use rustc_data_structures::fx::FxHashSet;
use rustc_hir as hir;
use rustc_hir::lang_items::LangItem;
use rustc_middle::ty::{self, Region, RegionVid, TypeFoldable};
use rustc_trait_selection::traits::auto_trait::{self, AutoTraitResult};

use std::fmt::Debug;

use super::*;

#[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
enum RegionTarget<'tcx> {
    Region(Region<'tcx>),
    RegionVid(RegionVid),
}

#[derive(Default, Debug, Clone)]
struct RegionDeps<'tcx> {
    larger: FxHashSet<RegionTarget<'tcx>>,
    smaller: FxHashSet<RegionTarget<'tcx>>,
}

pub struct AutoTraitFinder<'a, 'tcx> {
    pub cx: &'a core::DocContext<'tcx>,
    pub f: auto_trait::AutoTraitFinder<'tcx>,
}

impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
    pub fn new(cx: &'a core::DocContext<'tcx>) -> Self {
        let f = auto_trait::AutoTraitFinder::new(cx.tcx);

        AutoTraitFinder { cx, f }
    }

    // FIXME(eddyb) figure out a better way to pass information about
    // parametrization of `ty` than `param_env_def_id`.
    pub fn get_auto_trait_impls(&self, ty: Ty<'tcx>, param_env_def_id: DefId) -> Vec<Item> {
        let param_env = self.cx.tcx.param_env(param_env_def_id);

        debug!("get_auto_trait_impls({:?})", ty);
        let auto_traits = self.cx.auto_traits.iter().cloned();
        auto_traits
            .filter_map(|trait_def_id| {
                let trait_ref = ty::TraitRef {
                    def_id: trait_def_id,
                    substs: self.cx.tcx.mk_substs_trait(ty, &[]),
                };
                if !self.cx.generated_synthetics.borrow_mut().insert((ty, trait_def_id)) {
                    debug!("get_auto_trait_impl_for({:?}): already generated, aborting", trait_ref);
                    return None;
                }

                let result =
                    self.f.find_auto_trait_generics(ty, param_env, trait_def_id, |infcx, info| {
                        let region_data = info.region_data;

                        let names_map = self
                            .cx
                            .tcx
                            .generics_of(param_env_def_id)
                            .params
                            .iter()
                            .filter_map(|param| match param.kind {
                                ty::GenericParamDefKind::Lifetime => Some(param.name.to_string()),
                                _ => None,
                            })
                            .map(|name| (name.clone(), Lifetime(name)))
                            .collect();
                        let lifetime_predicates = self.handle_lifetimes(&region_data, &names_map);
                        let new_generics = self.param_env_to_generics(
                            infcx.tcx,
                            param_env_def_id,
                            info.full_user_env,
                            lifetime_predicates,
                            info.vid_to_region,
                        );

                        debug!(
                            "find_auto_trait_generics(param_env_def_id={:?}, trait_def_id={:?}): \
                            finished with {:?}",
                            param_env_def_id, trait_def_id, new_generics
                        );

                        new_generics
                    });

                let polarity;
                let new_generics = match result {
                    AutoTraitResult::PositiveImpl(new_generics) => {
                        polarity = None;
                        new_generics
                    }
                    AutoTraitResult::NegativeImpl => {
                        polarity = Some(ImplPolarity::Negative);

                        // For negative impls, we use the generic params, but *not* the predicates,
                        // from the original type. Otherwise, the displayed impl appears to be a
                        // conditional negative impl, when it's really unconditional.
                        //
                        // For example, consider the struct Foo<T: Copy>(*mut T). Using
                        // the original predicates in our impl would cause us to generate
                        // `impl !Send for Foo<T: Copy>`, which makes it appear that Foo
                        // implements Send where T is not copy.
                        //
                        // Instead, we generate `impl !Send for Foo<T>`, which better
                        // expresses the fact that `Foo<T>` never implements `Send`,
                        // regardless of the choice of `T`.
                        let params = (
                            self.cx.tcx.generics_of(param_env_def_id),
                            ty::GenericPredicates::default(),
                        )
                            .clean(self.cx)
                            .params;

                        Generics { params, where_predicates: Vec::new() }
                    }
                    AutoTraitResult::ExplicitImpl => return None,
                };

                Some(Item {
                    source: Span::empty(),
                    name: None,
                    attrs: Default::default(),
                    visibility: Inherited,
                    def_id: self.cx.next_def_id(param_env_def_id.krate),
                    stability: None,
                    deprecation: None,
                    inner: ImplItem(Impl {
                        unsafety: hir::Unsafety::Normal,
                        generics: new_generics,
                        provided_trait_methods: Default::default(),
                        trait_: Some(trait_ref.clean(self.cx).get_trait_type().unwrap()),
                        for_: ty.clean(self.cx),
                        items: Vec::new(),
                        polarity,
                        synthetic: true,
                        blanket_impl: None,
                    }),
                })
            })
            .collect()
    }

    fn get_lifetime(
        &self,
        region: Region<'_>,
        names_map: &FxHashMap<String, Lifetime>,
    ) -> Lifetime {
        self.region_name(region)
            .map(|name| {
                names_map.get(&name).unwrap_or_else(|| {
                    panic!("Missing lifetime with name {:?} for {:?}", name, region)
                })
            })
            .unwrap_or(&Lifetime::statik())
            .clone()
    }

    fn region_name(&self, region: Region<'_>) -> Option<String> {
        match region {
            &ty::ReEarlyBound(r) => Some(r.name.to_string()),
            _ => None,
        }
    }

    // This method calculates two things: Lifetime constraints of the form 'a: 'b,
    // and region constraints of the form ReVar: 'a
    //
    // This is essentially a simplified version of lexical_region_resolve. However,
    // handle_lifetimes determines what *needs be* true in order for an impl to hold.
    // lexical_region_resolve, along with much of the rest of the compiler, is concerned
    // with determining if a given set up constraints/predicates *are* met, given some
    // starting conditions (e.g., user-provided code). For this reason, it's easier
    // to perform the calculations we need on our own, rather than trying to make
    // existing inference/solver code do what we want.
    fn handle_lifetimes<'cx>(
        &self,
        regions: &RegionConstraintData<'cx>,
        names_map: &FxHashMap<String, Lifetime>,
    ) -> Vec<WherePredicate> {
        // Our goal is to 'flatten' the list of constraints by eliminating
        // all intermediate RegionVids. At the end, all constraints should
        // be between Regions (aka region variables). This gives us the information
        // we need to create the Generics.
        let mut finished: FxHashMap<_, Vec<_>> = Default::default();

        let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();

        // Flattening is done in two parts. First, we insert all of the constraints
        // into a map. Each RegionTarget (either a RegionVid or a Region) maps
        // to its smaller and larger regions. Note that 'larger' regions correspond
        // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
        for constraint in regions.constraints.keys() {
            match constraint {
                &Constraint::VarSubVar(r1, r2) => {
                    {
                        let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
                        deps1.larger.insert(RegionTarget::RegionVid(r2));
                    }

                    let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
                    deps2.smaller.insert(RegionTarget::RegionVid(r1));
                }
                &Constraint::RegSubVar(region, vid) => {
                    let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
                    deps.smaller.insert(RegionTarget::Region(region));
                }
                &Constraint::VarSubReg(vid, region) => {
                    let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
                    deps.larger.insert(RegionTarget::Region(region));
                }
                &Constraint::RegSubReg(r1, r2) => {
                    // The constraint is already in the form that we want, so we're done with it
                    // Desired order is 'larger, smaller', so flip then
                    if self.region_name(r1) != self.region_name(r2) {
                        finished
                            .entry(self.region_name(r2).expect("no region_name found"))
                            .or_default()
                            .push(r1);
                    }
                }
            }
        }

        // Here, we 'flatten' the map one element at a time.
        // All of the element's sub and super regions are connected
        // to each other. For example, if we have a graph that looks like this:
        //
        // (A, B) - C - (D, E)
        // Where (A, B) are subregions, and (D,E) are super-regions
        //
        // then after deleting 'C', the graph will look like this:
        //  ... - A - (D, E ...)
        //  ... - B - (D, E, ...)
        //  (A, B, ...) - D - ...
        //  (A, B, ...) - E - ...
        //
        //  where '...' signifies the existing sub and super regions of an entry
        //  When two adjacent ty::Regions are encountered, we've computed a final
        //  constraint, and add it to our list. Since we make sure to never re-add
        //  deleted items, this process will always finish.
        while !vid_map.is_empty() {
            let target = *vid_map.keys().next().expect("Keys somehow empty");
            let deps = vid_map.remove(&target).expect("Entry somehow missing");

            for smaller in deps.smaller.iter() {
                for larger in deps.larger.iter() {
                    match (smaller, larger) {
                        (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
                            if self.region_name(r1) != self.region_name(r2) {
                                finished
                                    .entry(self.region_name(r2).expect("no region name found"))
                                    .or_default()
                                    .push(r1) // Larger, smaller
                            }
                        }
                        (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
                            if let Entry::Occupied(v) = vid_map.entry(*smaller) {
                                let smaller_deps = v.into_mut();
                                smaller_deps.larger.insert(*larger);
                                smaller_deps.larger.remove(&target);
                            }
                        }
                        (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
                            if let Entry::Occupied(v) = vid_map.entry(*larger) {
                                let deps = v.into_mut();
                                deps.smaller.insert(*smaller);
                                deps.smaller.remove(&target);
                            }
                        }
                        (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
                            if let Entry::Occupied(v) = vid_map.entry(*smaller) {
                                let smaller_deps = v.into_mut();
                                smaller_deps.larger.insert(*larger);
                                smaller_deps.larger.remove(&target);
                            }

                            if let Entry::Occupied(v) = vid_map.entry(*larger) {
                                let larger_deps = v.into_mut();
                                larger_deps.smaller.insert(*smaller);
                                larger_deps.smaller.remove(&target);
                            }
                        }
                    }
                }
            }
        }

        let lifetime_predicates = names_map
            .iter()
            .flat_map(|(name, lifetime)| {
                let empty = Vec::new();
                let bounds: FxHashSet<GenericBound> = finished
                    .get(name)
                    .unwrap_or(&empty)
                    .iter()
                    .map(|region| GenericBound::Outlives(self.get_lifetime(region, names_map)))
                    .collect();

                if bounds.is_empty() {
                    return None;
                }
                Some(WherePredicate::RegionPredicate {
                    lifetime: lifetime.clone(),
                    bounds: bounds.into_iter().collect(),
                })
            })
            .collect();

        lifetime_predicates
    }

    fn extract_for_generics(
        &self,
        tcx: TyCtxt<'tcx>,
        pred: ty::Predicate<'tcx>,
    ) -> FxHashSet<GenericParamDef> {
        let regions = match pred.skip_binders() {
            ty::PredicateAtom::Trait(poly_trait_pred, _) => {
                tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(poly_trait_pred))
            }
            ty::PredicateAtom::Projection(poly_proj_pred) => {
                tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(poly_proj_pred))
            }
            _ => return FxHashSet::default(),
        };

        regions
            .into_iter()
            .filter_map(|br| {
                match br {
                    // We only care about named late bound regions, as we need to add them
                    // to the 'for<>' section
                    ty::BrNamed(_, name) => Some(GenericParamDef {
                        name: name.to_string(),
                        kind: GenericParamDefKind::Lifetime,
                    }),
                    _ => None,
                }
            })
            .collect()
    }

    fn make_final_bounds(
        &self,
        ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
        ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)>,
        lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
    ) -> Vec<WherePredicate> {
        ty_to_bounds
            .into_iter()
            .flat_map(|(ty, mut bounds)| {
                if let Some(data) = ty_to_fn.get(&ty) {
                    let (poly_trait, output) =
                        (data.0.as_ref().expect("as_ref failed").clone(), data.1.as_ref().cloned());
                    let new_ty = match &poly_trait.trait_ {
                        &Type::ResolvedPath {
                            ref path,
                            ref param_names,
                            ref did,
                            ref is_generic,
                        } => {
                            let mut new_path = path.clone();
                            let last_segment =
                                new_path.segments.pop().expect("segments were empty");

                            let (old_input, old_output) = match last_segment.args {
                                GenericArgs::AngleBracketed { args, .. } => {
                                    let types = args
                                        .iter()
                                        .filter_map(|arg| match arg {
                                            GenericArg::Type(ty) => Some(ty.clone()),
                                            _ => None,
                                        })
                                        .collect();
                                    (types, None)
                                }
                                GenericArgs::Parenthesized { inputs, output, .. } => {
                                    (inputs, output)
                                }
                            };

                            if old_output.is_some() && old_output != output {
                                panic!(
                                    "Output mismatch for {:?} {:?} {:?}",
                                    ty, old_output, data.1
                                );
                            }

                            let new_params =
                                GenericArgs::Parenthesized { inputs: old_input, output };

                            new_path
                                .segments
                                .push(PathSegment { name: last_segment.name, args: new_params });

                            Type::ResolvedPath {
                                path: new_path,
                                param_names: param_names.clone(),
                                did: *did,
                                is_generic: *is_generic,
                            }
                        }
                        _ => panic!("Unexpected data: {:?}, {:?}", ty, data),
                    };
                    bounds.insert(GenericBound::TraitBound(
                        PolyTrait { trait_: new_ty, generic_params: poly_trait.generic_params },
                        hir::TraitBoundModifier::None,
                    ));
                }
                if bounds.is_empty() {
                    return None;
                }

                let mut bounds_vec = bounds.into_iter().collect();
                self.sort_where_bounds(&mut bounds_vec);

                Some(WherePredicate::BoundPredicate { ty, bounds: bounds_vec })
            })
            .chain(
                lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
                    |(lifetime, bounds)| {
                        let mut bounds_vec = bounds.into_iter().collect();
                        self.sort_where_bounds(&mut bounds_vec);
                        WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
                    },
                ),
            )
            .collect()
    }

    // Converts the calculated ParamEnv and lifetime information to a clean::Generics, suitable for
    // display on the docs page. Cleaning the Predicates produces sub-optimal `WherePredicate`s,
    // so we fix them up:
    //
    // * Multiple bounds for the same type are coalesced into one: e.g., 'T: Copy', 'T: Debug'
    // becomes 'T: Copy + Debug'
    // * Fn bounds are handled specially - instead of leaving it as 'T: Fn(), <T as Fn::Output> =
    // K', we use the dedicated syntax 'T: Fn() -> K'
    // * We explicitly add a '?Sized' bound if we didn't find any 'Sized' predicates for a type
    fn param_env_to_generics(
        &self,
        tcx: TyCtxt<'tcx>,
        param_env_def_id: DefId,
        param_env: ty::ParamEnv<'tcx>,
        mut existing_predicates: Vec<WherePredicate>,
        vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
    ) -> Generics {
        debug!(
            "param_env_to_generics(param_env_def_id={:?}, param_env={:?}, \
             existing_predicates={:?})",
            param_env_def_id, param_env, existing_predicates
        );

        // The `Sized` trait must be handled specially, since we only display it when
        // it is *not* required (i.e., '?Sized')
        let sized_trait = self.cx.tcx.require_lang_item(LangItem::Sized, None);

        let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };

        let orig_bounds: FxHashSet<_> =
            self.cx.tcx.param_env(param_env_def_id).caller_bounds().iter().collect();
        let clean_where_predicates = param_env
            .caller_bounds()
            .iter()
            .filter(|p| {
                !orig_bounds.contains(p)
                    || match p.skip_binders() {
                        ty::PredicateAtom::Trait(pred, _) => pred.def_id() == sized_trait,
                        _ => false,
                    }
            })
            .map(|p| {
                let replaced = p.fold_with(&mut replacer);
                (replaced, replaced.clean(self.cx))
            });

        let mut generic_params =
            (tcx.generics_of(param_env_def_id), tcx.explicit_predicates_of(param_env_def_id))
                .clean(self.cx)
                .params;

        debug!(
            "param_env_to_generics({:?}): generic_params={:?}",
            param_env_def_id, generic_params
        );

        let mut has_sized = FxHashSet::default();
        let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
        let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
        let mut ty_to_traits: FxHashMap<Type, FxHashSet<Type>> = Default::default();

        let mut ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)> = Default::default();

        for (orig_p, p) in clean_where_predicates {
            if p.is_none() {
                continue;
            }
            let p = p.unwrap();
            match p {
                WherePredicate::BoundPredicate { ty, mut bounds } => {
                    // Writing a projection trait bound of the form
                    // <T as Trait>::Name : ?Sized
                    // is illegal, because ?Sized bounds can only
                    // be written in the (here, nonexistent) definition
                    // of the type.
                    // Therefore, we make sure that we never add a ?Sized
                    // bound for projections
                    if let Type::QPath { .. } = ty {
                        has_sized.insert(ty.clone());
                    }

                    if bounds.is_empty() {
                        continue;
                    }

                    let mut for_generics = self.extract_for_generics(tcx, orig_p);

                    assert!(bounds.len() == 1);
                    let mut b = bounds.pop().expect("bounds were empty");

                    if b.is_sized_bound(self.cx) {
                        has_sized.insert(ty.clone());
                    } else if !b
                        .get_trait_type()
                        .and_then(|t| {
                            ty_to_traits
                                .get(&ty)
                                .map(|bounds| bounds.contains(&strip_type(t.clone())))
                        })
                        .unwrap_or(false)
                    {
                        // If we've already added a projection bound for the same type, don't add
                        // this, as it would be a duplicate

                        // Handle any 'Fn/FnOnce/FnMut' bounds specially,
                        // as we want to combine them with any 'Output' qpaths
                        // later

                        let is_fn = match &mut b {
                            &mut GenericBound::TraitBound(ref mut p, _) => {
                                // Insert regions into the for_generics hash map first, to ensure
                                // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
                                for_generics.extend(p.generic_params.clone());
                                p.generic_params = for_generics.into_iter().collect();
                                self.is_fn_ty(tcx, &p.trait_)
                            }
                            _ => false,
                        };

                        let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");

                        if is_fn {
                            ty_to_fn
                                .entry(ty.clone())
                                .and_modify(|e| *e = (Some(poly_trait.clone()), e.1.clone()))
                                .or_insert(((Some(poly_trait.clone())), None));

                            ty_to_bounds.entry(ty.clone()).or_default();
                        } else {
                            ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
                        }
                    }
                }
                WherePredicate::RegionPredicate { lifetime, bounds } => {
                    lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
                }
                WherePredicate::EqPredicate { lhs, rhs } => {
                    match lhs {
                        Type::QPath { name: ref left_name, ref self_type, ref trait_ } => {
                            let ty = &*self_type;
                            match **trait_ {
                                Type::ResolvedPath {
                                    path: ref trait_path,
                                    ref param_names,
                                    ref did,
                                    ref is_generic,
                                } => {
                                    let mut new_trait_path = trait_path.clone();

                                    if self.is_fn_ty(tcx, trait_) && left_name == FN_OUTPUT_NAME {
                                        ty_to_fn
                                            .entry(*ty.clone())
                                            .and_modify(|e| *e = (e.0.clone(), Some(rhs.clone())))
                                            .or_insert((None, Some(rhs)));
                                        continue;
                                    }

                                    let args = &mut new_trait_path
                                        .segments
                                        .last_mut()
                                        .expect("segments were empty")
                                        .args;

                                    match args {
                                        // Convert something like '<T as Iterator::Item> = u8'
                                        // to 'T: Iterator<Item=u8>'
                                        GenericArgs::AngleBracketed {
                                            ref mut bindings, ..
                                        } => {
                                            bindings.push(TypeBinding {
                                                name: left_name.clone(),
                                                kind: TypeBindingKind::Equality { ty: rhs },
                                            });
                                        }
                                        GenericArgs::Parenthesized { .. } => {
                                            existing_predicates.push(WherePredicate::EqPredicate {
                                                lhs: lhs.clone(),
                                                rhs,
                                            });
                                            continue; // If something other than a Fn ends up
                                            // with parenthesis, leave it alone
                                        }
                                    }

                                    let bounds = ty_to_bounds.entry(*ty.clone()).or_default();

                                    bounds.insert(GenericBound::TraitBound(
                                        PolyTrait {
                                            trait_: Type::ResolvedPath {
                                                path: new_trait_path,
                                                param_names: param_names.clone(),
                                                did: *did,
                                                is_generic: *is_generic,
                                            },
                                            generic_params: Vec::new(),
                                        },
                                        hir::TraitBoundModifier::None,
                                    ));

                                    // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
                                    // that we don't see a
                                    // duplicate bound like `T: Iterator + Iterator<Item=u8>`
                                    // on the docs page.
                                    bounds.remove(&GenericBound::TraitBound(
                                        PolyTrait {
                                            trait_: *trait_.clone(),
                                            generic_params: Vec::new(),
                                        },
                                        hir::TraitBoundModifier::None,
                                    ));
                                    // Avoid creating any new duplicate bounds later in the outer
                                    // loop
                                    ty_to_traits
                                        .entry(*ty.clone())
                                        .or_default()
                                        .insert(*trait_.clone());
                                }
                                _ => panic!(
                                    "Unexpected trait {:?} for {:?}",
                                    trait_, param_env_def_id,
                                ),
                            }
                        }
                        _ => panic!("Unexpected LHS {:?} for {:?}", lhs, param_env_def_id),
                    }
                }
            };
        }

        let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);

        existing_predicates.extend(final_bounds);

        for param in generic_params.iter_mut() {
            match param.kind {
                GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
                    // We never want something like `impl<T=Foo>`.
                    default.take();
                    let generic_ty = Type::Generic(param.name.clone());
                    if !has_sized.contains(&generic_ty) {
                        bounds.insert(0, GenericBound::maybe_sized(self.cx));
                    }
                }
                GenericParamDefKind::Lifetime => {}
                GenericParamDefKind::Const { .. } => {}
            }
        }

        self.sort_where_predicates(&mut existing_predicates);

        Generics { params: generic_params, where_predicates: existing_predicates }
    }

    // Ensure that the predicates are in a consistent order. The precise
    // ordering doesn't actually matter, but it's important that
    // a given set of predicates always appears in the same order -
    // both for visual consistency between 'rustdoc' runs, and to
    // make writing tests much easier
    #[inline]
    fn sort_where_predicates(&self, mut predicates: &mut Vec<WherePredicate>) {
        // We should never have identical bounds - and if we do,
        // they're visually identical as well. Therefore, using
        // an unstable sort is fine.
        self.unstable_debug_sort(&mut predicates);
    }

    // Ensure that the bounds are in a consistent order. The precise
    // ordering doesn't actually matter, but it's important that
    // a given set of bounds always appears in the same order -
    // both for visual consistency between 'rustdoc' runs, and to
    // make writing tests much easier
    #[inline]
    fn sort_where_bounds(&self, mut bounds: &mut Vec<GenericBound>) {
        // We should never have identical bounds - and if we do,
        // they're visually identical as well. Therefore, using
        // an unstable sort is fine.
        self.unstable_debug_sort(&mut bounds);
    }

    // This might look horrendously hacky, but it's actually not that bad.
    //
    // For performance reasons, we use several different FxHashMaps
    // in the process of computing the final set of where predicates.
    // However, the iteration order of a HashMap is completely unspecified.
    // In fact, the iteration of an FxHashMap can even vary between platforms,
    // since FxHasher has different behavior for 32-bit and 64-bit platforms.
    //
    // Obviously, it's extremely undesirable for documentation rendering
    // to be dependent on the platform it's run on. Apart from being confusing
    // to end users, it makes writing tests much more difficult, as predicates
    // can appear in any order in the final result.
    //
    // To solve this problem, we sort WherePredicates and GenericBounds
    // by their Debug string. The thing to keep in mind is that we don't really
    // care what the final order is - we're synthesizing an impl or bound
    // ourselves, so any order can be considered equally valid. By sorting the
    // predicates and bounds, however, we ensure that for a given codebase, all
    // auto-trait impls always render in exactly the same way.
    //
    // Using the Debug implementation for sorting prevents us from needing to
    // write quite a bit of almost entirely useless code (e.g., how should two
    // Types be sorted relative to each other). It also allows us to solve the
    // problem for both WherePredicates and GenericBounds at the same time. This
    // approach is probably somewhat slower, but the small number of items
    // involved (impls rarely have more than a few bounds) means that it
    // shouldn't matter in practice.
    fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
        vec.sort_by_cached_key(|x| format!("{:?}", x))
    }

    fn is_fn_ty(&self, tcx: TyCtxt<'_>, ty: &Type) -> bool {
        match &ty {
            &&Type::ResolvedPath { ref did, .. } => {
                *did == tcx.require_lang_item(LangItem::Fn, None)
                    || *did == tcx.require_lang_item(LangItem::FnMut, None)
                    || *did == tcx.require_lang_item(LangItem::FnOnce, None)
            }
            _ => false,
        }
    }
}

// Replaces all ReVars in a type with ty::Region's, using the provided map
struct RegionReplacer<'a, 'tcx> {
    vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
    tcx: TyCtxt<'tcx>,
}

impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
        self.tcx
    }

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        (match r {
            &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
            _ => None,
        })
        .unwrap_or_else(|| r.super_fold_with(self))
    }
}