mod.rs

pub mod on_unimplemented;
pub mod suggestions;

use super::{
    ConstEvalFailure, EvaluationResult, FulfillmentError, FulfillmentErrorCode,
    MismatchedProjectionTypes, Obligation, ObligationCause, ObligationCauseCode,
    OnUnimplementedDirective, OnUnimplementedNote, OutputTypeParameterMismatch, Overflow,
    PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
};

use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use crate::infer::{self, InferCtxt, TyCtxtInferExt};
use rustc_data_structures::fx::FxHashMap;
use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder};
use rustc_hir as hir;
use rustc_hir::def_id::{DefId, LOCAL_CRATE};
use rustc_hir::{Node, QPath, TyKind, WhereBoundPredicate, WherePredicate};
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::ty::error::ExpectedFound;
use rustc_middle::ty::fast_reject;
use rustc_middle::ty::fold::TypeFolder;
use rustc_middle::ty::SubtypePredicate;
use rustc_middle::ty::{
    self, AdtKind, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
};
use rustc_session::DiagnosticMessageId;
use rustc_span::{BytePos, ExpnKind, Span, DUMMY_SP};
use std::fmt;

use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
use crate::traits::query::normalize::AtExt as _;
use on_unimplemented::InferCtxtExt as _;
use suggestions::InferCtxtExt as _;

pub use rustc_infer::traits::error_reporting::*;

pub trait InferCtxtExt<'tcx> {
    fn report_fulfillment_errors(
        &self,
        errors: &[FulfillmentError<'tcx>],
        body_id: Option<hir::BodyId>,
        fallback_has_occurred: bool,
    );

    fn report_overflow_error<T>(
        &self,
        obligation: &Obligation<'tcx, T>,
        suggest_increasing_limit: bool,
    ) -> !
    where
        T: fmt::Display + TypeFoldable<'tcx>;

    fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;

    fn report_selection_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        error: &SelectionError<'tcx>,
        fallback_has_occurred: bool,
        points_at_arg: bool,
    );

    /// Given some node representing a fn-like thing in the HIR map,
    /// returns a span and `ArgKind` information that describes the
    /// arguments it expects. This can be supplied to
    /// `report_arg_count_mismatch`.
    fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;

    /// Reports an error when the number of arguments needed by a
    /// trait match doesn't match the number that the expression
    /// provides.
    fn report_arg_count_mismatch(
        &self,
        span: Span,
        found_span: Option<Span>,
        expected_args: Vec<ArgKind>,
        found_args: Vec<ArgKind>,
        is_closure: bool,
    ) -> DiagnosticBuilder<'tcx>;
}

impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
    fn report_fulfillment_errors(
        &self,
        errors: &[FulfillmentError<'tcx>],
        body_id: Option<hir::BodyId>,
        fallback_has_occurred: bool,
    ) {
        #[derive(Debug)]
        struct ErrorDescriptor<'tcx> {
            predicate: ty::Predicate<'tcx>,
            index: Option<usize>, // None if this is an old error
        }

        let mut error_map: FxHashMap<_, Vec<_>> = self
            .reported_trait_errors
            .borrow()
            .iter()
            .map(|(&span, predicates)| {
                (
                    span,
                    predicates
                        .iter()
                        .map(|&predicate| ErrorDescriptor { predicate, index: None })
                        .collect(),
                )
            })
            .collect();

        for (index, error) in errors.iter().enumerate() {
            // We want to ignore desugarings here: spans are equivalent even
            // if one is the result of a desugaring and the other is not.
            let mut span = error.obligation.cause.span;
            let expn_data = span.ctxt().outer_expn_data();
            if let ExpnKind::Desugaring(_) = expn_data.kind {
                span = expn_data.call_site;
            }

            error_map.entry(span).or_default().push(ErrorDescriptor {
                predicate: error.obligation.predicate,
                index: Some(index),
            });

            self.reported_trait_errors
                .borrow_mut()
                .entry(span)
                .or_default()
                .push(error.obligation.predicate.clone());
        }

        // We do this in 2 passes because we want to display errors in order, though
        // maybe it *is* better to sort errors by span or something.
        let mut is_suppressed = vec![false; errors.len()];
        for (_, error_set) in error_map.iter() {
            // We want to suppress "duplicate" errors with the same span.
            for error in error_set {
                if let Some(index) = error.index {
                    // Suppress errors that are either:
                    // 1) strictly implied by another error.
                    // 2) implied by an error with a smaller index.
                    for error2 in error_set {
                        if error2.index.map_or(false, |index2| is_suppressed[index2]) {
                            // Avoid errors being suppressed by already-suppressed
                            // errors, to prevent all errors from being suppressed
                            // at once.
                            continue;
                        }

                        if self.error_implies(&error2.predicate, &error.predicate)
                            && !(error2.index >= error.index
                                && self.error_implies(&error.predicate, &error2.predicate))
                        {
                            info!("skipping {:?} (implied by {:?})", error, error2);
                            is_suppressed[index] = true;
                            break;
                        }
                    }
                }
            }
        }

        for (error, suppressed) in errors.iter().zip(is_suppressed) {
            if !suppressed {
                self.report_fulfillment_error(error, body_id, fallback_has_occurred);
            }
        }
    }

    /// Reports that an overflow has occurred and halts compilation. We
    /// halt compilation unconditionally because it is important that
    /// overflows never be masked -- they basically represent computations
    /// whose result could not be truly determined and thus we can't say
    /// if the program type checks or not -- and they are unusual
    /// occurrences in any case.
    fn report_overflow_error<T>(
        &self,
        obligation: &Obligation<'tcx, T>,
        suggest_increasing_limit: bool,
    ) -> !
    where
        T: fmt::Display + TypeFoldable<'tcx>,
    {
        let predicate = self.resolve_vars_if_possible(&obligation.predicate);
        let mut err = struct_span_err!(
            self.tcx.sess,
            obligation.cause.span,
            E0275,
            "overflow evaluating the requirement `{}`",
            predicate
        );

        if suggest_increasing_limit {
            self.suggest_new_overflow_limit(&mut err);
        }

        self.note_obligation_cause_code(
            &mut err,
            &obligation.predicate,
            &obligation.cause.code,
            &mut vec![],
        );

        err.emit();
        self.tcx.sess.abort_if_errors();
        bug!();
    }

    /// Reports that a cycle was detected which led to overflow and halts
    /// compilation. This is equivalent to `report_overflow_error` except
    /// that we can give a more helpful error message (and, in particular,
    /// we do not suggest increasing the overflow limit, which is not
    /// going to help).
    fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
        let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
        assert!(!cycle.is_empty());

        debug!("report_overflow_error_cycle: cycle={:?}", cycle);

        self.report_overflow_error(&cycle[0], false);
    }

    fn report_selection_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        error: &SelectionError<'tcx>,
        fallback_has_occurred: bool,
        points_at_arg: bool,
    ) {
        let tcx = self.tcx;
        let span = obligation.cause.span;

        let mut err = match *error {
            SelectionError::Unimplemented => {
                if let ObligationCauseCode::CompareImplMethodObligation {
                    item_name,
                    impl_item_def_id,
                    trait_item_def_id,
                }
                | ObligationCauseCode::CompareImplTypeObligation {
                    item_name,
                    impl_item_def_id,
                    trait_item_def_id,
                } = obligation.cause.code
                {
                    self.report_extra_impl_obligation(
                        span,
                        item_name,
                        impl_item_def_id,
                        trait_item_def_id,
                        &format!("`{}`", obligation.predicate),
                    )
                    .emit();
                    return;
                }
                match obligation.predicate {
                    ty::Predicate::Trait(ref trait_predicate, _) => {
                        let trait_predicate = self.resolve_vars_if_possible(trait_predicate);

                        if self.tcx.sess.has_errors() && trait_predicate.references_error() {
                            return;
                        }
                        let trait_ref = trait_predicate.to_poly_trait_ref();
                        let (post_message, pre_message, type_def) = self
                            .get_parent_trait_ref(&obligation.cause.code)
                            .map(|(t, s)| {
                                (
                                    format!(" in `{}`", t),
                                    format!("within `{}`, ", t),
                                    s.map(|s| (format!("within this `{}`", t), s)),
                                )
                            })
                            .unwrap_or_default();

                        let OnUnimplementedNote { message, label, note, enclosing_scope } =
                            self.on_unimplemented_note(trait_ref, obligation);
                        let have_alt_message = message.is_some() || label.is_some();
                        let is_try = self
                            .tcx
                            .sess
                            .source_map()
                            .span_to_snippet(span)
                            .map(|s| &s == "?")
                            .unwrap_or(false);
                        let is_from = format!("{}", trait_ref.print_only_trait_path())
                            .starts_with("std::convert::From<");
                        let (message, note) = if is_try && is_from {
                            (
                                Some(format!(
                                    "`?` couldn't convert the error to `{}`",
                                    trait_ref.self_ty(),
                                )),
                                Some(
                                    "the question mark operation (`?`) implicitly performs a \
                                     conversion on the error value using the `From` trait"
                                        .to_owned(),
                                ),
                            )
                        } else {
                            (message, note)
                        };

                        let mut err = struct_span_err!(
                            self.tcx.sess,
                            span,
                            E0277,
                            "{}",
                            message.unwrap_or_else(|| format!(
                                "the trait bound `{}` is not satisfied{}",
                                trait_ref.without_const().to_predicate(),
                                post_message,
                            ))
                        );

                        let explanation =
                            if obligation.cause.code == ObligationCauseCode::MainFunctionType {
                                "consider using `()`, or a `Result`".to_owned()
                            } else {
                                format!(
                                    "{}the trait `{}` is not implemented for `{}`",
                                    pre_message,
                                    trait_ref.print_only_trait_path(),
                                    trait_ref.self_ty(),
                                )
                            };

                        if self.suggest_add_reference_to_arg(
                            &obligation,
                            &mut err,
                            &trait_ref,
                            points_at_arg,
                            have_alt_message,
                        ) {
                            self.note_obligation_cause(&mut err, obligation);
                            err.emit();
                            return;
                        }
                        if let Some(ref s) = label {
                            // If it has a custom `#[rustc_on_unimplemented]`
                            // error message, let's display it as the label!
                            err.span_label(span, s.as_str());
                            err.help(&explanation);
                        } else {
                            err.span_label(span, explanation);
                        }
                        if let Some((msg, span)) = type_def {
                            err.span_label(span, &msg);
                        }
                        if let Some(ref s) = note {
                            // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
                            err.note(s.as_str());
                        }
                        if let Some(ref s) = enclosing_scope {
                            let enclosing_scope_span = tcx.def_span(
                                tcx.hir()
                                    .opt_local_def_id(obligation.cause.body_id)
                                    .unwrap_or_else(|| {
                                        tcx.hir().body_owner_def_id(hir::BodyId {
                                            hir_id: obligation.cause.body_id,
                                        })
                                    }),
                            );

                            err.span_label(enclosing_scope_span, s.as_str());
                        }

                        self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
                        self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
                        self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
                        self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
                        self.note_version_mismatch(&mut err, &trait_ref);
                        if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
                            err.emit();
                            return;
                        }

                        // Try to report a help message
                        if !trait_ref.has_infer_types_or_consts()
                            && self.predicate_can_apply(obligation.param_env, trait_ref)
                        {
                            // If a where-clause may be useful, remind the
                            // user that they can add it.
                            //
                            // don't display an on-unimplemented note, as
                            // these notes will often be of the form
                            //     "the type `T` can't be frobnicated"
                            // which is somewhat confusing.
                            self.suggest_restricting_param_bound(
                                &mut err,
                                &trait_ref,
                                obligation.cause.body_id,
                            );
                        } else {
                            if !have_alt_message {
                                // Can't show anything else useful, try to find similar impls.
                                let impl_candidates = self.find_similar_impl_candidates(trait_ref);
                                self.report_similar_impl_candidates(impl_candidates, &mut err);
                            }
                            self.suggest_change_mut(
                                &obligation,
                                &mut err,
                                &trait_ref,
                                points_at_arg,
                            );
                        }

                        // If this error is due to `!: Trait` not implemented but `(): Trait` is
                        // implemented, and fallback has occurred, then it could be due to a
                        // variable that used to fallback to `()` now falling back to `!`. Issue a
                        // note informing about the change in behaviour.
                        if trait_predicate.skip_binder().self_ty().is_never()
                            && fallback_has_occurred
                        {
                            let predicate = trait_predicate.map_bound(|mut trait_pred| {
                                trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
                                    self.tcx.mk_unit(),
                                    &trait_pred.trait_ref.substs[1..],
                                );
                                trait_pred
                            });
                            let unit_obligation = Obligation {
                                predicate: ty::Predicate::Trait(
                                    predicate,
                                    hir::Constness::NotConst,
                                ),
                                ..obligation.clone()
                            };
                            if self.predicate_may_hold(&unit_obligation) {
                                err.note(
                                    "the trait is implemented for `()`. \
                                     Possibly this error has been caused by changes to \
                                     Rust's type-inference algorithm (see issue #48950 \
                                     <https://github.com/rust-lang/rust/issues/48950> \
                                     for more information). Consider whether you meant to use \
                                     the type `()` here instead.",
                                );
                            }
                        }

                        err
                    }

                    ty::Predicate::Subtype(ref predicate) => {
                        // Errors for Subtype predicates show up as
                        // `FulfillmentErrorCode::CodeSubtypeError`,
                        // not selection error.
                        span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
                    }

                    ty::Predicate::RegionOutlives(ref predicate) => {
                        let predicate = self.resolve_vars_if_possible(predicate);
                        let err = self
                            .region_outlives_predicate(&obligation.cause, &predicate)
                            .err()
                            .unwrap();
                        struct_span_err!(
                            self.tcx.sess,
                            span,
                            E0279,
                            "the requirement `{}` is not satisfied (`{}`)",
                            predicate,
                            err,
                        )
                    }

                    ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
                        let predicate = self.resolve_vars_if_possible(&obligation.predicate);
                        struct_span_err!(
                            self.tcx.sess,
                            span,
                            E0280,
                            "the requirement `{}` is not satisfied",
                            predicate
                        )
                    }

                    ty::Predicate::ObjectSafe(trait_def_id) => {
                        let violations = self.tcx.object_safety_violations(trait_def_id);
                        report_object_safety_error(self.tcx, span, trait_def_id, violations)
                    }

                    ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
                        let found_kind = self.closure_kind(closure_substs).unwrap();
                        let closure_span =
                            self.tcx.sess.source_map().guess_head_span(
                                self.tcx.hir().span_if_local(closure_def_id).unwrap(),
                            );
                        let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
                        let mut err = struct_span_err!(
                            self.tcx.sess,
                            closure_span,
                            E0525,
                            "expected a closure that implements the `{}` trait, \
                             but this closure only implements `{}`",
                            kind,
                            found_kind
                        );

                        err.span_label(
                            closure_span,
                            format!("this closure implements `{}`, not `{}`", found_kind, kind),
                        );
                        err.span_label(
                            obligation.cause.span,
                            format!("the requirement to implement `{}` derives from here", kind),
                        );

                        // Additional context information explaining why the closure only implements
                        // a particular trait.
                        if let Some(tables) = self.in_progress_tables {
                            let tables = tables.borrow();
                            match (found_kind, tables.closure_kind_origins().get(hir_id)) {
                                (ty::ClosureKind::FnOnce, Some((span, name))) => {
                                    err.span_label(
                                        *span,
                                        format!(
                                            "closure is `FnOnce` because it moves the \
                                         variable `{}` out of its environment",
                                            name
                                        ),
                                    );
                                }
                                (ty::ClosureKind::FnMut, Some((span, name))) => {
                                    err.span_label(
                                        *span,
                                        format!(
                                            "closure is `FnMut` because it mutates the \
                                         variable `{}` here",
                                            name
                                        ),
                                    );
                                }
                                _ => {}
                            }
                        }

                        err.emit();
                        return;
                    }

                    ty::Predicate::WellFormed(ty) => {
                        // WF predicates cannot themselves make
                        // errors. They can only block due to
                        // ambiguity; otherwise, they always
                        // degenerate into other obligations
                        // (which may fail).
                        span_bug!(span, "WF predicate not satisfied for {:?}", ty);
                    }

                    ty::Predicate::ConstEvaluatable(..) => {
                        // Errors for `ConstEvaluatable` predicates show up as
                        // `SelectionError::ConstEvalFailure`,
                        // not `Unimplemented`.
                        span_bug!(
                            span,
                            "const-evaluatable requirement gave wrong error: `{:?}`",
                            obligation
                        )
                    }
                }
            }

            OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
                let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
                let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);

                if expected_trait_ref.self_ty().references_error() {
                    return;
                }

                let found_trait_ty = found_trait_ref.self_ty();

                let found_did = match found_trait_ty.kind {
                    ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
                    ty::Adt(def, _) => Some(def.did),
                    _ => None,
                };

                let found_span = found_did
                    .and_then(|did| self.tcx.hir().span_if_local(did))
                    .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def

                if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
                    // We check closures twice, with obligations flowing in different directions,
                    // but we want to complain about them only once.
                    return;
                }

                self.reported_closure_mismatch.borrow_mut().insert((span, found_span));

                let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
                    ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
                    _ => vec![ArgKind::empty()],
                };

                let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
                let expected = match expected_ty.kind {
                    ty::Tuple(ref tys) => tys
                        .iter()
                        .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
                        .collect(),
                    _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
                };

                if found.len() == expected.len() {
                    self.report_closure_arg_mismatch(
                        span,
                        found_span,
                        found_trait_ref,
                        expected_trait_ref,
                    )
                } else {
                    let (closure_span, found) = found_did
                        .and_then(|did| {
                            let node = self.tcx.hir().get_if_local(did)?;
                            let (found_span, found) = self.get_fn_like_arguments(node)?;
                            Some((Some(found_span), found))
                        })
                        .unwrap_or((found_span, found));

                    self.report_arg_count_mismatch(
                        span,
                        closure_span,
                        expected,
                        found,
                        found_trait_ty.is_closure(),
                    )
                }
            }

            TraitNotObjectSafe(did) => {
                let violations = self.tcx.object_safety_violations(did);
                report_object_safety_error(self.tcx, span, did, violations)
            }

            ConstEvalFailure(ErrorHandled::TooGeneric) => {
                // In this instance, we have a const expression containing an unevaluated
                // generic parameter. We have no idea whether this expression is valid or
                // not (e.g. it might result in an error), but we don't want to just assume
                // that it's okay, because that might result in post-monomorphisation time
                // errors. The onus is really on the caller to provide values that it can
                // prove are well-formed.
                let mut err = self
                    .tcx
                    .sess
                    .struct_span_err(span, "constant expression depends on a generic parameter");
                // FIXME(const_generics): we should suggest to the user how they can resolve this
                // issue. However, this is currently not actually possible
                // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
                err.note("this may fail depending on what value the parameter takes");
                err
            }

            // Already reported in the query.
            ConstEvalFailure(ErrorHandled::Reported) => {
                self.tcx.sess.delay_span_bug(span, "constant in type had an ignored error");
                return;
            }

            Overflow => {
                bug!("overflow should be handled before the `report_selection_error` path");
            }
        };

        self.note_obligation_cause(&mut err, obligation);
        self.point_at_returns_when_relevant(&mut err, &obligation);

        err.emit();
    }

    /// Given some node representing a fn-like thing in the HIR map,
    /// returns a span and `ArgKind` information that describes the
    /// arguments it expects. This can be supplied to
    /// `report_arg_count_mismatch`.
    fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
        let sm = self.tcx.sess.source_map();
        let hir = self.tcx.hir();
        Some(match node {
            Node::Expr(&hir::Expr {
                kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
                ..
            }) => (
                sm.guess_head_span(span),
                hir.body(id)
                    .params
                    .iter()
                    .map(|arg| {
                        if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
                            *arg.pat
                        {
                            Some(ArgKind::Tuple(
                                Some(span),
                                args.iter()
                                    .map(|pat| {
                                        sm.span_to_snippet(pat.span)
                                            .ok()
                                            .map(|snippet| (snippet, "_".to_owned()))
                                    })
                                    .collect::<Option<Vec<_>>>()?,
                            ))
                        } else {
                            let name = sm.span_to_snippet(arg.pat.span).ok()?;
                            Some(ArgKind::Arg(name, "_".to_owned()))
                        }
                    })
                    .collect::<Option<Vec<ArgKind>>>()?,
            ),
            Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
            | Node::ImplItem(&hir::ImplItem {
                span,
                kind: hir::ImplItemKind::Fn(ref sig, _),
                ..
            })
            | Node::TraitItem(&hir::TraitItem {
                span,
                kind: hir::TraitItemKind::Fn(ref sig, _),
                ..
            }) => (
                sm.guess_head_span(span),
                sig.decl
                    .inputs
                    .iter()
                    .map(|arg| match arg.clone().kind {
                        hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
                            Some(arg.span),
                            vec![("_".to_owned(), "_".to_owned()); tys.len()],
                        ),
                        _ => ArgKind::empty(),
                    })
                    .collect::<Vec<ArgKind>>(),
            ),
            Node::Ctor(ref variant_data) => {
                let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
                let span = sm.guess_head_span(span);
                (span, vec![ArgKind::empty(); variant_data.fields().len()])
            }
            _ => panic!("non-FnLike node found: {:?}", node),
        })
    }

    /// Reports an error when the number of arguments needed by a
    /// trait match doesn't match the number that the expression
    /// provides.
    fn report_arg_count_mismatch(
        &self,
        span: Span,
        found_span: Option<Span>,
        expected_args: Vec<ArgKind>,
        found_args: Vec<ArgKind>,
        is_closure: bool,
    ) -> DiagnosticBuilder<'tcx> {
        let kind = if is_closure { "closure" } else { "function" };

        let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
            let arg_length = arguments.len();
            let distinct = match &other[..] {
                &[ArgKind::Tuple(..)] => true,
                _ => false,
            };
            match (arg_length, arguments.get(0)) {
                (1, Some(&ArgKind::Tuple(_, ref fields))) => {
                    format!("a single {}-tuple as argument", fields.len())
                }
                _ => format!(
                    "{} {}argument{}",
                    arg_length,
                    if distinct && arg_length > 1 { "distinct " } else { "" },
                    pluralize!(arg_length)
                ),
            }
        };

        let expected_str = args_str(&expected_args, &found_args);
        let found_str = args_str(&found_args, &expected_args);

        let mut err = struct_span_err!(
            self.tcx.sess,
            span,
            E0593,
            "{} is expected to take {}, but it takes {}",
            kind,
            expected_str,
            found_str,
        );

        err.span_label(span, format!("expected {} that takes {}", kind, expected_str));

        if let Some(found_span) = found_span {
            err.span_label(found_span, format!("takes {}", found_str));

            // move |_| { ... }
            // ^^^^^^^^-- def_span
            //
            // move |_| { ... }
            // ^^^^^-- prefix
            let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
            // move |_| { ... }
            //      ^^^-- pipe_span
            let pipe_span =
                if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };

            // Suggest to take and ignore the arguments with expected_args_length `_`s if
            // found arguments is empty (assume the user just wants to ignore args in this case).
            // For example, if `expected_args_length` is 2, suggest `|_, _|`.
            if found_args.is_empty() && is_closure {
                let underscores = vec!["_"; expected_args.len()].join(", ");
                err.span_suggestion_verbose(
                    pipe_span,
                    &format!(
                        "consider changing the closure to take and ignore the expected argument{}",
                        pluralize!(expected_args.len())
                    ),
                    format!("|{}|", underscores),
                    Applicability::MachineApplicable,
                );
            }

            if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
                if fields.len() == expected_args.len() {
                    let sugg = fields
                        .iter()
                        .map(|(name, _)| name.to_owned())
                        .collect::<Vec<String>>()
                        .join(", ");
                    err.span_suggestion_verbose(
                        found_span,
                        "change the closure to take multiple arguments instead of a single tuple",
                        format!("|{}|", sugg),
                        Applicability::MachineApplicable,
                    );
                }
            }
            if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
                if fields.len() == found_args.len() && is_closure {
                    let sugg = format!(
                        "|({}){}|",
                        found_args
                            .iter()
                            .map(|arg| match arg {
                                ArgKind::Arg(name, _) => name.to_owned(),
                                _ => "_".to_owned(),
                            })
                            .collect::<Vec<String>>()
                            .join(", "),
                        // add type annotations if available
                        if found_args.iter().any(|arg| match arg {
                            ArgKind::Arg(_, ty) => ty != "_",
                            _ => false,
                        }) {
                            format!(
                                ": ({})",
                                fields
                                    .iter()
                                    .map(|(_, ty)| ty.to_owned())
                                    .collect::<Vec<String>>()
                                    .join(", ")
                            )
                        } else {
                            String::new()
                        },
                    );
                    err.span_suggestion_verbose(
                        found_span,
                        "change the closure to accept a tuple instead of individual arguments",
                        sugg,
                        Applicability::MachineApplicable,
                    );
                }
            }
        }

        err
    }
}

trait InferCtxtPrivExt<'tcx> {
    // returns if `cond` not occurring implies that `error` does not occur - i.e., that
    // `error` occurring implies that `cond` occurs.
    fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool;

    fn report_fulfillment_error(
        &self,
        error: &FulfillmentError<'tcx>,
        body_id: Option<hir::BodyId>,
        fallback_has_occurred: bool,
    );

    fn report_projection_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        error: &MismatchedProjectionTypes<'tcx>,
    );

    fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;

    fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;

    fn find_similar_impl_candidates(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
    ) -> Vec<ty::TraitRef<'tcx>>;

    fn report_similar_impl_candidates(
        &self,
        impl_candidates: Vec<ty::TraitRef<'tcx>>,
        err: &mut DiagnosticBuilder<'_>,
    );

    /// Gets the parent trait chain start
    fn get_parent_trait_ref(
        &self,
        code: &ObligationCauseCode<'tcx>,
    ) -> Option<(String, Option<Span>)>;

    /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
    /// with the same path as `trait_ref`, a help message about
    /// a probable version mismatch is added to `err`
    fn note_version_mismatch(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        trait_ref: &ty::PolyTraitRef<'tcx>,
    );

    fn mk_obligation_for_def_id(
        &self,
        def_id: DefId,
        output_ty: Ty<'tcx>,
        cause: ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> PredicateObligation<'tcx>;

    fn maybe_report_ambiguity(
        &self,
        obligation: &PredicateObligation<'tcx>,
        body_id: Option<hir::BodyId>,
    );

    fn predicate_can_apply(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        pred: ty::PolyTraitRef<'tcx>,
    ) -> bool;

    fn note_obligation_cause(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        obligation: &PredicateObligation<'tcx>,
    );

    fn suggest_unsized_bound_if_applicable(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        obligation: &PredicateObligation<'tcx>,
    );

    fn is_recursive_obligation(
        &self,
        obligated_types: &mut Vec<&ty::TyS<'tcx>>,
        cause_code: &ObligationCauseCode<'tcx>,
    ) -> bool;
}

impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
    // returns if `cond` not occurring implies that `error` does not occur - i.e., that
    // `error` occurring implies that `cond` occurs.
    fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool {
        if cond == error {
            return true;
        }

        let (cond, error) = match (cond, error) {
            (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error, _)) => (cond, error),
            _ => {
                // FIXME: make this work in other cases too.
                return false;
            }
        };

        for implication in super::elaborate_predicates(self.tcx, vec![*cond]) {
            if let ty::Predicate::Trait(implication, _) = implication {
                let error = error.to_poly_trait_ref();
                let implication = implication.to_poly_trait_ref();
                // FIXME: I'm just not taking associated types at all here.
                // Eventually I'll need to implement param-env-aware
                // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
                let param_env = ty::ParamEnv::empty();
                if self.can_sub(param_env, error, implication).is_ok() {
                    debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
                    return true;
                }
            }
        }

        false
    }

    fn report_fulfillment_error(
        &self,
        error: &FulfillmentError<'tcx>,
        body_id: Option<hir::BodyId>,
        fallback_has_occurred: bool,
    ) {
        debug!("report_fulfillment_error({:?})", error);
        match error.code {
            FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
                self.report_selection_error(
                    &error.obligation,
                    selection_error,
                    fallback_has_occurred,
                    error.points_at_arg_span,
                );
            }
            FulfillmentErrorCode::CodeProjectionError(ref e) => {
                self.report_projection_error(&error.obligation, e);
            }
            FulfillmentErrorCode::CodeAmbiguity => {
                self.maybe_report_ambiguity(&error.obligation, body_id);
            }
            FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
                self.report_mismatched_types(
                    &error.obligation.cause,
                    expected_found.expected,
                    expected_found.found,
                    err.clone(),
                )
                .emit();
            }
        }
    }

    fn report_projection_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        error: &MismatchedProjectionTypes<'tcx>,
    ) {
        let predicate = self.resolve_vars_if_possible(&obligation.predicate);

        if predicate.references_error() {
            return;
        }

        self.probe(|_| {
            let err_buf;
            let mut err = &error.err;
            let mut values = None;

            // try to find the mismatched types to report the error with.
            //
            // this can fail if the problem was higher-ranked, in which
            // cause I have no idea for a good error message.
            if let ty::Predicate::Projection(ref data) = predicate {
                let mut selcx = SelectionContext::new(self);
                let (data, _) = self.replace_bound_vars_with_fresh_vars(
                    obligation.cause.span,
                    infer::LateBoundRegionConversionTime::HigherRankedType,
                    data,
                );
                let mut obligations = vec![];
                let normalized_ty = super::normalize_projection_type(
                    &mut selcx,
                    obligation.param_env,
                    data.projection_ty,
                    obligation.cause.clone(),
                    0,
                    &mut obligations,
                );

                debug!(
                    "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
                    obligation.cause, obligation.param_env
                );

                debug!(
                    "report_projection_error normalized_ty={:?} data.ty={:?}",
                    normalized_ty, data.ty
                );

                let is_normalized_ty_expected = match &obligation.cause.code {
                    ObligationCauseCode::ItemObligation(_)
                    | ObligationCauseCode::BindingObligation(_, _)
                    | ObligationCauseCode::ObjectCastObligation(_) => false,
                    _ => true,
                };

                if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
                    is_normalized_ty_expected,
                    normalized_ty,
                    data.ty,
                ) {
                    values = Some(infer::ValuePairs::Types(ExpectedFound::new(
                        is_normalized_ty_expected,
                        normalized_ty,
                        data.ty,
                    )));

                    err_buf = error;
                    err = &err_buf;
                }
            }

            let msg = format!("type mismatch resolving `{}`", predicate);
            let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
            let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
            if fresh {
                let mut diag = struct_span_err!(
                    self.tcx.sess,
                    obligation.cause.span,
                    E0271,
                    "type mismatch resolving `{}`",
                    predicate
                );
                self.note_type_err(&mut diag, &obligation.cause, None, values, err);
                self.note_obligation_cause(&mut diag, obligation);
                diag.emit();
            }
        });
    }

    fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
        /// returns the fuzzy category of a given type, or None
        /// if the type can be equated to any type.
        fn type_category(t: Ty<'_>) -> Option<u32> {
            match t.kind {
                ty::Bool => Some(0),
                ty::Char => Some(1),
                ty::Str => Some(2),
                ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
                ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
                ty::Ref(..) | ty::RawPtr(..) => Some(5),
                ty::Array(..) | ty::Slice(..) => Some(6),
                ty::FnDef(..) | ty::FnPtr(..) => Some(7),
                ty::Dynamic(..) => Some(8),
                ty::Closure(..) => Some(9),
                ty::Tuple(..) => Some(10),
                ty::Projection(..) => Some(11),
                ty::Param(..) => Some(12),
                ty::Opaque(..) => Some(13),
                ty::Never => Some(14),
                ty::Adt(adt, ..) => match adt.adt_kind() {
                    AdtKind::Struct => Some(15),
                    AdtKind::Union => Some(16),
                    AdtKind::Enum => Some(17),
                },
                ty::Generator(..) => Some(18),
                ty::Foreign(..) => Some(19),
                ty::GeneratorWitness(..) => Some(20),
                ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
                ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
            }
        }

        match (type_category(a), type_category(b)) {
            (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
                (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
                _ => cat_a == cat_b,
            },
            // infer and error can be equated to all types
            _ => true,
        }
    }

    fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
        self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
            hir::GeneratorKind::Gen => "a generator",
            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
        })
    }

    fn find_similar_impl_candidates(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
    ) -> Vec<ty::TraitRef<'tcx>> {
        let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
        let all_impls = self.tcx.all_impls(trait_ref.def_id());

        match simp {
            Some(simp) => all_impls
                .iter()
                .filter_map(|&def_id| {
                    let imp = self.tcx.impl_trait_ref(def_id).unwrap();
                    let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
                    if let Some(imp_simp) = imp_simp {
                        if simp != imp_simp {
                            return None;
                        }
                    }

                    Some(imp)
                })
                .collect(),
            None => {
                all_impls.iter().map(|&def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect()
            }
        }
    }

    fn report_similar_impl_candidates(
        &self,
        impl_candidates: Vec<ty::TraitRef<'tcx>>,
        err: &mut DiagnosticBuilder<'_>,
    ) {
        if impl_candidates.is_empty() {
            return;
        }

        let len = impl_candidates.len();
        let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };

        let normalize = |candidate| {
            self.tcx.infer_ctxt().enter(|ref infcx| {
                let normalized = infcx
                    .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
                    .normalize(candidate)
                    .ok();
                match normalized {
                    Some(normalized) => format!("\n  {:?}", normalized.value),
                    None => format!("\n  {:?}", candidate),
                }
            })
        };

        // Sort impl candidates so that ordering is consistent for UI tests.
        let mut normalized_impl_candidates =
            impl_candidates.iter().map(normalize).collect::<Vec<String>>();

        // Sort before taking the `..end` range,
        // because the ordering of `impl_candidates` may not be deterministic:
        // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
        normalized_impl_candidates.sort();

        err.help(&format!(
            "the following implementations were found:{}{}",
            normalized_impl_candidates[..end].join(""),
            if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
        ));
    }

    /// Gets the parent trait chain start
    fn get_parent_trait_ref(
        &self,
        code: &ObligationCauseCode<'tcx>,
    ) -> Option<(String, Option<Span>)> {
        match code {
            &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
                let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
                match self.get_parent_trait_ref(&data.parent_code) {
                    Some(t) => Some(t),
                    None => {
                        let ty = parent_trait_ref.skip_binder().self_ty();
                        let span =
                            TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
                        Some((ty.to_string(), span))
                    }
                }
            }
            _ => None,
        }
    }

    /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
    /// with the same path as `trait_ref`, a help message about
    /// a probable version mismatch is added to `err`
    fn note_version_mismatch(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        trait_ref: &ty::PolyTraitRef<'tcx>,
    ) {
        let get_trait_impl = |trait_def_id| {
            let mut trait_impl = None;
            self.tcx.for_each_relevant_impl(trait_def_id, trait_ref.self_ty(), |impl_def_id| {
                if trait_impl.is_none() {
                    trait_impl = Some(impl_def_id);
                }
            });
            trait_impl
        };
        let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
        let all_traits = self.tcx.all_traits(LOCAL_CRATE);
        let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
            .iter()
            .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
            .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
            .collect();
        for trait_with_same_path in traits_with_same_path {
            if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
                let impl_span = self.tcx.def_span(impl_def_id);
                err.span_help(impl_span, "trait impl with same name found");
                let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
                let crate_msg = format!(
                    "perhaps two different versions of crate `{}` are being used?",
                    trait_crate
                );
                err.note(&crate_msg);
            }
        }
    }

    fn mk_obligation_for_def_id(
        &self,
        def_id: DefId,
        output_ty: Ty<'tcx>,
        cause: ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> PredicateObligation<'tcx> {
        let new_trait_ref =
            ty::TraitRef { def_id, substs: self.tcx.mk_substs_trait(output_ty, &[]) };
        Obligation::new(cause, param_env, new_trait_ref.without_const().to_predicate())
    }

    fn maybe_report_ambiguity(
        &self,
        obligation: &PredicateObligation<'tcx>,
        body_id: Option<hir::BodyId>,
    ) {
        // Unable to successfully determine, probably means
        // insufficient type information, but could mean
        // ambiguous impls. The latter *ought* to be a
        // coherence violation, so we don't report it here.

        let predicate = self.resolve_vars_if_possible(&obligation.predicate);
        let span = obligation.cause.span;

        debug!(
            "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
            predicate, obligation, body_id, obligation.cause.code,
        );

        // Ambiguity errors are often caused as fallout from earlier
        // errors. So just ignore them if this infcx is tainted.
        if self.is_tainted_by_errors() {
            return;
        }

        let mut err = match predicate {
            ty::Predicate::Trait(ref data, _) => {
                let trait_ref = data.to_poly_trait_ref();
                let self_ty = trait_ref.self_ty();
                debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);

                if predicate.references_error() {
                    return;
                }
                // Typically, this ambiguity should only happen if
                // there are unresolved type inference variables
                // (otherwise it would suggest a coherence
                // failure). But given #21974 that is not necessarily
                // the case -- we can have multiple where clauses that
                // are only distinguished by a region, which results
                // in an ambiguity even when all types are fully
                // known, since we don't dispatch based on region
                // relationships.

                // This is kind of a hack: it frequently happens that some earlier
                // error prevents types from being fully inferred, and then we get
                // a bunch of uninteresting errors saying something like "<generic
                // #0> doesn't implement Sized".  It may even be true that we
                // could just skip over all checks where the self-ty is an
                // inference variable, but I was afraid that there might be an
                // inference variable created, registered as an obligation, and
                // then never forced by writeback, and hence by skipping here we'd
                // be ignoring the fact that we don't KNOW the type works
                // out. Though even that would probably be harmless, given that
                // we're only talking about builtin traits, which are known to be
                // inhabited. We used to check for `self.tcx.sess.has_errors()` to
                // avoid inundating the user with unnecessary errors, but we now
                // check upstream for type errors and don't add the obligations to
                // begin with in those cases.
                if self
                    .tcx
                    .lang_items()
                    .sized_trait()
                    .map_or(false, |sized_id| sized_id == trait_ref.def_id())
                {
                    self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
                    return;
                }
                let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
                err.note(&format!("cannot resolve `{}`", predicate));
                if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
                    self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
                } else if let (
                    Ok(ref snippet),
                    ObligationCauseCode::BindingObligation(ref def_id, _),
                ) =
                    (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
                {
                    let generics = self.tcx.generics_of(*def_id);
                    if !generics.params.is_empty() && !snippet.ends_with('>') {
                        // FIXME: To avoid spurious suggestions in functions where type arguments
                        // where already supplied, we check the snippet to make sure it doesn't
                        // end with a turbofish. Ideally we would have access to a `PathSegment`
                        // instead. Otherwise we would produce the following output:
                        //
                        // error[E0283]: type annotations needed
                        //   --> $DIR/issue-54954.rs:3:24
                        //    |
                        // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
                        //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
                        //    |                        |
                        //    |                        cannot infer type
                        //    |                        help: consider specifying the type argument
                        //    |                        in the function call:
                        //    |                        `Tt::const_val::<[i8; 123]>::<T>`
                        // ...
                        // LL |     const fn const_val<T: Sized>() -> usize {
                        //    |              --------- - required by this bound in `Tt::const_val`
                        //    |
                        //    = note: cannot resolve `_: Tt`

                        err.span_suggestion_verbose(
                            span.shrink_to_hi(),
                            &format!(
                                "consider specifying the type argument{} in the function call",
                                pluralize!(generics.params.len()),
                            ),
                            format!(
                                "::<{}>",
                                generics
                                    .params
                                    .iter()
                                    .map(|p| p.name.to_string())
                                    .collect::<Vec<String>>()
                                    .join(", ")
                            ),
                            Applicability::HasPlaceholders,
                        );
                    }
                }
                err
            }

            ty::Predicate::WellFormed(ty) => {
                // Same hacky approach as above to avoid deluging user
                // with error messages.
                if ty.references_error() || self.tcx.sess.has_errors() {
                    return;
                }
                self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
            }

            ty::Predicate::Subtype(ref data) => {
                if data.references_error() || self.tcx.sess.has_errors() {
                    // no need to overload user in such cases
                    return;
                }
                let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
                // both must be type variables, or the other would've been instantiated
                assert!(a.is_ty_var() && b.is_ty_var());
                self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
            }
            ty::Predicate::Projection(ref data) => {
                let trait_ref = data.to_poly_trait_ref(self.tcx);
                let self_ty = trait_ref.self_ty();
                if predicate.references_error() {
                    return;
                }
                let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
                err.note(&format!("cannot resolve `{}`", predicate));
                err
            }

            _ => {
                if self.tcx.sess.has_errors() {
                    return;
                }
                let mut err = struct_span_err!(
                    self.tcx.sess,
                    span,
                    E0284,
                    "type annotations needed: cannot resolve `{}`",
                    predicate,
                );
                err.span_label(span, &format!("cannot resolve `{}`", predicate));
                err
            }
        };
        self.note_obligation_cause(&mut err, obligation);
        err.emit();
    }

    /// Returns `true` if the trait predicate may apply for *some* assignment
    /// to the type parameters.
    fn predicate_can_apply(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        pred: ty::PolyTraitRef<'tcx>,
    ) -> bool {
        struct ParamToVarFolder<'a, 'tcx> {
            infcx: &'a InferCtxt<'a, 'tcx>,
            var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
        }

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

            fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
                if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
                    let infcx = self.infcx;
                    self.var_map.entry(ty).or_insert_with(|| {
                        infcx.next_ty_var(TypeVariableOrigin {
                            kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
                            span: DUMMY_SP,
                        })
                    })
                } else {
                    ty.super_fold_with(self)
                }
            }
        }

        self.probe(|_| {
            let mut selcx = SelectionContext::new(self);

            let cleaned_pred =
                pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });

            let cleaned_pred = super::project::normalize(
                &mut selcx,
                param_env,
                ObligationCause::dummy(),
                &cleaned_pred,
            )
            .value;

            let obligation = Obligation::new(
                ObligationCause::dummy(),
                param_env,
                cleaned_pred.without_const().to_predicate(),
            );

            self.predicate_may_hold(&obligation)
        })
    }

    fn note_obligation_cause(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        obligation: &PredicateObligation<'tcx>,
    ) {
        // First, attempt to add note to this error with an async-await-specific
        // message, and fall back to regular note otherwise.
        if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
            self.note_obligation_cause_code(
                err,
                &obligation.predicate,
                &obligation.cause.code,
                &mut vec![],
            );
            self.suggest_unsized_bound_if_applicable(err, obligation);
        }
    }

    fn suggest_unsized_bound_if_applicable(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        obligation: &PredicateObligation<'tcx>,
    ) {
        if let (
            ty::Predicate::Trait(pred, _),
            ObligationCauseCode::BindingObligation(item_def_id, span),
        ) = (&obligation.predicate, &obligation.cause.code)
        {
            if let (Some(generics), true) = (
                self.tcx.hir().get_if_local(*item_def_id).as_ref().and_then(|n| n.generics()),
                Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
            ) {
                for param in generics.params {
                    if param.span == *span
                        && !param.bounds.iter().any(|bound| {
                            bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
                                == self.tcx.lang_items().sized_trait()
                        })
                    {
                        let (span, separator) = match param.bounds {
                            [] => (span.shrink_to_hi(), ":"),
                            [.., bound] => (bound.span().shrink_to_hi(), " + "),
                        };
                        err.span_suggestion_verbose(
                            span,
                            "consider relaxing the implicit `Sized` restriction",
                            format!("{} ?Sized", separator),
                            Applicability::MachineApplicable,
                        );
                        return;
                    }
                }
            }
        }
    }

    fn is_recursive_obligation(
        &self,
        obligated_types: &mut Vec<&ty::TyS<'tcx>>,
        cause_code: &ObligationCauseCode<'tcx>,
    ) -> bool {
        if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
            let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);

            if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
                return true;
            }
        }
        false
    }
}

pub fn recursive_type_with_infinite_size_error(
    tcx: TyCtxt<'tcx>,
    type_def_id: DefId,
) -> DiagnosticBuilder<'tcx> {
    assert!(type_def_id.is_local());
    let span = tcx.hir().span_if_local(type_def_id).unwrap();
    let span = tcx.sess.source_map().guess_head_span(span);
    let mut err = struct_span_err!(
        tcx.sess,
        span,
        E0072,
        "recursive type `{}` has infinite size",
        tcx.def_path_str(type_def_id)
    );
    err.span_label(span, "recursive type has infinite size");
    err.help(&format!(
        "insert indirection (e.g., a `Box`, `Rc`, or `&`) \
                           at some point to make `{}` representable",
        tcx.def_path_str(type_def_id)
    ));
    err
}

/// Summarizes information
#[derive(Clone)]
pub enum ArgKind {
    /// An argument of non-tuple type. Parameters are (name, ty)
    Arg(String, String),

    /// An argument of tuple type. For a "found" argument, the span is
    /// the locationo in the source of the pattern. For a "expected"
    /// argument, it will be None. The vector is a list of (name, ty)
    /// strings for the components of the tuple.
    Tuple(Option<Span>, Vec<(String, String)>),
}

impl ArgKind {
    fn empty() -> ArgKind {
        ArgKind::Arg("_".to_owned(), "_".to_owned())
    }

    /// Creates an `ArgKind` from the expected type of an
    /// argument. It has no name (`_`) and an optional source span.
    pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
        match t.kind {
            ty::Tuple(ref tys) => ArgKind::Tuple(
                span,
                tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
            ),
            _ => ArgKind::Arg("_".to_owned(), t.to_string()),
        }
    }
}

/// Suggest restricting a type param with a new bound.
pub fn suggest_constraining_type_param(
    tcx: TyCtxt<'_>,
    generics: &hir::Generics<'_>,
    err: &mut DiagnosticBuilder<'_>,
    param_name: &str,
    constraint: &str,
    def_id: Option<DefId>,
) -> bool {
    let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);

    let param = if let Some(param) = param {
        param
    } else {
        return false;
    };

    const MSG_RESTRICT_BOUND_FURTHER: &str = "consider further restricting this bound";
    let msg_restrict_type = format!("consider restricting type parameter `{}`", param_name);
    let msg_restrict_type_further =
        format!("consider further restricting type parameter `{}`", param_name);

    if def_id == tcx.lang_items().sized_trait() {
        // Type parameters are already `Sized` by default.
        err.span_label(param.span, &format!("this type parameter needs to be `{}`", constraint));
        return true;
    }
    let mut suggest_restrict = |span| {
        err.span_suggestion_verbose(
            span,
            MSG_RESTRICT_BOUND_FURTHER,
            format!(" + {}", constraint),
            Applicability::MachineApplicable,
        );
    };

    if param_name.starts_with("impl ") {
        // If there's an `impl Trait` used in argument position, suggest
        // restricting it:
        //
        //   fn foo(t: impl Foo) { ... }
        //             --------
        //             |
        //             help: consider further restricting this bound with `+ Bar`
        //
        // Suggestion for tools in this case is:
        //
        //   fn foo(t: impl Foo) { ... }
        //             --------
        //             |
        //             replace with: `impl Foo + Bar`

        suggest_restrict(param.span.shrink_to_hi());
        return true;
    }

    if generics.where_clause.predicates.is_empty()
        // Given `trait Base<T = String>: Super<T>` where `T: Copy`, suggest restricting in the
        // `where` clause instead of `trait Base<T: Copy = String>: Super<T>`.
        && !matches!(param.kind, hir::GenericParamKind::Type { default: Some(_), .. })
    {
        if let Some(bounds_span) = param.bounds_span() {
            // If user has provided some bounds, suggest restricting them:
            //
            //   fn foo<T: Foo>(t: T) { ... }
            //             ---
            //             |
            //             help: consider further restricting this bound with `+ Bar`
            //
            // Suggestion for tools in this case is:
            //
            //   fn foo<T: Foo>(t: T) { ... }
            //          --
            //          |
            //          replace with: `T: Bar +`
            suggest_restrict(bounds_span.shrink_to_hi());
        } else {
            // If user hasn't provided any bounds, suggest adding a new one:
            //
            //   fn foo<T>(t: T) { ... }
            //          - help: consider restricting this type parameter with `T: Foo`
            err.span_suggestion_verbose(
                param.span.shrink_to_hi(),
                &msg_restrict_type,
                format!(": {}", constraint),
                Applicability::MachineApplicable,
            );
        }

        true
    } else {
        // This part is a bit tricky, because using the `where` clause user can
        // provide zero, one or many bounds for the same type parameter, so we
        // have following cases to consider:
        //
        // 1) When the type parameter has been provided zero bounds
        //
        //    Message:
        //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
        //             - help: consider restricting this type parameter with `where X: Bar`
        //
        //    Suggestion:
        //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
        //                                           - insert: `, X: Bar`
        //
        //
        // 2) When the type parameter has been provided one bound
        //
        //    Message:
        //      fn foo<T>(t: T) where T: Foo { ... }
        //                            ^^^^^^
        //                            |
        //                            help: consider further restricting this bound with `+ Bar`
        //
        //    Suggestion:
        //      fn foo<T>(t: T) where T: Foo { ... }
        //                            ^^
        //                            |
        //                            replace with: `T: Bar +`
        //
        //
        // 3) When the type parameter has been provided many bounds
        //
        //    Message:
        //      fn foo<T>(t: T) where T: Foo, T: Bar {... }
        //             - help: consider further restricting this type parameter with `where T: Zar`
        //
        //    Suggestion:
        //      fn foo<T>(t: T) where T: Foo, T: Bar {... }
        //                                          - insert: `, T: Zar`

        let mut param_spans = Vec::new();

        for predicate in generics.where_clause.predicates {
            if let WherePredicate::BoundPredicate(WhereBoundPredicate {
                span, bounded_ty, ..
            }) = predicate
            {
                if let TyKind::Path(QPath::Resolved(_, path)) = &bounded_ty.kind {
                    if let Some(segment) = path.segments.first() {
                        if segment.ident.to_string() == param_name {
                            param_spans.push(span);
                        }
                    }
                }
            }
        }

        let where_clause_span = generics.where_clause.span_for_predicates_or_empty_place();
        // Account for `fn foo<T>(t: T) where T: Foo,` so we don't suggest two trailing commas.
        let mut trailing_comma = false;
        if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(where_clause_span) {
            trailing_comma = snippet.ends_with(",");
        }
        let where_clause_span = if trailing_comma {
            let hi = where_clause_span.hi();
            Span::new(hi - BytePos(1), hi, where_clause_span.ctxt())
        } else {
            where_clause_span.shrink_to_hi()
        };

        match &param_spans[..] {
            &[&param_span] => suggest_restrict(param_span.shrink_to_hi()),
            _ => {
                err.span_suggestion_verbose(
                    where_clause_span,
                    &msg_restrict_type_further,
                    format!(", {}: {}", param_name, constraint),
                    Applicability::MachineApplicable,
                );
            }
        }

        true
    }
}