1 files changed, 769 insertions, 0 deletions

diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
new file mode 100644
index 000000000..ec59145c7
--- /dev/null
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -0,0 +1,769 @@
+//! This module is concerned with finding methods that a given type provides.
+//! For details about how this works in rustc, see the method lookup page in the
+//! [rustc guide](https://rust-lang.github.io/rustc-guide/method-lookup.html)
+//! and the corresponding code mostly in librustc_typeck/check/method/probe.rs.
+use std::{iter, sync::Arc};
+
+use arrayvec::ArrayVec;
+use base_db::CrateId;
+use hir_def::{
+    builtin_type::{IntBitness, Signedness},
+    lang_item::LangItemTarget,
+    type_ref::Mutability,
+    AssocContainerId, AssocItemId, FunctionId, HasModule, ImplId, Lookup, TraitId,
+};
+use hir_expand::name::Name;
+use rustc_hash::{FxHashMap, FxHashSet};
+
+use super::Substs;
+use crate::{
+    autoderef,
+    db::HirDatabase,
+    primitive::{FloatBitness, FloatTy, IntTy},
+    utils::all_super_traits,
+    ApplicationTy, Canonical, DebruijnIndex, InEnvironment, TraitEnvironment, TraitRef, Ty, TyKind,
+    TypeCtor, TypeWalk,
+};
+
+/// This is used as a key for indexing impls.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum TyFingerprint {
+    Apply(TypeCtor),
+}
+
+impl TyFingerprint {
+    /// Creates a TyFingerprint for looking up an impl. Only certain types can
+    /// have impls: if we have some `struct S`, we can have an `impl S`, but not
+    /// `impl &S`. Hence, this will return `None` for reference types and such.
+    pub(crate) fn for_impl(ty: &Ty) -> Option<TyFingerprint> {
+        match ty {
+            Ty::Apply(a_ty) => Some(TyFingerprint::Apply(a_ty.ctor)),
+            _ => None,
+        }
+    }
+}
+
+pub(crate) const ALL_INT_FPS: [TyFingerprint; 12] = [
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Unsigned,
+        bitness: IntBitness::X8,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Unsigned,
+        bitness: IntBitness::X16,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Unsigned,
+        bitness: IntBitness::X32,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Unsigned,
+        bitness: IntBitness::X64,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Unsigned,
+        bitness: IntBitness::X128,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Unsigned,
+        bitness: IntBitness::Xsize,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Signed,
+        bitness: IntBitness::X8,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Signed,
+        bitness: IntBitness::X16,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Signed,
+        bitness: IntBitness::X32,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Signed,
+        bitness: IntBitness::X64,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Signed,
+        bitness: IntBitness::X128,
+    })),
+    TyFingerprint::Apply(TypeCtor::Int(IntTy {
+        signedness: Signedness::Signed,
+        bitness: IntBitness::Xsize,
+    })),
+];
+
+pub(crate) const ALL_FLOAT_FPS: [TyFingerprint; 2] = [
+    TyFingerprint::Apply(TypeCtor::Float(FloatTy { bitness: FloatBitness::X32 })),
+    TyFingerprint::Apply(TypeCtor::Float(FloatTy { bitness: FloatBitness::X64 })),
+];
+
+/// Trait impls defined or available in some crate.
+#[derive(Debug, Eq, PartialEq)]
+pub struct TraitImpls {
+    // If the `Option<TyFingerprint>` is `None`, the impl may apply to any self type.
+    map: FxHashMap<TraitId, FxHashMap<Option<TyFingerprint>, Vec<ImplId>>>,
+}
+
+impl TraitImpls {
+    pub(crate) fn trait_impls_in_crate_query(db: &dyn HirDatabase, krate: CrateId) -> Arc<Self> {
+        let _p = profile::span("trait_impls_in_crate_query");
+        let mut impls = Self { map: FxHashMap::default() };
+
+        let crate_def_map = db.crate_def_map(krate);
+        for (_module_id, module_data) in crate_def_map.modules.iter() {
+            for impl_id in module_data.scope.impls() {
+                let target_trait = match db.impl_trait(impl_id) {
+                    Some(tr) => tr.value.trait_,
+                    None => continue,
+                };
+                let self_ty = db.impl_self_ty(impl_id);
+                let self_ty_fp = TyFingerprint::for_impl(&self_ty.value);
+                impls
+                    .map
+                    .entry(target_trait)
+                    .or_default()
+                    .entry(self_ty_fp)
+                    .or_default()
+                    .push(impl_id);
+            }
+        }
+
+        Arc::new(impls)
+    }
+
+    pub(crate) fn trait_impls_in_deps_query(db: &dyn HirDatabase, krate: CrateId) -> Arc<Self> {
+        let _p = profile::span("trait_impls_in_deps_query");
+        let crate_graph = db.crate_graph();
+        let mut res = Self { map: FxHashMap::default() };
+
+        for krate in crate_graph.transitive_deps(krate) {
+            res.merge(&db.trait_impls_in_crate(krate));
+        }
+
+        Arc::new(res)
+    }
+
+    fn merge(&mut self, other: &Self) {
+        for (trait_, other_map) in &other.map {
+            let map = self.map.entry(*trait_).or_default();
+            for (fp, impls) in other_map {
+                let vec = map.entry(*fp).or_default();
+                vec.extend(impls);
+            }
+        }
+    }
+
+    /// Queries all impls of the given trait.
+    pub fn for_trait(&self, trait_: TraitId) -> impl Iterator<Item = ImplId> + '_ {
+        self.map
+            .get(&trait_)
+            .into_iter()
+            .flat_map(|map| map.values().flat_map(|v| v.iter().copied()))
+    }
+
+    /// Queries all impls of `trait_` that may apply to `self_ty`.
+    pub fn for_trait_and_self_ty(
+        &self,
+        trait_: TraitId,
+        self_ty: TyFingerprint,
+    ) -> impl Iterator<Item = ImplId> + '_ {
+        self.map
+            .get(&trait_)
+            .into_iter()
+            .flat_map(move |map| map.get(&None).into_iter().chain(map.get(&Some(self_ty))))
+            .flat_map(|v| v.iter().copied())
+    }
+
+    pub fn all_impls(&self) -> impl Iterator<Item = ImplId> + '_ {
+        self.map.values().flat_map(|map| map.values().flat_map(|v| v.iter().copied()))
+    }
+}
+
+/// Inherent impls defined in some crate.
+///
+/// Inherent impls can only be defined in the crate that also defines the self type of the impl
+/// (note that some primitives are considered to be defined by both libcore and liballoc).
+///
+/// This makes inherent impl lookup easier than trait impl lookup since we only have to consider a
+/// single crate.
+#[derive(Debug, Eq, PartialEq)]
+pub struct InherentImpls {
+    map: FxHashMap<TyFingerprint, Vec<ImplId>>,
+}
+
+impl InherentImpls {
+    pub(crate) fn inherent_impls_in_crate_query(db: &dyn HirDatabase, krate: CrateId) -> Arc<Self> {
+        let mut map: FxHashMap<_, Vec<_>> = FxHashMap::default();
+
+        let crate_def_map = db.crate_def_map(krate);
+        for (_module_id, module_data) in crate_def_map.modules.iter() {
+            for impl_id in module_data.scope.impls() {
+                let data = db.impl_data(impl_id);
+                if data.target_trait.is_some() {
+                    continue;
+                }
+
+                let self_ty = db.impl_self_ty(impl_id);
+                if let Some(fp) = TyFingerprint::for_impl(&self_ty.value) {
+                    map.entry(fp).or_default().push(impl_id);
+                }
+            }
+        }
+
+        Arc::new(Self { map })
+    }
+
+    pub fn for_self_ty(&self, self_ty: &Ty) -> &[ImplId] {
+        match TyFingerprint::for_impl(self_ty) {
+            Some(fp) => self.map.get(&fp).map(|vec| vec.as_ref()).unwrap_or(&[]),
+            None => &[],
+        }
+    }
+
+    pub fn all_impls(&self) -> impl Iterator<Item = ImplId> + '_ {
+        self.map.values().flat_map(|v| v.iter().copied())
+    }
+}
+
+impl Ty {
+    pub fn def_crates(
+        &self,
+        db: &dyn HirDatabase,
+        cur_crate: CrateId,
+    ) -> Option<ArrayVec<[CrateId; 2]>> {
+        // Types like slice can have inherent impls in several crates, (core and alloc).
+        // The corresponding impls are marked with lang items, so we can use them to find the required crates.
+        macro_rules! lang_item_crate {
+            ($($name:expr),+ $(,)?) => {{
+                let mut v = ArrayVec::<[LangItemTarget; 2]>::new();
+                $(
+                    v.extend(db.lang_item(cur_crate, $name.into()));
+                )+
+                v
+            }};
+        }
+
+        let lang_item_targets = match self {
+            Ty::Apply(a_ty) => match a_ty.ctor {
+                TypeCtor::Adt(def_id) => {
+                    return Some(std::iter::once(def_id.module(db.upcast()).krate).collect())
+                }
+                TypeCtor::Bool => lang_item_crate!("bool"),
+                TypeCtor::Char => lang_item_crate!("char"),
+                TypeCtor::Float(f) => match f.bitness {
+                    // There are two lang items: one in libcore (fXX) and one in libstd (fXX_runtime)
+                    FloatBitness::X32 => lang_item_crate!("f32", "f32_runtime"),
+                    FloatBitness::X64 => lang_item_crate!("f64", "f64_runtime"),
+                },
+                TypeCtor::Int(i) => lang_item_crate!(i.ty_to_string()),
+                TypeCtor::Str => lang_item_crate!("str_alloc", "str"),
+                TypeCtor::Slice => lang_item_crate!("slice_alloc", "slice"),
+                TypeCtor::RawPtr(Mutability::Shared) => lang_item_crate!("const_ptr"),
+                TypeCtor::RawPtr(Mutability::Mut) => lang_item_crate!("mut_ptr"),
+                _ => return None,
+            },
+            _ => return None,
+        };
+        let res = lang_item_targets
+            .into_iter()
+            .filter_map(|it| match it {
+                LangItemTarget::ImplDefId(it) => Some(it),
+                _ => None,
+            })
+            .map(|it| it.lookup(db.upcast()).container.module(db.upcast()).krate)
+            .collect();
+        Some(res)
+    }
+}
+/// Look up the method with the given name, returning the actual autoderefed
+/// receiver type (but without autoref applied yet).
+pub(crate) fn lookup_method(
+    ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    traits_in_scope: &FxHashSet<TraitId>,
+    name: &Name,
+) -> Option<(Ty, FunctionId)> {
+    iterate_method_candidates(
+        ty,
+        db,
+        env,
+        krate,
+        &traits_in_scope,
+        Some(name),
+        LookupMode::MethodCall,
+        |ty, f| match f {
+            AssocItemId::FunctionId(f) => Some((ty.clone(), f)),
+            _ => None,
+        },
+    )
+}
+
+/// Whether we're looking up a dotted method call (like `v.len()`) or a path
+/// (like `Vec::new`).
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum LookupMode {
+    /// Looking up a method call like `v.len()`: We only consider candidates
+    /// that have a `self` parameter, and do autoderef.
+    MethodCall,
+    /// Looking up a path like `Vec::new` or `Vec::default`: We consider all
+    /// candidates including associated constants, but don't do autoderef.
+    Path,
+}
+
+// This would be nicer if it just returned an iterator, but that runs into
+// lifetime problems, because we need to borrow temp `CrateImplDefs`.
+// FIXME add a context type here?
+pub fn iterate_method_candidates<T>(
+    ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    traits_in_scope: &FxHashSet<TraitId>,
+    name: Option<&Name>,
+    mode: LookupMode,
+    mut callback: impl FnMut(&Ty, AssocItemId) -> Option<T>,
+) -> Option<T> {
+    let mut slot = None;
+    iterate_method_candidates_impl(
+        ty,
+        db,
+        env,
+        krate,
+        traits_in_scope,
+        name,
+        mode,
+        &mut |ty, item| {
+            assert!(slot.is_none());
+            slot = callback(ty, item);
+            slot.is_some()
+        },
+    );
+    slot
+}
+
+fn iterate_method_candidates_impl(
+    ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    traits_in_scope: &FxHashSet<TraitId>,
+    name: Option<&Name>,
+    mode: LookupMode,
+    callback: &mut dyn FnMut(&Ty, AssocItemId) -> bool,
+) -> bool {
+    match mode {
+        LookupMode::MethodCall => {
+            // For method calls, rust first does any number of autoderef, and then one
+            // autoref (i.e. when the method takes &self or &mut self). We just ignore
+            // the autoref currently -- when we find a method matching the given name,
+            // we assume it fits.
+
+            // Also note that when we've got a receiver like &S, even if the method we
+            // find in the end takes &self, we still do the autoderef step (just as
+            // rustc does an autoderef and then autoref again).
+            let ty = InEnvironment { value: ty.clone(), environment: env.clone() };
+
+            // We have to be careful about the order we're looking at candidates
+            // in here. Consider the case where we're resolving `x.clone()`
+            // where `x: &Vec<_>`. This resolves to the clone method with self
+            // type `Vec<_>`, *not* `&_`. I.e. we need to consider methods where
+            // the receiver type exactly matches before cases where we have to
+            // do autoref. But in the autoderef steps, the `&_` self type comes
+            // up *before* the `Vec<_>` self type.
+            //
+            // On the other hand, we don't want to just pick any by-value method
+            // before any by-autoref method; it's just that we need to consider
+            // the methods by autoderef order of *receiver types*, not *self
+            // types*.
+
+            let deref_chain = autoderef_method_receiver(db, krate, ty);
+            for i in 0..deref_chain.len() {
+                if iterate_method_candidates_with_autoref(
+                    &deref_chain[i..],
+                    db,
+                    env.clone(),
+                    krate,
+                    traits_in_scope,
+                    name,
+                    callback,
+                ) {
+                    return true;
+                }
+            }
+            false
+        }
+        LookupMode::Path => {
+            // No autoderef for path lookups
+            iterate_method_candidates_for_self_ty(
+                &ty,
+                db,
+                env,
+                krate,
+                traits_in_scope,
+                name,
+                callback,
+            )
+        }
+    }
+}
+
+fn iterate_method_candidates_with_autoref(
+    deref_chain: &[Canonical<Ty>],
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    traits_in_scope: &FxHashSet<TraitId>,
+    name: Option<&Name>,
+    mut callback: &mut dyn FnMut(&Ty, AssocItemId) -> bool,
+) -> bool {
+    if iterate_method_candidates_by_receiver(
+        &deref_chain[0],
+        &deref_chain[1..],
+        db,
+        env.clone(),
+        krate,
+        &traits_in_scope,
+        name,
+        &mut callback,
+    ) {
+        return true;
+    }
+    let refed = Canonical {
+        kinds: deref_chain[0].kinds.clone(),
+        value: Ty::apply_one(TypeCtor::Ref(Mutability::Shared), deref_chain[0].value.clone()),
+    };
+    if iterate_method_candidates_by_receiver(
+        &refed,
+        deref_chain,
+        db,
+        env.clone(),
+        krate,
+        &traits_in_scope,
+        name,
+        &mut callback,
+    ) {
+        return true;
+    }
+    let ref_muted = Canonical {
+        kinds: deref_chain[0].kinds.clone(),
+        value: Ty::apply_one(TypeCtor::Ref(Mutability::Mut), deref_chain[0].value.clone()),
+    };
+    if iterate_method_candidates_by_receiver(
+        &ref_muted,
+        deref_chain,
+        db,
+        env,
+        krate,
+        &traits_in_scope,
+        name,
+        &mut callback,
+    ) {
+        return true;
+    }
+    false
+}
+
+fn iterate_method_candidates_by_receiver(
+    receiver_ty: &Canonical<Ty>,
+    rest_of_deref_chain: &[Canonical<Ty>],
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    traits_in_scope: &FxHashSet<TraitId>,
+    name: Option<&Name>,
+    mut callback: &mut dyn FnMut(&Ty, AssocItemId) -> bool,
+) -> bool {
+    // We're looking for methods with *receiver* type receiver_ty. These could
+    // be found in any of the derefs of receiver_ty, so we have to go through
+    // that.
+    for self_ty in std::iter::once(receiver_ty).chain(rest_of_deref_chain) {
+        if iterate_inherent_methods(self_ty, db, name, Some(receiver_ty), krate, &mut callback) {
+            return true;
+        }
+    }
+    for self_ty in std::iter::once(receiver_ty).chain(rest_of_deref_chain) {
+        if iterate_trait_method_candidates(
+            self_ty,
+            db,
+            env.clone(),
+            krate,
+            &traits_in_scope,
+            name,
+            Some(receiver_ty),
+            &mut callback,
+        ) {
+            return true;
+        }
+    }
+    false
+}
+
+fn iterate_method_candidates_for_self_ty(
+    self_ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    traits_in_scope: &FxHashSet<TraitId>,
+    name: Option<&Name>,
+    mut callback: &mut dyn FnMut(&Ty, AssocItemId) -> bool,
+) -> bool {
+    if iterate_inherent_methods(self_ty, db, name, None, krate, &mut callback) {
+        return true;
+    }
+    iterate_trait_method_candidates(self_ty, db, env, krate, traits_in_scope, name, None, callback)
+}
+
+fn iterate_trait_method_candidates(
+    self_ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    traits_in_scope: &FxHashSet<TraitId>,
+    name: Option<&Name>,
+    receiver_ty: Option<&Canonical<Ty>>,
+    callback: &mut dyn FnMut(&Ty, AssocItemId) -> bool,
+) -> bool {
+    // if ty is `dyn Trait`, the trait doesn't need to be in scope
+    let inherent_trait =
+        self_ty.value.dyn_trait().into_iter().flat_map(|t| all_super_traits(db.upcast(), t));
+    let env_traits = if let Ty::Placeholder(_) = self_ty.value {
+        // if we have `T: Trait` in the param env, the trait doesn't need to be in scope
+        env.trait_predicates_for_self_ty(&self_ty.value)
+            .map(|tr| tr.trait_)
+            .flat_map(|t| all_super_traits(db.upcast(), t))
+            .collect()
+    } else {
+        Vec::new()
+    };
+    let traits =
+        inherent_trait.chain(env_traits.into_iter()).chain(traits_in_scope.iter().copied());
+    'traits: for t in traits {
+        let data = db.trait_data(t);
+
+        // we'll be lazy about checking whether the type implements the
+        // trait, but if we find out it doesn't, we'll skip the rest of the
+        // iteration
+        let mut known_implemented = false;
+        for (_name, item) in data.items.iter() {
+            if !is_valid_candidate(db, name, receiver_ty, *item, self_ty) {
+                continue;
+            }
+            if !known_implemented {
+                let goal = generic_implements_goal(db, env.clone(), t, self_ty.clone());
+                if db.trait_solve(krate, goal).is_none() {
+                    continue 'traits;
+                }
+            }
+            known_implemented = true;
+            if callback(&self_ty.value, *item) {
+                return true;
+            }
+        }
+    }
+    false
+}
+
+fn iterate_inherent_methods(
+    self_ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    name: Option<&Name>,
+    receiver_ty: Option<&Canonical<Ty>>,
+    krate: CrateId,
+    callback: &mut dyn FnMut(&Ty, AssocItemId) -> bool,
+) -> bool {
+    let def_crates = match self_ty.value.def_crates(db, krate) {
+        Some(k) => k,
+        None => return false,
+    };
+    for krate in def_crates {
+        let impls = db.inherent_impls_in_crate(krate);
+
+        for &impl_def in impls.for_self_ty(&self_ty.value) {
+            for &item in db.impl_data(impl_def).items.iter() {
+                if !is_valid_candidate(db, name, receiver_ty, item, self_ty) {
+                    continue;
+                }
+                // we have to check whether the self type unifies with the type
+                // that the impl is for. If we have a receiver type, this
+                // already happens in `is_valid_candidate` above; if not, we
+                // check it here
+                if receiver_ty.is_none() && inherent_impl_substs(db, impl_def, self_ty).is_none() {
+                    test_utils::mark::hit!(impl_self_type_match_without_receiver);
+                    continue;
+                }
+                if callback(&self_ty.value, item) {
+                    return true;
+                }
+            }
+        }
+    }
+    false
+}
+
+/// Returns the self type for the index trait call.
+pub fn resolve_indexing_op(
+    db: &dyn HirDatabase,
+    ty: &Canonical<Ty>,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    index_trait: TraitId,
+) -> Option<Canonical<Ty>> {
+    let ty = InEnvironment { value: ty.clone(), environment: env.clone() };
+    let deref_chain = autoderef_method_receiver(db, krate, ty);
+    for ty in deref_chain {
+        let goal = generic_implements_goal(db, env.clone(), index_trait, ty.clone());
+        if db.trait_solve(krate, goal).is_some() {
+            return Some(ty);
+        }
+    }
+    None
+}
+
+fn is_valid_candidate(
+    db: &dyn HirDatabase,
+    name: Option<&Name>,
+    receiver_ty: Option<&Canonical<Ty>>,
+    item: AssocItemId,
+    self_ty: &Canonical<Ty>,
+) -> bool {
+    match item {
+        AssocItemId::FunctionId(m) => {
+            let data = db.function_data(m);
+            if let Some(name) = name {
+                if &data.name != name {
+                    return false;
+                }
+            }
+            if let Some(receiver_ty) = receiver_ty {
+                if !data.has_self_param {
+                    return false;
+                }
+                let transformed_receiver_ty = match transform_receiver_ty(db, m, self_ty) {
+                    Some(ty) => ty,
+                    None => return false,
+                };
+                if transformed_receiver_ty != receiver_ty.value {
+                    return false;
+                }
+            }
+            true
+        }
+        AssocItemId::ConstId(c) => {
+            let data = db.const_data(c);
+            name.map_or(true, |name| data.name.as_ref() == Some(name)) && receiver_ty.is_none()
+        }
+        _ => false,
+    }
+}
+
+pub(crate) fn inherent_impl_substs(
+    db: &dyn HirDatabase,
+    impl_id: ImplId,
+    self_ty: &Canonical<Ty>,
+) -> Option<Substs> {
+    // we create a var for each type parameter of the impl; we need to keep in
+    // mind here that `self_ty` might have vars of its own
+    let vars = Substs::build_for_def(db, impl_id)
+        .fill_with_bound_vars(DebruijnIndex::INNERMOST, self_ty.kinds.len())
+        .build();
+    let self_ty_with_vars = db.impl_self_ty(impl_id).subst(&vars);
+    let mut kinds = self_ty.kinds.to_vec();
+    kinds.extend(iter::repeat(TyKind::General).take(vars.len()));
+    let tys = Canonical { kinds: kinds.into(), value: (self_ty_with_vars, self_ty.value.clone()) };
+    let substs = super::infer::unify(&tys);
+    // We only want the substs for the vars we added, not the ones from self_ty.
+    // Also, if any of the vars we added are still in there, we replace them by
+    // Unknown. I think this can only really happen if self_ty contained
+    // Unknown, and in that case we want the result to contain Unknown in those
+    // places again.
+    substs.map(|s| fallback_bound_vars(s.suffix(vars.len()), self_ty.kinds.len()))
+}
+
+/// This replaces any 'free' Bound vars in `s` (i.e. those with indices past
+/// num_vars_to_keep) by `Ty::Unknown`.
+fn fallback_bound_vars(s: Substs, num_vars_to_keep: usize) -> Substs {
+    s.fold_binders(
+        &mut |ty, binders| {
+            if let Ty::Bound(bound) = &ty {
+                if bound.index >= num_vars_to_keep && bound.debruijn >= binders {
+                    Ty::Unknown
+                } else {
+                    ty
+                }
+            } else {
+                ty
+            }
+        },
+        DebruijnIndex::INNERMOST,
+    )
+}
+
+fn transform_receiver_ty(
+    db: &dyn HirDatabase,
+    function_id: FunctionId,
+    self_ty: &Canonical<Ty>,
+) -> Option<Ty> {
+    let substs = match function_id.lookup(db.upcast()).container {
+        AssocContainerId::TraitId(_) => Substs::build_for_def(db, function_id)
+            .push(self_ty.value.clone())
+            .fill_with_unknown()
+            .build(),
+        AssocContainerId::ImplId(impl_id) => inherent_impl_substs(db, impl_id, &self_ty)?,
+        AssocContainerId::ContainerId(_) => unreachable!(),
+    };
+    let sig = db.callable_item_signature(function_id.into());
+    Some(sig.value.params()[0].clone().subst_bound_vars(&substs))
+}
+
+pub fn implements_trait(
+    ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    trait_: TraitId,
+) -> bool {
+    let goal = generic_implements_goal(db, env, trait_, ty.clone());
+    let solution = db.trait_solve(krate, goal);
+
+    solution.is_some()
+}
+
+/// This creates Substs for a trait with the given Self type and type variables
+/// for all other parameters, to query Chalk with it.
+fn generic_implements_goal(
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    trait_: TraitId,
+    self_ty: Canonical<Ty>,
+) -> Canonical<InEnvironment<super::Obligation>> {
+    let mut kinds = self_ty.kinds.to_vec();
+    let substs = super::Substs::build_for_def(db, trait_)
+        .push(self_ty.value)
+        .fill_with_bound_vars(DebruijnIndex::INNERMOST, kinds.len())
+        .build();
+    kinds.extend(iter::repeat(TyKind::General).take(substs.len() - 1));
+    let trait_ref = TraitRef { trait_, substs };
+    let obligation = super::Obligation::Trait(trait_ref);
+    Canonical { kinds: kinds.into(), value: InEnvironment::new(env, obligation) }
+}
+
+fn autoderef_method_receiver(
+    db: &dyn HirDatabase,
+    krate: CrateId,
+    ty: InEnvironment<Canonical<Ty>>,
+) -> Vec<Canonical<Ty>> {
+    let mut deref_chain: Vec<_> = autoderef::autoderef(db, Some(krate), ty).collect();
+    // As a last step, we can do array unsizing (that's the only unsizing that rustc does for method receivers!)
+    if let Some(Ty::Apply(ApplicationTy { ctor: TypeCtor::Array, parameters })) =
+        deref_chain.last().map(|ty| &ty.value)
+    {
+        let kinds = deref_chain.last().unwrap().kinds.clone();
+        let unsized_ty = Ty::apply(TypeCtor::Slice, parameters.clone());
+        deref_chain.push(Canonical { value: unsized_ty, kinds })
+    }
+    deref_chain
+}