
+        .where_predicates_in_scope()
+        .flat_map(|pred| {
+            GenericPredicate::from_where_predicate(&ctx, pred)
+                .map(|p| Binders::new(generics.len(), p))
+        })
+        .collect()
+}
+
+/// Resolve the default type params from generics
+pub(crate) fn generic_defaults_query(
+    db: &dyn HirDatabase,
+    def: GenericDefId,
+) -> Arc<[Binders<Ty>]> {
+    let resolver = def.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    let generic_params = generics(db.upcast(), def);
+
+    let defaults = generic_params
+        .iter()
+        .enumerate()
+        .map(|(idx, (_, p))| {
+            let mut ty = p.default.as_ref().map_or(Ty::Unknown, |t| Ty::from_hir(&ctx, t));
+
+            // Each default can only refer to previous parameters.
+            ty.walk_mut_binders(
+                &mut |ty, binders| match ty {
+                    Ty::Bound(BoundVar { debruijn, index }) if *debruijn == binders => {
+                        if *index >= idx {
+                            // type variable default referring to parameter coming
+                            // after it. This is forbidden (FIXME: report
+                            // diagnostic)
+                            *ty = Ty::Unknown;
+                        }
+                    }
+                    _ => {}
+                },
+                DebruijnIndex::INNERMOST,
+            );
+
+            Binders::new(idx, ty)
+        })
+        .collect();
+
+    defaults
+}
+
+fn fn_sig_for_fn(db: &dyn HirDatabase, def: FunctionId) -> PolyFnSig {
+    let data = db.function_data(def);
+    let resolver = def.resolver(db.upcast());
+    let ctx_params = TyLoweringContext::new(db, &resolver)
+        .with_impl_trait_mode(ImplTraitLoweringMode::Variable)
+        .with_type_param_mode(TypeParamLoweringMode::Variable);
+    let params = data.params.iter().map(|tr| Ty::from_hir(&ctx_params, tr)).collect::<Vec<_>>();
+    let ctx_ret = TyLoweringContext::new(db, &resolver)
+        .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
+        .with_type_param_mode(TypeParamLoweringMode::Variable);
+    let ret = Ty::from_hir(&ctx_ret, &data.ret_type);
+    let generics = generics(db.upcast(), def.into());
+    let num_binders = generics.len();
+    Binders::new(num_binders, FnSig::from_params_and_return(params, ret, data.is_varargs))
+}
+
+/// Build the declared type of a function. This should not need to look at the
+/// function body.
+fn type_for_fn(db: &dyn HirDatabase, def: FunctionId) -> Binders<Ty> {
+    let generics = generics(db.upcast(), def.into());
+    let substs = Substs::bound_vars(&generics, DebruijnIndex::INNERMOST);
+    Binders::new(substs.len(), Ty::apply(TypeCtor::FnDef(def.into()), substs))
+}
+
+/// Build the declared type of a const.
+fn type_for_const(db: &dyn HirDatabase, def: ConstId) -> Binders<Ty> {
+    let data = db.const_data(def);
+    let generics = generics(db.upcast(), def.into());
+    let resolver = def.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+
+    Binders::new(generics.len(), Ty::from_hir(&ctx, &data.type_ref))
+}
+
+/// Build the declared type of a static.
+fn type_for_static(db: &dyn HirDatabase, def: StaticId) -> Binders<Ty> {
+    let data = db.static_data(def);
+    let resolver = def.resolver(db.upcast());
+    let ctx = TyLoweringContext::new(db, &resolver);
+
+    Binders::new(0, Ty::from_hir(&ctx, &data.type_ref))
+}
+
+/// Build the declared type of a static.
+fn type_for_builtin(def: BuiltinType) -> Ty {
+    Ty::simple(match def {
+        BuiltinType::Char => TypeCtor::Char,
+        BuiltinType::Bool => TypeCtor::Bool,
+        BuiltinType::Str => TypeCtor::Str,
+        BuiltinType::Int(t) => TypeCtor::Int(IntTy::from(t).into()),
+        BuiltinType::Float(t) => TypeCtor::Float(FloatTy::from(t).into()),
+    })
+}
+
+fn fn_sig_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> PolyFnSig {
+    let struct_data = db.struct_data(def);
+    let fields = struct_data.variant_data.fields();
+    let resolver = def.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    let params =
+        fields.iter().map(|(_, field)| Ty::from_hir(&ctx, &field.type_ref)).collect::<Vec<_>>();
+    let ret = type_for_adt(db, def.into());
+    Binders::new(ret.num_binders, FnSig::from_params_and_return(params, ret.value, false))
+}
+
+/// Build the type of a tuple struct constructor.
+fn type_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> Binders<Ty> {
+    let struct_data = db.struct_data(def);
+    if let StructKind::Unit = struct_data.variant_data.kind() {
+        return type_for_adt(db, def.into());
+    }
+    let generics = generics(db.upcast(), def.into());
+    let substs = Substs::bound_vars(&generics, DebruijnIndex::INNERMOST);
+    Binders::new(substs.len(), Ty::apply(TypeCtor::FnDef(def.into()), substs))
+}
+
+fn fn_sig_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId) -> PolyFnSig {
+    let enum_data = db.enum_data(def.parent);
+    let var_data = &enum_data.variants[def.local_id];
+    let fields = var_data.variant_data.fields();
+    let resolver = def.parent.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    let params =
+        fields.iter().map(|(_, field)| Ty::from_hir(&ctx, &field.type_ref)).collect::<Vec<_>>();
+    let ret = type_for_adt(db, def.parent.into());
+    Binders::new(ret.num_binders, FnSig::from_params_and_return(params, ret.value, false))
+}
+
+/// Build the type of a tuple enum variant constructor.
+fn type_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId) -> Binders<Ty> {
+    let enum_data = db.enum_data(def.parent);
+    let var_data = &enum_data.variants[def.local_id].variant_data;
+    if let StructKind::Unit = var_data.kind() {
+        return type_for_adt(db, def.parent.into());
+    }
+    let generics = generics(db.upcast(), def.parent.into());
+    let substs = Substs::bound_vars(&generics, DebruijnIndex::INNERMOST);
+    Binders::new(substs.len(), Ty::apply(TypeCtor::FnDef(def.into()), substs))
+}
+
+fn type_for_adt(db: &dyn HirDatabase, adt: AdtId) -> Binders<Ty> {
+    let generics = generics(db.upcast(), adt.into());
+    let substs = Substs::bound_vars(&generics, DebruijnIndex::INNERMOST);
+    Binders::new(substs.len(), Ty::apply(TypeCtor::Adt(adt), substs))
+}
+
+fn type_for_type_alias(db: &dyn HirDatabase, t: TypeAliasId) -> Binders<Ty> {
+    let generics = generics(db.upcast(), t.into());
+    let resolver = t.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    let type_ref = &db.type_alias_data(t).type_ref;
+    let substs = Substs::bound_vars(&generics, DebruijnIndex::INNERMOST);
+    let inner = Ty::from_hir(&ctx, type_ref.as_ref().unwrap_or(&TypeRef::Error));
+    Binders::new(substs.len(), inner)
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub enum CallableDefId {
+    FunctionId(FunctionId),
+    StructId(StructId),
+    EnumVariantId(EnumVariantId),
+}
+impl_from!(FunctionId, StructId, EnumVariantId for CallableDefId);
+
+impl CallableDefId {
+    pub fn krate(self, db: &dyn HirDatabase) -> CrateId {
+        let db = db.upcast();
+        match self {
+            CallableDefId::FunctionId(f) => f.lookup(db).module(db),
+            CallableDefId::StructId(s) => s.lookup(db).container.module(db),
+            CallableDefId::EnumVariantId(e) => e.parent.lookup(db).container.module(db),
+        }
+        .krate
+    }
+}
+
+impl From<CallableDefId> for GenericDefId {
+    fn from(def: CallableDefId) -> GenericDefId {
+        match def {
+            CallableDefId::FunctionId(f) => f.into(),
+            CallableDefId::StructId(s) => s.into(),
+            CallableDefId::EnumVariantId(e) => e.into(),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum TyDefId {
+    BuiltinType(BuiltinType),
+    AdtId(AdtId),
+    TypeAliasId(TypeAliasId),
+}
+impl_from!(BuiltinType, AdtId(StructId, EnumId, UnionId), TypeAliasId for TyDefId);
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum ValueTyDefId {
+    FunctionId(FunctionId),
+    StructId(StructId),
+    UnionId(UnionId),
+    EnumVariantId(EnumVariantId),
+    ConstId(ConstId),
+    StaticId(StaticId),
+}
+impl_from!(FunctionId, StructId, UnionId, EnumVariantId, ConstId, StaticId for ValueTyDefId);
+
+/// Build the declared type of an item. This depends on the namespace; e.g. for
+/// `struct Foo(usize)`, we have two types: The type of the struct itself, and
+/// the constructor function `(usize) -> Foo` which lives in the values
+/// namespace.
+pub(crate) fn ty_query(db: &dyn HirDatabase, def: TyDefId) -> Binders<Ty> {
+    match def {
+        TyDefId::BuiltinType(it) => Binders::new(0, type_for_builtin(it)),
+        TyDefId::AdtId(it) => type_for_adt(db, it),
+        TyDefId::TypeAliasId(it) => type_for_type_alias(db, it),
+    }
+}
+
+pub(crate) fn ty_recover(db: &dyn HirDatabase, _cycle: &[String], def: &TyDefId) -> Binders<Ty> {
+    let num_binders = match *def {
+        TyDefId::BuiltinType(_) => 0,
+        TyDefId::AdtId(it) => generics(db.upcast(), it.into()).len(),
+        TyDefId::TypeAliasId(it) => generics(db.upcast(), it.into()).len(),
+    };
+    Binders::new(num_binders, Ty::Unknown)
+}
+
+pub(crate) fn value_ty_query(db: &dyn HirDatabase, def: ValueTyDefId) -> Binders<Ty> {
+    match def {
+        ValueTyDefId::FunctionId(it) => type_for_fn(db, it),
+        ValueTyDefId::StructId(it) => type_for_struct_constructor(db, it),
+        ValueTyDefId::UnionId(it) => type_for_adt(db, it.into()),
+        ValueTyDefId::EnumVariantId(it) => type_for_enum_variant_constructor(db, it),
+        ValueTyDefId::ConstId(it) => type_for_const(db, it),
+        ValueTyDefId::StaticId(it) => type_for_static(db, it),
+    }
+}
+
+pub(crate) fn impl_self_ty_query(db: &dyn HirDatabase, impl_id: ImplId) -> Binders<Ty> {
+    let impl_data = db.impl_data(impl_id);
+    let resolver = impl_id.resolver(db.upcast());
+    let generics = generics(db.upcast(), impl_id.into());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    Binders::new(generics.len(), Ty::from_hir(&ctx, &impl_data.target_type))
+}
+
+pub(crate) fn impl_self_ty_recover(
+    db: &dyn HirDatabase,
+    _cycle: &[String],
+    impl_id: &ImplId,
+) -> Binders<Ty> {
+    let generics = generics(db.upcast(), (*impl_id).into());
+    Binders::new(generics.len(), Ty::Unknown)
+}
+
+pub(crate) fn impl_trait_query(db: &dyn HirDatabase, impl_id: ImplId) -> Option<Binders<TraitRef>> {
+    let impl_data = db.impl_data(impl_id);
+    let resolver = impl_id.resolver(db.upcast());
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+    let self_ty = db.impl_self_ty(impl_id);
+    let target_trait = impl_data.target_trait.as_ref()?;
+    Some(Binders::new(
+        self_ty.num_binders,
+        TraitRef::from_hir(&ctx, target_trait, Some(self_ty.value))?,
+    ))
+}
+
+pub(crate) fn return_type_impl_traits(
+    db: &dyn HirDatabase,
+    def: hir_def::FunctionId,
+) -> Option<Arc<Binders<ReturnTypeImplTraits>>> {
+    // FIXME unify with fn_sig_for_fn instead of doing lowering twice, maybe
+    let data = db.function_data(def);
+    let resolver = def.resolver(db.upcast());
+    let ctx_ret = TyLoweringContext::new(db, &resolver)
+        .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
+        .with_type_param_mode(TypeParamLoweringMode::Variable);
+    let _ret = Ty::from_hir(&ctx_ret, &data.ret_type);
+    let generics = generics(db.upcast(), def.into());
+    let num_binders = generics.len();
+    let return_type_impl_traits =
+        ReturnTypeImplTraits { impl_traits: ctx_ret.opaque_type_data.into_inner() };
+    if return_type_impl_traits.impl_traits.is_empty() {
+        None
+    } else {
+        Some(Arc::new(Binders::new(num_binders, return_type_impl_traits)))
+    }
+}