semverver 0.1.51

//! The traversal logic collecting changes in between crate versions.
//!
//! The changes get collected in multiple passes, and recorded in a `ChangeSet`.
//! The initial pass matches items by name in the module hierarchy, registering item removal
//! and addition, as well as structural changes to ADTs, type- or region parameters, and
//! function signatures. The second pass then proceeds to find non-public items that are named
//! differently, yet are compatible in their usage. The third pass now uses the information
//! collected in the previous passes to compare the types and/or trait bounds of all item pairs
//! that have been matched. Trait and inherent impls can't be matched by name, and are processed
//! in a fourth pass that uses trait bounds to find matching impls.

use crate::{
    changes::{ChangeSet, ChangeType},
    mapping::{IdMapping, NameMapping},
    mismatch::MismatchRelation,
    translate::TranslationContext,
    typeck::{BoundContext, TypeComparisonContext},
};
use log::{debug, info};
use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res, Res::Def};
use rustc_hir::def_id::DefId;
use rustc_hir::hir_id::HirId;
use rustc_hir::lang_items::LangItem;
use rustc_hir::Mutability;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_metadata::creader::CStore;
use rustc_middle::{
    metadata::ModChild,
    ty::{
        subst::{InternalSubsts, Subst},
        AssocItem, EarlyBinder, GenericParamDef, GenericParamDefKind, Generics, TraitRef, Ty,
        TyCtxt, TyKind, TypeAndMut, Visibility,
        Visibility::Public,
    },
};
use std::collections::{BTreeMap, HashSet, VecDeque};

/// The main entry point to our analysis passes.
///
/// Set up the necessary data structures and run the analysis passes and call the actual passes.
pub fn run_analysis(tcx: TyCtxt, old: DefId, new: DefId) -> ChangeSet {
    let mut changes = ChangeSet::default();
    let mut id_mapping = IdMapping::new(old.krate, new.krate);

    // first pass
    debug!("first pass started");
    diff_structure(&mut changes, &mut id_mapping, tcx, old, new);

    // second pass
    debug!("second pass started");
    {
        let mut mismatch = MismatchRelation::new(tcx, &mut id_mapping);
        debug!("constructed mismatch relation");
        mismatch.process();
    }

    // third pass
    debug!("third pass started");
    for (old, new) in id_mapping.items() {
        diff_types(&mut changes, &id_mapping, tcx, old, new);
    }

    // fourth pass on impls
    debug!("fourth pass started");
    diff_inherent_impls(&mut changes, &id_mapping, tcx);
    diff_trait_impls(&mut changes, &id_mapping, tcx);

    changes
}

// Get the visibility of the inner item, given the outer item's visibility.
fn get_vis(outer_vis: Visibility, def: ModChild) -> Visibility {
    if outer_vis == Public {
        def.vis
    } else {
        outer_vis
    }
}

pub fn run_traversal(tcx: TyCtxt, new: DefId) {
    use rustc_hir::def::DefKind::*;
    let mut visited = HashSet::new();
    let mut mod_queue = VecDeque::new();

    // Start off with the root module.
    mod_queue.push_back((new, Vec::new(), Public));

    // Pull a module from the queue, with its global visibility.
    while let Some((new_def_id, idents, new_vis)) = mod_queue.pop_front() {
        for item in tcx.module_children(new_def_id).iter().copied() {
            let n_vis = get_vis(new_vis, item);
            match item.res {
                Def(Mod, n_def_id) => {
                    if visited.insert(n_def_id) {
                        let mut idents = idents.clone();
                        idents.push(format!("{}", item.ident));

                        mod_queue.push_back((n_def_id, idents, n_vis));
                    }
                }
                Def(n_kind, _) if n_vis == Public => {
                    match n_kind {
                        TyAlias | Struct | Union | Enum | Trait => {
                            let mut idents = idents.clone();
                            idents.push(format!("{}", item.ident));

                            println!("{}", idents.join("::"));
                        }
                        _ => (),
                    };
                }
                _ => (),
            }
        }
    }
}

// Below functions constitute the first pass of analysis, in which module structure, ADT
// structure, public and private status of items, and generics are examined for changes.

/// Given two crate root modules, compare their exports and structure.
///
/// Traverse the two root modules in an interleaved manner, matching up pairs of modules
/// from the two crate versions and compare for changes. Matching children get processed
/// in the same fashion.
#[allow(clippy::cognitive_complexity)]
fn diff_structure<'tcx>(
    changes: &mut ChangeSet,
    id_mapping: &mut IdMapping,
    tcx: TyCtxt<'tcx>,
    old: DefId,
    new: DefId,
) {
    use rustc_hir::def::DefKind::*;

    let mut visited = HashSet::new();
    let mut children = NameMapping::default();
    let mut mod_queue = VecDeque::new();
    let mut traits = Vec::new();
    // Additions and removals are processed with a delay to avoid creating multiple path change
    // entries. This is necessary, since the order in which added or removed paths are found wrt
    // each other and their item's definition can't be relied upon.
    let mut removals = Vec::new();
    let mut additions = Vec::new();

    // Start off with the root module pair.
    mod_queue.push_back((old, new, Public, Public));

    // Pull a matched module pair from the queue, with the modules' global visibility.
    while let Some((old_def_id, new_def_id, old_vis, new_vis)) = mod_queue.pop_front() {
        children.add(
            tcx.module_children(old_def_id).to_vec(), // TODO: clean up
            tcx.module_children(new_def_id).to_vec(),
        );

        for items in children.drain() {
            match items {
                // an item pair is found
                (Some(o), Some(n)) => {
                    let (o_res, n_res) = (o.res.expect_non_local(), n.res.expect_non_local());
                    if let (Def(Mod, o_def_id), Def(Mod, n_def_id)) = (o_res, n_res) {
                        if visited.insert((o_def_id, n_def_id)) {
                            let o_vis = get_vis(old_vis, o);
                            let n_vis = get_vis(new_vis, n);

                            if o_vis != n_vis {
                                changes.new_change(
                                    o_def_id,
                                    n_def_id,
                                    o.ident.name,
                                    tcx.def_span(o_def_id),
                                    tcx.def_span(n_def_id),
                                    true,
                                );

                                // this seemingly overly complex condition is needed to handle
                                // `Restricted` visibility correctly.
                                if o_vis == Public && n_vis != Public {
                                    changes.add_change(ChangeType::ItemMadePrivate, o_def_id, None);
                                } else if o_vis != Public && n_vis == Public {
                                    changes.add_change(ChangeType::ItemMadePublic, o_def_id, None);
                                }
                            }

                            mod_queue.push_back((o_def_id, n_def_id, o_vis, n_vis));
                        }
                    } else if id_mapping.add_export(o_res, n_res) {
                        // struct constructors are weird/hard - let's go shopping!
                        if let (Def(Ctor(CtorOf::Struct, _), _), Def(Ctor(CtorOf::Struct, _), _)) =
                            (o_res, n_res)
                        {
                            continue;
                        }

                        let o_def_id = o_res.def_id();
                        let n_def_id = n_res.def_id();
                        let o_vis = get_vis(old_vis, o);
                        let n_vis = get_vis(new_vis, n);

                        let output = o_vis == Public || n_vis == Public;
                        changes.new_change(
                            o_def_id,
                            n_def_id,
                            o.ident.name,
                            tcx.def_span(o_def_id),
                            tcx.def_span(n_def_id),
                            output,
                        );

                        if o_vis == Public && n_vis != Public {
                            changes.add_change(ChangeType::ItemMadePrivate, o_def_id, None);
                        } else if o_vis != Public && n_vis == Public {
                            changes.add_change(ChangeType::ItemMadePublic, o_def_id, None);
                        }

                        let (o_kind, n_kind) = match (o_res, n_res) {
                            (Res::Def(o_kind, _), Res::Def(n_kind, _)) => (o_kind, n_kind),
                            _ => {
                                // a non-matching item pair (seriously broken though) -
                                // register the change and abort further analysis of it
                                // changes.add_change(ChangeType::KindDifference, o_def_id, None);
                                continue;
                            }
                        };

                        match (o_kind, n_kind) {
                            // TODO: update comment
                            // matching items we don't care about because they are either
                            // impossible to encounter at this stage (Mod, AssocTy, PrimTy,
                            // TyParam, SelfTy, Ctor, AssocConst, Local, Upvar,
                            // Variant, Method, Err), whose analysis is out scope
                            // for us (GlobalAsm), or which don't requite further
                            // analysis at this stage (Const).
                            (Mod, Mod)
                            | (AssocTy, AssocTy)
                            | (TyParam, TyParam)
                            | (Ctor(CtorOf::Struct, _), Ctor(CtorOf::Struct, _))
                            | (Ctor(CtorOf::Variant, _), Ctor(CtorOf::Variant, _))
                            | (AssocConst, AssocConst)
                            | (Variant, Variant)
                            | (Const, Const)
                            | (AssocFn, AssocFn)
                            | (Macro(_), Macro(_))
                            | (TraitAlias, TraitAlias)
                            | (ForeignTy, ForeignTy)
                            | (ConstParam, ConstParam) => {}
                            // statics are subject to mutability comparison
                            (Static(old_mut), Static(new_mut)) => {
                                if old_mut != new_mut {
                                    let change_type = ChangeType::StaticMutabilityChanged {
                                        now_mut: new_mut == Mutability::Mut,
                                    };

                                    changes.add_change(change_type, o_def_id, None);
                                }
                            }
                            // functions can declare generics and have structural properties
                            // that need to be compared
                            (Fn, Fn) => {
                                diff_generics(changes, id_mapping, tcx, true, o_def_id, n_def_id);
                                diff_fn(changes, tcx, o_res, n_res);
                            }
                            // type aliases can declare generics, too
                            (TyAlias, TyAlias) => {
                                diff_generics(changes, id_mapping, tcx, false, o_def_id, n_def_id);
                            }
                            // ADTs can declare generics and have lots of structural properties
                            // to check, most notably the number and name of variants and/or
                            // fields
                            (Struct, Struct) | (Union, Union) | (Enum, Enum) => {
                                diff_generics(changes, id_mapping, tcx, false, o_def_id, n_def_id);
                                diff_adts(changes, id_mapping, tcx, o_res, n_res);
                            }
                            // trait definitions can declare generics and require us to check
                            // for trait item addition and removal, as well as changes to their
                            // kinds and defaultness
                            (Trait, Trait) => {
                                if o_vis != Public {
                                    debug!("private trait: {:?}", o_def_id);
                                    id_mapping.add_private_trait(o_def_id);
                                }

                                if n_vis != Public {
                                    debug!("private trait: {:?}", n_def_id);
                                    id_mapping.add_private_trait(n_def_id);
                                }

                                diff_generics(changes, id_mapping, tcx, false, o_def_id, n_def_id);
                                traits.push((o_def_id, n_def_id, output));
                            }
                            // a non-matching item pair - register the change and abort further
                            // analysis of it
                            _ => {
                                changes.add_change(ChangeType::KindDifference, o_def_id, None);
                            }
                        }
                    }
                }
                // only an old item is found
                (Some(o), None) => {
                    // struct constructors are weird/hard - let's go shopping!
                    if let Def(Ctor(CtorOf::Struct, _), _) = o.res.expect_non_local::<HirId>() {
                        continue;
                    }

                    if get_vis(old_vis, o) == Public {
                        // delay the handling of removals until the id mapping is complete
                        removals.push(o);
                    }
                }
                // only a new item is found
                (None, Some(n)) => {
                    // struct constructors are weird/hard - let's go shopping!
                    if let Def(Ctor(CtorOf::Struct, _), _) = n.res.expect_non_local::<HirId>() {
                        continue;
                    }

                    if get_vis(new_vis, n) == Public {
                        debug!("addition: {:?} ({:?})", new_vis, n);
                        // delay the handling of additions until the id mapping is complete
                        additions.push(n);
                    }
                }
                (None, None) => unreachable!(),
            }
        }
    }

    // finally, process item additions and removals
    for n in additions {
        let n_def_id = n.res.expect_non_local::<HirId>().def_id();

        if !id_mapping.contains_new_id(n_def_id) {
            id_mapping.add_non_mapped(n_def_id);
        }

        changes.new_path_change(n_def_id, n.ident.name, tcx.def_span(n_def_id));
        changes.add_path_addition(n_def_id, n.span);
    }

    for o in removals {
        let o_def_id = o.res.expect_non_local::<HirId>().def_id();

        // reuse an already existing path change entry, if possible
        if id_mapping.contains_old_id(o_def_id) {
            let n_def_id = id_mapping.get_new_id(o_def_id).unwrap();
            changes.new_path_change(n_def_id, o.ident.name, tcx.def_span(n_def_id));
            changes.add_path_removal(n_def_id, o.span);
        } else {
            id_mapping.add_non_mapped(o_def_id);
            changes.new_path_change(o_def_id, o.ident.name, tcx.def_span(o_def_id));
            changes.add_path_removal(o_def_id, o.span);
        }
    }

    for (o_def_id, n_def_id, output) in traits {
        diff_traits(changes, id_mapping, tcx, o_def_id, n_def_id, output);
    }
}

/// Given two fn items, perform structural checks.
fn diff_fn<'tcx>(changes: &mut ChangeSet, tcx: TyCtxt<'tcx>, old: Res, new: Res) {
    let old_def_id = old.def_id();
    let new_def_id = new.def_id();

    let old_const = tcx.is_const_fn(old_def_id);
    let new_const = tcx.is_const_fn(new_def_id);

    if old_const != new_const {
        changes.add_change(
            ChangeType::FnConstChanged {
                now_const: new_const,
            },
            old_def_id,
            None,
        );
    }
}

/// Given two method items, perform structural checks.
fn diff_method<'tcx>(changes: &mut ChangeSet, tcx: TyCtxt<'tcx>, old: AssocItem, new: AssocItem) {
    if old.fn_has_self_parameter != new.fn_has_self_parameter {
        changes.add_change(
            ChangeType::MethodSelfChanged {
                now_self: new.fn_has_self_parameter,
            },
            old.def_id,
            None,
        );
    }

    let old_pub = old.vis == Public;
    let new_pub = new.vis == Public;

    if old_pub && !new_pub {
        changes.add_change(ChangeType::ItemMadePrivate, old.def_id, None);
    } else if !old_pub && new_pub {
        changes.add_change(ChangeType::ItemMadePublic, old.def_id, None);
    }

    diff_fn(
        changes,
        tcx,
        Def(DefKind::Fn, old.def_id),
        Def(DefKind::Fn, new.def_id),
    );
}

/// Given two ADT items, perform structural checks.
///
/// This establishes the needed correspondence between non-toplevel items such as enum variants,
/// struct- and enum fields etc.
fn diff_adts(changes: &mut ChangeSet, id_mapping: &mut IdMapping, tcx: TyCtxt, old: Res, new: Res) {
    use rustc_hir::def::DefKind::*;

    let old_def_id = old.def_id();
    let new_def_id = new.def_id();

    let (old_def, new_def) = match (old, new) {
        (Def(Struct, _), Def(Struct, _))
        | (Def(Union, _), Def(Union, _))
        | (Def(Enum, _), Def(Enum, _)) => (tcx.adt_def(old_def_id), tcx.adt_def(new_def_id)),
        _ => return,
    };

    let is_enum = matches!(old, Def(Enum, _));

    let mut variants = BTreeMap::new();
    let mut fields = BTreeMap::new();

    for variant in old_def.variants() {
        variants
            .entry(variant.ident(tcx).name)
            .or_insert((None, None))
            .0 = Some(variant);
    }

    for variant in new_def.variants() {
        variants
            .entry(variant.ident(tcx).name)
            .or_insert((None, None))
            .1 = Some(variant);
    }

    for items in variants.values() {
        match *items {
            (Some(old), Some(new)) => {
                for field in &old.fields {
                    fields
                        .entry(field.ident(tcx).name)
                        .or_insert((None, None))
                        .0 = Some(field);
                }

                for field in &new.fields {
                    fields
                        .entry(field.ident(tcx).name)
                        .or_insert((None, None))
                        .1 = Some(field);
                }

                let mut total_private = true;
                let mut total_public = true;

                for items2 in fields.values() {
                    if let Some(o) = items2.0 {
                        let public = o.vis == Public;
                        total_public &= public;
                        total_private &= !public;
                    }
                }

                if old.ctor_kind != new.ctor_kind {
                    let c = ChangeType::VariantStyleChanged {
                        now_struct: new.ctor_kind == CtorKind::Fictive,
                        total_private,
                        is_enum,
                    };
                    changes.add_change(c, old_def_id, Some(tcx.def_span(new.def_id)));

                    continue;
                }

                for items2 in fields.values() {
                    match *items2 {
                        (Some(o), Some(n)) => {
                            if o.vis == Public && n.vis == Public {
                                id_mapping.add_subitem(old_def_id, o.did, n.did);
                            } else if o.vis != Public && n.vis == Public {
                                changes.add_change(
                                    ChangeType::ItemMadePublic,
                                    old_def_id,
                                    Some(tcx.def_span(n.did)),
                                );
                            } else if o.vis == Public && n.vis != Public {
                                changes.add_change(
                                    ChangeType::ItemMadePrivate,
                                    old_def_id,
                                    Some(tcx.def_span(n.did)),
                                );
                            }
                        }
                        (Some(o), None) => {
                            let c = ChangeType::VariantFieldRemoved {
                                public: o.vis == Public,
                                total_public,
                                is_enum,
                            };
                            changes.add_change(c, old_def_id, Some(tcx.def_span(o.did)));
                        }
                        (None, Some(n)) => {
                            let c = ChangeType::VariantFieldAdded {
                                public: n.vis == Public,
                                total_public,
                                is_enum,
                            };
                            changes.add_change(c, old_def_id, Some(tcx.def_span(n.did)));
                        }
                        (None, None) => unreachable!(),
                    }
                }

                fields.clear();
            }
            (Some(old), None) => {
                changes.add_change(
                    ChangeType::VariantRemoved,
                    old_def_id,
                    Some(tcx.def_span(old.def_id)),
                );
            }
            (None, Some(new)) => {
                changes.add_change(
                    ChangeType::VariantAdded,
                    old_def_id,
                    Some(tcx.def_span(new.def_id)),
                );
            }
            (None, None) => unreachable!(),
        }
    }

    for impl_def_id in tcx.inherent_impls(old_def_id).iter() {
        for item_def_id in tcx.associated_item_def_ids(*impl_def_id).iter() {
            let item = tcx.associated_item(*item_def_id);
            id_mapping.add_inherent_item(
                old_def_id,
                item.kind,
                item.ident(tcx).name,
                *impl_def_id,
                *item_def_id,
            );
        }
    }

    for impl_def_id in tcx.inherent_impls(new_def_id).iter() {
        for item_def_id in tcx.associated_item_def_ids(*impl_def_id).iter() {
            let item = tcx.associated_item(*item_def_id);
            id_mapping.add_inherent_item(
                new_def_id,
                item.kind,
                item.ident(tcx).name,
                *impl_def_id,
                *item_def_id,
            );
        }
    }
}

/// Given two trait items, perform structural checks.
///
/// This establishes the needed correspondence between non-toplevel items found in the trait
/// definition.
fn diff_traits<'tcx>(
    changes: &mut ChangeSet,
    id_mapping: &mut IdMapping,
    tcx: TyCtxt<'tcx>,
    old: DefId,
    new: DefId,
    output: bool,
) {
    use rustc_hir::Unsafety::Unsafe;
    use rustc_middle::ty::subst::GenericArgKind::Type;
    use rustc_middle::ty::{ParamTy, PredicateKind};

    debug!(
        "diff_traits: old: {:?}, new: {:?}, output: {:?}",
        old, new, output
    );

    let old_unsafety = tcx.trait_def(old).unsafety;
    let new_unsafety = tcx.trait_def(new).unsafety;

    if old_unsafety != new_unsafety {
        let change_type = ChangeType::TraitUnsafetyChanged {
            now_unsafe: new_unsafety == Unsafe,
        };

        changes.add_change(change_type, old, None);
    }

    let mut old_sealed = false;
    let old_param_env = tcx.param_env(old);

    for bound in old_param_env.caller_bounds() {
        if let PredicateKind::Trait(pred) = bound.kind().skip_binder() {
            let trait_ref = pred.trait_ref;

            debug!("trait_ref substs (old): {:?}", trait_ref.substs);

            if id_mapping.is_private_trait(trait_ref.def_id) && trait_ref.substs.len() == 1 {
                if let Type(typ) = trait_ref.substs[0].unpack() {
                    if let TyKind::Param(ParamTy { index: 0, .. }) = typ.kind() {
                        old_sealed = true;
                    }
                }
            }
        }
    }

    let mut items = BTreeMap::new();

    for old_def_id in tcx.associated_item_def_ids(old).iter() {
        let item = tcx.associated_item(*old_def_id);
        items.entry(item.ident(tcx).name).or_insert((None, None)).0 = Some(item);
        // tcx.describe_def(*old_def_id).map(|d| (d, item));
    }

    for new_def_id in tcx.associated_item_def_ids(new).iter() {
        let item = tcx.associated_item(*new_def_id);
        items.entry(item.ident(tcx).name).or_insert((None, None)).1 = Some(item);
        // tcx.describe_def(*new_def_id).map(|d| (d, item));
    }

    for (name, item_pair) in &items {
        match *item_pair {
            (Some(old_item), Some(new_item)) => {
                let old_def_id = old_item.def_id;
                let new_def_id = new_item.def_id;
                let old_res = Res::Def(old_item.kind.as_def_kind(), old_def_id);
                let new_res = Res::Def(new_item.kind.as_def_kind(), new_def_id);

                id_mapping.add_trait_item(old_res, new_res, old);
                changes.new_change(
                    old_def_id,
                    new_def_id,
                    *name,
                    tcx.def_span(old_def_id),
                    tcx.def_span(new_def_id),
                    output,
                );

                diff_generics(changes, id_mapping, tcx, true, old_def_id, new_def_id);
                diff_method(changes, tcx, *old_item, *new_item);
            }
            (Some(old_item), None) => {
                let change_type = ChangeType::TraitItemRemoved {
                    defaulted: old_item.defaultness.has_value(),
                };
                changes.add_change(change_type, old, Some(tcx.def_span(old_item.def_id)));
                id_mapping.add_non_mapped(old_item.def_id);
            }
            (None, Some(new_item)) => {
                let change_type = ChangeType::TraitItemAdded {
                    defaulted: new_item.defaultness.has_value(),
                    sealed_trait: old_sealed,
                };
                changes.add_change(change_type, old, Some(tcx.def_span(new_item.def_id)));
                id_mapping.add_non_mapped(new_item.def_id);
            }
            (None, None) => unreachable!(),
        }
    }
}

/// Given two items, compare their type and region parameter sets.
fn diff_generics(
    changes: &mut ChangeSet,
    id_mapping: &mut IdMapping,
    tcx: TyCtxt,
    is_fn: bool,
    old: DefId,
    new: DefId,
) {
    use rustc_middle::ty::Variance;
    use rustc_middle::ty::Variance::*;
    use std::cmp::max;

    fn diff_variance<'tcx>(old_var: Variance, new_var: Variance) -> Option<ChangeType<'tcx>> {
        match (old_var, new_var) {
            (Covariant, Covariant)
            | (Invariant, Invariant)
            | (Contravariant, Contravariant)
            | (Bivariant, Bivariant) => None,
            (Invariant, _) | (_, Bivariant) => Some(ChangeType::VarianceLoosened),
            (_, Invariant) | (Bivariant, _) => Some(ChangeType::VarianceTightened),
            (Covariant, Contravariant) => Some(ChangeType::VarianceChanged {
                now_contravariant: true,
            }),
            (Contravariant, Covariant) => Some(ChangeType::VarianceChanged {
                now_contravariant: false,
            }),
        }
    }

    // guarantee that the return value's kind is `GenericParamDefKind::Lifetime`
    fn get_region_from_params(gen: &Generics, idx: usize) -> Option<&GenericParamDef> {
        let param = gen.params.get(idx)?;

        match param.kind {
            GenericParamDefKind::Lifetime => Some(param),
            _ => None,
        }
    }

    // guarantee that the return value's kind is `GenericParamDefKind::Type`
    fn get_type_from_params(gen: &Generics, idx: usize) -> Option<&GenericParamDef> {
        let param = &gen.params.get(idx)?;

        match param.kind {
            GenericParamDefKind::Type { .. } => Some(param),
            _ => None,
        }
    }

    debug!("diff_generics: old: {:?}, new: {:?}", old, new);

    let mut found = Vec::new();

    let old_gen = tcx.generics_of(old);
    let new_gen = tcx.generics_of(new);

    let old_var = tcx.variances_of(old);
    let new_var = tcx.variances_of(new);

    let old_count = old_gen.own_counts();
    let new_count = new_gen.own_counts();

    // NEEDSWORK: proper detection of what's going on here..
    let was_async_trait_transformed = old_count.lifetimes > 0
        && old_gen.params[old_count.lifetimes - 1].name.as_str() == "'async_trait";

    let self_add = if old_gen.has_self && new_gen.has_self {
        if was_async_trait_transformed {
            // async_trait transformation undoes the self lifetime?
            0
        } else {
            1
        }
    } else if !old_gen.has_self && !new_gen.has_self {
        0
    } else {
        unreachable!()
    };

    // TODO: we might need to track the number of parameters in the parent.

    debug!("old_gen: {:?}, new_gen: {:?}", old_gen, new_gen);
    debug!(
        "old_count.lifetimes: {:?}, new_count.lifetimes: {:?}",
        old_count.lifetimes, new_count.lifetimes
    );

    for i in 0..max(old_count.lifetimes, new_count.lifetimes) {
        match (
            get_region_from_params(old_gen, i + self_add),
            get_region_from_params(new_gen, i + self_add),
        ) {
            (Some(old_region), Some(new_region)) => {
                // type aliases don't have inferred variance, so we have to ignore that.
                if let (Some(old_var), Some(new_var)) = (old_var.get(i), new_var.get(i)) {
                    if let Some(t) = diff_variance(*old_var, *new_var) {
                        found.push(t)
                    };
                }

                id_mapping.add_internal_item(old_region.def_id, new_region.def_id);
            }
            (Some(_), None) => {
                found.push(ChangeType::RegionParameterRemoved);
            }
            (None, Some(_)) => {
                found.push(ChangeType::RegionParameterAdded);
            }
            (None, None) => unreachable!(),
        }
    }

    for i in 0..max(old_count.types, new_count.types) {
        let pair = if i == 0 && self_add == 1 {
            (
                get_type_from_params(old_gen, 0),
                get_type_from_params(new_gen, 0),
            )
        } else {
            (
                get_type_from_params(old_gen, old_count.lifetimes + i),
                get_type_from_params(new_gen, new_count.lifetimes + i),
            )
        };

        match pair {
            (Some(old_type), Some(new_type)) => {
                // type aliases don't have inferred variance, so we have to ignore that.
                if let (Some(old_var), Some(new_var)) = (
                    old_var.get(old_count.lifetimes + i),
                    new_var.get(new_count.lifetimes + i),
                ) {
                    if let Some(t) = diff_variance(*old_var, *new_var) {
                        found.push(t)
                    };
                }

                let old_default = match old_type.kind {
                    GenericParamDefKind::Type { has_default, .. } => has_default,
                    _ => unreachable!(),
                };
                let new_default = match new_type.kind {
                    GenericParamDefKind::Type { has_default, .. } => has_default,
                    _ => unreachable!(),
                };

                if old_default && !new_default {
                    found.push(ChangeType::TypeParameterRemoved { defaulted: true });
                    found.push(ChangeType::TypeParameterAdded { defaulted: false });
                } else if !old_default && new_default {
                    found.push(ChangeType::TypeParameterRemoved { defaulted: false });
                    found.push(ChangeType::TypeParameterAdded { defaulted: true });
                }

                debug!(
                    "in item {:?} / {:?}:\n  type param pair: {:?}, {:?}",
                    old, new, old_type, new_type
                );

                id_mapping.add_internal_item(old_type.def_id, new_type.def_id);
                id_mapping.add_type_param(old_type);
                id_mapping.add_type_param(new_type);
            }
            (Some(old_type), None) => {
                let old_default = match old_type.kind {
                    GenericParamDefKind::Type { has_default, .. } => has_default,
                    _ => unreachable!(),
                };

                found.push(ChangeType::TypeParameterRemoved {
                    defaulted: old_default,
                });
                id_mapping.add_type_param(old_type);
                id_mapping.add_non_mapped(old_type.def_id);
            }
            (None, Some(new_type)) => {
                let new_default = match new_type.kind {
                    GenericParamDefKind::Type { has_default, .. } => has_default,
                    _ => unreachable!(),
                };

                found.push(ChangeType::TypeParameterAdded {
                    defaulted: new_default || is_fn,
                });
                id_mapping.add_type_param(new_type);
                id_mapping.add_non_mapped(new_type.def_id);
            }
            (None, None) => unreachable!(),
        }
    }

    for change_type in found.drain(..) {
        changes.add_change(change_type, old, None);
    }
}

// Below functions constitute the third pass of analysis, in which the types and/or trait bounds
// of matching items are compared for changes.

/// Given two items, compare their types.
fn diff_types<'tcx>(
    changes: &mut ChangeSet<'tcx>,
    id_mapping: &IdMapping,
    tcx: TyCtxt<'tcx>,
    old: Res,
    new: Res,
) {
    use rustc_hir::def::DefKind::*;

    let old_def_id = old.def_id();
    let new_def_id = new.def_id();

    // bail out of analysis of already broken items
    if changes.item_breaking(old_def_id)
        || id_mapping
            .get_trait_def(old_def_id)
            .map_or(false, |did| changes.trait_item_breaking(did))
    {
        return;
    }

    match old {
        // type aliases, consts and statics just need their type to be checked
        Def(TyAlias | Const | Static(_), _) => {
            cmp_types(
                changes,
                id_mapping,
                tcx,
                old_def_id,
                new_def_id,
                tcx.type_of(old_def_id),
                tcx.type_of(new_def_id),
            );
        }
        // functions and methods require us to compare their signatures, not types
        Def(Fn | AssocFn, _) => {
            let old_fn_sig = tcx.type_of(old_def_id).fn_sig(tcx);
            let new_fn_sig = tcx.type_of(new_def_id).fn_sig(tcx);

            cmp_types(
                changes,
                id_mapping,
                tcx,
                old_def_id,
                new_def_id,
                tcx.mk_fn_ptr(old_fn_sig),
                tcx.mk_fn_ptr(new_fn_sig),
            );
        }
        // ADTs' types are compared field-wise
        Def(Struct | Enum | Union, _) => {
            if let Some(children) = id_mapping.children_of(old_def_id) {
                for (o_def_id, n_def_id) in children {
                    let o_ty = tcx.type_of(o_def_id);
                    let n_ty = tcx.type_of(n_def_id);

                    cmp_types(changes, id_mapping, tcx, old_def_id, new_def_id, o_ty, n_ty);
                }
            }
        }
        // a trait definition has no type, so only it's trait bounds are compared
        Def(Trait, _) => {
            cmp_bounds(changes, id_mapping, tcx, old_def_id, new_def_id);
        }
        _ => (),
    }
}

/// Compare two types and their trait bounds, possibly registering the resulting change.
fn cmp_types<'tcx>(
    changes: &mut ChangeSet<'tcx>,
    id_mapping: &IdMapping,
    tcx: TyCtxt<'tcx>,
    orig_def_id: DefId,
    target_def_id: DefId,
    orig: Ty<'tcx>,
    target: Ty<'tcx>,
) {
    info!(
        "comparing types and bounds of {:?} / {:?}:\n  {:?} / {:?}",
        orig_def_id, target_def_id, orig, target
    );

    tcx.infer_ctxt().enter(|infcx| {
        let compcx = TypeComparisonContext::target_new(&infcx, id_mapping, false);

        let orig_substs = InternalSubsts::identity_for_item(infcx.tcx, target_def_id);
        let orig = compcx.forward_trans.translate_item_type(orig_def_id, orig);
        // let orig = orig.subst(infcx.tcx, orig_substs);

        // functions need inference, other items can declare defaulted type parameters
        let target_substs = if target.is_fn() {
            compcx.compute_target_infer_substs(target_def_id)
        } else {
            compcx.compute_target_default_substs(target_def_id)
        };
        let target = EarlyBinder(target).subst(infcx.tcx, target_substs);

        let target_param_env =
            EarlyBinder(infcx.tcx.param_env(target_def_id)).subst(infcx.tcx, target_substs);

        if let Some(err) =
            compcx.check_type_error(tcx, target_def_id, target_param_env, orig, target)
        {
            changes.add_change(ChangeType::TypeChanged { error: err }, orig_def_id, None);
        } else {
            // check the bounds if no type error has been found
            compcx.check_bounds_bidirectional(
                changes,
                tcx,
                orig_def_id,
                target_def_id,
                orig_substs,
                target_substs,
            );
        }
    });
}

/// Compare the trait bounds of two items, possibly registering the resulting change.
fn cmp_bounds<'tcx>(
    changes: &mut ChangeSet<'tcx>,
    id_mapping: &IdMapping,
    tcx: TyCtxt<'tcx>,
    orig_def_id: DefId,
    target_def_id: DefId,
) {
    info!(
        "comparing bounds of {:?} / {:?}",
        orig_def_id, target_def_id
    );

    tcx.infer_ctxt().enter(|infcx| {
        let compcx = TypeComparisonContext::target_new(&infcx, id_mapping, true);

        let orig_substs = InternalSubsts::identity_for_item(infcx.tcx, target_def_id);
        let target_substs = compcx.compute_target_default_substs(target_def_id);

        compcx.check_bounds_bidirectional(
            changes,
            tcx,
            orig_def_id,
            target_def_id,
            orig_substs,
            target_substs,
        );
    })
}

// Below functions constitute the fourth pass of analysis, in which impls are matched up based on
// their trait bounds and compared for changes, if applicable.

/// Compare the inherent implementations of all matching items.
fn diff_inherent_impls<'tcx>(
    changes: &mut ChangeSet<'tcx>,
    id_mapping: &IdMapping,
    tcx: TyCtxt<'tcx>,
) {
    debug!("diffing inherent impls");

    let to_new = TranslationContext::target_new(tcx, id_mapping, false);
    let to_old = TranslationContext::target_old(tcx, id_mapping, false);

    for (orig_item, orig_impls) in id_mapping.inherent_impls() {
        // determine where the item comes from
        let (forward_trans, err_type) = if id_mapping.in_old_crate(orig_item.parent_def_id) {
            (&to_new, ChangeType::AssociatedItemRemoved)
        } else if id_mapping.in_new_crate(orig_item.parent_def_id) {
            (&to_old, ChangeType::AssociatedItemAdded)
        } else {
            unreachable!()
        };

        // determine item visibility
        let parent_output = changes.get_output(orig_item.parent_def_id);

        // for each original impl the item is present in, ...
        for &(orig_impl_def_id, orig_item_def_id) in orig_impls {
            let orig_assoc_item = tcx.associated_item(orig_item_def_id);

            let item_span = tcx.def_span(orig_item_def_id);
            changes.new_change(
                orig_item_def_id,
                orig_item_def_id,
                orig_item.name,
                item_span,
                item_span,
                parent_output && orig_assoc_item.vis == Public,
            );

            // ... determine the set of target impls that serve as candidates
            let target_impls = if let Some(impls) = forward_trans
                .translate_inherent_entry(orig_item)
                .and_then(|item| id_mapping.get_inherent_impls(&item))
            {
                impls
            } else {
                changes.add_change(err_type.clone(), orig_item_def_id, None);
                continue;
            };

            // if any of the candidates matches, the item is compatible across versions
            let match_found =
                target_impls
                    .iter()
                    .any(|&(target_impl_def_id, target_item_def_id)| {
                        let target_assoc_item = tcx.associated_item(target_item_def_id);

                        if parent_output && target_assoc_item.vis == Public {
                            changes.set_output(orig_item.parent_def_id);
                        }

                        match_inherent_impl(
                            changes,
                            id_mapping,
                            tcx,
                            orig_impl_def_id,
                            target_impl_def_id,
                            *orig_assoc_item,
                            *target_assoc_item,
                        )
                    });

            // otherwise, it has been essentially added/removed
            if !match_found {
                changes.add_change(err_type.clone(), orig_item_def_id, None);
            }
        }
    }
}

// There doesn't seem to be a way to get the visibility of impl traits from rustc
// (CC rust-lang/rust#61464), so we implement the logic here.  Note that this implementation is far
// from perfect and will cause false positives in some cases (see comment in the inner function).
#[allow(clippy::let_and_return)]
#[allow(clippy::match_same_arms)]
fn is_impl_trait_public<'tcx>(tcx: TyCtxt<'tcx>, impl_def_id: DefId) -> bool {
    fn type_visibility(tcx: TyCtxt, ty: Ty) -> Visibility {
        match ty.kind() {
            TyKind::Adt(def, _) => tcx.visibility(def.did()),

            TyKind::Array(t, _)
            | TyKind::Slice(t)
            | TyKind::RawPtr(TypeAndMut { ty: t, .. })
            | TyKind::Ref(_, t, _) => type_visibility(tcx, *t),

            TyKind::Bool
            | TyKind::Char
            | TyKind::Int(_)
            | TyKind::Uint(_)
            | TyKind::Float(_)
            | TyKind::Str
            | TyKind::Never => Visibility::Public,

            // FIXME We assume everything else is public which is, of course, not always the case.
            //       This means we will have false positives.  We need to improve this.
            _ => Visibility::Public,
        }
    }

    let trait_ref: TraitRef<'tcx> = tcx.impl_trait_ref(impl_def_id).unwrap();
    let trait_def_id = trait_ref.def_id;

    if tcx.visibility(trait_def_id) != Visibility::Public {
        return false;
    }

    // Check if all input types of the trait implementation are public (including `Self`).
    let is_public = trait_ref
        .substs
        .types()
        .map(|t| type_visibility(tcx, t))
        .all(|v| v == Visibility::Public);

    is_public
}

/// Compare the implementations of all matching traits.
fn diff_trait_impls<'tcx>(
    changes: &mut ChangeSet<'tcx>,
    id_mapping: &IdMapping,
    tcx: TyCtxt<'tcx>,
) {
    debug!("diffing trait impls");

    let to_new = TranslationContext::target_new(tcx, id_mapping, false);
    let to_old = TranslationContext::target_old(tcx, id_mapping, false);

    // NOTE: Ignore for now core::marker::Structural{Eq, PartialEq} since
    // these are impl'd via *Eq traits but can we want users to see regular
    // *Eq traits here as the former are a bit auto-magical for the user
    let structural_teq_def_id = tcx.require_lang_item(LangItem::StructuralTeq, None);
    let structural_peq_def_id = tcx.require_lang_item(LangItem::StructuralPeq, None);
    let structural_trait_def_ids = [structural_peq_def_id, structural_teq_def_id];

    let cstore = CStore::from_tcx(tcx);
    for (old_trait_def_id, old_impl_def_id, _) in
        cstore.trait_impls_in_crate_untracked(id_mapping.get_old_crate())
    {
        if structural_trait_def_ids.contains(&old_trait_def_id) {
            continue;
        }

        if !to_new.can_translate(old_trait_def_id) || !is_impl_trait_public(tcx, old_impl_def_id) {
            continue;
        }

        if !match_trait_impl(tcx, &to_new, old_impl_def_id) {
            changes.new_change_impl(
                old_impl_def_id,
                tcx.def_path_str(old_impl_def_id),
                tcx.def_span(old_impl_def_id),
            );
            changes.add_change(ChangeType::TraitImplTightened, old_impl_def_id, None);
        }
    }

    for (new_trait_def_id, new_impl_def_id, _) in
        cstore.trait_impls_in_crate_untracked(id_mapping.get_new_crate())
    {
        if structural_trait_def_ids.contains(&new_trait_def_id) {
            continue;
        }

        if !to_old.can_translate(new_trait_def_id) || !is_impl_trait_public(tcx, new_impl_def_id) {
            continue;
        }

        if !match_trait_impl(tcx, &to_old, new_impl_def_id) {
            changes.new_change_impl(
                new_impl_def_id,
                tcx.def_path_str(new_impl_def_id),
                tcx.def_span(new_impl_def_id),
            );
            changes.add_change(ChangeType::TraitImplLoosened, new_impl_def_id, None);
        }
    }
}

/// Compare an item pair in two inherent implementations and indicate whether the target one is
/// compatible with the original one.
fn match_inherent_impl<'tcx>(
    changes: &mut ChangeSet<'tcx>,
    id_mapping: &IdMapping,
    tcx: TyCtxt<'tcx>,
    orig_impl_def_id: DefId,
    target_impl_def_id: DefId,
    orig_item: AssocItem,
    target_item: AssocItem,
) -> bool {
    use rustc_middle::ty::AssocKind;

    debug!(
        "match_inherent_impl: orig_impl/item: {:?}/{:?}, target_impl/item: {:?}/{:?}",
        orig_impl_def_id, orig_item, target_impl_def_id, target_item
    );

    let orig_item_def_id = orig_item.def_id;
    let target_item_def_id = target_item.def_id;

    tcx.infer_ctxt().enter(|infcx| {
        let (compcx, register_errors) = if id_mapping.in_old_crate(orig_impl_def_id) {
            (
                TypeComparisonContext::target_new(&infcx, id_mapping, false),
                true,
            )
        } else {
            (
                TypeComparisonContext::target_old(&infcx, id_mapping, false),
                false,
            )
        };

        let orig_substs = InternalSubsts::identity_for_item(infcx.tcx, target_item_def_id);
        let orig_self = compcx
            .forward_trans
            .translate_item_type(orig_impl_def_id, infcx.tcx.type_of(orig_impl_def_id));

        let target_substs = compcx.compute_target_infer_substs(target_item_def_id);
        let target_self =
            EarlyBinder(infcx.tcx.type_of(target_impl_def_id)).subst(infcx.tcx, target_substs);

        let target_param_env = infcx.tcx.param_env(target_impl_def_id);

        let error = compcx.check_type_error(
            tcx,
            target_impl_def_id,
            target_param_env,
            orig_self,
            target_self,
        );

        if error.is_some() {
            // `Self` on the impls isn't equal - no impl match.
            return false;
        }

        let orig_param_env = compcx
            .forward_trans
            .translate_param_env(orig_impl_def_id, tcx.param_env(orig_impl_def_id));

        if let Some(orig_param_env) = orig_param_env {
            let errors =
                compcx.check_bounds_error(tcx, orig_param_env, target_impl_def_id, target_substs);
            if errors.is_some() {
                // The bounds on the impls have been tightened - no impl match.
                return false;
            }
        } else {
            // The bounds could not have been translated - no impl match.
            return false;
        }

        // at this point we have an impl match, so the return value is always `true`.

        if !register_errors {
            // checking backwards, impls match.
            return true;
        }

        // prepare the item type for comparison, as we do for toplevel items' types
        let (orig, target) = match (orig_item.kind, target_item.kind) {
            (AssocKind::Const, AssocKind::Const) | (AssocKind::Type, AssocKind::Type) => (
                infcx.tcx.type_of(orig_item_def_id),
                infcx.tcx.type_of(target_item_def_id),
            ),
            (AssocKind::Fn, AssocKind::Fn) => {
                diff_method(changes, tcx, orig_item, target_item);
                let orig_sig = infcx.tcx.type_of(orig_item_def_id).fn_sig(tcx);
                let target_sig = infcx.tcx.type_of(target_item_def_id).fn_sig(tcx);
                (tcx.mk_fn_ptr(orig_sig), tcx.mk_fn_ptr(target_sig))
            }
            _ => unreachable!(),
        };

        let orig = compcx
            .forward_trans
            .translate_item_type(orig_item_def_id, orig);
        let target = EarlyBinder(target).subst(infcx.tcx, target_substs);

        let error =
            compcx.check_type_error(tcx, target_item_def_id, target_param_env, orig, target);

        if let Some(err) = error {
            changes.add_change(
                ChangeType::TypeChanged { error: err },
                orig_item_def_id,
                None,
            );
        } else {
            // check the bounds if no type error has been found
            compcx.check_bounds_bidirectional(
                changes,
                tcx,
                orig_item_def_id,
                target_item_def_id,
                orig_substs,
                target_substs,
            );
        }

        true
    })
}

/// Compare two implementations and indicate whether the target one is compatible with the
/// original one.
fn match_trait_impl<'a, 'tcx>(
    tcx: TyCtxt<'tcx>,
    trans: &TranslationContext<'a, 'tcx>,
    orig_def_id: DefId,
) -> bool {
    debug!("matching: {:?}", orig_def_id);

    tcx.infer_ctxt().enter(|infcx| {
        let old_param_env =
            if let Some(env) = trans.translate_param_env(orig_def_id, tcx.param_env(orig_def_id)) {
                env
            } else {
                return false;
            };

        debug!("env: {:?}", old_param_env);

        let orig = tcx.impl_trait_ref(orig_def_id).unwrap();
        debug!("trait ref: {:?}", orig);
        debug!(
            "translated ref: {:?}",
            trans.translate_trait_ref(orig_def_id, &orig)
        );

        let mut bound_cx = BoundContext::new(&infcx, old_param_env);
        bound_cx.register_trait_ref(trans.translate_trait_ref(orig_def_id, &orig));
        bound_cx.get_errors().is_none()
    })
}