Auto merge of #69550 - RalfJung:scalar, r=oli-obk

[rust.git] / src / librustc_typeck / check / wfcheck.rs

use crate::check::{FnCtxt, Inherited};
use crate::constrained_generic_params::{identify_constrained_generic_params, Parameter};

use rustc::middle::lang_items;
use rustc::session::parse::feature_err;
use rustc::ty::subst::{InternalSubsts, Subst};
use rustc::ty::{
    self, AdtKind, GenericParamDefKind, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
};
use rustc_ast::ast;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
use rustc_hir::def_id::DefId;
use rustc_hir::ItemKind;
use rustc_infer::infer::opaque_types::may_define_opaque_type;
use rustc_infer::traits::{self, ObligationCause, ObligationCauseCode};
use rustc_span::symbol::sym;
use rustc_span::Span;

use rustc_hir as hir;
use rustc_hir::itemlikevisit::ParItemLikeVisitor;

/// Helper type of a temporary returned by `.for_item(...)`.
/// This is necessary because we can't write the following bound:
///
/// ```rust
/// F: for<'b, 'tcx> where 'tcx FnOnce(FnCtxt<'b, 'tcx>)
/// ```
struct CheckWfFcxBuilder<'tcx> {
    inherited: super::InheritedBuilder<'tcx>,
    id: hir::HirId,
    span: Span,
    param_env: ty::ParamEnv<'tcx>,
}

impl<'tcx> CheckWfFcxBuilder<'tcx> {
    fn with_fcx<F>(&mut self, f: F)
    where
        F: for<'b> FnOnce(&FnCtxt<'b, 'tcx>, TyCtxt<'tcx>) -> Vec<Ty<'tcx>>,
    {
        let id = self.id;
        let span = self.span;
        let param_env = self.param_env;
        self.inherited.enter(|inh| {
            let fcx = FnCtxt::new(&inh, param_env, id);
            if !inh.tcx.features().trivial_bounds {
                // As predicates are cached rather than obligations, this
                // needsto be called first so that they are checked with an
                // empty `param_env`.
                check_false_global_bounds(&fcx, span, id);
            }
            let wf_tys = f(&fcx, fcx.tcx);
            fcx.select_all_obligations_or_error();
            fcx.regionck_item(id, span, &wf_tys);
        });
    }
}

/// Checks that the field types (in a struct def'n) or argument types (in an enum def'n) are
/// well-formed, meaning that they do not require any constraints not declared in the struct
/// definition itself. For example, this definition would be illegal:
///
/// ```rust
/// struct Ref<'a, T> { x: &'a T }
/// ```
///
/// because the type did not declare that `T:'a`.
///
/// We do this check as a pre-pass before checking fn bodies because if these constraints are
/// not included it frequently leads to confusing errors in fn bodies. So it's better to check
/// the types first.
pub fn check_item_well_formed(tcx: TyCtxt<'_>, def_id: DefId) {
    let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
    let item = tcx.hir().expect_item(hir_id);

    debug!(
        "check_item_well_formed(it.hir_id={:?}, it.name={})",
        item.hir_id,
        tcx.def_path_str(def_id)
    );

    match item.kind {
        // Right now we check that every default trait implementation
        // has an implementation of itself. Basically, a case like:
        //
        //     impl Trait for T {}
        //
        // has a requirement of `T: Trait` which was required for default
        // method implementations. Although this could be improved now that
        // there's a better infrastructure in place for this, it's being left
        // for a follow-up work.
        //
        // Since there's such a requirement, we need to check *just* positive
        // implementations, otherwise things like:
        //
        //     impl !Send for T {}
        //
        // won't be allowed unless there's an *explicit* implementation of `Send`
        // for `T`
        hir::ItemKind::Impl { defaultness, ref of_trait, ref self_ty, .. } => {
            let is_auto = tcx
                .impl_trait_ref(tcx.hir().local_def_id(item.hir_id))
                .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
            let polarity = tcx.impl_polarity(def_id);
            if let (hir::Defaultness::Default { .. }, true) = (defaultness, is_auto) {
                tcx.sess.span_err(item.span, "impls of auto traits cannot be default");
            }
            match polarity {
                ty::ImplPolarity::Positive => {
                    check_impl(tcx, item, self_ty, of_trait);
                }
                ty::ImplPolarity::Negative => {
                    // FIXME(#27579): what amount of WF checking do we need for neg impls?
                    if of_trait.is_some() && !is_auto {
                        struct_span_err!(
                            tcx.sess,
                            item.span,
                            E0192,
                            "negative impls are only allowed for \
                                   auto traits (e.g., `Send` and `Sync`)"
                        )
                        .emit()
                    }
                }
                ty::ImplPolarity::Reservation => {
                    // FIXME: what amount of WF checking do we need for reservation impls?
                }
            }
        }
        hir::ItemKind::Fn(..) => {
            check_item_fn(tcx, item);
        }
        hir::ItemKind::Static(ref ty, ..) => {
            check_item_type(tcx, item.hir_id, ty.span, false);
        }
        hir::ItemKind::Const(ref ty, ..) => {
            check_item_type(tcx, item.hir_id, ty.span, false);
        }
        hir::ItemKind::ForeignMod(ref module) => {
            for it in module.items.iter() {
                if let hir::ForeignItemKind::Static(ref ty, ..) = it.kind {
                    check_item_type(tcx, it.hir_id, ty.span, true);
                }
            }
        }
        hir::ItemKind::Struct(ref struct_def, ref ast_generics) => {
            check_type_defn(tcx, item, false, |fcx| vec![fcx.non_enum_variant(struct_def)]);

            check_variances_for_type_defn(tcx, item, ast_generics);
        }
        hir::ItemKind::Union(ref struct_def, ref ast_generics) => {
            check_type_defn(tcx, item, true, |fcx| vec![fcx.non_enum_variant(struct_def)]);

            check_variances_for_type_defn(tcx, item, ast_generics);
        }
        hir::ItemKind::Enum(ref enum_def, ref ast_generics) => {
            check_type_defn(tcx, item, true, |fcx| fcx.enum_variants(enum_def));

            check_variances_for_type_defn(tcx, item, ast_generics);
        }
        hir::ItemKind::Trait(..) => {
            check_trait(tcx, item);
        }
        hir::ItemKind::TraitAlias(..) => {
            check_trait(tcx, item);
        }
        _ => {}
    }
}

pub fn check_trait_item(tcx: TyCtxt<'_>, def_id: DefId) {
    let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
    let trait_item = tcx.hir().expect_trait_item(hir_id);

    let method_sig = match trait_item.kind {
        hir::TraitItemKind::Method(ref sig, _) => Some(sig),
        _ => None,
    };
    check_object_unsafe_self_trait_by_name(tcx, &trait_item);
    check_associated_item(tcx, trait_item.hir_id, trait_item.span, method_sig);
}

fn could_be_self(trait_def_id: DefId, ty: &hir::Ty<'_>) -> bool {
    match ty.kind {
        hir::TyKind::TraitObject([trait_ref], ..) => match trait_ref.trait_ref.path.segments {
            [s] => s.res.and_then(|r| r.opt_def_id()) == Some(trait_def_id),
            _ => false,
        },
        _ => false,
    }
}

/// Detect when an object unsafe trait is referring to itself in one of its associated items.
/// When this is done, suggest using `Self` instead.
fn check_object_unsafe_self_trait_by_name(tcx: TyCtxt<'_>, item: &hir::TraitItem<'_>) {
    let (trait_name, trait_def_id) = match tcx.hir().get(tcx.hir().get_parent_item(item.hir_id)) {
        hir::Node::Item(item) => match item.kind {
            hir::ItemKind::Trait(..) => (item.ident, tcx.hir().local_def_id(item.hir_id)),
            _ => return,
        },
        _ => return,
    };
    let mut trait_should_be_self = vec![];
    match &item.kind {
        hir::TraitItemKind::Const(ty, _) | hir::TraitItemKind::Type(_, Some(ty))
            if could_be_self(trait_def_id, ty) =>
        {
            trait_should_be_self.push(ty.span)
        }
        hir::TraitItemKind::Method(sig, _) => {
            for ty in sig.decl.inputs {
                if could_be_self(trait_def_id, ty) {
                    trait_should_be_self.push(ty.span);
                }
            }
            match sig.decl.output {
                hir::FnRetTy::Return(ty) if could_be_self(trait_def_id, ty) => {
                    trait_should_be_self.push(ty.span);
                }
                _ => {}
            }
        }
        _ => {}
    }
    if !trait_should_be_self.is_empty() {
        if tcx.object_safety_violations(trait_def_id).is_empty() {
            return;
        }
        let sugg = trait_should_be_self.iter().map(|span| (*span, "Self".to_string())).collect();
        tcx.sess
            .struct_span_err(
                trait_should_be_self,
                "associated item referring to unboxed trait object for its own trait",
            )
            .span_label(trait_name.span, "in this trait")
            .multipart_suggestion(
                "you might have meant to use `Self` to refer to the implementing type",
                sugg,
                Applicability::MachineApplicable,
            )
            .emit();
    }
}

pub fn check_impl_item(tcx: TyCtxt<'_>, def_id: DefId) {
    let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
    let impl_item = tcx.hir().expect_impl_item(hir_id);

    let method_sig = match impl_item.kind {
        hir::ImplItemKind::Method(ref sig, _) => Some(sig),
        _ => None,
    };

    check_associated_item(tcx, impl_item.hir_id, impl_item.span, method_sig);
}

fn check_associated_item(
    tcx: TyCtxt<'_>,
    item_id: hir::HirId,
    span: Span,
    sig_if_method: Option<&hir::FnSig<'_>>,
) {
    debug!("check_associated_item: {:?}", item_id);

    let code = ObligationCauseCode::MiscObligation;
    for_id(tcx, item_id, span).with_fcx(|fcx, tcx| {
        let item = fcx.tcx.associated_item(fcx.tcx.hir().local_def_id(item_id));

        let (mut implied_bounds, self_ty) = match item.container {
            ty::TraitContainer(_) => (vec![], fcx.tcx.types.self_param),
            ty::ImplContainer(def_id) => {
                (fcx.impl_implied_bounds(def_id, span), fcx.tcx.type_of(def_id))
            }
        };

        match item.kind {
            ty::AssocKind::Const => {
                let ty = fcx.tcx.type_of(item.def_id);
                let ty = fcx.normalize_associated_types_in(span, &ty);
                fcx.register_wf_obligation(ty, span, code.clone());
            }
            ty::AssocKind::Method => {
                let sig = fcx.tcx.fn_sig(item.def_id);
                let sig = fcx.normalize_associated_types_in(span, &sig);
                let hir_sig = sig_if_method.expect("bad signature for method");
                check_fn_or_method(
                    tcx,
                    fcx,
                    item.ident.span,
                    sig,
                    hir_sig,
                    item.def_id,
                    &mut implied_bounds,
                );
                check_method_receiver(fcx, hir_sig, &item, self_ty);
            }
            ty::AssocKind::Type => {
                if item.defaultness.has_value() {
                    let ty = fcx.tcx.type_of(item.def_id);
                    let ty = fcx.normalize_associated_types_in(span, &ty);
                    fcx.register_wf_obligation(ty, span, code.clone());
                }
            }
            ty::AssocKind::OpaqueTy => {
                // Do nothing: opaque types check themselves.
            }
        }

        implied_bounds
    })
}

fn for_item<'tcx>(tcx: TyCtxt<'tcx>, item: &hir::Item<'_>) -> CheckWfFcxBuilder<'tcx> {
    for_id(tcx, item.hir_id, item.span)
}

fn for_id(tcx: TyCtxt<'_>, id: hir::HirId, span: Span) -> CheckWfFcxBuilder<'_> {
    let def_id = tcx.hir().local_def_id(id);
    CheckWfFcxBuilder {
        inherited: Inherited::build(tcx, def_id),
        id,
        span,
        param_env: tcx.param_env(def_id),
    }
}

fn item_adt_kind(kind: &ItemKind<'_>) -> Option<AdtKind> {
    match kind {
        ItemKind::Struct(..) => Some(AdtKind::Struct),
        ItemKind::Union(..) => Some(AdtKind::Union),
        ItemKind::Enum(..) => Some(AdtKind::Enum),
        _ => None,
    }
}

/// In a type definition, we check that to ensure that the types of the fields are well-formed.
fn check_type_defn<'tcx, F>(
    tcx: TyCtxt<'tcx>,
    item: &hir::Item<'tcx>,
    all_sized: bool,
    mut lookup_fields: F,
) where
    F: for<'fcx> FnMut(&FnCtxt<'fcx, 'tcx>) -> Vec<AdtVariant<'tcx>>,
{
    for_item(tcx, item).with_fcx(|fcx, fcx_tcx| {
        let variants = lookup_fields(fcx);
        let def_id = fcx.tcx.hir().local_def_id(item.hir_id);
        let packed = fcx.tcx.adt_def(def_id).repr.packed();

        for variant in &variants {
            // For DST, or when drop needs to copy things around, all
            // intermediate types must be sized.
            let needs_drop_copy = || {
                packed && {
                    let ty = variant.fields.last().unwrap().ty;
                    let ty = fcx.tcx.erase_regions(&ty);
                    if ty.has_local_value() {
                        fcx_tcx
                            .sess
                            .delay_span_bug(item.span, &format!("inference variables in {:?}", ty));
                        // Just treat unresolved type expression as if it needs drop.
                        true
                    } else {
                        ty.needs_drop(fcx_tcx, fcx_tcx.param_env(def_id))
                    }
                }
            };
            let all_sized = all_sized || variant.fields.is_empty() || needs_drop_copy();
            let unsized_len = if all_sized { 0 } else { 1 };
            for (idx, field) in
                variant.fields[..variant.fields.len() - unsized_len].iter().enumerate()
            {
                let last = idx == variant.fields.len() - 1;
                fcx.register_bound(
                    field.ty,
                    fcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
                    traits::ObligationCause::new(
                        field.span,
                        fcx.body_id,
                        traits::FieldSized {
                            adt_kind: match item_adt_kind(&item.kind) {
                                Some(i) => i,
                                None => bug!(),
                            },
                            last,
                        },
                    ),
                );
            }

            // All field types must be well-formed.
            for field in &variant.fields {
                fcx.register_wf_obligation(
                    field.ty,
                    field.span,
                    ObligationCauseCode::MiscObligation,
                )
            }
        }

        check_where_clauses(tcx, fcx, item.span, def_id, None);

        // No implied bounds in a struct definition.
        vec![]
    });
}

fn check_trait(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
    debug!("check_trait: {:?}", item.hir_id);

    let trait_def_id = tcx.hir().local_def_id(item.hir_id);

    let trait_def = tcx.trait_def(trait_def_id);
    if trait_def.is_marker {
        for associated_def_id in &*tcx.associated_item_def_ids(trait_def_id) {
            struct_span_err!(
                tcx.sess,
                tcx.def_span(*associated_def_id),
                E0714,
                "marker traits cannot have associated items",
            )
            .emit();
        }
    }

    for_item(tcx, item).with_fcx(|fcx, _| {
        check_where_clauses(tcx, fcx, item.span, trait_def_id, None);
        check_associated_type_defaults(fcx, trait_def_id);

        vec![]
    });
}

/// Checks all associated type defaults of trait `trait_def_id`.
///
/// Assuming the defaults are used, check that all predicates (bounds on the
/// assoc type and where clauses on the trait) hold.
fn check_associated_type_defaults(fcx: &FnCtxt<'_, '_>, trait_def_id: DefId) {
    let tcx = fcx.tcx;
    let substs = InternalSubsts::identity_for_item(tcx, trait_def_id);

    // For all assoc. types with defaults, build a map from
    // `<Self as Trait<...>>::Assoc` to the default type.
    let map = tcx
        .associated_items(trait_def_id)
        .in_definition_order()
        .filter_map(|item| {
            if item.kind == ty::AssocKind::Type && item.defaultness.has_value() {
                // `<Self as Trait<...>>::Assoc`
                let proj = ty::ProjectionTy { substs, item_def_id: item.def_id };
                let default_ty = tcx.type_of(item.def_id);
                debug!("assoc. type default mapping: {} -> {}", proj, default_ty);
                Some((proj, default_ty))
            } else {
                None
            }
        })
        .collect::<FxHashMap<_, _>>();

    /// Replaces projections of associated types with their default types.
    ///
    /// This does a "shallow substitution", meaning that defaults that refer to
    /// other defaulted assoc. types will still refer to the projection
    /// afterwards, not to the other default. For example:
    ///
    /// ```compile_fail
    /// trait Tr {
    ///     type A: Clone = Vec<Self::B>;
    ///     type B = u8;
    /// }
    /// ```
    ///
    /// This will end up replacing the bound `Self::A: Clone` with
    /// `Vec<Self::B>: Clone`, not with `Vec<u8>: Clone`. If we did a deep
    /// substitution and ended up with the latter, the trait would be accepted.
    /// If an `impl` then replaced `B` with something that isn't `Clone`,
    /// suddenly the default for `A` is no longer valid. The shallow
    /// substitution forces the trait to add a `B: Clone` bound to be accepted,
    /// which means that an `impl` can replace any default without breaking
    /// others.
    ///
    /// Note that this isn't needed for soundness: The defaults would still be
    /// checked in any impl that doesn't override them.
    struct DefaultNormalizer<'tcx> {
        tcx: TyCtxt<'tcx>,
        map: FxHashMap<ty::ProjectionTy<'tcx>, Ty<'tcx>>,
    }

    impl<'tcx> ty::fold::TypeFolder<'tcx> for DefaultNormalizer<'tcx> {
        fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
            self.tcx
        }

        fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
            match t.kind {
                ty::Projection(proj_ty) => {
                    if let Some(default) = self.map.get(&proj_ty) {
                        default
                    } else {
                        t.super_fold_with(self)
                    }
                }
                _ => t.super_fold_with(self),
            }
        }
    }

    // Now take all predicates defined on the trait, replace any mention of
    // the assoc. types with their default, and prove them.
    // We only consider predicates that directly mention the assoc. type.
    let mut norm = DefaultNormalizer { tcx, map };
    let predicates = fcx.tcx.predicates_of(trait_def_id);
    for &(orig_pred, span) in predicates.predicates.iter() {
        let pred = orig_pred.fold_with(&mut norm);
        if pred != orig_pred {
            // Mentions one of the defaulted assoc. types
            debug!("default suitability check: proving predicate: {} -> {}", orig_pred, pred);
            let pred = fcx.normalize_associated_types_in(span, &pred);
            let cause = traits::ObligationCause::new(
                span,
                fcx.body_id,
                traits::ItemObligation(trait_def_id),
            );
            let obligation = traits::Obligation::new(cause, fcx.param_env, pred);

            fcx.register_predicate(obligation);
        }
    }
}

fn check_item_fn(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
    for_item(tcx, item).with_fcx(|fcx, tcx| {
        let def_id = fcx.tcx.hir().local_def_id(item.hir_id);
        let sig = fcx.tcx.fn_sig(def_id);
        let sig = fcx.normalize_associated_types_in(item.span, &sig);
        let mut implied_bounds = vec![];
        let hir_sig = match &item.kind {
            ItemKind::Fn(sig, ..) => sig,
            _ => bug!("expected `ItemKind::Fn`, found `{:?}`", item.kind),
        };
        check_fn_or_method(tcx, fcx, item.ident.span, sig, hir_sig, def_id, &mut implied_bounds);
        implied_bounds
    })
}

fn check_item_type(tcx: TyCtxt<'_>, item_id: hir::HirId, ty_span: Span, allow_foreign_ty: bool) {
    debug!("check_item_type: {:?}", item_id);

    for_id(tcx, item_id, ty_span).with_fcx(|fcx, tcx| {
        let ty = tcx.type_of(tcx.hir().local_def_id(item_id));
        let item_ty = fcx.normalize_associated_types_in(ty_span, &ty);

        let mut forbid_unsized = true;
        if allow_foreign_ty {
            let tail = fcx.tcx.struct_tail_erasing_lifetimes(item_ty, fcx.param_env);
            if let ty::Foreign(_) = tail.kind {
                forbid_unsized = false;
            }
        }

        fcx.register_wf_obligation(item_ty, ty_span, ObligationCauseCode::MiscObligation);
        if forbid_unsized {
            fcx.register_bound(
                item_ty,
                fcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
                traits::ObligationCause::new(ty_span, fcx.body_id, traits::MiscObligation),
            );
        }

        // No implied bounds in a const, etc.
        vec![]
    });
}

fn check_impl<'tcx>(
    tcx: TyCtxt<'tcx>,
    item: &'tcx hir::Item<'tcx>,
    ast_self_ty: &hir::Ty<'_>,
    ast_trait_ref: &Option<hir::TraitRef<'_>>,
) {
    debug!("check_impl: {:?}", item);

    for_item(tcx, item).with_fcx(|fcx, tcx| {
        let item_def_id = fcx.tcx.hir().local_def_id(item.hir_id);

        match *ast_trait_ref {
            Some(ref ast_trait_ref) => {
                // `#[rustc_reservation_impl]` impls are not real impls and
                // therefore don't need to be WF (the trait's `Self: Trait` predicate
                // won't hold).
                let trait_ref = fcx.tcx.impl_trait_ref(item_def_id).unwrap();
                let trait_ref =
                    fcx.normalize_associated_types_in(ast_trait_ref.path.span, &trait_ref);
                let obligations = traits::wf::trait_obligations(
                    fcx,
                    fcx.param_env,
                    fcx.body_id,
                    &trait_ref,
                    ast_trait_ref.path.span,
                    Some(item),
                );
                for obligation in obligations {
                    fcx.register_predicate(obligation);
                }
            }
            None => {
                let self_ty = fcx.tcx.type_of(item_def_id);
                let self_ty = fcx.normalize_associated_types_in(item.span, &self_ty);
                fcx.register_wf_obligation(
                    self_ty,
                    ast_self_ty.span,
                    ObligationCauseCode::MiscObligation,
                );
            }
        }

        check_where_clauses(tcx, fcx, item.span, item_def_id, None);

        fcx.impl_implied_bounds(item_def_id, item.span)
    });
}

/// Checks where-clauses and inline bounds that are declared on `def_id`.
fn check_where_clauses<'tcx, 'fcx>(
    tcx: TyCtxt<'tcx>,
    fcx: &FnCtxt<'fcx, 'tcx>,
    span: Span,
    def_id: DefId,
    return_ty: Option<(Ty<'tcx>, Span)>,
) {
    debug!("check_where_clauses(def_id={:?}, return_ty={:?})", def_id, return_ty);

    let predicates = fcx.tcx.predicates_of(def_id);
    let generics = tcx.generics_of(def_id);

    let is_our_default = |def: &ty::GenericParamDef| match def.kind {
        GenericParamDefKind::Type { has_default, .. } => {
            has_default && def.index >= generics.parent_count as u32
        }
        _ => unreachable!(),
    };

    // Check that concrete defaults are well-formed. See test `type-check-defaults.rs`.
    // For example, this forbids the declaration:
    //
    //     struct Foo<T = Vec<[u32]>> { .. }
    //
    // Here, the default `Vec<[u32]>` is not WF because `[u32]: Sized` does not hold.
    for param in &generics.params {
        if let GenericParamDefKind::Type { .. } = param.kind {
            if is_our_default(&param) {
                let ty = fcx.tcx.type_of(param.def_id);
                // Ignore dependent defaults -- that is, where the default of one type
                // parameter includes another (e.g., `<T, U = T>`). In those cases, we can't
                // be sure if it will error or not as user might always specify the other.
                if !ty.needs_subst() {
                    fcx.register_wf_obligation(
                        ty,
                        fcx.tcx.def_span(param.def_id),
                        ObligationCauseCode::MiscObligation,
                    );
                }
            }
        }
    }

    // Check that trait predicates are WF when params are substituted by their defaults.
    // We don't want to overly constrain the predicates that may be written but we want to
    // catch cases where a default my never be applied such as `struct Foo<T: Copy = String>`.
    // Therefore we check if a predicate which contains a single type param
    // with a concrete default is WF with that default substituted.
    // For more examples see tests `defaults-well-formedness.rs` and `type-check-defaults.rs`.
    //
    // First we build the defaulted substitution.
    let substs = InternalSubsts::for_item(fcx.tcx, def_id, |param, _| {
        match param.kind {
            GenericParamDefKind::Lifetime => {
                // All regions are identity.
                fcx.tcx.mk_param_from_def(param)
            }

            GenericParamDefKind::Type { .. } => {
                // If the param has a default, ...
                if is_our_default(param) {
                    let default_ty = fcx.tcx.type_of(param.def_id);
                    // ... and it's not a dependent default, ...
                    if !default_ty.needs_subst() {
                        // ... then substitute it with the default.
                        return default_ty.into();
                    }
                }
                // Mark unwanted params as error.
                fcx.tcx.types.err.into()
            }

            GenericParamDefKind::Const => {
                // FIXME(const_generics:defaults)
                fcx.tcx.consts.err.into()
            }
        }
    });

    // Now we build the substituted predicates.
    let default_obligations = predicates
        .predicates
        .iter()
        .flat_map(|&(pred, sp)| {
            #[derive(Default)]
            struct CountParams {
                params: FxHashSet<u32>,
            }
            impl<'tcx> ty::fold::TypeVisitor<'tcx> for CountParams {
                fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
                    if let ty::Param(param) = t.kind {
                        self.params.insert(param.index);
                    }
                    t.super_visit_with(self)
                }

                fn visit_region(&mut self, _: ty::Region<'tcx>) -> bool {
                    true
                }

                fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
                    if let ty::ConstKind::Param(param) = c.val {
                        self.params.insert(param.index);
                    }
                    c.super_visit_with(self)
                }
            }
            let mut param_count = CountParams::default();
            let has_region = pred.visit_with(&mut param_count);
            let substituted_pred = pred.subst(fcx.tcx, substs);
            // Don't check non-defaulted params, dependent defaults (including lifetimes)
            // or preds with multiple params.
            if substituted_pred.references_error() || param_count.params.len() > 1 || has_region {
                None
            } else if predicates.predicates.iter().any(|&(p, _)| p == substituted_pred) {
                // Avoid duplication of predicates that contain no parameters, for example.
                None
            } else {
                Some((substituted_pred, sp))
            }
        })
        .map(|(pred, sp)| {
            // Convert each of those into an obligation. So if you have
            // something like `struct Foo<T: Copy = String>`, we would
            // take that predicate `T: Copy`, substitute to `String: Copy`
            // (actually that happens in the previous `flat_map` call),
            // and then try to prove it (in this case, we'll fail).
            //
            // Note the subtle difference from how we handle `predicates`
            // below: there, we are not trying to prove those predicates
            // to be *true* but merely *well-formed*.
            let pred = fcx.normalize_associated_types_in(sp, &pred);
            let cause =
                traits::ObligationCause::new(sp, fcx.body_id, traits::ItemObligation(def_id));
            traits::Obligation::new(cause, fcx.param_env, pred)
        });

    let mut predicates = predicates.instantiate_identity(fcx.tcx);

    if let Some((return_ty, span)) = return_ty {
        let opaque_types = check_opaque_types(tcx, fcx, def_id, span, return_ty);
        for _ in 0..opaque_types.len() {
            predicates.spans.push(span);
        }
        predicates.predicates.extend(opaque_types);
    }

    let predicates = fcx.normalize_associated_types_in(span, &predicates);

    debug!("check_where_clauses: predicates={:?}", predicates.predicates);
    assert_eq!(predicates.predicates.len(), predicates.spans.len());
    let wf_obligations =
        predicates.predicates.iter().zip(predicates.spans.iter()).flat_map(|(p, sp)| {
            traits::wf::predicate_obligations(fcx, fcx.param_env, fcx.body_id, p, *sp)
        });

    for obligation in wf_obligations.chain(default_obligations) {
        debug!("next obligation cause: {:?}", obligation.cause);
        fcx.register_predicate(obligation);
    }
}

fn check_fn_or_method<'fcx, 'tcx>(
    tcx: TyCtxt<'tcx>,
    fcx: &FnCtxt<'fcx, 'tcx>,
    span: Span,
    sig: ty::PolyFnSig<'tcx>,
    hir_sig: &hir::FnSig<'_>,
    def_id: DefId,
    implied_bounds: &mut Vec<Ty<'tcx>>,
) {
    let sig = fcx.normalize_associated_types_in(span, &sig);
    let sig = fcx.tcx.liberate_late_bound_regions(def_id, &sig);

    for (input_ty, span) in sig.inputs().iter().zip(hir_sig.decl.inputs.iter().map(|t| t.span)) {
        fcx.register_wf_obligation(&input_ty, span, ObligationCauseCode::MiscObligation);
    }
    implied_bounds.extend(sig.inputs());

    fcx.register_wf_obligation(
        sig.output(),
        hir_sig.decl.output.span(),
        ObligationCauseCode::ReturnType,
    );

    // FIXME(#25759) return types should not be implied bounds
    implied_bounds.push(sig.output());

    check_where_clauses(tcx, fcx, span, def_id, Some((sig.output(), hir_sig.decl.output.span())));
}

/// Checks "defining uses" of opaque `impl Trait` types to ensure that they meet the restrictions
/// laid for "higher-order pattern unification".
/// This ensures that inference is tractable.
/// In particular, definitions of opaque types can only use other generics as arguments,
/// and they cannot repeat an argument. Example:
///
/// ```rust
/// type Foo<A, B> = impl Bar<A, B>;
///
/// // Okay -- `Foo` is applied to two distinct, generic types.
/// fn a<T, U>() -> Foo<T, U> { .. }
///
/// // Not okay -- `Foo` is applied to `T` twice.
/// fn b<T>() -> Foo<T, T> { .. }
///
/// // Not okay -- `Foo` is applied to a non-generic type.
/// fn b<T>() -> Foo<T, u32> { .. }
/// ```
///
fn check_opaque_types<'fcx, 'tcx>(
    tcx: TyCtxt<'tcx>,
    fcx: &FnCtxt<'fcx, 'tcx>,
    fn_def_id: DefId,
    span: Span,
    ty: Ty<'tcx>,
) -> Vec<ty::Predicate<'tcx>> {
    trace!("check_opaque_types(ty={:?})", ty);
    let mut substituted_predicates = Vec::new();
    ty.fold_with(&mut ty::fold::BottomUpFolder {
        tcx: fcx.tcx,
        ty_op: |ty| {
            if let ty::Opaque(def_id, substs) = ty.kind {
                trace!("check_opaque_types: opaque_ty, {:?}, {:?}", def_id, substs);
                let generics = tcx.generics_of(def_id);
                // Only check named `impl Trait` types defined in this crate.
                if generics.parent.is_none() && def_id.is_local() {
                    let opaque_hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
                    if may_define_opaque_type(tcx, fn_def_id, opaque_hir_id) {
                        trace!("check_opaque_types: may define, generics={:#?}", generics);
                        let mut seen: FxHashMap<_, Vec<_>> = FxHashMap::default();
                        for (subst, param) in substs.iter().zip(&generics.params) {
                            match subst.unpack() {
                                ty::subst::GenericArgKind::Type(ty) => match ty.kind {
                                    ty::Param(..) => {}
                                    // Prevent `fn foo() -> Foo<u32>` from being defining.
                                    _ => {
                                        tcx.sess
                                            .struct_span_err(
                                                span,
                                                "non-defining opaque type use \
                                                 in defining scope",
                                            )
                                            .span_note(
                                                tcx.def_span(param.def_id),
                                                &format!(
                                                    "used non-generic type {} for \
                                                     generic parameter",
                                                    ty,
                                                ),
                                            )
                                            .emit();
                                    }
                                },

                                ty::subst::GenericArgKind::Lifetime(region) => {
                                    let param_span = tcx.def_span(param.def_id);
                                    if let ty::ReStatic = region {
                                        tcx.sess
                                            .struct_span_err(
                                                span,
                                                "non-defining opaque type use \
                                                    in defining scope",
                                            )
                                            .span_label(
                                                param_span,
                                                "cannot use static lifetime; use a bound lifetime \
                                                instead or remove the lifetime parameter from the \
                                                opaque type",
                                            )
                                            .emit();
                                    } else {
                                        seen.entry(region).or_default().push(param_span);
                                    }
                                }

                                ty::subst::GenericArgKind::Const(ct) => match ct.val {
                                    ty::ConstKind::Param(_) => {}
                                    _ => {
                                        tcx.sess
                                            .struct_span_err(
                                                span,
                                                "non-defining opaque type use \
                                                in defining scope",
                                            )
                                            .span_note(
                                                tcx.def_span(param.def_id),
                                                &format!(
                                                    "used non-generic const {} for \
                                                    generic parameter",
                                                    ty,
                                                ),
                                            )
                                            .emit();
                                    }
                                },
                            } // match subst
                        } // for (subst, param)
                        for (_, spans) in seen {
                            if spans.len() > 1 {
                                tcx.sess
                                    .struct_span_err(
                                        span,
                                        "non-defining opaque type use \
                                            in defining scope",
                                    )
                                    .span_note(spans, "lifetime used multiple times")
                                    .emit();
                            }
                        }
                    } // if may_define_opaque_type

                    // Now register the bounds on the parameters of the opaque type
                    // so the parameters given by the function need to fulfill them.
                    //
                    //     type Foo<T: Bar> = impl Baz + 'static;
                    //     fn foo<U>() -> Foo<U> { .. *}
                    //
                    // becomes
                    //
                    //     type Foo<T: Bar> = impl Baz + 'static;
                    //     fn foo<U: Bar>() -> Foo<U> { .. *}
                    let predicates = tcx.predicates_of(def_id);
                    trace!("check_opaque_types: may define, predicates={:#?}", predicates,);
                    for &(pred, _) in predicates.predicates {
                        let substituted_pred = pred.subst(fcx.tcx, substs);
                        // Avoid duplication of predicates that contain no parameters, for example.
                        if !predicates.predicates.iter().any(|&(p, _)| p == substituted_pred) {
                            substituted_predicates.push(substituted_pred);
                        }
                    }
                } // if is_named_opaque_type
            } // if let Opaque
            ty
        },
        lt_op: |lt| lt,
        ct_op: |ct| ct,
    });
    substituted_predicates
}

const HELP_FOR_SELF_TYPE: &str = "consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, \
     `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one \
     of the previous types except `Self`)";

fn check_method_receiver<'fcx, 'tcx>(
    fcx: &FnCtxt<'fcx, 'tcx>,
    fn_sig: &hir::FnSig<'_>,
    method: &ty::AssocItem,
    self_ty: Ty<'tcx>,
) {
    // Check that the method has a valid receiver type, given the type `Self`.
    debug!("check_method_receiver({:?}, self_ty={:?})", method, self_ty);

    if !method.method_has_self_argument {
        return;
    }

    let span = fn_sig.decl.inputs[0].span;

    let sig = fcx.tcx.fn_sig(method.def_id);
    let sig = fcx.normalize_associated_types_in(span, &sig);
    let sig = fcx.tcx.liberate_late_bound_regions(method.def_id, &sig);

    debug!("check_method_receiver: sig={:?}", sig);

    let self_ty = fcx.normalize_associated_types_in(span, &self_ty);
    let self_ty = fcx.tcx.liberate_late_bound_regions(method.def_id, &ty::Binder::bind(self_ty));

    let receiver_ty = sig.inputs()[0];

    let receiver_ty = fcx.normalize_associated_types_in(span, &receiver_ty);
    let receiver_ty =
        fcx.tcx.liberate_late_bound_regions(method.def_id, &ty::Binder::bind(receiver_ty));

    if fcx.tcx.features().arbitrary_self_types {
        if !receiver_is_valid(fcx, span, receiver_ty, self_ty, true) {
            // Report error; `arbitrary_self_types` was enabled.
            e0307(fcx, span, receiver_ty);
        }
    } else {
        if !receiver_is_valid(fcx, span, receiver_ty, self_ty, false) {
            if receiver_is_valid(fcx, span, receiver_ty, self_ty, true) {
                // Report error; would have worked with `arbitrary_self_types`.
                feature_err(
                    &fcx.tcx.sess.parse_sess,
                    sym::arbitrary_self_types,
                    span,
                    &format!(
                        "`{}` cannot be used as the type of `self` without \
                            the `arbitrary_self_types` feature",
                        receiver_ty,
                    ),
                )
                .help(HELP_FOR_SELF_TYPE)
                .emit();
            } else {
                // Report error; would not have worked with `arbitrary_self_types`.
                e0307(fcx, span, receiver_ty);
            }
        }
    }
}

fn e0307(fcx: &FnCtxt<'fcx, 'tcx>, span: Span, receiver_ty: Ty<'_>) {
    struct_span_err!(
        fcx.tcx.sess.diagnostic(),
        span,
        E0307,
        "invalid `self` parameter type: {:?}",
        receiver_ty,
    )
    .note("type of `self` must be `Self` or a type that dereferences to it")
    .help(HELP_FOR_SELF_TYPE)
    .emit();
}

/// Returns whether `receiver_ty` would be considered a valid receiver type for `self_ty`. If
/// `arbitrary_self_types` is enabled, `receiver_ty` must transitively deref to `self_ty`, possibly
/// through a `*const/mut T` raw pointer. If the feature is not enabled, the requirements are more
/// strict: `receiver_ty` must implement `Receiver` and directly implement
/// `Deref<Target = self_ty>`.
///
/// N.B., there are cases this function returns `true` but causes an error to be emitted,
/// particularly when `receiver_ty` derefs to a type that is the same as `self_ty` but has the
/// wrong lifetime. Be careful of this if you are calling this function speculatively.
fn receiver_is_valid<'fcx, 'tcx>(
    fcx: &FnCtxt<'fcx, 'tcx>,
    span: Span,
    receiver_ty: Ty<'tcx>,
    self_ty: Ty<'tcx>,
    arbitrary_self_types_enabled: bool,
) -> bool {
    let cause = fcx.cause(span, traits::ObligationCauseCode::MethodReceiver);

    let can_eq_self = |ty| fcx.infcx.can_eq(fcx.param_env, self_ty, ty).is_ok();

    // `self: Self` is always valid.
    if can_eq_self(receiver_ty) {
        if let Some(mut err) = fcx.demand_eqtype_with_origin(&cause, self_ty, receiver_ty) {
            err.emit();
        }
        return true;
    }

    let mut autoderef = fcx.autoderef(span, receiver_ty);

    // The `arbitrary_self_types` feature allows raw pointer receivers like `self: *const Self`.
    if arbitrary_self_types_enabled {
        autoderef = autoderef.include_raw_pointers();
    }

    // The first type is `receiver_ty`, which we know its not equal to `self_ty`; skip it.
    autoderef.next();

    let receiver_trait_def_id = fcx.tcx.require_lang_item(lang_items::ReceiverTraitLangItem, None);

    // Keep dereferencing `receiver_ty` until we get to `self_ty`.
    loop {
        if let Some((potential_self_ty, _)) = autoderef.next() {
            debug!(
                "receiver_is_valid: potential self type `{:?}` to match `{:?}`",
                potential_self_ty, self_ty
            );

            if can_eq_self(potential_self_ty) {
                autoderef.finalize(fcx);

                if let Some(mut err) =
                    fcx.demand_eqtype_with_origin(&cause, self_ty, potential_self_ty)
                {
                    err.emit();
                }

                break;
            } else {
                // Without `feature(arbitrary_self_types)`, we require that each step in the
                // deref chain implement `receiver`
                if !arbitrary_self_types_enabled
                    && !receiver_is_implemented(
                        fcx,
                        receiver_trait_def_id,
                        cause.clone(),
                        potential_self_ty,
                    )
                {
                    return false;
                }
            }
        } else {
            debug!("receiver_is_valid: type `{:?}` does not deref to `{:?}`", receiver_ty, self_ty);
            // If he receiver already has errors reported due to it, consider it valid to avoid
            // unnecessary errors (#58712).
            return receiver_ty.references_error();
        }
    }

    // Without `feature(arbitrary_self_types)`, we require that `receiver_ty` implements `Receiver`.
    if !arbitrary_self_types_enabled
        && !receiver_is_implemented(fcx, receiver_trait_def_id, cause.clone(), receiver_ty)
    {
        return false;
    }

    true
}

fn receiver_is_implemented(
    fcx: &FnCtxt<'_, 'tcx>,
    receiver_trait_def_id: DefId,
    cause: ObligationCause<'tcx>,
    receiver_ty: Ty<'tcx>,
) -> bool {
    let trait_ref = ty::TraitRef {
        def_id: receiver_trait_def_id,
        substs: fcx.tcx.mk_substs_trait(receiver_ty, &[]),
    };

    let obligation =
        traits::Obligation::new(cause, fcx.param_env, trait_ref.without_const().to_predicate());

    if fcx.predicate_must_hold_modulo_regions(&obligation) {
        true
    } else {
        debug!(
            "receiver_is_implemented: type `{:?}` does not implement `Receiver` trait",
            receiver_ty
        );
        false
    }
}

fn check_variances_for_type_defn<'tcx>(
    tcx: TyCtxt<'tcx>,
    item: &hir::Item<'tcx>,
    hir_generics: &hir::Generics<'_>,
) {
    let item_def_id = tcx.hir().local_def_id(item.hir_id);
    let ty = tcx.type_of(item_def_id);
    if tcx.has_error_field(ty) {
        return;
    }

    let ty_predicates = tcx.predicates_of(item_def_id);
    assert_eq!(ty_predicates.parent, None);
    let variances = tcx.variances_of(item_def_id);

    let mut constrained_parameters: FxHashSet<_> = variances
        .iter()
        .enumerate()
        .filter(|&(_, &variance)| variance != ty::Bivariant)
        .map(|(index, _)| Parameter(index as u32))
        .collect();

    identify_constrained_generic_params(tcx, ty_predicates, None, &mut constrained_parameters);

    for (index, _) in variances.iter().enumerate() {
        if constrained_parameters.contains(&Parameter(index as u32)) {
            continue;
        }

        let param = &hir_generics.params[index];

        match param.name {
            hir::ParamName::Error => {}
            _ => report_bivariance(tcx, param.span, param.name.ident().name),
        }
    }
}

fn report_bivariance(tcx: TyCtxt<'_>, span: Span, param_name: ast::Name) {
    let mut err = error_392(tcx, span, param_name);

    let suggested_marker_id = tcx.lang_items().phantom_data();
    // Help is available only in presence of lang items.
    let msg = if let Some(def_id) = suggested_marker_id {
        format!(
            "consider removing `{}`, referring to it in a field, or using a marker such as `{}`",
            param_name,
            tcx.def_path_str(def_id),
        )
    } else {
        format!("consider removing `{}` or referring to it in a field", param_name)
    };
    err.help(&msg);
    err.emit();
}

/// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
/// aren't true.
fn check_false_global_bounds(fcx: &FnCtxt<'_, '_>, span: Span, id: hir::HirId) {
    let empty_env = ty::ParamEnv::empty();

    let def_id = fcx.tcx.hir().local_def_id(id);
    let predicates = fcx.tcx.predicates_of(def_id).predicates.iter().map(|(p, _)| *p).collect();
    // Check elaborated bounds.
    let implied_obligations = traits::elaborate_predicates(fcx.tcx, predicates);

    for pred in implied_obligations {
        // Match the existing behavior.
        if pred.is_global() && !pred.has_late_bound_regions() {
            let pred = fcx.normalize_associated_types_in(span, &pred);
            let obligation = traits::Obligation::new(
                traits::ObligationCause::new(span, id, traits::TrivialBound),
                empty_env,
                pred,
            );
            fcx.register_predicate(obligation);
        }
    }

    fcx.select_all_obligations_or_error();
}

pub struct CheckTypeWellFormedVisitor<'tcx> {
    tcx: TyCtxt<'tcx>,
}

impl CheckTypeWellFormedVisitor<'tcx> {
    pub fn new(tcx: TyCtxt<'tcx>) -> CheckTypeWellFormedVisitor<'tcx> {
        CheckTypeWellFormedVisitor { tcx }
    }
}

impl ParItemLikeVisitor<'tcx> for CheckTypeWellFormedVisitor<'tcx> {
    fn visit_item(&self, i: &'tcx hir::Item<'tcx>) {
        debug!("visit_item: {:?}", i);
        let def_id = self.tcx.hir().local_def_id(i.hir_id);
        self.tcx.ensure().check_item_well_formed(def_id);
    }

    fn visit_trait_item(&self, trait_item: &'tcx hir::TraitItem<'tcx>) {
        debug!("visit_trait_item: {:?}", trait_item);
        let def_id = self.tcx.hir().local_def_id(trait_item.hir_id);
        self.tcx.ensure().check_trait_item_well_formed(def_id);
    }

    fn visit_impl_item(&self, impl_item: &'tcx hir::ImplItem<'tcx>) {
        debug!("visit_impl_item: {:?}", impl_item);
        let def_id = self.tcx.hir().local_def_id(impl_item.hir_id);
        self.tcx.ensure().check_impl_item_well_formed(def_id);
    }
}

///////////////////////////////////////////////////////////////////////////
// ADT

struct AdtVariant<'tcx> {
    fields: Vec<AdtField<'tcx>>,
}

struct AdtField<'tcx> {
    ty: Ty<'tcx>,
    span: Span,
}

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    fn non_enum_variant(&self, struct_def: &hir::VariantData<'_>) -> AdtVariant<'tcx> {
        let fields = struct_def
            .fields()
            .iter()
            .map(|field| {
                let field_ty = self.tcx.type_of(self.tcx.hir().local_def_id(field.hir_id));
                let field_ty = self.normalize_associated_types_in(field.span, &field_ty);
                let field_ty = self.resolve_vars_if_possible(&field_ty);
                debug!("non_enum_variant: type of field {:?} is {:?}", field, field_ty);
                AdtField { ty: field_ty, span: field.span }
            })
            .collect();
        AdtVariant { fields }
    }

    fn enum_variants(&self, enum_def: &hir::EnumDef<'_>) -> Vec<AdtVariant<'tcx>> {
        enum_def.variants.iter().map(|variant| self.non_enum_variant(&variant.data)).collect()
    }

    fn impl_implied_bounds(&self, impl_def_id: DefId, span: Span) -> Vec<Ty<'tcx>> {
        match self.tcx.impl_trait_ref(impl_def_id) {
            Some(ref trait_ref) => {
                // Trait impl: take implied bounds from all types that
                // appear in the trait reference.
                let trait_ref = self.normalize_associated_types_in(span, trait_ref);
                trait_ref.substs.types().collect()
            }

            None => {
                // Inherent impl: take implied bounds from the `self` type.
                let self_ty = self.tcx.type_of(impl_def_id);
                let self_ty = self.normalize_associated_types_in(span, &self_ty);
                vec![self_ty]
            }
        }
    }
}

fn error_392(tcx: TyCtxt<'_>, span: Span, param_name: ast::Name) -> DiagnosticBuilder<'_> {
    let mut err =
        struct_span_err!(tcx.sess, span, E0392, "parameter `{}` is never used", param_name);
    err.span_label(span, "unused parameter");
    err
}