rustc_trait_selection/error_reporting/traits/

ambiguity.rs

use std::ops::ControlFlow;

use rustc_errors::{Applicability, Diag, E0283, E0284, E0790, MultiSpan, struct_span_code_err};
use rustc_hir as hir;
use rustc_hir::attrs::lang_items::LangItem;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::def_id::{CRATE_DEF_ID, DefId};
use rustc_hir::intravisit::Visitor as _;
use rustc_infer::infer::{BoundRegionConversionTime, InferCtxt};
use rustc_infer::traits::util::elaborate;
use rustc_infer::traits::{
    Obligation, ObligationCause, ObligationCauseCode, PolyTraitObligation, PredicateObligation,
};
use rustc_middle::ty::print::PrintPolyTraitClauseExt;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable as _, TypeVisitableExt as _, Unnormalized};
use rustc_session::diagnostics::feature_err_unstable_feature_bound;
use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
use tracing::{debug, instrument};

use crate::error_reporting::TypeErrCtxt;
use crate::error_reporting::infer::need_type_info::TypeAnnotationNeeded;
use crate::error_reporting::traits::{FindExprBySpan, to_pretty_impl_header};
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::{FulfillmentError, ObligationCtxt};

#[derive(Debug)]
pub enum CandidateSource {
    DefId(DefId),
    ParamEnv(Span),
}

pub fn compute_applicable_impls_for_diagnostics<'tcx>(
    infcx: &InferCtxt<'tcx>,
    obligation: &PolyTraitObligation<'tcx>,
    ignore_predicates_of_impls: bool,
) -> Vec<CandidateSource> {
    let tcx = infcx.tcx;
    let param_env = obligation.param_env;

    let predicate_polarity = obligation.predicate.skip_binder().polarity;

    let impl_may_apply = |impl_def_id| {
        let ocx = ObligationCtxt::new(infcx);
        infcx.enter_forall(obligation.predicate, |placeholder_obligation| {
            let obligation_trait_ref = ocx.normalize(
                &ObligationCause::dummy(),
                param_env,
                Unnormalized::new_wip(placeholder_obligation.trait_ref),
            );

            let impl_args = infcx.fresh_args_for_item(DUMMY_SP, impl_def_id);
            let impl_trait_ref =
                tcx.impl_trait_ref(impl_def_id).instantiate(tcx, impl_args).skip_norm_wip();
            let impl_trait_ref = ocx.normalize(
                &ObligationCause::dummy(),
                param_env,
                Unnormalized::new_wip(impl_trait_ref),
            );

            if let Err(_) =
                ocx.eq(&ObligationCause::dummy(), param_env, obligation_trait_ref, impl_trait_ref)
            {
                return false;
            }

            let impl_trait_header = tcx.impl_trait_header(impl_def_id);
            let impl_polarity = impl_trait_header.polarity;

            match (impl_polarity, predicate_polarity) {
                (ty::ImplPolarity::Positive, ty::ClausePolarity::Positive)
                | (ty::ImplPolarity::Negative, ty::ClausePolarity::Negative) => {}
                _ => return false,
            }

            if !ignore_predicates_of_impls {
                let obligations = tcx
                    .clauses_of(impl_def_id)
                    .instantiate(tcx, impl_args)
                    .into_iter()
                    .map(|(clause, _)| {
                        Obligation::new(
                            tcx,
                            ObligationCause::dummy(),
                            param_env,
                            clause.skip_norm_wip(),
                        )
                    })
                    // Kinda hacky, but let's just throw away obligations that overflow.
                    // This may reduce the accuracy of this check (if the obligation guides
                    // inference or it actually resulted in error after others are processed)
                    // ... but this is diagnostics code.
                    .filter(|obligation| {
                        infcx.next_trait_solver() || infcx.evaluate_obligation(obligation).is_ok()
                    });
                ocx.register_obligations(obligations);
            }

            ocx.try_evaluate_obligations().no_errors()
        })
    };

    let param_env_candidate_may_apply = |poly_trait_predicate: ty::PolyTraitClause<'tcx>| {
        let ocx = ObligationCtxt::new(infcx);
        infcx.enter_forall(obligation.predicate, |placeholder_obligation| {
            let obligation_trait_ref = ocx.normalize(
                &ObligationCause::dummy(),
                param_env,
                Unnormalized::new_wip(placeholder_obligation.trait_ref),
            );

            let param_env_predicate = infcx.instantiate_binder_with_fresh_vars(
                DUMMY_SP,
                BoundRegionConversionTime::HigherRankedType,
                poly_trait_predicate,
            );
            let param_env_trait_ref = ocx.normalize(
                &ObligationCause::dummy(),
                param_env,
                Unnormalized::new_wip(param_env_predicate.trait_ref),
            );

            if let Err(_) = ocx.eq(
                &ObligationCause::dummy(),
                param_env,
                obligation_trait_ref,
                param_env_trait_ref,
            ) {
                return false;
            }

            ocx.try_evaluate_obligations().no_errors()
        })
    };

    let mut ambiguities = Vec::new();

    tcx.for_each_relevant_impl(
        obligation.predicate.def_id(),
        obligation.predicate.skip_binder().trait_ref.self_ty(),
        |impl_def_id| {
            if infcx.probe(|_| impl_may_apply(impl_def_id)) {
                ambiguities.push(CandidateSource::DefId(impl_def_id))
            }
        },
    );

    // If our `body_def_id` has been set (and isn't just from a dummy obligation cause),
    // then try to look for a param-env clause that would apply. The way we compute
    // this is somewhat manual, since we need the spans, so we elaborate this directly
    // from `clauses_of` rather than actually looking at the param-env which
    // otherwise would be more appropriate.
    let body_def_id = obligation.cause.body_def_id;
    if body_def_id != CRATE_DEF_ID {
        let clauses = tcx.clauses_of(body_def_id.to_def_id()).instantiate_identity(tcx);
        for (clause, span) in
            elaborate(tcx, clauses.into_iter().map(|(c, s)| (c.skip_norm_wip(), s)))
        {
            let kind = clause.kind();
            if let ty::ClauseKind::Trait(trait_pred) = kind.skip_binder()
                && param_env_candidate_may_apply(kind.rebind(trait_pred))
            {
                if kind.rebind(trait_pred.trait_ref)
                    == ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_pred.def_id()))
                {
                    ambiguities.push(CandidateSource::ParamEnv(tcx.def_span(trait_pred.def_id())))
                } else {
                    ambiguities.push(CandidateSource::ParamEnv(span))
                }
            }
        }
    }

    ambiguities
}

impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
    /// The term of an ambiguous obligation's predicate that gets blamed for the
    /// missing type annotation: the first one still containing inference variables.
    ///
    /// Besides `maybe_report_ambiguity` pointing its diagnostics at this term,
    /// `report_fulfillment_errors` merges the ambiguity errors whose blamed terms
    /// share an inference variable into a single diagnostic.
    pub(super) fn ambiguity_term(&self, predicate: ty::Predicate<'tcx>) -> Option<ty::Term<'tcx>> {
        match predicate.kind().skip_binder() {
            ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
                data.trait_ref.args.terms().find(|term| term.has_non_region_infer())
            }
            ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) => data
                .projection_term
                .args
                .terms()
                .chain([data.term])
                .find(|term| term.has_non_region_infer()),
            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(term)) => Some(term),
            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(data)) => {
                data.walk().filter_map(ty::GenericArg::as_term).find(|term| term.is_infer())
            }
            ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, _)) => Some(ct.into()),
            ty::PredicateKind::Subtype(data) => Some(data.a.into()),
            ty::PredicateKind::NormalizesTo(data) if data.term.is_infer() => Some(data.term),
            _ => None,
        }
    }

    #[instrument(skip(self), level = "debug")]
    pub(super) fn maybe_report_ambiguity(
        &self,
        obligation: &PredicateObligation<'tcx>,
        related: &[&FulfillmentError<'tcx>],
    ) -> ErrorGuaranteed {
        // 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;
        let mut long_ty_path = None;

        debug!(?predicate, obligation.cause.code = ?obligation.cause.code());

        // Ambiguity errors are often caused as fallout from earlier errors.
        // We ignore them if this `infcx` is tainted in some cases below.

        let bound_predicate = predicate.kind();
        let mut err = match bound_predicate.skip_binder() {
            ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
                let trait_pred = bound_predicate.rebind(data);
                debug!(?trait_pred);

                if let Err(e) = predicate.error_reported() {
                    return e;
                }

                if let Err(guar) = self.tcx.ensure_result().coherent_trait(trait_pred.def_id()) {
                    // Avoid bogus "type annotations needed `Foo: Bar`" errors on `impl Bar for
                    // Foo` in case other `Foo` impls are incoherent.
                    return guar;
                }

                // 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.tainted_by_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 matches!(
                    self.tcx.as_lang_item(trait_pred.def_id()),
                    Some(LangItem::Sized | LangItem::MetaSized)
                ) {
                    return match self.tainted_by_errors() {
                        None => self
                            .emit_inference_failure_err(
                                obligation.cause.body_def_id,
                                span,
                                trait_pred.self_ty().skip_binder().into(),
                                TypeAnnotationNeeded::E0282,
                                false,
                            )
                            .emit(),
                        Some(e) => e,
                    };
                }

                // 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.

                // Pick the first generic parameter that still contains inference variables as the one
                // we're going to emit an error for. If there are none (see above), fall back to
                // a more general error.
                let term = self.ambiguity_term(predicate);

                let mut err = if let Some(term) = term {
                    let candidates: Vec<_> = self
                        .tcx
                        .all_impls(trait_pred.def_id())
                        .filter_map(|def_id| {
                            let imp = self.tcx.impl_trait_header(def_id);
                            if imp.polarity != ty::ImplPolarity::Positive
                                || !self.tcx.is_user_visible_dep(def_id.krate)
                            {
                                return None;
                            }
                            let imp = imp.trait_ref.skip_binder();
                            if imp
                                .with_replaced_self_ty(self.tcx, trait_pred.skip_binder().self_ty())
                                == trait_pred.skip_binder().trait_ref
                            {
                                Some(imp.self_ty())
                            } else {
                                None
                            }
                        })
                        .collect();
                    self.emit_inference_failure_err_with_type_hint(
                        obligation.cause.body_def_id,
                        span,
                        term,
                        TypeAnnotationNeeded::E0283,
                        true,
                        match &candidates[..] {
                            [candidate] => Some(*candidate),
                            _ => None,
                        },
                    )
                } else {
                    struct_span_code_err!(
                        self.dcx(),
                        span,
                        E0283,
                        "type annotations needed: cannot satisfy `{}`",
                        self.tcx.short_string(predicate, &mut long_ty_path),
                    )
                    .with_long_ty_path(long_ty_path)
                };

                if let Some(ambiguities) = self.applicable_impls_to_mention(obligation, trait_pred)
                {
                    if let Some(e) = self.tainted_by_errors()
                        && term.is_none()
                    {
                        // If `arg.is_none()`, then this is probably two param-env
                        // candidates or impl candidates that are equal modulo lifetimes.
                        // Therefore, if we've already emitted an error, just skip this
                        // one, since it's not particularly actionable.
                        err.cancel();
                        return e;
                    }
                    self.annotate_source_of_ambiguity(&mut err, &ambiguities, predicate);
                } else {
                    if let Some(e) = self.tainted_by_errors() {
                        err.cancel();
                        return e;
                    }
                    if let Some(clause) = predicate.as_trait_clause()
                        && let ty::Infer(_) = clause.self_ty().skip_binder().kind()
                    {
                        let tr = self.tcx.short_string(
                            clause.print_modifiers_and_trait_path(),
                            &mut err.long_ty_path(),
                        );
                        err.note(format!("the type must implement `{tr}`"));
                    } else {
                        let pred = self.tcx.short_string(predicate, &mut err.long_ty_path());
                        err.note(format!("cannot satisfy `{pred}`"));
                    }
                    let impl_candidates =
                        self.find_similar_impl_candidates(predicate.as_trait_clause().unwrap());
                    if impl_candidates.len() < 40 {
                        self.report_similar_impl_candidates(
                            impl_candidates.as_slice(),
                            obligation,
                            trait_pred,
                            obligation.cause.body_def_id,
                            &mut err,
                            false,
                            obligation.param_env,
                        );
                    }
                }

                if let ObligationCauseCode::WhereClause(def_id, _)
                | ObligationCauseCode::WhereClauseInExpr(def_id, ..) = *obligation.cause.code()
                {
                    self.suggest_fully_qualified_path(&mut err, def_id, span, trait_pred.def_id());
                }

                if term.is_some_and(|term| term.as_type().is_some())
                    && let Some(body) =
                        self.tcx.hir_maybe_body_owned_by(obligation.cause.body_def_id)
                {
                    let mut expr_finder = FindExprBySpan::new(span, self.tcx);
                    expr_finder.visit_expr(&body.value);

                    if let Some(hir::Expr {
                        kind:
                            hir::ExprKind::Call(
                                hir::Expr {
                                    kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)),
                                    ..
                                },
                                _,
                            )
                            | hir::ExprKind::Path(hir::QPath::Resolved(None, path)),
                        ..
                    }) = expr_finder.result
                        && let [
                            ..,
                            trait_path_segment @ hir::PathSegment {
                                res: Res::Def(DefKind::Trait, trait_id),
                                ..
                            },
                            hir::PathSegment {
                                ident: assoc_item_ident,
                                res: Res::Def(_, item_id),
                                ..
                            },
                        ] = path.segments
                        && data.trait_ref.def_id == *trait_id
                        && self.tcx.trait_of_assoc(*item_id) == Some(*trait_id)
                        && let None = self.tainted_by_errors()
                    {
                        let assoc_item = self.tcx.associated_item(*item_id);
                        let (verb, noun) = match assoc_item.kind {
                            ty::AssocKind::Const { .. } => ("refer to the", "constant"),
                            ty::AssocKind::Fn { .. } => ("call", "function"),
                            // This is already covered by E0223, but this following single match
                            // arm doesn't hurt here.
                            ty::AssocKind::Type { .. } => ("refer to the", "type"),
                        };

                        // Replace the more general E0283 with a more specific error
                        err.cancel();
                        err = self.dcx().struct_span_err(
                            span,
                            format!(
                                "cannot {verb} associated {noun} on trait without specifying the \
                                 corresponding `impl` type",
                            ),
                        );
                        err.code(E0790);

                        if item_id.is_local() {
                            let trait_ident = self.tcx.item_name(*trait_id);
                            err.span_label(
                                self.tcx.def_span(*item_id),
                                format!("`{trait_ident}::{assoc_item_ident}` defined here"),
                            );
                        }

                        err.span_label(span, format!("cannot {verb} associated {noun} of trait"));

                        let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);

                        if let Some(&impl_def_id) =
                            trait_impls.non_blanket_impls().values().flatten().next()
                        {
                            let non_blanket_impl_count =
                                trait_impls.non_blanket_impls().values().flatten().count();
                            // If there is only one implementation of the trait, suggest using it.
                            // Otherwise, use a placeholder comment for the implementation.
                            let (message, self_types) = if non_blanket_impl_count == 1 {
                                (
                                    "use the fully-qualified path to the only available \
                                     implementation",
                                    vec![format!(
                                        "{}",
                                        self.tcx
                                            .type_of(impl_def_id)
                                            .instantiate_identity()
                                            .skip_norm_wip()
                                    )],
                                )
                            } else if non_blanket_impl_count < 20 {
                                (
                                    "use a fully-qualified path to one of the available \
                                     implementations",
                                    trait_impls
                                        .non_blanket_impls()
                                        .values()
                                        .flatten()
                                        .map(|&id| {
                                            format!(
                                                "{}",
                                                self.tcx
                                                    .type_of(id)
                                                    .instantiate_identity()
                                                    .skip_norm_wip()
                                            )
                                        })
                                        .collect::<Vec<String>>(),
                                )
                            } else {
                                (
                                    "use a fully-qualified path to a specific available \
                                     implementation",
                                    vec!["/* self type */".to_string()],
                                )
                            };
                            let suggestions: Vec<_> = self_types
                                .into_iter()
                                .map(|self_type| {
                                    let mut suggestions = vec![(
                                        path.span.shrink_to_lo(),
                                        format!("<{self_type} as "),
                                    )];
                                    if let Some(generic_arg) = trait_path_segment.args {
                                        let between_span = trait_path_segment
                                            .ident
                                            .span
                                            .between(generic_arg.span_ext);
                                        // get rid of :: between Trait and <type>
                                        // must be '::' between them, otherwise the parser won't accept the code
                                        suggestions.push((between_span, "".to_string()));
                                        suggestions.push((
                                            generic_arg.span_ext.shrink_to_hi(),
                                            ">".to_string(),
                                        ));
                                    } else {
                                        suggestions.push((
                                            trait_path_segment.ident.span.shrink_to_hi(),
                                            ">".to_string(),
                                        ));
                                    }
                                    suggestions
                                })
                                .collect();
                            err.multipart_suggestions(
                                message,
                                suggestions,
                                Applicability::MaybeIncorrect,
                            );
                        }
                    }
                };

                err
            }

            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(term)) => {
                // Same hacky approach as above to avoid deluging user
                // with error messages.

                if let Err(e) = term.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }

                self.emit_inference_failure_err(
                    obligation.cause.body_def_id,
                    span,
                    term,
                    TypeAnnotationNeeded::E0282,
                    false,
                )
            }

            ty::PredicateKind::Subtype(data) => {
                if let Err(e) = data.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }
                let ty::SubtypePredicate { a_is_expected: _, a, b } = data;
                // both must be type variables, or the other would've been instantiated
                assert!(a.is_ty_var() && b.is_ty_var());
                self.emit_inference_failure_err(
                    obligation.cause.body_def_id,
                    span,
                    a.into(),
                    TypeAnnotationNeeded::E0282,
                    true,
                )
            }

            ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) => {
                if let Err(e) = predicate.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }

                if data.projection_term.kind.is_trait_projection()
                    && let Err(guar) =
                        self.tcx.ensure_result().coherent_trait(self.tcx.parent(data.def_id()))
                {
                    // Avoid bogus "type annotations needed `Foo: Bar`" errors on `impl Bar for Foo` in case
                    // other `Foo` impls are incoherent.
                    return guar;
                }
                let term = self.ambiguity_term(predicate);
                let predicate = self.tcx.short_string(predicate, &mut long_ty_path);
                if let Some(term) = term {
                    self.emit_inference_failure_err(
                        obligation.cause.body_def_id,
                        span,
                        term,
                        TypeAnnotationNeeded::E0284,
                        true,
                    )
                    .with_note(format!("cannot satisfy `{predicate}`"))
                    .with_long_ty_path(long_ty_path)
                } else {
                    // If we can't find a generic parameter, just print a generic error
                    struct_span_code_err!(
                        self.dcx(),
                        span,
                        E0284,
                        "type annotations needed: cannot satisfy `{predicate}`",
                    )
                    .with_span_label(span, format!("cannot satisfy `{predicate}`"))
                    .with_long_ty_path(long_ty_path)
                }
            }

            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(_)) => {
                if let Err(e) = predicate.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }
                if let Some(term) = self.ambiguity_term(predicate) {
                    self.emit_inference_failure_err(
                        obligation.cause.body_def_id,
                        span,
                        term,
                        TypeAnnotationNeeded::E0284,
                        true,
                    )
                } else {
                    // If we can't find a generic parameter, just print a generic error
                    let predicate = self.tcx.short_string(predicate, &mut long_ty_path);
                    struct_span_code_err!(
                        self.dcx(),
                        span,
                        E0284,
                        "type annotations needed: cannot satisfy `{predicate}`",
                    )
                    .with_span_label(span, format!("cannot satisfy `{predicate}`"))
                    .with_long_ty_path(long_ty_path)
                }
            }

            ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ..)) => self
                .emit_inference_failure_err(
                    obligation.cause.body_def_id,
                    span,
                    ct.into(),
                    TypeAnnotationNeeded::E0284,
                    true,
                ),

            ty::PredicateKind::NormalizesTo(ty::NormalizesTo { alias, term })
                if term.is_infer() =>
            {
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }
                let alias = self.tcx.short_string(alias, &mut long_ty_path);
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0284,
                    "type annotations needed: cannot normalize `{alias}`",
                )
                .with_span_label(span, format!("cannot normalize `{alias}`"))
                .with_long_ty_path(long_ty_path)
            }

            ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(sym)) => {
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }

                if self.tcx.features().staged_api() {
                    self.dcx().struct_span_err(
                        span,
                        format!("unstable feature `{sym}` is used without being enabled."),
                    ).with_help(format!("The feature can be enabled by marking the current item with `#[unstable_feature_bound({sym})]`"))
                } else {
                    feature_err_unstable_feature_bound(
                        &self.tcx.sess,
                        sym,
                        span,
                        format!("use of unstable library feature `{sym}`"),
                    )
                }
            }

            _ => {
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }
                let predicate = self.tcx.short_string(predicate, &mut long_ty_path);
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0284,
                    "type annotations needed: cannot satisfy `{predicate}`",
                )
                .with_span_label(span, format!("cannot satisfy `{predicate}`"))
                .with_long_ty_path(long_ty_path)
            }
        };

        // The related obligations are ambiguous because of the same inference variable,
        // so they belong to this diagnostic: annotating the variable has to satisfy all
        // of them at once. Mention their requirements, except for bookkeeping predicates
        // (`WellFormed`, sizedness, ...) whose mention wouldn't be actionable.
        let mut mentioned = vec![predicate];
        let mut mentioned_strs: Vec<String> = vec![];
        for &error in related {
            let related_pred = self.resolve_vars_if_possible(error.obligation.predicate);
            if mentioned.contains(&related_pred) {
                continue;
            }
            let note = match related_pred.kind().skip_binder() {
                ty::PredicateKind::Clause(ty::ClauseKind::Trait(data))
                    if !matches!(
                        self.tcx.as_lang_item(data.def_id()),
                        Some(LangItem::Sized | LangItem::MetaSized | LangItem::PointeeSized)
                    ) =>
                {
                    let clause = related_pred.kind().rebind(data);
                    if let ty::Infer(_) = clause.self_ty().skip_binder().kind() {
                        let tr = self.tcx.short_string(
                            clause.print_modifiers_and_trait_path(),
                            &mut err.long_ty_path(),
                        );
                        format!("the type must also implement `{tr}`")
                    } else {
                        let pred = self.tcx.short_string(related_pred, &mut err.long_ty_path());
                        let note = format!("cannot satisfy `{pred}`");
                        // The self type is known, so the `impl`s that could have applied to it are
                        // few and worth pointing at, like the blamed bound does. When it is still
                        // an inference variable the list is every `impl` of the trait, which is
                        // why the branch above only names the trait.
                        //
                        // `tainted_by_errors` is checked because `annotate_source_of_ambiguity`
                        // downgrades the whole diagnostic once an error was already emitted.
                        if !mentioned_strs.contains(&note)
                            && self.tainted_by_errors().is_none()
                            && let Some(ambiguities) =
                                self.applicable_impls_to_mention(&error.obligation, clause)
                        {
                            self.annotate_source_of_ambiguity(&mut err, &ambiguities, related_pred);
                            mentioned_strs.push(note);
                            mentioned.push(related_pred);
                            continue;
                        }
                        note
                    }
                }
                ty::PredicateKind::Clause(ty::ClauseKind::Projection(_)) => {
                    let pred = self.tcx.short_string(related_pred, &mut err.long_ty_path());
                    format!("cannot satisfy `{pred}`")
                }
                _ => {
                    mentioned.push(related_pred);
                    continue;
                }
            };
            // Two predicates can print identically (e.g. `From<?0>` and `From<?1>` both show as
            // `From<_>`); only emit each unique note string once.
            if !mentioned_strs.contains(&note) {
                err.note(note.clone());
                mentioned_strs.push(note);
            }
            mentioned.push(related_pred);
        }

        self.note_obligation_cause(&mut err, obligation);
        // The merged errors are not reported on their own anymore, so the bounds they came from
        // have to be explained here too. Causes shared with the blamed obligation are already
        // described by the call above.
        for &error in related {
            if error.obligation.cause.code() != obligation.cause.code() {
                self.note_obligation_cause(&mut err, &error.obligation);
            }
        }
        err.emit()
    }

    /// The `impl`s and `where` clauses that could have satisfied `trait_pred`, when listing them
    /// is likely to help. `None` means the caller should describe the bound some other way.
    fn applicable_impls_to_mention(
        &self,
        obligation: &PredicateObligation<'tcx>,
        trait_pred: ty::PolyTraitClause<'tcx>,
    ) -> Option<Vec<CandidateSource>> {
        let mut ambiguities = compute_applicable_impls_for_diagnostics(
            self.infcx,
            &obligation.with(self.tcx, trait_pred),
            false,
        );
        let has_non_region_infer =
            trait_pred.skip_binder().trait_ref.args.types().any(|t| !t.is_ty_or_numeric_infer());
        // It doesn't make sense to talk about applicable impls if there are more than a
        // handful of them. If there are a lot of them, but only a few of them have no type
        // params, we only show those, as they are more likely to be useful/intended.
        if ambiguities.len() > 5 {
            let infcx = self.infcx;
            if !ambiguities.iter().all(|option| match option {
                CandidateSource::DefId(did) => infcx.tcx.generics_of(*did).count() == 0,
                CandidateSource::ParamEnv(_) => true,
            }) {
                // If not all are blanket impls, we filter blanked impls out.
                ambiguities.retain(|option| match option {
                    CandidateSource::DefId(did) => infcx.tcx.generics_of(*did).count() == 0,
                    CandidateSource::ParamEnv(_) => true,
                });
            }
        }
        (ambiguities.len() > 1 && ambiguities.len() < 10 && has_non_region_infer)
            .then_some(ambiguities)
    }

    fn annotate_source_of_ambiguity(
        &self,
        err: &mut Diag<'_>,
        ambiguities: &[CandidateSource],
        predicate: ty::Predicate<'tcx>,
    ) {
        let mut spans = vec![];
        let mut crates = vec![];
        let mut post = vec![];
        let mut has_param_env = false;
        for ambiguity in ambiguities {
            match ambiguity {
                CandidateSource::DefId(impl_def_id) => match self.tcx.span_of_impl(*impl_def_id) {
                    Ok(span) => spans.push(span),
                    Err(name) => {
                        crates.push(name);
                        if let Some(header) = to_pretty_impl_header(self.tcx, *impl_def_id) {
                            post.push(header);
                        }
                    }
                },
                CandidateSource::ParamEnv(span) => {
                    has_param_env = true;
                    spans.push(*span);
                }
            }
        }
        let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{n}`")).collect();
        crate_names.sort();
        crate_names.dedup();
        post.sort();
        post.dedup();

        if self.tainted_by_errors().is_some()
            && (crate_names.len() == 1
                && spans.len() == 0
                && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
                || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
        {
            // Avoid complaining about other inference issues for expressions like
            // `42 >> 1`, where the types are still `{integer}`, but we want to
            // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
            // NOTE(eddyb) this was `.cancel()`, but `err`
            // is borrowed, so we can't fully defuse it.
            err.downgrade_to_delayed_bug();
            return;
        }

        let msg = format!(
            "multiple `impl`s{} satisfying `{}` found",
            if has_param_env { " or `where` clauses" } else { "" },
            predicate
        );
        let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
            format!(":\n{}", post.iter().map(|p| format!("- {p}")).collect::<Vec<_>>().join("\n"))
        } else if post.len() == 1 {
            format!(": `{}`", post[0])
        } else {
            String::new()
        };

        match (spans.len(), crates.len(), crate_names.len()) {
            (0, 0, 0) => {
                err.note(format!("cannot satisfy `{predicate}`"));
            }
            (0, _, 1) => {
                err.note(format!("{msg} in the `{}` crate{post}", crates[0]));
            }
            (0, _, _) => {
                err.note(format!(
                    "{} in the following crates: {}{}",
                    msg,
                    crate_names.join(", "),
                    post,
                ));
            }
            (_, 0, 0) => {
                let span: MultiSpan = spans.into();
                err.span_note(span, msg);
            }
            (_, 1, 1) => {
                let span: MultiSpan = spans.into();
                err.span_note(span, msg);
                err.note(format!("and another `impl` found in the `{}` crate{post}", crates[0]));
            }
            _ => {
                let span: MultiSpan = spans.into();
                err.span_note(span, msg);
                err.note(format!(
                    "and more `impl`s found in the following crates: {}{}",
                    crate_names.join(", "),
                    post,
                ));
            }
        }
    }
}

struct HasNumericInferVisitor;

impl<'tcx> ty::TypeVisitor<TyCtxt<'tcx>> for HasNumericInferVisitor {
    type Result = ControlFlow<()>;

    fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
        if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
            ControlFlow::Break(())
        } else {
            ControlFlow::Continue(())
        }
    }
}