Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 6156 insertions, 0 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
new file mode 100644
index 000000000..e442c5c91
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -0,0 +1,2765 @@
+pub mod on_unimplemented;
+pub mod suggestions;
+
+use super::{
+    EvaluationResult, FulfillmentContext, 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, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+    MultiSpan, Style,
+};
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_hir::intravisit::Visitor;
+use rustc_hir::GenericParam;
+use rustc_hir::Item;
+use rustc_hir::Node;
+use rustc_infer::traits::TraitEngine;
+use rustc_middle::traits::select::OverflowError;
+use rustc_middle::ty::abstract_const::NotConstEvaluatable;
+use rustc_middle::ty::error::ExpectedFound;
+use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
+use rustc_middle::ty::{
+    self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
+    TypeVisitable,
+};
+use rustc_span::symbol::{kw, sym};
+use rustc_span::{ExpnKind, Span, DUMMY_SP};
+use std::fmt;
+use std::iter;
+use std::ops::ControlFlow;
+
+use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
+use crate::traits::query::normalize::AtExt as _;
+use crate::traits::specialize::to_pretty_impl_header;
+use on_unimplemented::InferCtxtExt as _;
+use suggestions::InferCtxtExt as _;
+
+pub use rustc_infer::traits::error_reporting::*;
+
+// When outputting impl candidates, prefer showing those that are more similar.
+//
+// We also compare candidates after skipping lifetimes, which has a lower
+// priority than exact matches.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub enum CandidateSimilarity {
+    Exact { ignoring_lifetimes: bool },
+    Fuzzy { ignoring_lifetimes: bool },
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct ImplCandidate<'tcx> {
+    pub trait_ref: ty::TraitRef<'tcx>,
+    pub similarity: CandidateSimilarity,
+}
+
+pub trait InferCtxtExt<'tcx> {
+    fn report_fulfillment_errors(
+        &self,
+        errors: &[FulfillmentError<'tcx>],
+        body_id: Option<hir::BodyId>,
+        fallback_has_occurred: bool,
+    ) -> ErrorGuaranteed;
+
+    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>]) -> !;
+
+    /// The `root_obligation` parameter should be the `root_obligation` field
+    /// from a `FulfillmentError`. If no `FulfillmentError` is available,
+    /// then it should be the same as `obligation`.
+    fn report_selection_error(
+        &self,
+        obligation: PredicateObligation<'tcx>,
+        root_obligation: &PredicateObligation<'tcx>,
+        error: &SelectionError<'tcx>,
+        fallback_has_occurred: 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, ErrorGuaranteed>;
+
+    /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
+    /// in that order, and returns the generic type corresponding to the
+    /// argument of that trait (corresponding to the closure arguments).
+    fn type_implements_fn_trait(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: ty::Binder<'tcx, Ty<'tcx>>,
+        constness: ty::BoundConstness,
+        polarity: ty::ImplPolarity,
+    ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'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,
+    ) -> ErrorGuaranteed {
+        #[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);
+        }
+
+        // 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 iter::zip(errors, is_suppressed) {
+            if !suppressed {
+                self.report_fulfillment_error(error, body_id, fallback_has_occurred);
+            }
+        }
+
+        self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
+    }
+
+    /// 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.clone());
+        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.param_env,
+            obligation.cause.code(),
+            &mut vec![],
+            &mut Default::default(),
+        );
+
+        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!(?cycle, "report_overflow_error_cycle");
+
+        // The 'deepest' obligation is most likely to have a useful
+        // cause 'backtrace'
+        self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
+    }
+
+    fn report_selection_error(
+        &self,
+        mut obligation: PredicateObligation<'tcx>,
+        root_obligation: &PredicateObligation<'tcx>,
+        error: &SelectionError<'tcx>,
+        fallback_has_occurred: bool,
+    ) {
+        self.set_tainted_by_errors();
+        let tcx = self.tcx;
+        let mut span = obligation.cause.span;
+
+        let mut err = match *error {
+            SelectionError::Ambiguous(ref impls) => {
+                let mut err = self.tcx.sess.struct_span_err(
+                    obligation.cause.span,
+                    &format!("multiple applicable `impl`s for `{}`", obligation.predicate),
+                );
+                self.annotate_source_of_ambiguity(&mut err, impls, obligation.predicate);
+                err.emit();
+                return;
+            }
+            SelectionError::Unimplemented => {
+                // If this obligation was generated as a result of well-formedness checking, see if we
+                // can get a better error message by performing HIR-based well-formedness checking.
+                if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
+                    root_obligation.cause.code().peel_derives()
+                {
+                    if let Some(cause) = self
+                        .tcx
+                        .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
+                    {
+                        obligation.cause = cause.clone();
+                        span = obligation.cause.span;
+                    }
+                }
+                if let ObligationCauseCode::CompareImplItemObligation {
+                    impl_item_def_id,
+                    trait_item_def_id,
+                    kind: _,
+                } = *obligation.cause.code()
+                {
+                    self.report_extra_impl_obligation(
+                        span,
+                        impl_item_def_id,
+                        trait_item_def_id,
+                        &format!("`{}`", obligation.predicate),
+                    )
+                    .emit();
+                    return;
+                }
+
+                let bound_predicate = obligation.predicate.kind();
+                match bound_predicate.skip_binder() {
+                    ty::PredicateKind::Trait(trait_predicate) => {
+                        let trait_predicate = bound_predicate.rebind(trait_predicate);
+                        let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
+
+                        trait_predicate.remap_constness_diag(obligation.param_env);
+                        let predicate_is_const = ty::BoundConstness::ConstIfConst
+                            == trait_predicate.skip_binder().constness;
+
+                        if self.tcx.sess.has_errors().is_some()
+                            && 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,
+                            append_const_msg,
+                        } = self.on_unimplemented_note(trait_ref, &obligation);
+                        let have_alt_message = message.is_some() || label.is_some();
+                        let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
+                        let is_unsize =
+                            Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
+                        let (message, note, append_const_msg) = if is_try_conversion {
+                            (
+                                Some(format!(
+                                    "`?` couldn't convert the error to `{}`",
+                                    trait_ref.skip_binder().self_ty(),
+                                )),
+                                Some(
+                                    "the question mark operation (`?`) implicitly performs a \
+                                     conversion on the error value using the `From` trait"
+                                        .to_owned(),
+                                ),
+                                Some(None),
+                            )
+                        } else {
+                            (message, note, append_const_msg)
+                        };
+
+                        let mut err = struct_span_err!(
+                            self.tcx.sess,
+                            span,
+                            E0277,
+                            "{}",
+                            message
+                                .and_then(|cannot_do_this| {
+                                    match (predicate_is_const, append_const_msg) {
+                                        // do nothing if predicate is not const
+                                        (false, _) => Some(cannot_do_this),
+                                        // suggested using default post message
+                                        (true, Some(None)) => {
+                                            Some(format!("{cannot_do_this} in const contexts"))
+                                        }
+                                        // overridden post message
+                                        (true, Some(Some(post_message))) => {
+                                            Some(format!("{cannot_do_this}{post_message}"))
+                                        }
+                                        // fallback to generic message
+                                        (true, None) => None,
+                                    }
+                                })
+                                .unwrap_or_else(|| format!(
+                                    "the trait bound `{}` is not satisfied{}",
+                                    trait_predicate, post_message,
+                                ))
+                        );
+
+                        if is_try_conversion {
+                            let none_error = self
+                                .tcx
+                                .get_diagnostic_item(sym::none_error)
+                                .map(|def_id| tcx.type_of(def_id));
+                            let should_convert_option_to_result =
+                                Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
+                            let should_convert_result_to_option =
+                                Some(trait_ref.self_ty().skip_binder()) == none_error;
+                            if should_convert_option_to_result {
+                                err.span_suggestion_verbose(
+                                    span.shrink_to_lo(),
+                                    "consider converting the `Option<T>` into a `Result<T, _>` \
+                                     using `Option::ok_or` or `Option::ok_or_else`",
+                                    ".ok_or_else(|| /* error value */)",
+                                    Applicability::HasPlaceholders,
+                                );
+                            } else if should_convert_result_to_option {
+                                err.span_suggestion_verbose(
+                                    span.shrink_to_lo(),
+                                    "consider converting the `Result<T, _>` into an `Option<T>` \
+                                     using `Result::ok`",
+                                    ".ok()",
+                                    Applicability::MachineApplicable,
+                                );
+                            }
+                            if let Some(ret_span) = self.return_type_span(&obligation) {
+                                err.span_label(
+                                    ret_span,
+                                    &format!(
+                                        "expected `{}` because of this",
+                                        trait_ref.skip_binder().self_ty()
+                                    ),
+                                );
+                            }
+                        }
+
+                        if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
+                            && predicate_is_const
+                        {
+                            err.note("`~const Drop` was renamed to `~const Destruct`");
+                            err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
+                        }
+
+                        let explanation = if let ObligationCauseCode::MainFunctionType =
+                            obligation.cause.code()
+                        {
+                            "consider using `()`, or a `Result`".to_owned()
+                        } else {
+                            format!(
+                                "{}the trait `{}` is not implemented for `{}`",
+                                pre_message,
+                                trait_predicate.print_modifiers_and_trait_path(),
+                                trait_ref.skip_binder().self_ty(),
+                            )
+                        };
+
+                        if self.suggest_add_reference_to_arg(
+                            &obligation,
+                            &mut err,
+                            trait_predicate,
+                            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);
+                            if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
+                                // When the self type is a type param We don't need to "the trait
+                                // `std::marker::Sized` is not implemented for `T`" as we will point
+                                // at the type param with a label to suggest constraining it.
+                                err.help(&explanation);
+                            }
+                        } else {
+                            err.span_label(span, explanation);
+                        }
+
+                        if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
+                           Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
+                            self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
+                        }
+
+                        if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
+                            let non_const_predicate = trait_ref.without_const();
+                            let non_const_obligation = Obligation {
+                                cause: obligation.cause.clone(),
+                                param_env: obligation.param_env.without_const(),
+                                predicate: non_const_predicate.to_predicate(tcx),
+                                recursion_depth: obligation.recursion_depth,
+                            };
+                            if self.predicate_may_hold(&non_const_obligation) {
+                                err.span_note(
+                                    span,
+                                    &format!(
+                                        "the trait `{}` is implemented for `{}`, \
+                                        but that implementation is not `const`",
+                                        non_const_predicate.print_modifiers_and_trait_path(),
+                                        trait_ref.skip_binder().self_ty(),
+                                    ),
+                                );
+                            }
+                        }
+
+                        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 body = 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,
+                                    })
+                                });
+
+                            let enclosing_scope_span =
+                                tcx.hir().span_with_body(tcx.hir().local_def_id_to_hir_id(body));
+
+                            err.span_label(enclosing_scope_span, s);
+                        }
+
+                        self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
+                        self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
+                        let mut suggested =
+                            self.suggest_dereferences(&obligation, &mut err, trait_predicate);
+                        suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
+                        suggested |=
+                            self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
+                        suggested |= self.suggest_semicolon_removal(
+                            &obligation,
+                            &mut err,
+                            span,
+                            trait_predicate,
+                        );
+                        self.note_version_mismatch(&mut err, &trait_ref);
+                        self.suggest_remove_await(&obligation, &mut err);
+                        self.suggest_derive(&obligation, &mut err, trait_predicate);
+
+                        if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
+                            self.suggest_await_before_try(
+                                &mut err,
+                                &obligation,
+                                trait_predicate,
+                                span,
+                            );
+                        }
+
+                        if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
+                            err.emit();
+                            return;
+                        }
+
+                        if is_unsize {
+                            // If the obligation failed due to a missing implementation of the
+                            // `Unsize` trait, give a pointer to why that might be the case
+                            err.note(
+                                "all implementations of `Unsize` are provided \
+                                automatically by the compiler, see \
+                                <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
+                                for more information",
+                            );
+                        }
+
+                        let is_fn_trait = [
+                            self.tcx.lang_items().fn_trait(),
+                            self.tcx.lang_items().fn_mut_trait(),
+                            self.tcx.lang_items().fn_once_trait(),
+                        ]
+                        .contains(&Some(trait_ref.def_id()));
+                        let is_target_feature_fn = if let ty::FnDef(def_id, _) =
+                            *trait_ref.skip_binder().self_ty().kind()
+                        {
+                            !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
+                        } else {
+                            false
+                        };
+                        if is_fn_trait && is_target_feature_fn {
+                            err.note(
+                                "`#[target_feature]` functions do not implement the `Fn` traits",
+                            );
+                        }
+
+                        // Try to report a help message
+                        if is_fn_trait
+                            && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
+                                obligation.param_env,
+                                trait_ref.self_ty(),
+                                trait_predicate.skip_binder().constness,
+                                trait_predicate.skip_binder().polarity,
+                            )
+                        {
+                            // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
+                            // suggestion to add trait bounds for the type, since we only typically implement
+                            // these traits once.
+
+                            // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
+                            // to implement.
+                            let selected_kind =
+                                ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
+                                    .expect("expected to map DefId to ClosureKind");
+                            if !implemented_kind.extends(selected_kind) {
+                                err.note(
+                                    &format!(
+                                        "`{}` implements `{}`, but it must implement `{}`, which is more general",
+                                        trait_ref.skip_binder().self_ty(),
+                                        implemented_kind,
+                                        selected_kind
+                                    )
+                                );
+                            }
+
+                            // Note any argument mismatches
+                            let given_ty = params.skip_binder();
+                            let expected_ty = trait_ref.skip_binder().substs.type_at(1);
+                            if let ty::Tuple(given) = given_ty.kind()
+                                && let ty::Tuple(expected) = expected_ty.kind()
+                            {
+                                if expected.len() != given.len() {
+                                    // Note number of types that were expected and given
+                                    err.note(
+                                        &format!(
+                                            "expected a closure taking {} argument{}, but one taking {} argument{} was given",
+                                            given.len(),
+                                            pluralize!(given.len()),
+                                            expected.len(),
+                                            pluralize!(expected.len()),
+                                        )
+                                    );
+                                } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
+                                    // Print type mismatch
+                                    let (expected_args, given_args) =
+                                        self.cmp(given_ty, expected_ty);
+                                    err.note_expected_found(
+                                        &"a closure with arguments",
+                                        expected_args,
+                                        &"a closure with arguments",
+                                        given_args,
+                                    );
+                                }
+                            }
+                        } else 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_predicate,
+                                None,
+                                obligation.cause.body_id,
+                            );
+                        } else if !suggested {
+                            // Can't show anything else useful, try to find similar impls.
+                            let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
+                            if !self.report_similar_impl_candidates(
+                                impl_candidates,
+                                trait_ref,
+                                obligation.cause.body_id,
+                                &mut err,
+                            ) {
+                                // This is *almost* equivalent to
+                                // `obligation.cause.code().peel_derives()`, but it gives us the
+                                // trait predicate for that corresponding root obligation. This
+                                // lets us get a derived obligation from a type parameter, like
+                                // when calling `string.strip_suffix(p)` where `p` is *not* an
+                                // implementer of `Pattern<'_>`.
+                                let mut code = obligation.cause.code();
+                                let mut trait_pred = trait_predicate;
+                                let mut peeled = false;
+                                while let Some((parent_code, parent_trait_pred)) = code.parent() {
+                                    code = parent_code;
+                                    if let Some(parent_trait_pred) = parent_trait_pred {
+                                        trait_pred = parent_trait_pred;
+                                        peeled = true;
+                                    }
+                                }
+                                let def_id = trait_pred.def_id();
+                                // Mention *all* the `impl`s for the *top most* obligation, the
+                                // user might have meant to use one of them, if any found. We skip
+                                // auto-traits or fundamental traits that might not be exactly what
+                                // the user might expect to be presented with. Instead this is
+                                // useful for less general traits.
+                                if peeled
+                                    && !self.tcx.trait_is_auto(def_id)
+                                    && !self.tcx.lang_items().items().contains(&Some(def_id))
+                                {
+                                    let trait_ref = trait_pred.to_poly_trait_ref();
+                                    let impl_candidates =
+                                        self.find_similar_impl_candidates(trait_pred);
+                                    self.report_similar_impl_candidates(
+                                        impl_candidates,
+                                        trait_ref,
+                                        obligation.cause.body_id,
+                                        &mut err,
+                                    );
+                                }
+                            }
+                        }
+
+                        // Changing mutability doesn't make a difference to whether we have
+                        // an `Unsize` impl (Fixes ICE in #71036)
+                        if !is_unsize {
+                            self.suggest_change_mut(&obligation, &mut err, trait_predicate);
+                        }
+
+                        // 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.with(predicate.to_predicate(tcx));
+                            if self.predicate_may_hold(&unit_obligation) {
+                                err.note(
+                                    "this error might have been caused by changes to \
+                                    Rust's type-inference algorithm (see issue #48950 \
+                                    <https://github.com/rust-lang/rust/issues/48950> \
+                                    for more information)",
+                                );
+                                err.help("did you intend to use the type `()` here instead?");
+                            }
+                        }
+
+                        // Return early if the trait is Debug or Display and the invocation
+                        // originates within a standard library macro, because the output
+                        // is otherwise overwhelming and unhelpful (see #85844 for an
+                        // example).
+
+                        let in_std_macro =
+                            match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
+                                Some(macro_def_id) => {
+                                    let crate_name = tcx.crate_name(macro_def_id.krate);
+                                    crate_name == sym::std || crate_name == sym::core
+                                }
+                                None => false,
+                            };
+
+                        if in_std_macro
+                            && matches!(
+                                self.tcx.get_diagnostic_name(trait_ref.def_id()),
+                                Some(sym::Debug | sym::Display)
+                            )
+                        {
+                            err.emit();
+                            return;
+                        }
+
+                        err
+                    }
+
+                    ty::PredicateKind::Subtype(predicate) => {
+                        // Errors for Subtype predicates show up as
+                        // `FulfillmentErrorCode::CodeSubtypeError`,
+                        // not selection error.
+                        span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
+                    }
+
+                    ty::PredicateKind::Coerce(predicate) => {
+                        // Errors for Coerce predicates show up as
+                        // `FulfillmentErrorCode::CodeSubtypeError`,
+                        // not selection error.
+                        span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
+                    }
+
+                    ty::PredicateKind::RegionOutlives(..)
+                    | ty::PredicateKind::Projection(..)
+                    | ty::PredicateKind::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::PredicateKind::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::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
+                        let found_kind = self.closure_kind(closure_substs).unwrap();
+                        let closure_span = self.tcx.def_span(closure_def_id);
+                        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(typeck_results) = self.in_progress_typeck_results {
+                            let hir_id = self
+                                .tcx
+                                .hir()
+                                .local_def_id_to_hir_id(closure_def_id.expect_local());
+                            let typeck_results = typeck_results.borrow();
+                            match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
+                                (ty::ClosureKind::FnOnce, Some((span, place))) => {
+                                    err.span_label(
+                                        *span,
+                                        format!(
+                                            "closure is `FnOnce` because it moves the \
+                                         variable `{}` out of its environment",
+                                            ty::place_to_string_for_capture(tcx, place)
+                                        ),
+                                    );
+                                }
+                                (ty::ClosureKind::FnMut, Some((span, place))) => {
+                                    err.span_label(
+                                        *span,
+                                        format!(
+                                            "closure is `FnMut` because it mutates the \
+                                         variable `{}` here",
+                                            ty::place_to_string_for_capture(tcx, place)
+                                        ),
+                                    );
+                                }
+                                _ => {}
+                            }
+                        }
+
+                        err.emit();
+                        return;
+                    }
+
+                    ty::PredicateKind::WellFormed(ty) => {
+                        if !self.tcx.sess.opts.unstable_opts.chalk {
+                            // 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);
+                        } else {
+                            // FIXME: we'll need a better message which takes into account
+                            // which bounds actually failed to hold.
+                            self.tcx.sess.struct_span_err(
+                                span,
+                                &format!("the type `{}` is not well-formed (chalk)", ty),
+                            )
+                        }
+                    }
+
+                    ty::PredicateKind::ConstEvaluatable(..) => {
+                        // Errors for `ConstEvaluatable` predicates show up as
+                        // `SelectionError::ConstEvalFailure`,
+                        // not `Unimplemented`.
+                        span_bug!(
+                            span,
+                            "const-evaluatable requirement gave wrong error: `{:?}`",
+                            obligation
+                        )
+                    }
+
+                    ty::PredicateKind::ConstEquate(..) => {
+                        // Errors for `ConstEquate` predicates show up as
+                        // `SelectionError::ConstEvalFailure`,
+                        // not `Unimplemented`.
+                        span_bug!(
+                            span,
+                            "const-equate requirement gave wrong error: `{:?}`",
+                            obligation
+                        )
+                    }
+
+                    ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
+                        span,
+                        "TypeWellFormedFromEnv predicate should only exist in the environment"
+                    ),
+                }
+            }
+
+            OutputTypeParameterMismatch(found_trait_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 Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
+                    return;
+                };
+
+                let found_did = match *found_trait_ty.kind() {
+                    ty::Closure(did, _)
+                    | ty::Foreign(did)
+                    | ty::FnDef(did, _)
+                    | ty::Generator(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));
+
+                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, 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)
+            }
+
+            SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
+                bug!(
+                    "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
+                )
+            }
+            SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
+                if !self.tcx.features().generic_const_exprs {
+                    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).
+                    //
+                    // Note that with `feature(generic_const_exprs)` this case should not
+                    // be reachable.
+                    err.note("this may fail depending on what value the parameter takes");
+                    err.emit();
+                    return;
+                }
+
+                match obligation.predicate.kind().skip_binder() {
+                    ty::PredicateKind::ConstEvaluatable(uv) => {
+                        let mut err =
+                            self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
+                        let const_span = self.tcx.def_span(uv.def.did);
+                        match self.tcx.sess.source_map().span_to_snippet(const_span) {
+                            Ok(snippet) => err.help(&format!(
+                                "try adding a `where` bound using this expression: `where [(); {}]:`",
+                                snippet
+                            )),
+                            _ => err.help("consider adding a `where` bound using this expression"),
+                        };
+                        err
+                    }
+                    _ => {
+                        span_bug!(
+                            span,
+                            "unexpected non-ConstEvaluatable predicate, this should not be reachable"
+                        )
+                    }
+                }
+            }
+
+            // Already reported in the query.
+            SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
+                // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
+                self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
+                return;
+            }
+            // Already reported.
+            Overflow(OverflowError::Error(_)) => {
+                self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
+                return;
+            }
+            Overflow(_) => {
+                bug!("overflow should be handled before the `report_selection_error` path");
+            }
+            SelectionError::ErrorReporting => {
+                bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
+            }
+        };
+
+        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(&hir::Closure { body, fn_decl_span, .. }),
+                ..
+            }) => (
+                fn_decl_span,
+                hir.body(body)
+                    .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 { kind: hir::ItemKind::Fn(ref sig, ..), .. })
+            | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
+            | Node::TraitItem(&hir::TraitItem {
+                kind: hir::TraitItemKind::Fn(ref sig, _), ..
+            }) => (
+                sig.span,
+                sig.decl
+                    .inputs
+                    .iter()
+                    .map(|arg| match arg.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_or(DUMMY_SP, |id| hir.span(id));
+                (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, ErrorGuaranteed> {
+        let kind = if is_closure { "closure" } else { "function" };
+
+        let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
+            let arg_length = arguments.len();
+            let distinct = matches!(other, &[ArgKind::Tuple(..)]);
+            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[..]
+                && 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
+    }
+
+    fn type_implements_fn_trait(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: ty::Binder<'tcx, Ty<'tcx>>,
+        constness: ty::BoundConstness,
+        polarity: ty::ImplPolarity,
+    ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
+        self.commit_if_ok(|_| {
+            for trait_def_id in [
+                self.tcx.lang_items().fn_trait(),
+                self.tcx.lang_items().fn_mut_trait(),
+                self.tcx.lang_items().fn_once_trait(),
+            ] {
+                let Some(trait_def_id) = trait_def_id else { continue };
+                // Make a fresh inference variable so we can determine what the substitutions
+                // of the trait are.
+                let var = self.next_ty_var(TypeVariableOrigin {
+                    span: DUMMY_SP,
+                    kind: TypeVariableOriginKind::MiscVariable,
+                });
+                let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
+                let obligation = Obligation::new(
+                    ObligationCause::dummy(),
+                    param_env,
+                    ty.rebind(ty::TraitPredicate {
+                        trait_ref: ty::TraitRef::new(trait_def_id, substs),
+                        constness,
+                        polarity,
+                    })
+                    .to_predicate(self.tcx),
+                );
+                let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
+                fulfill_cx.register_predicate_obligation(self, obligation);
+                if fulfill_cx.select_all_or_error(self).is_empty() {
+                    return Ok((
+                        ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
+                            .expect("expected to map DefId to ClosureKind"),
+                        ty.rebind(self.resolve_vars_if_possible(var)),
+                    ));
+                }
+            }
+
+            Err(())
+        })
+    }
+}
+
+trait InferCtxtPrivExt<'hir, '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>,
+        ignoring_lifetimes: bool,
+    ) -> Option<CandidateSimilarity>;
+
+    fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
+
+    fn find_similar_impl_candidates(
+        &self,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> Vec<ImplCandidate<'tcx>>;
+
+    fn report_similar_impl_candidates(
+        &self,
+        impl_candidates: Vec<ImplCandidate<'tcx>>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        body_id: hir::HirId,
+        err: &mut Diagnostic,
+    ) -> bool;
+
+    /// 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 Diagnostic,
+        trait_ref: &ty::PolyTraitRef<'tcx>,
+    ) -> bool;
+
+    /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
+    /// `trait_ref`.
+    ///
+    /// For this to work, `new_self_ty` must have no escaping bound variables.
+    fn mk_trait_obligation_with_new_self_ty(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'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 Diagnostic, obligation: &PredicateObligation<'tcx>);
+
+    fn suggest_unsized_bound_if_applicable(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+    );
+
+    fn annotate_source_of_ambiguity(
+        &self,
+        err: &mut Diagnostic,
+        impls: &[DefId],
+        predicate: ty::Predicate<'tcx>,
+    );
+
+    fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'hir>);
+
+    fn maybe_indirection_for_unsized(
+        &self,
+        err: &mut Diagnostic,
+        item: &'hir Item<'hir>,
+        param: &'hir GenericParam<'hir>,
+    ) -> bool;
+
+    fn is_recursive_obligation(
+        &self,
+        obligated_types: &mut Vec<Ty<'tcx>>,
+        cause_code: &ObligationCauseCode<'tcx>,
+    ) -> bool;
+}
+
+impl<'a, 'tcx> InferCtxtPrivExt<'a, '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;
+        }
+
+        // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
+        let bound_error = error.kind();
+        let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
+            (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
+                (cond, bound_error.rebind(error))
+            }
+            _ => {
+                // FIXME: make this work in other cases too.
+                return false;
+            }
+        };
+
+        for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
+            let bound_predicate = obligation.predicate.kind();
+            if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
+                let error = error.to_poly_trait_ref();
+                let implication = bound_predicate.rebind(implication.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
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn report_fulfillment_error(
+        &self,
+        error: &FulfillmentError<'tcx>,
+        body_id: Option<hir::BodyId>,
+        fallback_has_occurred: bool,
+    ) {
+        match error.code {
+            FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
+                self.report_selection_error(
+                    error.obligation.clone(),
+                    &error.root_obligation,
+                    selection_error,
+                    fallback_has_occurred,
+                );
+            }
+            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();
+            }
+            FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
+                self.report_mismatched_consts(
+                    &error.obligation.cause,
+                    expected_found.expected,
+                    expected_found.found,
+                    err.clone(),
+                )
+                .emit();
+            }
+        }
+    }
+
+    #[instrument(level = "debug", skip_all)]
+    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.
+            let bound_predicate = predicate.kind();
+            if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
+                let mut selcx = SelectionContext::new(self);
+                let data = self.replace_bound_vars_with_fresh_vars(
+                    obligation.cause.span,
+                    infer::LateBoundRegionConversionTime::HigherRankedType,
+                    bound_predicate.rebind(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!(?obligation.cause, ?obligation.param_env);
+
+                debug!(?normalized_ty, data.ty = ?data.term);
+
+                let is_normalized_ty_expected = !matches!(
+                    obligation.cause.code().peel_derives(),
+                    ObligationCauseCode::ItemObligation(_)
+                        | ObligationCauseCode::BindingObligation(_, _)
+                        | ObligationCauseCode::ObjectCastObligation(..)
+                        | ObligationCauseCode::OpaqueType
+                );
+                if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
+                    is_normalized_ty_expected,
+                    normalized_ty,
+                    data.term,
+                ) {
+                    values = Some(infer::ValuePairs::Terms(ExpectedFound::new(
+                        is_normalized_ty_expected,
+                        normalized_ty,
+                        data.term,
+                    )));
+                    err_buf = error;
+                    err = &err_buf;
+                }
+            }
+
+            let mut diag = struct_span_err!(
+                self.tcx.sess,
+                obligation.cause.span,
+                E0271,
+                "type mismatch resolving `{}`",
+                predicate
+            );
+            let secondary_span = match predicate.kind().skip_binder() {
+                ty::PredicateKind::Projection(proj) => self
+                    .tcx
+                    .opt_associated_item(proj.projection_ty.item_def_id)
+                    .and_then(|trait_assoc_item| {
+                        self.tcx
+                            .trait_of_item(proj.projection_ty.item_def_id)
+                            .map(|id| (trait_assoc_item, id))
+                    })
+                    .and_then(|(trait_assoc_item, id)| {
+                        let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
+                        self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
+                            self.tcx
+                                .associated_items(did)
+                                .in_definition_order()
+                                .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
+                        })
+                    })
+                    .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
+                        Some(
+                            hir::Node::TraitItem(hir::TraitItem {
+                                kind: hir::TraitItemKind::Type(_, Some(ty)),
+                                ..
+                            })
+                            | hir::Node::ImplItem(hir::ImplItem {
+                                kind: hir::ImplItemKind::TyAlias(ty),
+                                ..
+                            }),
+                        ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
+                        _ => None,
+                    }),
+                _ => None,
+            };
+            self.note_type_err(
+                &mut diag,
+                &obligation.cause,
+                secondary_span,
+                values,
+                err,
+                true,
+                false,
+            );
+            self.note_obligation_cause(&mut diag, obligation);
+            diag.emit();
+        });
+    }
+
+    fn fuzzy_match_tys(
+        &self,
+        mut a: Ty<'tcx>,
+        mut b: Ty<'tcx>,
+        ignoring_lifetimes: bool,
+    ) -> Option<CandidateSimilarity> {
+        /// returns the fuzzy category of a given type, or None
+        /// if the type can be equated to any type.
+        fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
+            match t.kind() {
+                ty::Bool => Some(0),
+                ty::Char => Some(1),
+                ty::Str => Some(2),
+                ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
+                ty::Int(..)
+                | ty::Uint(..)
+                | ty::Float(..)
+                | ty::Infer(ty::IntVar(..) | 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::Param(..) => Some(11),
+                ty::Projection(..) => Some(12),
+                ty::Opaque(..) => Some(13),
+                ty::Never => Some(14),
+                ty::Adt(..) => Some(15),
+                ty::Generator(..) => Some(16),
+                ty::Foreign(..) => Some(17),
+                ty::GeneratorWitness(..) => Some(18),
+                ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
+            }
+        }
+
+        let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
+            loop {
+                match t.kind() {
+                    ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
+                        t = *inner
+                    }
+                    _ => break t,
+                }
+            }
+        };
+
+        if !ignoring_lifetimes {
+            a = strip_references(a);
+            b = strip_references(b);
+        }
+
+        let cat_a = type_category(self.tcx, a)?;
+        let cat_b = type_category(self.tcx, b)?;
+        if a == b {
+            Some(CandidateSimilarity::Exact { ignoring_lifetimes })
+        } else if cat_a == cat_b {
+            match (a.kind(), b.kind()) {
+                (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
+                (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
+                // Matching on references results in a lot of unhelpful
+                // suggestions, so let's just not do that for now.
+                //
+                // We still upgrade successful matches to `ignoring_lifetimes: true`
+                // to prioritize that impl.
+                (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
+                    self.fuzzy_match_tys(a, b, true).is_some()
+                }
+                _ => true,
+            }
+            .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
+        } else if ignoring_lifetimes {
+            None
+        } else {
+            self.fuzzy_match_tys(a, b, 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_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> Vec<ImplCandidate<'tcx>> {
+        self.tcx
+            .all_impls(trait_pred.def_id())
+            .filter_map(|def_id| {
+                if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
+                    || !trait_pred
+                        .skip_binder()
+                        .is_constness_satisfied_by(self.tcx.constness(def_id))
+                {
+                    return None;
+                }
+
+                let imp = self.tcx.impl_trait_ref(def_id).unwrap();
+
+                self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
+                    .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
+            })
+            .collect()
+    }
+
+    fn report_similar_impl_candidates(
+        &self,
+        impl_candidates: Vec<ImplCandidate<'tcx>>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        body_id: hir::HirId,
+        err: &mut Diagnostic,
+    ) -> bool {
+        let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
+            candidates.sort();
+            candidates.dedup();
+            let len = candidates.len();
+            if candidates.len() == 0 {
+                return false;
+            }
+            if candidates.len() == 1 {
+                err.highlighted_help(vec![
+                    (
+                        format!("the trait `{}` ", candidates[0].print_only_trait_path()),
+                        Style::NoStyle,
+                    ),
+                    ("is".to_string(), Style::Highlight),
+                    (" implemented for `".to_string(), Style::NoStyle),
+                    (candidates[0].self_ty().to_string(), Style::Highlight),
+                    ("`".to_string(), Style::NoStyle),
+                ]);
+                return true;
+            }
+            let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
+            // Check if the trait is the same in all cases. If so, we'll only show the type.
+            let mut traits: Vec<_> =
+                candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
+            traits.sort();
+            traits.dedup();
+
+            let mut candidates: Vec<String> = candidates
+                .into_iter()
+                .map(|c| {
+                    if traits.len() == 1 {
+                        format!("\n  {}", c.self_ty())
+                    } else {
+                        format!("\n  {}", c)
+                    }
+                })
+                .collect();
+
+            candidates.sort();
+            candidates.dedup();
+            let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
+            err.help(&format!(
+                "the following other types implement trait `{}`:{}{}",
+                trait_ref.print_only_trait_path(),
+                candidates[..end].join(""),
+                if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
+            ));
+            true
+        };
+
+        let def_id = trait_ref.def_id();
+        if impl_candidates.is_empty() {
+            if self.tcx.trait_is_auto(def_id)
+                || self.tcx.lang_items().items().contains(&Some(def_id))
+                || self.tcx.get_diagnostic_name(def_id).is_some()
+            {
+                // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
+                return false;
+            }
+            let normalized_impl_candidates: Vec<_> = self
+                .tcx
+                .all_impls(def_id)
+                // Ignore automatically derived impls and `!Trait` impls.
+                .filter(|&def_id| {
+                    self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
+                        || self.tcx.is_builtin_derive(def_id)
+                })
+                .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
+                .filter(|trait_ref| {
+                    let self_ty = trait_ref.self_ty();
+                    // Avoid mentioning type parameters.
+                    if let ty::Param(_) = self_ty.kind() {
+                        false
+                    }
+                    // Avoid mentioning types that are private to another crate
+                    else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
+                        // FIXME(compiler-errors): This could be generalized, both to
+                        // be more granular, and probably look past other `#[fundamental]`
+                        // types, too.
+                        self.tcx
+                            .visibility(def.did())
+                            .is_accessible_from(body_id.owner.to_def_id(), self.tcx)
+                    } else {
+                        true
+                    }
+                })
+                .collect();
+            return report(normalized_impl_candidates, err);
+        }
+
+        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) => normalized.value,
+                    None => candidate,
+                }
+            })
+        };
+
+        // Sort impl candidates so that ordering is consistent for UI tests.
+        // because the ordering of `impl_candidates` may not be deterministic:
+        // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
+        //
+        // Prefer more similar candidates first, then sort lexicographically
+        // by their normalized string representation.
+        let mut normalized_impl_candidates_and_similarities = impl_candidates
+            .into_iter()
+            .map(|ImplCandidate { trait_ref, similarity }| {
+                let normalized = normalize(trait_ref);
+                (similarity, normalized)
+            })
+            .collect::<Vec<_>>();
+        normalized_impl_candidates_and_similarities.sort();
+        normalized_impl_candidates_and_similarities.dedup();
+
+        let normalized_impl_candidates = normalized_impl_candidates_and_similarities
+            .into_iter()
+            .map(|(_, normalized)| normalized)
+            .collect::<Vec<_>>();
+
+        report(normalized_impl_candidates, err)
+    }
+
+    /// Gets the parent trait chain start
+    fn get_parent_trait_ref(
+        &self,
+        code: &ObligationCauseCode<'tcx>,
+    ) -> Option<(String, Option<Span>)> {
+        match code {
+            ObligationCauseCode::BuiltinDerivedObligation(data) => {
+                let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
+                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(self.tcx, ty)
+                            .map(|(_, def_id)| self.tcx.def_span(def_id));
+                        Some((ty.to_string(), span))
+                    }
+                }
+            }
+            ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
+                self.get_parent_trait_ref(&parent_code)
+            }
+            _ => 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 Diagnostic,
+        trait_ref: &ty::PolyTraitRef<'tcx>,
+    ) -> bool {
+        let get_trait_impl = |trait_def_id| {
+            self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
+        };
+        let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
+        let traits_with_same_path: std::collections::BTreeSet<_> = self
+            .tcx
+            .all_traits()
+            .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();
+        let mut suggested = false;
+        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);
+                suggested = true;
+            }
+        }
+        suggested
+    }
+
+    fn mk_trait_obligation_with_new_self_ty(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
+    ) -> PredicateObligation<'tcx> {
+        let trait_pred = trait_ref_and_ty.map_bound_ref(|(tr, new_self_ty)| ty::TraitPredicate {
+            trait_ref: ty::TraitRef {
+                substs: self.tcx.mk_substs_trait(*new_self_ty, &tr.trait_ref.substs[1..]),
+                ..tr.trait_ref
+            },
+            ..*tr
+        });
+
+        Obligation::new(ObligationCause::dummy(), param_env, trait_pred.to_predicate(self.tcx))
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    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!(?predicate, obligation.cause.code = tracing::field::debug(&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::Trait(data) => {
+                let trait_ref = bound_predicate.rebind(data.trait_ref);
+                debug!(?trait_ref);
+
+                if predicate.references_error() {
+                    return;
+                }
+
+                // 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() == Some(trait_ref.def_id()) {
+                    if !self.is_tainted_by_errors() {
+                        self.emit_inference_failure_err(
+                            body_id,
+                            span,
+                            trait_ref.self_ty().skip_binder().into(),
+                            ErrorCode::E0282,
+                            false,
+                        )
+                        .emit();
+                    }
+                    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.
+
+                // Pick the first substitution 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 subst = data.trait_ref.substs.iter().find(|s| s.has_infer_types_or_consts());
+
+                let mut err = if let Some(subst) = subst {
+                    self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
+                } else {
+                    struct_span_err!(
+                        self.tcx.sess,
+                        span,
+                        E0283,
+                        "type annotations needed: cannot satisfy `{}`",
+                        predicate,
+                    )
+                };
+
+                let obligation = Obligation::new(
+                    obligation.cause.clone(),
+                    obligation.param_env,
+                    trait_ref.to_poly_trait_predicate(),
+                );
+                let mut selcx = SelectionContext::with_query_mode(
+                    &self,
+                    crate::traits::TraitQueryMode::Standard,
+                );
+                match selcx.select_from_obligation(&obligation) {
+                    Err(SelectionError::Ambiguous(impls)) if impls.len() > 1 => {
+                        self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
+                    }
+                    _ => {
+                        if self.is_tainted_by_errors() {
+                            err.cancel();
+                            return;
+                        }
+                        err.note(&format!("cannot satisfy `{}`", 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(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.iter().any(|p| p.name != kw::SelfUpper)
+                        && !snippet.ends_with('>')
+                        && !generics.has_impl_trait()
+                        && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
+                    {
+                        // 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 satisfy `_: Tt`
+
+                        // Clear any more general suggestions in favor of our specific one
+                        err.clear_suggestions();
+
+                        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,
+                        );
+                    }
+                }
+
+                if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
+                    (body_id, subst.map(|subst| subst.unpack()))
+                {
+                    struct FindExprBySpan<'hir> {
+                        span: Span,
+                        result: Option<&'hir hir::Expr<'hir>>,
+                    }
+
+                    impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
+                        fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
+                            if self.span == ex.span {
+                                self.result = Some(ex);
+                            } else {
+                                hir::intravisit::walk_expr(self, ex);
+                            }
+                        }
+                    }
+
+                    let mut expr_finder = FindExprBySpan { span, result: None };
+
+                    expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
+
+                    if let Some(hir::Expr {
+                        kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
+                    ) = expr_finder.result
+                        && let [
+                            ..,
+                            trait_path_segment @ hir::PathSegment {
+                                res: Some(rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id)),
+                                ..
+                            },
+                            hir::PathSegment {
+                                ident: assoc_item_name,
+                                res: Some(rustc_hir::def::Res::Def(_, item_id)),
+                                ..
+                            }
+                        ] = path.segments
+                        && data.trait_ref.def_id == *trait_id
+                        && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
+                        && !self.is_tainted_by_errors()
+                    {
+                        let (verb, noun) = match self.tcx.associated_item(item_id).kind {
+                            ty::AssocKind::Const => ("refer to the", "constant"),
+                            ty::AssocKind::Fn => ("call", "function"),
+                            ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
+                        };
+
+                        // Replace the more general E0283 with a more specific error
+                        err.cancel();
+                        err = self.tcx.sess.struct_span_err_with_code(
+                            span,
+                            &format!(
+                                "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
+                             ),
+                            rustc_errors::error_code!(E0790),
+                        );
+
+                        if let Some(local_def_id) = data.trait_ref.def_id.as_local()
+                            && let Some(hir::Node::Item(hir::Item { ident: trait_name, kind: hir::ItemKind::Trait(_, _, _, _, trait_item_refs), .. })) = self.tcx.hir().find_by_def_id(local_def_id)
+                            && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
+                            err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
+                        }
+
+                        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 trait_impls.blanket_impls().is_empty()
+                            && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
+                            && let Some(impl_def_id) = impl_ty.def() {
+                            let message = if trait_impls.non_blanket_impls().len() == 1 {
+                                "use the fully-qualified path to the only available implementation".to_string()
+                            } else {
+                                format!(
+                                    "use a fully-qualified path to a specific available implementation ({} found)",
+                                    trait_impls.non_blanket_impls().len()
+                                )
+                            };
+
+                            err.multipart_suggestion(
+                                message,
+                                vec![
+                                    (trait_path_segment.ident.span.shrink_to_lo(), format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())),
+                                    (trait_path_segment.ident.span.shrink_to_hi(), format!(">"))
+                                ],
+                                Applicability::MaybeIncorrect
+                            );
+                        }
+                    }
+                };
+
+                err
+            }
+
+            ty::PredicateKind::WellFormed(arg) => {
+                // Same hacky approach as above to avoid deluging user
+                // with error messages.
+                if arg.references_error()
+                    || self.tcx.sess.has_errors().is_some()
+                    || self.is_tainted_by_errors()
+                {
+                    return;
+                }
+
+                self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
+            }
+
+            ty::PredicateKind::Subtype(data) => {
+                if data.references_error()
+                    || self.tcx.sess.has_errors().is_some()
+                    || self.is_tainted_by_errors()
+                {
+                    // no need to overload user in such cases
+                    return;
+                }
+                let 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(body_id, span, a.into(), ErrorCode::E0282, true)
+            }
+            ty::PredicateKind::Projection(data) => {
+                if predicate.references_error() || self.is_tainted_by_errors() {
+                    return;
+                }
+                let subst = data
+                    .projection_ty
+                    .substs
+                    .iter()
+                    .chain(Some(data.term.into_arg()))
+                    .find(|g| g.has_infer_types_or_consts());
+                if let Some(subst) = subst {
+                    let mut err = self.emit_inference_failure_err(
+                        body_id,
+                        span,
+                        subst,
+                        ErrorCode::E0284,
+                        true,
+                    );
+                    err.note(&format!("cannot satisfy `{}`", predicate));
+                    err
+                } else {
+                    // If we can't find a substitution, just print a generic error
+                    let mut err = struct_span_err!(
+                        self.tcx.sess,
+                        span,
+                        E0284,
+                        "type annotations needed: cannot satisfy `{}`",
+                        predicate,
+                    );
+                    err.span_label(span, &format!("cannot satisfy `{}`", predicate));
+                    err
+                }
+            }
+
+            ty::PredicateKind::ConstEvaluatable(data) => {
+                if predicate.references_error() || self.is_tainted_by_errors() {
+                    return;
+                }
+                let subst = data.substs.iter().find(|g| g.has_infer_types_or_consts());
+                if let Some(subst) = subst {
+                    let err = self.emit_inference_failure_err(
+                        body_id,
+                        span,
+                        subst,
+                        ErrorCode::E0284,
+                        true,
+                    );
+                    err
+                } else {
+                    // If we can't find a substitution, just print a generic error
+                    let mut err = struct_span_err!(
+                        self.tcx.sess,
+                        span,
+                        E0284,
+                        "type annotations needed: cannot satisfy `{}`",
+                        predicate,
+                    );
+                    err.span_label(span, &format!("cannot satisfy `{}`", predicate));
+                    err
+                }
+            }
+            _ => {
+                if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
+                    return;
+                }
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0284,
+                    "type annotations needed: cannot satisfy `{}`",
+                    predicate,
+                );
+                err.span_label(span, &format!("cannot satisfy `{}`", predicate));
+                err
+            }
+        };
+        self.note_obligation_cause(&mut err, obligation);
+        err.emit();
+    }
+
+    fn annotate_source_of_ambiguity(
+        &self,
+        err: &mut Diagnostic,
+        impls: &[DefId],
+        predicate: ty::Predicate<'tcx>,
+    ) {
+        let mut spans = vec![];
+        let mut crates = vec![];
+        let mut post = vec![];
+        for def_id in impls {
+            match self.tcx.span_of_impl(*def_id) {
+                Ok(span) => spans.push(span),
+                Err(name) => {
+                    crates.push(name);
+                    if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
+                        post.push(header);
+                    }
+                }
+            }
+        }
+        let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
+        let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
+        crate_names.sort();
+        crate_names.dedup();
+        post.sort();
+        post.dedup();
+
+        if self.is_tainted_by_errors()
+            && (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 post = if post.len() > 4 {
+            format!(
+                ":\n{}\nand {} more",
+                post.iter().map(|p| format!("- {}", p)).take(4).collect::<Vec<_>>().join("\n"),
+                post.len() - 4,
+            )
+        } else 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!("{} in the `{}` crate{}", msg, crates[0], post,));
+            }
+            (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{}", crates[0], post,),
+                );
+            }
+            _ => {
+                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,
+                ));
+            }
+        }
+    }
+
+    /// 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(selcx.tcx()),
+            );
+
+            self.predicate_may_hold(&obligation)
+        })
+    }
+
+    fn note_obligation_cause(&self, err: &mut Diagnostic, 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.param_env,
+                obligation.cause.code(),
+                &mut vec![],
+                &mut Default::default(),
+            );
+            self.suggest_unsized_bound_if_applicable(err, obligation);
+        }
+    }
+
+    #[instrument(level = "debug", skip_all)]
+    fn suggest_unsized_bound_if_applicable(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+    ) {
+        let (
+            ty::PredicateKind::Trait(pred),
+            &ObligationCauseCode::BindingObligation(item_def_id, span),
+        ) = (
+            obligation.predicate.kind().skip_binder(),
+            obligation.cause.code().peel_derives(),
+        )  else {
+            return;
+        };
+        debug!(?pred, ?item_def_id, ?span);
+
+        let (Some(node), true) = (
+            self.tcx.hir().get_if_local(item_def_id),
+            Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
+        ) else {
+            return;
+        };
+        self.maybe_suggest_unsized_generics(err, span, node);
+    }
+
+    #[instrument(level = "debug", skip_all)]
+    fn maybe_suggest_unsized_generics<'hir>(
+        &self,
+        err: &mut Diagnostic,
+        span: Span,
+        node: Node<'hir>,
+    ) {
+        let Some(generics) = node.generics() else {
+            return;
+        };
+        let sized_trait = self.tcx.lang_items().sized_trait();
+        debug!(?generics.params);
+        debug!(?generics.predicates);
+        let Some(param) = generics.params.iter().find(|param| param.span == span) else {
+            return;
+        };
+        let param_def_id = self.tcx.hir().local_def_id(param.hir_id);
+        // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
+        // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
+        let explicitly_sized = generics
+            .bounds_for_param(param_def_id)
+            .flat_map(|bp| bp.bounds)
+            .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
+        if explicitly_sized {
+            return;
+        }
+        debug!(?param);
+        match node {
+            hir::Node::Item(
+                item @ hir::Item {
+                    // Only suggest indirection for uses of type parameters in ADTs.
+                    kind:
+                        hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
+                    ..
+                },
+            ) => {
+                if self.maybe_indirection_for_unsized(err, item, param) {
+                    return;
+                }
+            }
+            _ => {}
+        };
+        // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
+        let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param_def_id)
+        {
+            (s, " +")
+        } else {
+            (span.shrink_to_hi(), ":")
+        };
+        err.span_suggestion_verbose(
+            span,
+            "consider relaxing the implicit `Sized` restriction",
+            format!("{} ?Sized", separator),
+            Applicability::MachineApplicable,
+        );
+    }
+
+    fn maybe_indirection_for_unsized<'hir>(
+        &self,
+        err: &mut Diagnostic,
+        item: &'hir Item<'hir>,
+        param: &'hir GenericParam<'hir>,
+    ) -> bool {
+        // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
+        // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
+        // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
+        let mut visitor =
+            FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
+        visitor.visit_item(item);
+        if visitor.invalid_spans.is_empty() {
+            return false;
+        }
+        let mut multispan: MultiSpan = param.span.into();
+        multispan.push_span_label(
+            param.span,
+            format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
+        );
+        for sp in visitor.invalid_spans {
+            multispan.push_span_label(
+                sp,
+                format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
+            );
+        }
+        err.span_help(
+            multispan,
+            &format!(
+                "you could relax the implicit `Sized` bound on `{T}` if it were \
+                used through indirection like `&{T}` or `Box<{T}>`",
+                T = param.name.ident(),
+            ),
+        );
+        true
+    }
+
+    fn is_recursive_obligation(
+        &self,
+        obligated_types: &mut Vec<Ty<'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_pred);
+            let self_ty = parent_trait_ref.skip_binder().self_ty();
+            if obligated_types.iter().any(|ot| ot == &self_ty) {
+                return true;
+            }
+            if let ty::Adt(def, substs) = self_ty.kind()
+                && let [arg] = &substs[..]
+                && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
+                && let ty::Adt(inner_def, _) = ty.kind()
+                && inner_def == def
+            {
+                return true;
+            }
+        }
+        false
+    }
+}
+
+/// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
+/// `param: ?Sized` would be a valid constraint.
+struct FindTypeParam {
+    param: rustc_span::Symbol,
+    invalid_spans: Vec<Span>,
+    nested: bool,
+}
+
+impl<'v> Visitor<'v> for FindTypeParam {
+    fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
+        // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
+    }
+
+    fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
+        // We collect the spans of all uses of the "bare" type param, like in `field: T` or
+        // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
+        // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
+        // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
+        // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
+        // in that case should make what happened clear enough.
+        match ty.kind {
+            hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
+            hir::TyKind::Path(hir::QPath::Resolved(None, path))
+                if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
+            {
+                if !self.nested {
+                    debug!(?ty, "FindTypeParam::visit_ty");
+                    self.invalid_spans.push(ty.span);
+                }
+            }
+            hir::TyKind::Path(_) => {
+                let prev = self.nested;
+                self.nested = true;
+                hir::intravisit::walk_ty(self, ty);
+                self.nested = prev;
+            }
+            _ => {
+                hir::intravisit::walk_ty(self, ty);
+            }
+        }
+    }
+}
+
+pub fn recursive_type_with_infinite_size_error<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    type_def_id: DefId,
+    spans: Vec<(Span, Option<hir::HirId>)>,
+) {
+    assert!(type_def_id.is_local());
+    let span = tcx.def_span(type_def_id);
+    let path = tcx.def_path_str(type_def_id);
+    let mut err =
+        struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
+    err.span_label(span, "recursive type has infinite size");
+    for &(span, _) in &spans {
+        err.span_label(span, "recursive without indirection");
+    }
+    let msg = format!(
+        "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
+        path,
+    );
+    if spans.len() <= 4 {
+        // FIXME(compiler-errors): This suggestion might be erroneous if Box is shadowed
+        err.multipart_suggestion(
+            &msg,
+            spans
+                .into_iter()
+                .flat_map(|(span, field_id)| {
+                    if let Some(generic_span) = get_option_generic_from_field_id(tcx, field_id) {
+                        // If we match an `Option` and can grab the span of the Option's generic, then
+                        // suggest boxing the generic arg for a non-null niche optimization.
+                        vec![
+                            (generic_span.shrink_to_lo(), "Box<".to_string()),
+                            (generic_span.shrink_to_hi(), ">".to_string()),
+                        ]
+                    } else {
+                        vec![
+                            (span.shrink_to_lo(), "Box<".to_string()),
+                            (span.shrink_to_hi(), ">".to_string()),
+                        ]
+                    }
+                })
+                .collect(),
+            Applicability::HasPlaceholders,
+        );
+    } else {
+        err.help(&msg);
+    }
+    err.emit();
+}
+
+/// Extract the span for the generic type `T` of `Option<T>` in a field definition
+fn get_option_generic_from_field_id(tcx: TyCtxt<'_>, field_id: Option<hir::HirId>) -> Option<Span> {
+    let node = tcx.hir().find(field_id?);
+
+    // Expect a field from our field_id
+    let Some(hir::Node::Field(field_def)) = node
+        else { bug!("Expected HirId corresponding to FieldDef, found: {:?}", node) };
+
+    // Match a type that is a simple QPath with no Self
+    let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = &field_def.ty.kind
+        else { return None };
+
+    // Check if the path we're checking resolves to Option
+    let hir::def::Res::Def(_, did) = path.res
+        else { return None };
+
+    // Bail if this path doesn't describe `::core::option::Option`
+    if !tcx.is_diagnostic_item(sym::Option, did) {
+        return None;
+    }
+
+    // Match a single generic arg in the 0th path segment
+    let generic_arg = path.segments.last()?.args?.args.get(0)?;
+
+    // Take the span out of the type, if it's a type
+    if let hir::GenericArg::Type(generic_ty) = generic_arg { Some(generic_ty.span) } else { None }
+}
+
+/// 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 location in the source of the pattern. For an "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(tys) => ArgKind::Tuple(
+                span,
+                tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
+            ),
+            _ => ArgKind::Arg("_".to_owned(), t.to_string()),
+        }
+    }
+}
+
+struct HasNumericInferVisitor;
+
+impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
+    type BreakTy = ();
+
+    fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
+            ControlFlow::Break(())
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
new file mode 100644
index 000000000..e6907637c
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
@@ -0,0 +1,272 @@
+use super::{
+    ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote, PredicateObligation,
+};
+use crate::infer::InferCtxt;
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_middle::ty::subst::{Subst, SubstsRef};
+use rustc_middle::ty::{self, GenericParamDefKind};
+use rustc_span::symbol::sym;
+use std::iter;
+
+use super::InferCtxtPrivExt;
+
+pub trait InferCtxtExt<'tcx> {
+    /*private*/
+    fn impl_similar_to(
+        &self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> Option<(DefId, SubstsRef<'tcx>)>;
+
+    /*private*/
+    fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str>;
+
+    fn on_unimplemented_note(
+        &self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> OnUnimplementedNote;
+}
+
+impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
+    fn impl_similar_to(
+        &self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> Option<(DefId, SubstsRef<'tcx>)> {
+        let tcx = self.tcx;
+        let param_env = obligation.param_env;
+        let trait_ref = tcx.erase_late_bound_regions(trait_ref);
+        let trait_self_ty = trait_ref.self_ty();
+
+        let mut self_match_impls = vec![];
+        let mut fuzzy_match_impls = vec![];
+
+        self.tcx.for_each_relevant_impl(trait_ref.def_id, trait_self_ty, |def_id| {
+            let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
+            let impl_trait_ref = tcx.bound_impl_trait_ref(def_id).unwrap().subst(tcx, impl_substs);
+
+            let impl_self_ty = impl_trait_ref.self_ty();
+
+            if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
+                self_match_impls.push((def_id, impl_substs));
+
+                if iter::zip(
+                    trait_ref.substs.types().skip(1),
+                    impl_trait_ref.substs.types().skip(1),
+                )
+                .all(|(u, v)| self.fuzzy_match_tys(u, v, false).is_some())
+                {
+                    fuzzy_match_impls.push((def_id, impl_substs));
+                }
+            }
+        });
+
+        let impl_def_id_and_substs = if self_match_impls.len() == 1 {
+            self_match_impls[0]
+        } else if fuzzy_match_impls.len() == 1 {
+            fuzzy_match_impls[0]
+        } else {
+            return None;
+        };
+
+        tcx.has_attr(impl_def_id_and_substs.0, sym::rustc_on_unimplemented)
+            .then_some(impl_def_id_and_substs)
+    }
+
+    /// Used to set on_unimplemented's `ItemContext`
+    /// to be the enclosing (async) block/function/closure
+    fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str> {
+        let hir = self.tcx.hir();
+        let node = hir.find(hir_id)?;
+        match &node {
+            hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. }) => {
+                self.describe_generator(*body_id).or_else(|| {
+                    Some(match sig.header {
+                        hir::FnHeader { asyncness: hir::IsAsync::Async, .. } => "an async function",
+                        _ => "a function",
+                    })
+                })
+            }
+            hir::Node::TraitItem(hir::TraitItem {
+                kind: hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)),
+                ..
+            }) => self.describe_generator(*body_id).or_else(|| Some("a trait method")),
+            hir::Node::ImplItem(hir::ImplItem {
+                kind: hir::ImplItemKind::Fn(sig, body_id),
+                ..
+            }) => self.describe_generator(*body_id).or_else(|| {
+                Some(match sig.header {
+                    hir::FnHeader { asyncness: hir::IsAsync::Async, .. } => "an async method",
+                    _ => "a method",
+                })
+            }),
+            hir::Node::Expr(hir::Expr {
+                kind: hir::ExprKind::Closure(hir::Closure { body, movability, .. }),
+                ..
+            }) => self.describe_generator(*body).or_else(|| {
+                Some(if movability.is_some() { "an async closure" } else { "a closure" })
+            }),
+            hir::Node::Expr(hir::Expr { .. }) => {
+                let parent_hid = hir.get_parent_node(hir_id);
+                if parent_hid != hir_id { self.describe_enclosure(parent_hid) } else { None }
+            }
+            _ => None,
+        }
+    }
+
+    fn on_unimplemented_note(
+        &self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> OnUnimplementedNote {
+        let (def_id, substs) = self
+            .impl_similar_to(trait_ref, obligation)
+            .unwrap_or_else(|| (trait_ref.def_id(), trait_ref.skip_binder().substs));
+        let trait_ref = trait_ref.skip_binder();
+
+        let mut flags = vec![(
+            sym::ItemContext,
+            self.describe_enclosure(obligation.cause.body_id).map(|s| s.to_owned()),
+        )];
+
+        match obligation.cause.code() {
+            ObligationCauseCode::BuiltinDerivedObligation(..)
+            | ObligationCauseCode::ImplDerivedObligation(..)
+            | ObligationCauseCode::DerivedObligation(..) => {}
+            _ => {
+                // this is a "direct", user-specified, rather than derived,
+                // obligation.
+                flags.push((sym::direct, None));
+            }
+        }
+
+        if let ObligationCauseCode::ItemObligation(item)
+        | ObligationCauseCode::BindingObligation(item, _) = *obligation.cause.code()
+        {
+            // FIXME: maybe also have some way of handling methods
+            // from other traits? That would require name resolution,
+            // which we might want to be some sort of hygienic.
+            //
+            // Currently I'm leaving it for what I need for `try`.
+            if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
+                let method = self.tcx.item_name(item);
+                flags.push((sym::from_method, None));
+                flags.push((sym::from_method, Some(method.to_string())));
+            }
+        }
+
+        if let Some(k) = obligation.cause.span.desugaring_kind() {
+            flags.push((sym::from_desugaring, None));
+            flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
+        }
+
+        // Add all types without trimmed paths.
+        ty::print::with_no_trimmed_paths!({
+            let generics = self.tcx.generics_of(def_id);
+            let self_ty = trait_ref.self_ty();
+            // This is also included through the generics list as `Self`,
+            // but the parser won't allow you to use it
+            flags.push((sym::_Self, Some(self_ty.to_string())));
+            if let Some(def) = self_ty.ty_adt_def() {
+                // We also want to be able to select self's original
+                // signature with no type arguments resolved
+                flags.push((sym::_Self, Some(self.tcx.type_of(def.did()).to_string())));
+            }
+
+            for param in generics.params.iter() {
+                let value = match param.kind {
+                    GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
+                        substs[param.index as usize].to_string()
+                    }
+                    GenericParamDefKind::Lifetime => continue,
+                };
+                let name = param.name;
+                flags.push((name, Some(value)));
+
+                if let GenericParamDefKind::Type { .. } = param.kind {
+                    let param_ty = substs[param.index as usize].expect_ty();
+                    if let Some(def) = param_ty.ty_adt_def() {
+                        // We also want to be able to select the parameter's
+                        // original signature with no type arguments resolved
+                        flags.push((name, Some(self.tcx.type_of(def.did()).to_string())));
+                    }
+                }
+            }
+
+            if let Some(true) = self_ty.ty_adt_def().map(|def| def.did().is_local()) {
+                flags.push((sym::crate_local, None));
+            }
+
+            // Allow targeting all integers using `{integral}`, even if the exact type was resolved
+            if self_ty.is_integral() {
+                flags.push((sym::_Self, Some("{integral}".to_owned())));
+            }
+
+            if self_ty.is_array_slice() {
+                flags.push((sym::_Self, Some("&[]".to_owned())));
+            }
+
+            if self_ty.is_fn() {
+                let fn_sig = self_ty.fn_sig(self.tcx);
+                let shortname = match fn_sig.unsafety() {
+                    hir::Unsafety::Normal => "fn",
+                    hir::Unsafety::Unsafe => "unsafe fn",
+                };
+                flags.push((sym::_Self, Some(shortname.to_owned())));
+            }
+
+            // Slices give us `[]`, `[{ty}]`
+            if let ty::Slice(aty) = self_ty.kind() {
+                flags.push((sym::_Self, Some("[]".to_string())));
+                if let Some(def) = aty.ty_adt_def() {
+                    // We also want to be able to select the slice's type's original
+                    // signature with no type arguments resolved
+                    flags.push((sym::_Self, Some(format!("[{}]", self.tcx.type_of(def.did())))));
+                }
+                if aty.is_integral() {
+                    flags.push((sym::_Self, Some("[{integral}]".to_string())));
+                }
+            }
+
+            // Arrays give us `[]`, `[{ty}; _]` and `[{ty}; N]`
+            if let ty::Array(aty, len) = self_ty.kind() {
+                flags.push((sym::_Self, Some("[]".to_string())));
+                let len = len.kind().try_to_value().and_then(|v| v.try_to_machine_usize(self.tcx));
+                flags.push((sym::_Self, Some(format!("[{}; _]", aty))));
+                if let Some(n) = len {
+                    flags.push((sym::_Self, Some(format!("[{}; {}]", aty, n))));
+                }
+                if let Some(def) = aty.ty_adt_def() {
+                    // We also want to be able to select the array's type's original
+                    // signature with no type arguments resolved
+                    let def_ty = self.tcx.type_of(def.did());
+                    flags.push((sym::_Self, Some(format!("[{def_ty}; _]"))));
+                    if let Some(n) = len {
+                        flags.push((sym::_Self, Some(format!("[{def_ty}; {n}]"))));
+                    }
+                }
+                if aty.is_integral() {
+                    flags.push((sym::_Self, Some("[{integral}; _]".to_string())));
+                    if let Some(n) = len {
+                        flags.push((sym::_Self, Some(format!("[{{integral}}; {n}]"))));
+                    }
+                }
+            }
+            if let ty::Dynamic(traits, _) = self_ty.kind() {
+                for t in traits.iter() {
+                    if let ty::ExistentialPredicate::Trait(trait_ref) = t.skip_binder() {
+                        flags.push((sym::_Self, Some(self.tcx.def_path_str(trait_ref.def_id))))
+                    }
+                }
+            }
+        });
+
+        if let Ok(Some(command)) = OnUnimplementedDirective::of_item(self.tcx, def_id) {
+            command.evaluate(self.tcx, trait_ref, &flags)
+        } else {
+            OnUnimplementedNote::default()
+        }
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
new file mode 100644
index 000000000..219413121
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -0,0 +1,3119 @@
+use super::{
+    EvaluationResult, Obligation, ObligationCause, ObligationCauseCode, PredicateObligation,
+    SelectionContext,
+};
+
+use crate::autoderef::Autoderef;
+use crate::infer::InferCtxt;
+use crate::traits::normalize_to;
+
+use hir::HirId;
+use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::stack::ensure_sufficient_stack;
+use rustc_errors::{
+    error_code, pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder,
+    ErrorGuaranteed, MultiSpan, Style,
+};
+use rustc_hir as hir;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::DefId;
+use rustc_hir::intravisit::Visitor;
+use rustc_hir::lang_items::LangItem;
+use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_middle::hir::map;
+use rustc_middle::ty::{
+    self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
+    GeneratorDiagnosticData, GeneratorInteriorTypeCause, Infer, InferTy, IsSuggestable,
+    ProjectionPredicate, ToPredicate, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
+    TypeVisitable,
+};
+use rustc_middle::ty::{TypeAndMut, TypeckResults};
+use rustc_session::Limit;
+use rustc_span::def_id::LOCAL_CRATE;
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::{BytePos, DesugaringKind, ExpnKind, Span, DUMMY_SP};
+use rustc_target::spec::abi;
+use std::fmt;
+
+use super::InferCtxtPrivExt;
+use crate::infer::InferCtxtExt as _;
+use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
+use rustc_middle::ty::print::with_no_trimmed_paths;
+
+#[derive(Debug)]
+pub enum GeneratorInteriorOrUpvar {
+    // span of interior type
+    Interior(Span),
+    // span of upvar
+    Upvar(Span),
+}
+
+// This type provides a uniform interface to retrieve data on generators, whether it originated from
+// the local crate being compiled or from a foreign crate.
+#[derive(Debug)]
+pub enum GeneratorData<'tcx, 'a> {
+    Local(&'a TypeckResults<'tcx>),
+    Foreign(&'tcx GeneratorDiagnosticData<'tcx>),
+}
+
+impl<'tcx, 'a> GeneratorData<'tcx, 'a> {
+    // Try to get information about variables captured by the generator that matches a type we are
+    // looking for with `ty_matches` function. We uses it to find upvar which causes a failure to
+    // meet an obligation
+    fn try_get_upvar_span<F>(
+        &self,
+        infer_context: &InferCtxt<'a, 'tcx>,
+        generator_did: DefId,
+        ty_matches: F,
+    ) -> Option<GeneratorInteriorOrUpvar>
+    where
+        F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
+    {
+        match self {
+            GeneratorData::Local(typeck_results) => {
+                infer_context.tcx.upvars_mentioned(generator_did).and_then(|upvars| {
+                    upvars.iter().find_map(|(upvar_id, upvar)| {
+                        let upvar_ty = typeck_results.node_type(*upvar_id);
+                        let upvar_ty = infer_context.resolve_vars_if_possible(upvar_ty);
+                        if ty_matches(ty::Binder::dummy(upvar_ty)) {
+                            Some(GeneratorInteriorOrUpvar::Upvar(upvar.span))
+                        } else {
+                            None
+                        }
+                    })
+                })
+            }
+            GeneratorData::Foreign(_) => None,
+        }
+    }
+
+    // Try to get the span of a type being awaited on that matches the type we are looking with the
+    // `ty_matches` function. We uses it to find awaited type which causes a failure to meet an
+    // obligation
+    fn get_from_await_ty<F>(
+        &self,
+        visitor: AwaitsVisitor,
+        hir: map::Map<'tcx>,
+        ty_matches: F,
+    ) -> Option<Span>
+    where
+        F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
+    {
+        match self {
+            GeneratorData::Local(typeck_results) => visitor
+                .awaits
+                .into_iter()
+                .map(|id| hir.expect_expr(id))
+                .find(|await_expr| {
+                    ty_matches(ty::Binder::dummy(typeck_results.expr_ty_adjusted(&await_expr)))
+                })
+                .map(|expr| expr.span),
+            GeneratorData::Foreign(generator_diagnostic_data) => visitor
+                .awaits
+                .into_iter()
+                .map(|id| hir.expect_expr(id))
+                .find(|await_expr| {
+                    ty_matches(ty::Binder::dummy(
+                        generator_diagnostic_data
+                            .adjustments
+                            .get(&await_expr.hir_id.local_id)
+                            .map_or::<&[ty::adjustment::Adjustment<'tcx>], _>(&[], |a| &a[..])
+                            .last()
+                            .map_or_else::<Ty<'tcx>, _, _>(
+                                || {
+                                    generator_diagnostic_data
+                                        .nodes_types
+                                        .get(&await_expr.hir_id.local_id)
+                                        .cloned()
+                                        .unwrap_or_else(|| {
+                                            bug!(
+                                                "node_type: no type for node `{}`",
+                                                ty::tls::with(|tcx| tcx
+                                                    .hir()
+                                                    .node_to_string(await_expr.hir_id))
+                                            )
+                                        })
+                                },
+                                |adj| adj.target,
+                            ),
+                    ))
+                })
+                .map(|expr| expr.span),
+        }
+    }
+
+    /// Get the type, expression, span and optional scope span of all types
+    /// that are live across the yield of this generator
+    fn get_generator_interior_types(
+        &self,
+    ) -> ty::Binder<'tcx, &[GeneratorInteriorTypeCause<'tcx>]> {
+        match self {
+            GeneratorData::Local(typeck_result) => {
+                typeck_result.generator_interior_types.as_deref()
+            }
+            GeneratorData::Foreign(generator_diagnostic_data) => {
+                generator_diagnostic_data.generator_interior_types.as_deref()
+            }
+        }
+    }
+
+    // Used to get the source of the data, note we don't have as much information for generators
+    // originated from foreign crates
+    fn is_foreign(&self) -> bool {
+        match self {
+            GeneratorData::Local(_) => false,
+            GeneratorData::Foreign(_) => true,
+        }
+    }
+}
+
+// This trait is public to expose the diagnostics methods to clippy.
+pub trait InferCtxtExt<'tcx> {
+    fn suggest_restricting_param_bound(
+        &self,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+        proj_pred: Option<ty::PolyProjectionPredicate<'tcx>>,
+        body_id: hir::HirId,
+    );
+
+    fn suggest_dereferences(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool;
+
+    fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol>;
+
+    fn suggest_fn_call(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool;
+
+    fn suggest_add_reference_to_arg(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+        has_custom_message: bool,
+    ) -> bool;
+
+    fn suggest_borrowing_for_object_cast(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+        self_ty: Ty<'tcx>,
+        object_ty: Ty<'tcx>,
+    );
+
+    fn suggest_remove_reference(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool;
+
+    fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic);
+
+    fn suggest_change_mut(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    );
+
+    fn suggest_semicolon_removal(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        span: Span,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool;
+
+    fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span>;
+
+    fn suggest_impl_trait(
+        &self,
+        err: &mut Diagnostic,
+        span: Span,
+        obligation: &PredicateObligation<'tcx>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool;
+
+    fn point_at_returns_when_relevant(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+    );
+
+    fn report_closure_arg_mismatch(
+        &self,
+        span: Span,
+        found_span: Option<Span>,
+        found: ty::PolyTraitRef<'tcx>,
+        expected: ty::PolyTraitRef<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
+
+    fn suggest_fully_qualified_path(
+        &self,
+        err: &mut Diagnostic,
+        item_def_id: DefId,
+        span: Span,
+        trait_ref: DefId,
+    );
+
+    fn maybe_note_obligation_cause_for_async_await(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> bool;
+
+    fn note_obligation_cause_for_async_await(
+        &self,
+        err: &mut Diagnostic,
+        interior_or_upvar_span: GeneratorInteriorOrUpvar,
+        interior_extra_info: Option<(Option<Span>, Span, Option<hir::HirId>, Option<Span>)>,
+        is_async: bool,
+        outer_generator: Option<DefId>,
+        trait_pred: ty::TraitPredicate<'tcx>,
+        target_ty: Ty<'tcx>,
+        typeck_results: Option<&ty::TypeckResults<'tcx>>,
+        obligation: &PredicateObligation<'tcx>,
+        next_code: Option<&ObligationCauseCode<'tcx>>,
+    );
+
+    fn note_obligation_cause_code<T>(
+        &self,
+        err: &mut Diagnostic,
+        predicate: &T,
+        param_env: ty::ParamEnv<'tcx>,
+        cause_code: &ObligationCauseCode<'tcx>,
+        obligated_types: &mut Vec<Ty<'tcx>>,
+        seen_requirements: &mut FxHashSet<DefId>,
+    ) where
+        T: fmt::Display;
+
+    fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
+
+    /// Suggest to await before try: future? => future.await?
+    fn suggest_await_before_try(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+        span: Span,
+    );
+
+    fn suggest_floating_point_literal(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_ref: &ty::PolyTraitRef<'tcx>,
+    );
+
+    fn suggest_derive(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    );
+
+    fn suggest_dereferencing_index(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    );
+}
+
+fn predicate_constraint(generics: &hir::Generics<'_>, pred: String) -> (Span, String) {
+    (
+        generics.tail_span_for_predicate_suggestion(),
+        format!("{} {}", generics.add_where_or_trailing_comma(), pred),
+    )
+}
+
+/// Type parameter needs more bounds. The trivial case is `T` `where T: Bound`, but
+/// it can also be an `impl Trait` param that needs to be decomposed to a type
+/// param for cleaner code.
+fn suggest_restriction<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    hir_id: HirId,
+    hir_generics: &hir::Generics<'tcx>,
+    msg: &str,
+    err: &mut Diagnostic,
+    fn_sig: Option<&hir::FnSig<'_>>,
+    projection: Option<&ty::ProjectionTy<'_>>,
+    trait_pred: ty::PolyTraitPredicate<'tcx>,
+    // When we are dealing with a trait, `super_traits` will be `Some`:
+    // Given `trait T: A + B + C {}`
+    //              -  ^^^^^^^^^ GenericBounds
+    //              |
+    //              &Ident
+    super_traits: Option<(&Ident, &hir::GenericBounds<'_>)>,
+) {
+    if hir_generics.where_clause_span.from_expansion()
+        || hir_generics.where_clause_span.desugaring_kind().is_some()
+    {
+        return;
+    }
+    let Some(item_id) = hir_id.as_owner() else { return; };
+    let generics = tcx.generics_of(item_id);
+    // Given `fn foo(t: impl Trait)` where `Trait` requires assoc type `A`...
+    if let Some((param, bound_str, fn_sig)) =
+        fn_sig.zip(projection).and_then(|(sig, p)| match p.self_ty().kind() {
+            // Shenanigans to get the `Trait` from the `impl Trait`.
+            ty::Param(param) => {
+                let param_def = generics.type_param(param, tcx);
+                if param_def.kind.is_synthetic() {
+                    let bound_str =
+                        param_def.name.as_str().strip_prefix("impl ")?.trim_start().to_string();
+                    return Some((param_def, bound_str, sig));
+                }
+                None
+            }
+            _ => None,
+        })
+    {
+        let type_param_name = hir_generics.params.next_type_param_name(Some(&bound_str));
+        let trait_pred = trait_pred.fold_with(&mut ReplaceImplTraitFolder {
+            tcx,
+            param,
+            replace_ty: ty::ParamTy::new(generics.count() as u32, Symbol::intern(&type_param_name))
+                .to_ty(tcx),
+        });
+        if !trait_pred.is_suggestable(tcx, false) {
+            return;
+        }
+        // We know we have an `impl Trait` that doesn't satisfy a required projection.
+
+        // Find all of the occurrences of `impl Trait` for `Trait` in the function arguments'
+        // types. There should be at least one, but there might be *more* than one. In that
+        // case we could just ignore it and try to identify which one needs the restriction,
+        // but instead we choose to suggest replacing all instances of `impl Trait` with `T`
+        // where `T: Trait`.
+        let mut ty_spans = vec![];
+        for input in fn_sig.decl.inputs {
+            ReplaceImplTraitVisitor { ty_spans: &mut ty_spans, param_did: param.def_id }
+                .visit_ty(input);
+        }
+        // The type param `T: Trait` we will suggest to introduce.
+        let type_param = format!("{}: {}", type_param_name, bound_str);
+
+        let mut sugg = vec![
+            if let Some(span) = hir_generics.span_for_param_suggestion() {
+                (span, format!(", {}", type_param))
+            } else {
+                (hir_generics.span, format!("<{}>", type_param))
+            },
+            // `fn foo(t: impl Trait)`
+            //                       ^ suggest `where <T as Trait>::A: Bound`
+            predicate_constraint(hir_generics, trait_pred.to_predicate(tcx).to_string()),
+        ];
+        sugg.extend(ty_spans.into_iter().map(|s| (s, type_param_name.to_string())));
+
+        // Suggest `fn foo<T: Trait>(t: T) where <T as Trait>::A: Bound`.
+        // FIXME: once `#![feature(associated_type_bounds)]` is stabilized, we should suggest
+        // `fn foo(t: impl Trait<A: Bound>)` instead.
+        err.multipart_suggestion(
+            "introduce a type parameter with a trait bound instead of using `impl Trait`",
+            sugg,
+            Applicability::MaybeIncorrect,
+        );
+    } else {
+        if !trait_pred.is_suggestable(tcx, false) {
+            return;
+        }
+        // Trivial case: `T` needs an extra bound: `T: Bound`.
+        let (sp, suggestion) = match (
+            hir_generics
+                .params
+                .iter()
+                .find(|p| !matches!(p.kind, hir::GenericParamKind::Type { synthetic: true, .. })),
+            super_traits,
+        ) {
+            (_, None) => {
+                predicate_constraint(hir_generics, trait_pred.to_predicate(tcx).to_string())
+            }
+            (None, Some((ident, []))) => (
+                ident.span.shrink_to_hi(),
+                format!(": {}", trait_pred.print_modifiers_and_trait_path()),
+            ),
+            (_, Some((_, [.., bounds]))) => (
+                bounds.span().shrink_to_hi(),
+                format!(" + {}", trait_pred.print_modifiers_and_trait_path()),
+            ),
+            (Some(_), Some((_, []))) => (
+                hir_generics.span.shrink_to_hi(),
+                format!(": {}", trait_pred.print_modifiers_and_trait_path()),
+            ),
+        };
+
+        err.span_suggestion_verbose(
+            sp,
+            &format!("consider further restricting {}", msg),
+            suggestion,
+            Applicability::MachineApplicable,
+        );
+    }
+}
+
+impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
+    fn suggest_restricting_param_bound(
+        &self,
+        mut err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+        proj_pred: Option<ty::PolyProjectionPredicate<'tcx>>,
+        body_id: hir::HirId,
+    ) {
+        let trait_pred = self.resolve_numeric_literals_with_default(trait_pred);
+
+        let self_ty = trait_pred.skip_binder().self_ty();
+        let (param_ty, projection) = match self_ty.kind() {
+            ty::Param(_) => (true, None),
+            ty::Projection(projection) => (false, Some(projection)),
+            _ => (false, None),
+        };
+
+        // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
+        //        don't suggest `T: Sized + ?Sized`.
+        let mut hir_id = body_id;
+        while let Some(node) = self.tcx.hir().find(hir_id) {
+            match node {
+                hir::Node::Item(hir::Item {
+                    ident,
+                    kind: hir::ItemKind::Trait(_, _, generics, bounds, _),
+                    ..
+                }) if self_ty == self.tcx.types.self_param => {
+                    assert!(param_ty);
+                    // Restricting `Self` for a single method.
+                    suggest_restriction(
+                        self.tcx,
+                        hir_id,
+                        &generics,
+                        "`Self`",
+                        err,
+                        None,
+                        projection,
+                        trait_pred,
+                        Some((ident, bounds)),
+                    );
+                    return;
+                }
+
+                hir::Node::TraitItem(hir::TraitItem {
+                    generics,
+                    kind: hir::TraitItemKind::Fn(..),
+                    ..
+                }) if self_ty == self.tcx.types.self_param => {
+                    assert!(param_ty);
+                    // Restricting `Self` for a single method.
+                    suggest_restriction(
+                        self.tcx, hir_id, &generics, "`Self`", err, None, projection, trait_pred,
+                        None,
+                    );
+                    return;
+                }
+
+                hir::Node::TraitItem(hir::TraitItem {
+                    generics,
+                    kind: hir::TraitItemKind::Fn(fn_sig, ..),
+                    ..
+                })
+                | hir::Node::ImplItem(hir::ImplItem {
+                    generics,
+                    kind: hir::ImplItemKind::Fn(fn_sig, ..),
+                    ..
+                })
+                | hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Fn(fn_sig, generics, _), ..
+                }) if projection.is_some() => {
+                    // Missing restriction on associated type of type parameter (unmet projection).
+                    suggest_restriction(
+                        self.tcx,
+                        hir_id,
+                        &generics,
+                        "the associated type",
+                        err,
+                        Some(fn_sig),
+                        projection,
+                        trait_pred,
+                        None,
+                    );
+                    return;
+                }
+                hir::Node::Item(hir::Item {
+                    kind:
+                        hir::ItemKind::Trait(_, _, generics, ..)
+                        | hir::ItemKind::Impl(hir::Impl { generics, .. }),
+                    ..
+                }) if projection.is_some() => {
+                    // Missing restriction on associated type of type parameter (unmet projection).
+                    suggest_restriction(
+                        self.tcx,
+                        hir_id,
+                        &generics,
+                        "the associated type",
+                        err,
+                        None,
+                        projection,
+                        trait_pred,
+                        None,
+                    );
+                    return;
+                }
+
+                hir::Node::Item(hir::Item {
+                    kind:
+                        hir::ItemKind::Struct(_, generics)
+                        | hir::ItemKind::Enum(_, generics)
+                        | hir::ItemKind::Union(_, generics)
+                        | hir::ItemKind::Trait(_, _, generics, ..)
+                        | hir::ItemKind::Impl(hir::Impl { generics, .. })
+                        | hir::ItemKind::Fn(_, generics, _)
+                        | hir::ItemKind::TyAlias(_, generics)
+                        | hir::ItemKind::TraitAlias(generics, _)
+                        | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
+                    ..
+                })
+                | hir::Node::TraitItem(hir::TraitItem { generics, .. })
+                | hir::Node::ImplItem(hir::ImplItem { generics, .. })
+                    if param_ty =>
+                {
+                    // We skip the 0'th subst (self) because we do not want
+                    // to consider the predicate as not suggestible if the
+                    // self type is an arg position `impl Trait` -- instead,
+                    // we handle that by adding ` + Bound` below.
+                    // FIXME(compiler-errors): It would be nice to do the same
+                    // this that we do in `suggest_restriction` and pull the
+                    // `impl Trait` into a new generic if it shows up somewhere
+                    // else in the predicate.
+                    if !trait_pred.skip_binder().trait_ref.substs[1..]
+                        .iter()
+                        .all(|g| g.is_suggestable(self.tcx, false))
+                    {
+                        return;
+                    }
+                    // Missing generic type parameter bound.
+                    let param_name = self_ty.to_string();
+                    let mut constraint = with_no_trimmed_paths!(
+                        trait_pred.print_modifiers_and_trait_path().to_string()
+                    );
+
+                    if let Some(proj_pred) = proj_pred {
+                        let ProjectionPredicate { projection_ty, term } = proj_pred.skip_binder();
+                        let item = self.tcx.associated_item(projection_ty.item_def_id);
+
+                        // FIXME: this case overlaps with code in TyCtxt::note_and_explain_type_err.
+                        // That should be extracted into a helper function.
+                        if constraint.ends_with('>') {
+                            constraint = format!(
+                                "{}, {}={}>",
+                                &constraint[..constraint.len() - 1],
+                                item.name,
+                                term
+                            );
+                        } else {
+                            constraint.push_str(&format!("<{}={}>", item.name, term));
+                        }
+                    }
+
+                    if suggest_constraining_type_param(
+                        self.tcx,
+                        generics,
+                        &mut err,
+                        &param_name,
+                        &constraint,
+                        Some(trait_pred.def_id()),
+                    ) {
+                        return;
+                    }
+                }
+
+                hir::Node::Item(hir::Item {
+                    kind:
+                        hir::ItemKind::Struct(_, generics)
+                        | hir::ItemKind::Enum(_, generics)
+                        | hir::ItemKind::Union(_, generics)
+                        | hir::ItemKind::Trait(_, _, generics, ..)
+                        | hir::ItemKind::Impl(hir::Impl { generics, .. })
+                        | hir::ItemKind::Fn(_, generics, _)
+                        | hir::ItemKind::TyAlias(_, generics)
+                        | hir::ItemKind::TraitAlias(generics, _)
+                        | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
+                    ..
+                }) if !param_ty => {
+                    // Missing generic type parameter bound.
+                    if suggest_arbitrary_trait_bound(self.tcx, generics, &mut err, trait_pred) {
+                        return;
+                    }
+                }
+                hir::Node::Crate(..) => return,
+
+                _ => {}
+            }
+
+            hir_id = self.tcx.hir().local_def_id_to_hir_id(self.tcx.hir().get_parent_item(hir_id));
+        }
+    }
+
+    /// When after several dereferencing, the reference satisfies the trait
+    /// binding. This function provides dereference suggestion for this
+    /// specific situation.
+    fn suggest_dereferences(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool {
+        // It only make sense when suggesting dereferences for arguments
+        let ObligationCauseCode::FunctionArgumentObligation { .. } = obligation.cause.code() else {
+            return false;
+        };
+        let param_env = obligation.param_env;
+        let body_id = obligation.cause.body_id;
+        let span = obligation.cause.span;
+        let mut real_trait_pred = trait_pred;
+        let mut code = obligation.cause.code();
+        while let Some((parent_code, parent_trait_pred)) = code.parent() {
+            code = parent_code;
+            if let Some(parent_trait_pred) = parent_trait_pred {
+                real_trait_pred = parent_trait_pred;
+            }
+
+            // Skipping binder here, remapping below
+            let real_ty = real_trait_pred.self_ty().skip_binder();
+
+            if let ty::Ref(region, base_ty, mutbl) = *real_ty.kind() {
+                let mut autoderef = Autoderef::new(self, param_env, body_id, span, base_ty, span);
+                if let Some(steps) = autoderef.find_map(|(ty, steps)| {
+                    // Re-add the `&`
+                    let ty = self.tcx.mk_ref(region, TypeAndMut { ty, mutbl });
+
+                    // Remapping bound vars here
+                    let real_trait_pred_and_ty =
+                        real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, ty));
+                    let obligation = self
+                        .mk_trait_obligation_with_new_self_ty(param_env, real_trait_pred_and_ty);
+                    Some(steps).filter(|_| self.predicate_may_hold(&obligation))
+                }) {
+                    if steps > 0 {
+                        if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(span) {
+                            // Don't care about `&mut` because `DerefMut` is used less
+                            // often and user will not expect autoderef happens.
+                            if src.starts_with('&') && !src.starts_with("&mut ") {
+                                let derefs = "*".repeat(steps);
+                                err.span_suggestion(
+                                    span,
+                                    "consider dereferencing here",
+                                    format!("&{}{}", derefs, &src[1..]),
+                                    Applicability::MachineApplicable,
+                                );
+                                return true;
+                            }
+                        }
+                    }
+                } else if real_trait_pred != trait_pred {
+                    // This branch addresses #87437.
+
+                    // Remapping bound vars here
+                    let real_trait_pred_and_base_ty =
+                        real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, base_ty));
+                    let obligation = self.mk_trait_obligation_with_new_self_ty(
+                        param_env,
+                        real_trait_pred_and_base_ty,
+                    );
+                    if self.predicate_may_hold(&obligation) {
+                        err.span_suggestion_verbose(
+                            span.shrink_to_lo(),
+                            "consider dereferencing here",
+                            "*",
+                            Applicability::MachineApplicable,
+                        );
+                        return true;
+                    }
+                }
+            }
+        }
+        false
+    }
+
+    /// Given a closure's `DefId`, return the given name of the closure.
+    ///
+    /// This doesn't account for reassignments, but it's only used for suggestions.
+    fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol> {
+        let get_name = |err: &mut Diagnostic, kind: &hir::PatKind<'_>| -> Option<Symbol> {
+            // Get the local name of this closure. This can be inaccurate because
+            // of the possibility of reassignment, but this should be good enough.
+            match &kind {
+                hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, None) => {
+                    Some(ident.name)
+                }
+                _ => {
+                    err.note(msg);
+                    None
+                }
+            }
+        };
+
+        let hir = self.tcx.hir();
+        let hir_id = hir.local_def_id_to_hir_id(def_id.as_local()?);
+        let parent_node = hir.get_parent_node(hir_id);
+        match hir.find(parent_node) {
+            Some(hir::Node::Stmt(hir::Stmt { kind: hir::StmtKind::Local(local), .. })) => {
+                get_name(err, &local.pat.kind)
+            }
+            // Different to previous arm because one is `&hir::Local` and the other
+            // is `P<hir::Local>`.
+            Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
+            _ => None,
+        }
+    }
+
+    /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
+    /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
+    /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
+    fn suggest_fn_call(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool {
+        // Skipping binder here, remapping below
+        let self_ty = trait_pred.self_ty().skip_binder();
+
+        let (def_id, output_ty, callable) = match *self_ty.kind() {
+            ty::Closure(def_id, substs) => (def_id, substs.as_closure().sig().output(), "closure"),
+            ty::FnDef(def_id, _) => (def_id, self_ty.fn_sig(self.tcx).output(), "function"),
+            _ => return false,
+        };
+        let msg = format!("use parentheses to call the {}", callable);
+
+        // "We should really create a single list of bound vars from the combined vars
+        // from the predicate and function, but instead we just liberate the function bound vars"
+        let output_ty = self.tcx.liberate_late_bound_regions(def_id, output_ty);
+
+        // Remapping bound vars here
+        let trait_pred_and_self = trait_pred.map_bound(|trait_pred| (trait_pred, output_ty));
+
+        let new_obligation =
+            self.mk_trait_obligation_with_new_self_ty(obligation.param_env, trait_pred_and_self);
+
+        match self.evaluate_obligation(&new_obligation) {
+            Ok(
+                EvaluationResult::EvaluatedToOk
+                | EvaluationResult::EvaluatedToOkModuloRegions
+                | EvaluationResult::EvaluatedToOkModuloOpaqueTypes
+                | EvaluationResult::EvaluatedToAmbig,
+            ) => {}
+            _ => return false,
+        }
+        let hir = self.tcx.hir();
+        // Get the name of the callable and the arguments to be used in the suggestion.
+        let (snippet, sugg) = match hir.get_if_local(def_id) {
+            Some(hir::Node::Expr(hir::Expr {
+                kind: hir::ExprKind::Closure(hir::Closure { fn_decl, fn_decl_span, .. }),
+                ..
+            })) => {
+                err.span_label(*fn_decl_span, "consider calling this closure");
+                let Some(name) = self.get_closure_name(def_id, err, &msg) else {
+                    return false;
+                };
+                let args = fn_decl.inputs.iter().map(|_| "_").collect::<Vec<_>>().join(", ");
+                let sugg = format!("({})", args);
+                (format!("{}{}", name, sugg), sugg)
+            }
+            Some(hir::Node::Item(hir::Item {
+                ident,
+                kind: hir::ItemKind::Fn(.., body_id),
+                ..
+            })) => {
+                err.span_label(ident.span, "consider calling this function");
+                let body = hir.body(*body_id);
+                let args = body
+                    .params
+                    .iter()
+                    .map(|arg| match &arg.pat.kind {
+                        hir::PatKind::Binding(_, _, ident, None)
+                        // FIXME: provide a better suggestion when encountering `SelfLower`, it
+                        // should suggest a method call.
+                        if ident.name != kw::SelfLower => ident.to_string(),
+                        _ => "_".to_string(),
+                    })
+                    .collect::<Vec<_>>()
+                    .join(", ");
+                let sugg = format!("({})", args);
+                (format!("{}{}", ident, sugg), sugg)
+            }
+            _ => return false,
+        };
+        if matches!(obligation.cause.code(), ObligationCauseCode::FunctionArgumentObligation { .. })
+        {
+            // When the obligation error has been ensured to have been caused by
+            // an argument, the `obligation.cause.span` points at the expression
+            // of the argument, so we can provide a suggestion. Otherwise, we give
+            // a more general note.
+            err.span_suggestion_verbose(
+                obligation.cause.span.shrink_to_hi(),
+                &msg,
+                sugg,
+                Applicability::HasPlaceholders,
+            );
+        } else {
+            err.help(&format!("{}: `{}`", msg, snippet));
+        }
+        true
+    }
+
+    fn suggest_add_reference_to_arg(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        poly_trait_pred: ty::PolyTraitPredicate<'tcx>,
+        has_custom_message: bool,
+    ) -> bool {
+        let span = obligation.cause.span;
+
+        let code = if let ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } =
+            obligation.cause.code()
+        {
+            &parent_code
+        } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
+            span.ctxt().outer_expn_data().kind
+        {
+            obligation.cause.code()
+        } else {
+            return false;
+        };
+
+        // List of traits for which it would be nonsensical to suggest borrowing.
+        // For instance, immutable references are always Copy, so suggesting to
+        // borrow would always succeed, but it's probably not what the user wanted.
+        let mut never_suggest_borrow: Vec<_> =
+            [LangItem::Copy, LangItem::Clone, LangItem::Unpin, LangItem::Sized]
+                .iter()
+                .filter_map(|lang_item| self.tcx.lang_items().require(*lang_item).ok())
+                .collect();
+
+        if let Some(def_id) = self.tcx.get_diagnostic_item(sym::Send) {
+            never_suggest_borrow.push(def_id);
+        }
+
+        let param_env = obligation.param_env;
+
+        // Try to apply the original trait binding obligation by borrowing.
+        let mut try_borrowing =
+            |old_pred: ty::PolyTraitPredicate<'tcx>, blacklist: &[DefId]| -> bool {
+                if blacklist.contains(&old_pred.def_id()) {
+                    return false;
+                }
+                // We map bounds to `&T` and `&mut T`
+                let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
+                    (
+                        trait_pred,
+                        self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
+                    )
+                });
+                let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
+                    (
+                        trait_pred,
+                        self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
+                    )
+                });
+
+                let mk_result = |trait_pred_and_new_ty| {
+                    let obligation =
+                        self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
+                    self.predicate_must_hold_modulo_regions(&obligation)
+                };
+                let imm_result = mk_result(trait_pred_and_imm_ref);
+                let mut_result = mk_result(trait_pred_and_mut_ref);
+
+                if imm_result || mut_result {
+                    if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                        // We have a very specific type of error, where just borrowing this argument
+                        // might solve the problem. In cases like this, the important part is the
+                        // original type obligation, not the last one that failed, which is arbitrary.
+                        // Because of this, we modify the error to refer to the original obligation and
+                        // return early in the caller.
+
+                        let msg = format!("the trait bound `{}` is not satisfied", old_pred);
+                        if has_custom_message {
+                            err.note(&msg);
+                        } else {
+                            err.message =
+                                vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
+                        }
+                        if snippet.starts_with('&') {
+                            // This is already a literal borrow and the obligation is failing
+                            // somewhere else in the obligation chain. Do not suggest non-sense.
+                            return false;
+                        }
+                        err.span_label(
+                            span,
+                            &format!(
+                                "expected an implementor of trait `{}`",
+                                old_pred.print_modifiers_and_trait_path(),
+                            ),
+                        );
+
+                        // This if is to prevent a special edge-case
+                        if matches!(
+                            span.ctxt().outer_expn_data().kind,
+                            ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
+                        ) {
+                            // We don't want a borrowing suggestion on the fields in structs,
+                            // ```
+                            // struct Foo {
+                            //  the_foos: Vec<Foo>
+                            // }
+                            // ```
+
+                            if imm_result && mut_result {
+                                err.span_suggestions(
+                                    span.shrink_to_lo(),
+                                    "consider borrowing here",
+                                    ["&".to_string(), "&mut ".to_string()].into_iter(),
+                                    Applicability::MaybeIncorrect,
+                                );
+                            } else {
+                                err.span_suggestion_verbose(
+                                    span.shrink_to_lo(),
+                                    &format!(
+                                        "consider{} borrowing here",
+                                        if mut_result { " mutably" } else { "" }
+                                    ),
+                                    format!("&{}", if mut_result { "mut " } else { "" }),
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+                        }
+                        return true;
+                    }
+                }
+                return false;
+            };
+
+        if let ObligationCauseCode::ImplDerivedObligation(cause) = &*code {
+            try_borrowing(cause.derived.parent_trait_pred, &[])
+        } else if let ObligationCauseCode::BindingObligation(_, _)
+        | ObligationCauseCode::ItemObligation(_) = code
+        {
+            try_borrowing(poly_trait_pred, &never_suggest_borrow)
+        } else {
+            false
+        }
+    }
+
+    // Suggest borrowing the type
+    fn suggest_borrowing_for_object_cast(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+        self_ty: Ty<'tcx>,
+        object_ty: Ty<'tcx>,
+    ) {
+        let ty::Dynamic(predicates, _) = object_ty.kind() else { return; };
+        let self_ref_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, self_ty);
+
+        for predicate in predicates.iter() {
+            if !self.predicate_must_hold_modulo_regions(
+                &obligation.with(predicate.with_self_ty(self.tcx, self_ref_ty)),
+            ) {
+                return;
+            }
+        }
+
+        err.span_suggestion(
+            obligation.cause.span.shrink_to_lo(),
+            &format!(
+                "consider borrowing the value, since `&{self_ty}` can be coerced into `{object_ty}`"
+            ),
+            "&",
+            Applicability::MaybeIncorrect,
+        );
+    }
+
+    /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
+    /// suggest removing these references until we reach a type that implements the trait.
+    fn suggest_remove_reference(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool {
+        let span = obligation.cause.span;
+
+        let mut suggested = false;
+        if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+            let refs_number =
+                snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
+            if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
+                // Do not suggest removal of borrow from type arguments.
+                return false;
+            }
+
+            // Skipping binder here, remapping below
+            let mut suggested_ty = trait_pred.self_ty().skip_binder();
+
+            for refs_remaining in 0..refs_number {
+                let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {
+                    break;
+                };
+                suggested_ty = *inner_ty;
+
+                // Remapping bound vars here
+                let trait_pred_and_suggested_ty =
+                    trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
+
+                let new_obligation = self.mk_trait_obligation_with_new_self_ty(
+                    obligation.param_env,
+                    trait_pred_and_suggested_ty,
+                );
+
+                if self.predicate_may_hold(&new_obligation) {
+                    let sp = self
+                        .tcx
+                        .sess
+                        .source_map()
+                        .span_take_while(span, |c| c.is_whitespace() || *c == '&');
+
+                    let remove_refs = refs_remaining + 1;
+
+                    let msg = if remove_refs == 1 {
+                        "consider removing the leading `&`-reference".to_string()
+                    } else {
+                        format!("consider removing {} leading `&`-references", remove_refs)
+                    };
+
+                    err.span_suggestion_short(sp, &msg, "", Applicability::MachineApplicable);
+                    suggested = true;
+                    break;
+                }
+            }
+        }
+        suggested
+    }
+
+    fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic) {
+        let span = obligation.cause.span;
+
+        if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code().peel_derives() {
+            let hir = self.tcx.hir();
+            if let Some(node) = hir_id.and_then(|hir_id| hir.find(hir_id)) {
+                if let hir::Node::Expr(expr) = node {
+                    // FIXME: use `obligation.predicate.kind()...trait_ref.self_ty()` to see if we have `()`
+                    // and if not maybe suggest doing something else? If we kept the expression around we
+                    // could also check if it is an fn call (very likely) and suggest changing *that*, if
+                    // it is from the local crate.
+                    err.span_suggestion_verbose(
+                        expr.span.shrink_to_hi().with_hi(span.hi()),
+                        "remove the `.await`",
+                        "",
+                        Applicability::MachineApplicable,
+                    );
+                    // FIXME: account for associated `async fn`s.
+                    if let hir::Expr { span, kind: hir::ExprKind::Call(base, _), .. } = expr {
+                        if let ty::PredicateKind::Trait(pred) =
+                            obligation.predicate.kind().skip_binder()
+                        {
+                            err.span_label(
+                                *span,
+                                &format!("this call returns `{}`", pred.self_ty()),
+                            );
+                        }
+                        if let Some(typeck_results) =
+                            self.in_progress_typeck_results.map(|t| t.borrow())
+                            && let ty = typeck_results.expr_ty_adjusted(base)
+                            && let ty::FnDef(def_id, _substs) = ty.kind()
+                            && let Some(hir::Node::Item(hir::Item { ident, span, vis_span, .. })) =
+                                hir.get_if_local(*def_id)
+                        {
+                            let msg = format!(
+                                "alternatively, consider making `fn {}` asynchronous",
+                                ident
+                            );
+                            if vis_span.is_empty() {
+                                err.span_suggestion_verbose(
+                                    span.shrink_to_lo(),
+                                    &msg,
+                                    "async ",
+                                    Applicability::MaybeIncorrect,
+                                );
+                            } else {
+                                err.span_suggestion_verbose(
+                                    vis_span.shrink_to_hi(),
+                                    &msg,
+                                    " async",
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    /// Check if the trait bound is implemented for a different mutability and note it in the
+    /// final error.
+    fn suggest_change_mut(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) {
+        let points_at_arg = matches!(
+            obligation.cause.code(),
+            ObligationCauseCode::FunctionArgumentObligation { .. },
+        );
+
+        let span = obligation.cause.span;
+        if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+            let refs_number =
+                snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
+            if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
+                // Do not suggest removal of borrow from type arguments.
+                return;
+            }
+            let trait_pred = self.resolve_vars_if_possible(trait_pred);
+            if trait_pred.has_infer_types_or_consts() {
+                // Do not ICE while trying to find if a reborrow would succeed on a trait with
+                // unresolved bindings.
+                return;
+            }
+
+            // Skipping binder here, remapping below
+            if let ty::Ref(region, t_type, mutability) = *trait_pred.skip_binder().self_ty().kind()
+            {
+                let suggested_ty = match mutability {
+                    hir::Mutability::Mut => self.tcx.mk_imm_ref(region, t_type),
+                    hir::Mutability::Not => self.tcx.mk_mut_ref(region, t_type),
+                };
+
+                // Remapping bound vars here
+                let trait_pred_and_suggested_ty =
+                    trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
+
+                let new_obligation = self.mk_trait_obligation_with_new_self_ty(
+                    obligation.param_env,
+                    trait_pred_and_suggested_ty,
+                );
+                let suggested_ty_would_satisfy_obligation = self
+                    .evaluate_obligation_no_overflow(&new_obligation)
+                    .must_apply_modulo_regions();
+                if suggested_ty_would_satisfy_obligation {
+                    let sp = self
+                        .tcx
+                        .sess
+                        .source_map()
+                        .span_take_while(span, |c| c.is_whitespace() || *c == '&');
+                    if points_at_arg && mutability == hir::Mutability::Not && refs_number > 0 {
+                        err.span_suggestion_verbose(
+                            sp,
+                            "consider changing this borrow's mutability",
+                            "&mut ",
+                            Applicability::MachineApplicable,
+                        );
+                    } else {
+                        err.note(&format!(
+                            "`{}` is implemented for `{:?}`, but not for `{:?}`",
+                            trait_pred.print_modifiers_and_trait_path(),
+                            suggested_ty,
+                            trait_pred.skip_binder().self_ty(),
+                        ));
+                    }
+                }
+            }
+        }
+    }
+
+    fn suggest_semicolon_removal(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        span: Span,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool {
+        let hir = self.tcx.hir();
+        let parent_node = hir.get_parent_node(obligation.cause.body_id);
+        let node = hir.find(parent_node);
+        if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. })) = node
+            && let hir::ExprKind::Block(blk, _) = &hir.body(*body_id).value.kind
+            && sig.decl.output.span().overlaps(span)
+            && blk.expr.is_none()
+            && trait_pred.self_ty().skip_binder().is_unit()
+            && let Some(stmt) = blk.stmts.last()
+            && let hir::StmtKind::Semi(expr) = stmt.kind
+            // Only suggest this if the expression behind the semicolon implements the predicate
+            && let Some(typeck_results) = self.in_progress_typeck_results
+            && let Some(ty) = typeck_results.borrow().expr_ty_opt(expr)
+            && self.predicate_may_hold(&self.mk_trait_obligation_with_new_self_ty(
+                obligation.param_env, trait_pred.map_bound(|trait_pred| (trait_pred, ty))
+            ))
+        {
+            err.span_label(
+                expr.span,
+                &format!(
+                    "this expression has type `{}`, which implements `{}`",
+                    ty,
+                    trait_pred.print_modifiers_and_trait_path()
+                )
+            );
+            err.span_suggestion(
+                self.tcx.sess.source_map().end_point(stmt.span),
+                "remove this semicolon",
+                "",
+                Applicability::MachineApplicable
+            );
+            return true;
+        }
+        false
+    }
+
+    fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span> {
+        let hir = self.tcx.hir();
+        let parent_node = hir.get_parent_node(obligation.cause.body_id);
+        let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, ..), .. })) = hir.find(parent_node) else {
+            return None;
+        };
+
+        if let hir::FnRetTy::Return(ret_ty) = sig.decl.output { Some(ret_ty.span) } else { None }
+    }
+
+    /// If all conditions are met to identify a returned `dyn Trait`, suggest using `impl Trait` if
+    /// applicable and signal that the error has been expanded appropriately and needs to be
+    /// emitted.
+    fn suggest_impl_trait(
+        &self,
+        err: &mut Diagnostic,
+        span: Span,
+        obligation: &PredicateObligation<'tcx>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool {
+        match obligation.cause.code().peel_derives() {
+            // Only suggest `impl Trait` if the return type is unsized because it is `dyn Trait`.
+            ObligationCauseCode::SizedReturnType => {}
+            _ => return false,
+        }
+
+        let hir = self.tcx.hir();
+        let fn_hir_id = hir.get_parent_node(obligation.cause.body_id);
+        let node = hir.find(fn_hir_id);
+        let Some(hir::Node::Item(hir::Item {
+            kind: hir::ItemKind::Fn(sig, _, body_id),
+            ..
+        })) = node
+        else {
+            return false;
+        };
+        let body = hir.body(*body_id);
+        let trait_pred = self.resolve_vars_if_possible(trait_pred);
+        let ty = trait_pred.skip_binder().self_ty();
+        let is_object_safe = match ty.kind() {
+            ty::Dynamic(predicates, _) => {
+                // If the `dyn Trait` is not object safe, do not suggest `Box<dyn Trait>`.
+                predicates
+                    .principal_def_id()
+                    .map_or(true, |def_id| self.tcx.object_safety_violations(def_id).is_empty())
+            }
+            // We only want to suggest `impl Trait` to `dyn Trait`s.
+            // For example, `fn foo() -> str` needs to be filtered out.
+            _ => return false,
+        };
+
+        let hir::FnRetTy::Return(ret_ty) = sig.decl.output else {
+            return false;
+        };
+
+        // Use `TypeVisitor` instead of the output type directly to find the span of `ty` for
+        // cases like `fn foo() -> (dyn Trait, i32) {}`.
+        // Recursively look for `TraitObject` types and if there's only one, use that span to
+        // suggest `impl Trait`.
+
+        // Visit to make sure there's a single `return` type to suggest `impl Trait`,
+        // otherwise suggest using `Box<dyn Trait>` or an enum.
+        let mut visitor = ReturnsVisitor::default();
+        visitor.visit_body(&body);
+
+        let typeck_results = self.in_progress_typeck_results.map(|t| t.borrow()).unwrap();
+        let Some(liberated_sig) = typeck_results.liberated_fn_sigs().get(fn_hir_id).copied() else { return false; };
+
+        let ret_types = visitor
+            .returns
+            .iter()
+            .filter_map(|expr| Some((expr.span, typeck_results.node_type_opt(expr.hir_id)?)))
+            .map(|(expr_span, ty)| (expr_span, self.resolve_vars_if_possible(ty)));
+        let (last_ty, all_returns_have_same_type, only_never_return) = ret_types.clone().fold(
+            (None, true, true),
+            |(last_ty, mut same, only_never_return): (std::option::Option<Ty<'_>>, bool, bool),
+             (_, ty)| {
+                let ty = self.resolve_vars_if_possible(ty);
+                same &=
+                    !matches!(ty.kind(), ty::Error(_))
+                        && last_ty.map_or(true, |last_ty| {
+                            // FIXME: ideally we would use `can_coerce` here instead, but `typeck` comes
+                            // *after* in the dependency graph.
+                            match (ty.kind(), last_ty.kind()) {
+                                (Infer(InferTy::IntVar(_)), Infer(InferTy::IntVar(_)))
+                                | (Infer(InferTy::FloatVar(_)), Infer(InferTy::FloatVar(_)))
+                                | (Infer(InferTy::FreshIntTy(_)), Infer(InferTy::FreshIntTy(_)))
+                                | (
+                                    Infer(InferTy::FreshFloatTy(_)),
+                                    Infer(InferTy::FreshFloatTy(_)),
+                                ) => true,
+                                _ => ty == last_ty,
+                            }
+                        });
+                (Some(ty), same, only_never_return && matches!(ty.kind(), ty::Never))
+            },
+        );
+        let mut spans_and_needs_box = vec![];
+
+        match liberated_sig.output().kind() {
+            ty::Dynamic(predicates, _) => {
+                let cause = ObligationCause::misc(ret_ty.span, fn_hir_id);
+                let param_env = ty::ParamEnv::empty();
+
+                if !only_never_return {
+                    for (expr_span, return_ty) in ret_types {
+                        let self_ty_satisfies_dyn_predicates = |self_ty| {
+                            predicates.iter().all(|predicate| {
+                                let pred = predicate.with_self_ty(self.tcx, self_ty);
+                                let obl = Obligation::new(cause.clone(), param_env, pred);
+                                self.predicate_may_hold(&obl)
+                            })
+                        };
+
+                        if let ty::Adt(def, substs) = return_ty.kind()
+                            && def.is_box()
+                            && self_ty_satisfies_dyn_predicates(substs.type_at(0))
+                        {
+                            spans_and_needs_box.push((expr_span, false));
+                        } else if self_ty_satisfies_dyn_predicates(return_ty) {
+                            spans_and_needs_box.push((expr_span, true));
+                        } else {
+                            return false;
+                        }
+                    }
+                }
+            }
+            _ => return false,
+        };
+
+        let sm = self.tcx.sess.source_map();
+        if !ret_ty.span.overlaps(span) {
+            return false;
+        }
+        let snippet = if let hir::TyKind::TraitObject(..) = ret_ty.kind {
+            if let Ok(snippet) = sm.span_to_snippet(ret_ty.span) {
+                snippet
+            } else {
+                return false;
+            }
+        } else {
+            // Substitute the type, so we can print a fixup given `type Alias = dyn Trait`
+            let name = liberated_sig.output().to_string();
+            let name =
+                name.strip_prefix('(').and_then(|name| name.strip_suffix(')')).unwrap_or(&name);
+            if !name.starts_with("dyn ") {
+                return false;
+            }
+            name.to_owned()
+        };
+
+        err.code(error_code!(E0746));
+        err.set_primary_message("return type cannot have an unboxed trait object");
+        err.children.clear();
+        let impl_trait_msg = "for information on `impl Trait`, see \
+            <https://doc.rust-lang.org/book/ch10-02-traits.html\
+            #returning-types-that-implement-traits>";
+        let trait_obj_msg = "for information on trait objects, see \
+            <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
+            #using-trait-objects-that-allow-for-values-of-different-types>";
+
+        let has_dyn = snippet.split_whitespace().next().map_or(false, |s| s == "dyn");
+        let trait_obj = if has_dyn { &snippet[4..] } else { &snippet };
+        if only_never_return {
+            // No return paths, probably using `panic!()` or similar.
+            // Suggest `-> T`, `-> impl Trait`, and if `Trait` is object safe, `-> Box<dyn Trait>`.
+            suggest_trait_object_return_type_alternatives(
+                err,
+                ret_ty.span,
+                trait_obj,
+                is_object_safe,
+            );
+        } else if let (Some(last_ty), true) = (last_ty, all_returns_have_same_type) {
+            // Suggest `-> impl Trait`.
+            err.span_suggestion(
+                ret_ty.span,
+                &format!(
+                    "use `impl {1}` as the return type, as all return paths are of type `{}`, \
+                     which implements `{1}`",
+                    last_ty, trait_obj,
+                ),
+                format!("impl {}", trait_obj),
+                Applicability::MachineApplicable,
+            );
+            err.note(impl_trait_msg);
+        } else {
+            if is_object_safe {
+                // Suggest `-> Box<dyn Trait>` and `Box::new(returned_value)`.
+                err.multipart_suggestion(
+                    "return a boxed trait object instead",
+                    vec![
+                        (ret_ty.span.shrink_to_lo(), "Box<".to_string()),
+                        (span.shrink_to_hi(), ">".to_string()),
+                    ],
+                    Applicability::MaybeIncorrect,
+                );
+                for (span, needs_box) in spans_and_needs_box {
+                    if needs_box {
+                        err.multipart_suggestion(
+                            "... and box this value",
+                            vec![
+                                (span.shrink_to_lo(), "Box::new(".to_string()),
+                                (span.shrink_to_hi(), ")".to_string()),
+                            ],
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+            } else {
+                // This is currently not possible to trigger because E0038 takes precedence, but
+                // leave it in for completeness in case anything changes in an earlier stage.
+                err.note(&format!(
+                    "if trait `{}` were object-safe, you could return a trait object",
+                    trait_obj,
+                ));
+            }
+            err.note(trait_obj_msg);
+            err.note(&format!(
+                "if all the returned values were of the same type you could use `impl {}` as the \
+                 return type",
+                trait_obj,
+            ));
+            err.note(impl_trait_msg);
+            err.note("you can create a new `enum` with a variant for each returned type");
+        }
+        true
+    }
+
+    fn point_at_returns_when_relevant(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+    ) {
+        match obligation.cause.code().peel_derives() {
+            ObligationCauseCode::SizedReturnType => {}
+            _ => return,
+        }
+
+        let hir = self.tcx.hir();
+        let parent_node = hir.get_parent_node(obligation.cause.body_id);
+        let node = hir.find(parent_node);
+        if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })) =
+            node
+        {
+            let body = hir.body(*body_id);
+            // Point at all the `return`s in the function as they have failed trait bounds.
+            let mut visitor = ReturnsVisitor::default();
+            visitor.visit_body(&body);
+            let typeck_results = self.in_progress_typeck_results.map(|t| t.borrow()).unwrap();
+            for expr in &visitor.returns {
+                if let Some(returned_ty) = typeck_results.node_type_opt(expr.hir_id) {
+                    let ty = self.resolve_vars_if_possible(returned_ty);
+                    err.span_label(expr.span, &format!("this returned value is of type `{}`", ty));
+                }
+            }
+        }
+    }
+
+    fn report_closure_arg_mismatch(
+        &self,
+        span: Span,
+        found_span: Option<Span>,
+        found: ty::PolyTraitRef<'tcx>,
+        expected: ty::PolyTraitRef<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        pub(crate) fn build_fn_sig_ty<'tcx>(
+            tcx: TyCtxt<'tcx>,
+            trait_ref: ty::PolyTraitRef<'tcx>,
+        ) -> Ty<'tcx> {
+            let inputs = trait_ref.skip_binder().substs.type_at(1);
+            let sig = match inputs.kind() {
+                ty::Tuple(inputs)
+                    if tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some() =>
+                {
+                    tcx.mk_fn_sig(
+                        inputs.iter(),
+                        tcx.mk_ty_infer(ty::TyVar(ty::TyVid::from_u32(0))),
+                        false,
+                        hir::Unsafety::Normal,
+                        abi::Abi::Rust,
+                    )
+                }
+                _ => tcx.mk_fn_sig(
+                    std::iter::once(inputs),
+                    tcx.mk_ty_infer(ty::TyVar(ty::TyVid::from_u32(0))),
+                    false,
+                    hir::Unsafety::Normal,
+                    abi::Abi::Rust,
+                ),
+            };
+
+            tcx.mk_fn_ptr(trait_ref.rebind(sig))
+        }
+
+        let argument_kind = match expected.skip_binder().self_ty().kind() {
+            ty::Closure(..) => "closure",
+            ty::Generator(..) => "generator",
+            _ => "function",
+        };
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            span,
+            E0631,
+            "type mismatch in {argument_kind} arguments",
+        );
+
+        err.span_label(span, "expected due to this");
+
+        let found_span = found_span.unwrap_or(span);
+        err.span_label(found_span, "found signature defined here");
+
+        let expected = build_fn_sig_ty(self.tcx, expected);
+        let found = build_fn_sig_ty(self.tcx, found);
+
+        let (expected_str, found_str) =
+            self.tcx.infer_ctxt().enter(|infcx| infcx.cmp(expected, found));
+
+        let signature_kind = format!("{argument_kind} signature");
+        err.note_expected_found(&signature_kind, expected_str, &signature_kind, found_str);
+
+        err
+    }
+
+    fn suggest_fully_qualified_path(
+        &self,
+        err: &mut Diagnostic,
+        item_def_id: DefId,
+        span: Span,
+        trait_ref: DefId,
+    ) {
+        if let Some(assoc_item) = self.tcx.opt_associated_item(item_def_id) {
+            if let ty::AssocKind::Const | ty::AssocKind::Type = assoc_item.kind {
+                err.note(&format!(
+                    "{}s cannot be accessed directly on a `trait`, they can only be \
+                        accessed through a specific `impl`",
+                    assoc_item.kind.as_def_kind().descr(item_def_id)
+                ));
+                err.span_suggestion(
+                    span,
+                    "use the fully qualified path to an implementation",
+                    format!("<Type as {}>::{}", self.tcx.def_path_str(trait_ref), assoc_item.name),
+                    Applicability::HasPlaceholders,
+                );
+            }
+        }
+    }
+
+    /// Adds an async-await specific note to the diagnostic when the future does not implement
+    /// an auto trait because of a captured type.
+    ///
+    /// ```text
+    /// note: future does not implement `Qux` as this value is used across an await
+    ///   --> $DIR/issue-64130-3-other.rs:17:5
+    ///    |
+    /// LL |     let x = Foo;
+    ///    |         - has type `Foo`
+    /// LL |     baz().await;
+    ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+    /// LL | }
+    ///    | - `x` is later dropped here
+    /// ```
+    ///
+    /// When the diagnostic does not implement `Send` or `Sync` specifically, then the diagnostic
+    /// is "replaced" with a different message and a more specific error.
+    ///
+    /// ```text
+    /// error: future cannot be sent between threads safely
+    ///   --> $DIR/issue-64130-2-send.rs:21:5
+    ///    |
+    /// LL | fn is_send<T: Send>(t: T) { }
+    ///    |               ---- required by this bound in `is_send`
+    /// ...
+    /// LL |     is_send(bar());
+    ///    |     ^^^^^^^ future returned by `bar` is not send
+    ///    |
+    ///    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not
+    ///            implemented for `Foo`
+    /// note: future is not send as this value is used across an await
+    ///   --> $DIR/issue-64130-2-send.rs:15:5
+    ///    |
+    /// LL |     let x = Foo;
+    ///    |         - has type `Foo`
+    /// LL |     baz().await;
+    ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+    /// LL | }
+    ///    | - `x` is later dropped here
+    /// ```
+    ///
+    /// Returns `true` if an async-await specific note was added to the diagnostic.
+    #[instrument(level = "debug", skip_all, fields(?obligation.predicate, ?obligation.cause.span))]
+    fn maybe_note_obligation_cause_for_async_await(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> bool {
+        let hir = self.tcx.hir();
+
+        // Attempt to detect an async-await error by looking at the obligation causes, looking
+        // for a generator to be present.
+        //
+        // When a future does not implement a trait because of a captured type in one of the
+        // generators somewhere in the call stack, then the result is a chain of obligations.
+        //
+        // Given an `async fn` A that calls an `async fn` B which captures a non-send type and that
+        // future is passed as an argument to a function C which requires a `Send` type, then the
+        // chain looks something like this:
+        //
+        // - `BuiltinDerivedObligation` with a generator witness (B)
+        // - `BuiltinDerivedObligation` with a generator (B)
+        // - `BuiltinDerivedObligation` with `std::future::GenFuture` (B)
+        // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
+        // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
+        // - `BuiltinDerivedObligation` with a generator witness (A)
+        // - `BuiltinDerivedObligation` with a generator (A)
+        // - `BuiltinDerivedObligation` with `std::future::GenFuture` (A)
+        // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
+        // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
+        // - `BindingObligation` with `impl_send (Send requirement)
+        //
+        // The first obligation in the chain is the most useful and has the generator that captured
+        // the type. The last generator (`outer_generator` below) has information about where the
+        // bound was introduced. At least one generator should be present for this diagnostic to be
+        // modified.
+        let (mut trait_ref, mut target_ty) = match obligation.predicate.kind().skip_binder() {
+            ty::PredicateKind::Trait(p) => (Some(p), Some(p.self_ty())),
+            _ => (None, None),
+        };
+        let mut generator = None;
+        let mut outer_generator = None;
+        let mut next_code = Some(obligation.cause.code());
+
+        let mut seen_upvar_tys_infer_tuple = false;
+
+        while let Some(code) = next_code {
+            debug!(?code);
+            match code {
+                ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
+                    next_code = Some(parent_code);
+                }
+                ObligationCauseCode::ImplDerivedObligation(cause) => {
+                    let ty = cause.derived.parent_trait_pred.skip_binder().self_ty();
+                    debug!(
+                        parent_trait_ref = ?cause.derived.parent_trait_pred,
+                        self_ty.kind = ?ty.kind(),
+                        "ImplDerived",
+                    );
+
+                    match *ty.kind() {
+                        ty::Generator(did, ..) => {
+                            generator = generator.or(Some(did));
+                            outer_generator = Some(did);
+                        }
+                        ty::GeneratorWitness(..) => {}
+                        ty::Tuple(_) if !seen_upvar_tys_infer_tuple => {
+                            // By introducing a tuple of upvar types into the chain of obligations
+                            // of a generator, the first non-generator item is now the tuple itself,
+                            // we shall ignore this.
+
+                            seen_upvar_tys_infer_tuple = true;
+                        }
+                        _ if generator.is_none() => {
+                            trait_ref = Some(cause.derived.parent_trait_pred.skip_binder());
+                            target_ty = Some(ty);
+                        }
+                        _ => {}
+                    }
+
+                    next_code = Some(&cause.derived.parent_code);
+                }
+                ObligationCauseCode::DerivedObligation(derived_obligation)
+                | ObligationCauseCode::BuiltinDerivedObligation(derived_obligation) => {
+                    let ty = derived_obligation.parent_trait_pred.skip_binder().self_ty();
+                    debug!(
+                        parent_trait_ref = ?derived_obligation.parent_trait_pred,
+                        self_ty.kind = ?ty.kind(),
+                    );
+
+                    match *ty.kind() {
+                        ty::Generator(did, ..) => {
+                            generator = generator.or(Some(did));
+                            outer_generator = Some(did);
+                        }
+                        ty::GeneratorWitness(..) => {}
+                        ty::Tuple(_) if !seen_upvar_tys_infer_tuple => {
+                            // By introducing a tuple of upvar types into the chain of obligations
+                            // of a generator, the first non-generator item is now the tuple itself,
+                            // we shall ignore this.
+
+                            seen_upvar_tys_infer_tuple = true;
+                        }
+                        _ if generator.is_none() => {
+                            trait_ref = Some(derived_obligation.parent_trait_pred.skip_binder());
+                            target_ty = Some(ty);
+                        }
+                        _ => {}
+                    }
+
+                    next_code = Some(&derived_obligation.parent_code);
+                }
+                _ => break,
+            }
+        }
+
+        // Only continue if a generator was found.
+        debug!(?generator, ?trait_ref, ?target_ty);
+        let (Some(generator_did), Some(trait_ref), Some(target_ty)) = (generator, trait_ref, target_ty) else {
+            return false;
+        };
+
+        let span = self.tcx.def_span(generator_did);
+
+        let in_progress_typeck_results = self.in_progress_typeck_results.map(|t| t.borrow());
+        let generator_did_root = self.tcx.typeck_root_def_id(generator_did);
+        debug!(
+            ?generator_did,
+            ?generator_did_root,
+            in_progress_typeck_results.hir_owner = ?in_progress_typeck_results.as_ref().map(|t| t.hir_owner),
+            ?span,
+        );
+
+        let generator_body = generator_did
+            .as_local()
+            .and_then(|def_id| hir.maybe_body_owned_by(def_id))
+            .map(|body_id| hir.body(body_id));
+        let is_async = match generator_did.as_local() {
+            Some(_) => generator_body
+                .and_then(|body| body.generator_kind())
+                .map(|generator_kind| matches!(generator_kind, hir::GeneratorKind::Async(..)))
+                .unwrap_or(false),
+            None => self
+                .tcx
+                .generator_kind(generator_did)
+                .map(|generator_kind| matches!(generator_kind, hir::GeneratorKind::Async(..)))
+                .unwrap_or(false),
+        };
+        let mut visitor = AwaitsVisitor::default();
+        if let Some(body) = generator_body {
+            visitor.visit_body(body);
+        }
+        debug!(awaits = ?visitor.awaits);
+
+        // Look for a type inside the generator interior that matches the target type to get
+        // a span.
+        let target_ty_erased = self.tcx.erase_regions(target_ty);
+        let ty_matches = |ty| -> bool {
+            // Careful: the regions for types that appear in the
+            // generator interior are not generally known, so we
+            // want to erase them when comparing (and anyway,
+            // `Send` and other bounds are generally unaffected by
+            // the choice of region).  When erasing regions, we
+            // also have to erase late-bound regions. This is
+            // because the types that appear in the generator
+            // interior generally contain "bound regions" to
+            // represent regions that are part of the suspended
+            // generator frame. Bound regions are preserved by
+            // `erase_regions` and so we must also call
+            // `erase_late_bound_regions`.
+            let ty_erased = self.tcx.erase_late_bound_regions(ty);
+            let ty_erased = self.tcx.erase_regions(ty_erased);
+            let eq = ty_erased == target_ty_erased;
+            debug!(?ty_erased, ?target_ty_erased, ?eq);
+            eq
+        };
+
+        let mut interior_or_upvar_span = None;
+        let mut interior_extra_info = None;
+
+        // Get the typeck results from the infcx if the generator is the function we are currently
+        // type-checking; otherwise, get them by performing a query.  This is needed to avoid
+        // cycles. If we can't use resolved types because the generator comes from another crate,
+        // we still provide a targeted error but without all the relevant spans.
+        let generator_data: Option<GeneratorData<'tcx, '_>> = match &in_progress_typeck_results {
+            Some(t) if t.hir_owner.to_def_id() == generator_did_root => {
+                Some(GeneratorData::Local(&t))
+            }
+            _ if generator_did.is_local() => {
+                Some(GeneratorData::Local(self.tcx.typeck(generator_did.expect_local())))
+            }
+            _ => self
+                .tcx
+                .generator_diagnostic_data(generator_did)
+                .as_ref()
+                .map(|generator_diag_data| GeneratorData::Foreign(generator_diag_data)),
+        };
+
+        if let Some(generator_data) = generator_data.as_ref() {
+            interior_or_upvar_span =
+                generator_data.try_get_upvar_span(&self, generator_did, ty_matches);
+
+            // The generator interior types share the same binders
+            if let Some(cause) =
+                generator_data.get_generator_interior_types().skip_binder().iter().find(
+                    |ty::GeneratorInteriorTypeCause { ty, .. }| {
+                        ty_matches(generator_data.get_generator_interior_types().rebind(*ty))
+                    },
+                )
+            {
+                let from_awaited_ty = generator_data.get_from_await_ty(visitor, hir, ty_matches);
+                let ty::GeneratorInteriorTypeCause { span, scope_span, yield_span, expr, .. } =
+                    cause;
+
+                interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(*span));
+                interior_extra_info = Some((*scope_span, *yield_span, *expr, from_awaited_ty));
+            }
+
+            if interior_or_upvar_span.is_none() && generator_data.is_foreign() {
+                interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(span));
+            }
+        }
+
+        if let Some(interior_or_upvar_span) = interior_or_upvar_span {
+            let typeck_results = generator_data.and_then(|generator_data| match generator_data {
+                GeneratorData::Local(typeck_results) => Some(typeck_results),
+                GeneratorData::Foreign(_) => None,
+            });
+            self.note_obligation_cause_for_async_await(
+                err,
+                interior_or_upvar_span,
+                interior_extra_info,
+                is_async,
+                outer_generator,
+                trait_ref,
+                target_ty,
+                typeck_results,
+                obligation,
+                next_code,
+            );
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Unconditionally adds the diagnostic note described in
+    /// `maybe_note_obligation_cause_for_async_await`'s documentation comment.
+    #[instrument(level = "debug", skip_all)]
+    fn note_obligation_cause_for_async_await(
+        &self,
+        err: &mut Diagnostic,
+        interior_or_upvar_span: GeneratorInteriorOrUpvar,
+        interior_extra_info: Option<(Option<Span>, Span, Option<hir::HirId>, Option<Span>)>,
+        is_async: bool,
+        outer_generator: Option<DefId>,
+        trait_pred: ty::TraitPredicate<'tcx>,
+        target_ty: Ty<'tcx>,
+        typeck_results: Option<&ty::TypeckResults<'tcx>>,
+        obligation: &PredicateObligation<'tcx>,
+        next_code: Option<&ObligationCauseCode<'tcx>>,
+    ) {
+        let source_map = self.tcx.sess.source_map();
+
+        let (await_or_yield, an_await_or_yield) =
+            if is_async { ("await", "an await") } else { ("yield", "a yield") };
+        let future_or_generator = if is_async { "future" } else { "generator" };
+
+        // Special case the primary error message when send or sync is the trait that was
+        // not implemented.
+        let hir = self.tcx.hir();
+        let trait_explanation = if let Some(name @ (sym::Send | sym::Sync)) =
+            self.tcx.get_diagnostic_name(trait_pred.def_id())
+        {
+            let (trait_name, trait_verb) =
+                if name == sym::Send { ("`Send`", "sent") } else { ("`Sync`", "shared") };
+
+            err.clear_code();
+            err.set_primary_message(format!(
+                "{} cannot be {} between threads safely",
+                future_or_generator, trait_verb
+            ));
+
+            let original_span = err.span.primary_span().unwrap();
+            let mut span = MultiSpan::from_span(original_span);
+
+            let message = outer_generator
+                .and_then(|generator_did| {
+                    Some(match self.tcx.generator_kind(generator_did).unwrap() {
+                        GeneratorKind::Gen => format!("generator is not {}", trait_name),
+                        GeneratorKind::Async(AsyncGeneratorKind::Fn) => self
+                            .tcx
+                            .parent(generator_did)
+                            .as_local()
+                            .map(|parent_did| hir.local_def_id_to_hir_id(parent_did))
+                            .and_then(|parent_hir_id| hir.opt_name(parent_hir_id))
+                            .map(|name| {
+                                format!("future returned by `{}` is not {}", name, trait_name)
+                            })?,
+                        GeneratorKind::Async(AsyncGeneratorKind::Block) => {
+                            format!("future created by async block is not {}", trait_name)
+                        }
+                        GeneratorKind::Async(AsyncGeneratorKind::Closure) => {
+                            format!("future created by async closure is not {}", trait_name)
+                        }
+                    })
+                })
+                .unwrap_or_else(|| format!("{} is not {}", future_or_generator, trait_name));
+
+            span.push_span_label(original_span, message);
+            err.set_span(span);
+
+            format!("is not {}", trait_name)
+        } else {
+            format!("does not implement `{}`", trait_pred.print_modifiers_and_trait_path())
+        };
+
+        let mut explain_yield = |interior_span: Span,
+                                 yield_span: Span,
+                                 scope_span: Option<Span>| {
+            let mut span = MultiSpan::from_span(yield_span);
+            if let Ok(snippet) = source_map.span_to_snippet(interior_span) {
+                // #70935: If snippet contains newlines, display "the value" instead
+                // so that we do not emit complex diagnostics.
+                let snippet = &format!("`{}`", snippet);
+                let snippet = if snippet.contains('\n') { "the value" } else { snippet };
+                // note: future is not `Send` as this value is used across an await
+                //   --> $DIR/issue-70935-complex-spans.rs:13:9
+                //    |
+                // LL |            baz(|| async {
+                //    |  ______________-
+                //    | |
+                //    | |
+                // LL | |              foo(tx.clone());
+                // LL | |          }).await;
+                //    | |          - ^^^^^^ await occurs here, with value maybe used later
+                //    | |__________|
+                //    |            has type `closure` which is not `Send`
+                // note: value is later dropped here
+                // LL | |          }).await;
+                //    | |                  ^
+                //
+                span.push_span_label(
+                    yield_span,
+                    format!("{} occurs here, with {} maybe used later", await_or_yield, snippet),
+                );
+                span.push_span_label(
+                    interior_span,
+                    format!("has type `{}` which {}", target_ty, trait_explanation),
+                );
+                // If available, use the scope span to annotate the drop location.
+                let mut scope_note = None;
+                if let Some(scope_span) = scope_span {
+                    let scope_span = source_map.end_point(scope_span);
+
+                    let msg = format!("{} is later dropped here", snippet);
+                    if source_map.is_multiline(yield_span.between(scope_span)) {
+                        span.push_span_label(scope_span, msg);
+                    } else {
+                        scope_note = Some((scope_span, msg));
+                    }
+                }
+                err.span_note(
+                    span,
+                    &format!(
+                        "{} {} as this value is used across {}",
+                        future_or_generator, trait_explanation, an_await_or_yield
+                    ),
+                );
+                if let Some((span, msg)) = scope_note {
+                    err.span_note(span, &msg);
+                }
+            }
+        };
+        match interior_or_upvar_span {
+            GeneratorInteriorOrUpvar::Interior(interior_span) => {
+                if let Some((scope_span, yield_span, expr, from_awaited_ty)) = interior_extra_info {
+                    if let Some(await_span) = from_awaited_ty {
+                        // The type causing this obligation is one being awaited at await_span.
+                        let mut span = MultiSpan::from_span(await_span);
+                        span.push_span_label(
+                            await_span,
+                            format!(
+                                "await occurs here on type `{}`, which {}",
+                                target_ty, trait_explanation
+                            ),
+                        );
+                        err.span_note(
+                            span,
+                            &format!(
+                                "future {not_trait} as it awaits another future which {not_trait}",
+                                not_trait = trait_explanation
+                            ),
+                        );
+                    } else {
+                        // Look at the last interior type to get a span for the `.await`.
+                        debug!(
+                            generator_interior_types = ?format_args!(
+                                "{:#?}", typeck_results.as_ref().map(|t| &t.generator_interior_types)
+                            ),
+                        );
+                        explain_yield(interior_span, yield_span, scope_span);
+                    }
+
+                    if let Some(expr_id) = expr {
+                        let expr = hir.expect_expr(expr_id);
+                        debug!("target_ty evaluated from {:?}", expr);
+
+                        let parent = hir.get_parent_node(expr_id);
+                        if let Some(hir::Node::Expr(e)) = hir.find(parent) {
+                            let parent_span = hir.span(parent);
+                            let parent_did = parent.owner.to_def_id();
+                            // ```rust
+                            // impl T {
+                            //     fn foo(&self) -> i32 {}
+                            // }
+                            // T.foo();
+                            // ^^^^^^^ a temporary `&T` created inside this method call due to `&self`
+                            // ```
+                            //
+                            let is_region_borrow = if let Some(typeck_results) = typeck_results {
+                                typeck_results
+                                    .expr_adjustments(expr)
+                                    .iter()
+                                    .any(|adj| adj.is_region_borrow())
+                            } else {
+                                false
+                            };
+
+                            // ```rust
+                            // struct Foo(*const u8);
+                            // bar(Foo(std::ptr::null())).await;
+                            //     ^^^^^^^^^^^^^^^^^^^^^ raw-ptr `*T` created inside this struct ctor.
+                            // ```
+                            debug!(parent_def_kind = ?self.tcx.def_kind(parent_did));
+                            let is_raw_borrow_inside_fn_like_call =
+                                match self.tcx.def_kind(parent_did) {
+                                    DefKind::Fn | DefKind::Ctor(..) => target_ty.is_unsafe_ptr(),
+                                    _ => false,
+                                };
+                            if let Some(typeck_results) = typeck_results {
+                                if (typeck_results.is_method_call(e) && is_region_borrow)
+                                    || is_raw_borrow_inside_fn_like_call
+                                {
+                                    err.span_help(
+                                        parent_span,
+                                        "consider moving this into a `let` \
+                        binding to create a shorter lived borrow",
+                                    );
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+            GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
+                // `Some(ref_ty)` if `target_ty` is `&T` and `T` fails to impl `Sync`
+                let refers_to_non_sync = match target_ty.kind() {
+                    ty::Ref(_, ref_ty, _) => match self.evaluate_obligation(&obligation) {
+                        Ok(eval) if !eval.may_apply() => Some(ref_ty),
+                        _ => None,
+                    },
+                    _ => None,
+                };
+
+                let (span_label, span_note) = match refers_to_non_sync {
+                    // if `target_ty` is `&T` and `T` fails to impl `Sync`,
+                    // include suggestions to make `T: Sync` so that `&T: Send`
+                    Some(ref_ty) => (
+                        format!(
+                            "has type `{}` which {}, because `{}` is not `Sync`",
+                            target_ty, trait_explanation, ref_ty
+                        ),
+                        format!(
+                            "captured value {} because `&` references cannot be sent unless their referent is `Sync`",
+                            trait_explanation
+                        ),
+                    ),
+                    None => (
+                        format!("has type `{}` which {}", target_ty, trait_explanation),
+                        format!("captured value {}", trait_explanation),
+                    ),
+                };
+
+                let mut span = MultiSpan::from_span(upvar_span);
+                span.push_span_label(upvar_span, span_label);
+                err.span_note(span, &span_note);
+            }
+        }
+
+        // Add a note for the item obligation that remains - normally a note pointing to the
+        // bound that introduced the obligation (e.g. `T: Send`).
+        debug!(?next_code);
+        self.note_obligation_cause_code(
+            err,
+            &obligation.predicate,
+            obligation.param_env,
+            next_code.unwrap(),
+            &mut Vec::new(),
+            &mut Default::default(),
+        );
+    }
+
+    fn note_obligation_cause_code<T>(
+        &self,
+        err: &mut Diagnostic,
+        predicate: &T,
+        param_env: ty::ParamEnv<'tcx>,
+        cause_code: &ObligationCauseCode<'tcx>,
+        obligated_types: &mut Vec<Ty<'tcx>>,
+        seen_requirements: &mut FxHashSet<DefId>,
+    ) where
+        T: fmt::Display,
+    {
+        let tcx = self.tcx;
+        match *cause_code {
+            ObligationCauseCode::ExprAssignable
+            | ObligationCauseCode::MatchExpressionArm { .. }
+            | ObligationCauseCode::Pattern { .. }
+            | ObligationCauseCode::IfExpression { .. }
+            | ObligationCauseCode::IfExpressionWithNoElse
+            | ObligationCauseCode::MainFunctionType
+            | ObligationCauseCode::StartFunctionType
+            | ObligationCauseCode::IntrinsicType
+            | ObligationCauseCode::MethodReceiver
+            | ObligationCauseCode::ReturnNoExpression
+            | ObligationCauseCode::UnifyReceiver(..)
+            | ObligationCauseCode::OpaqueType
+            | ObligationCauseCode::MiscObligation
+            | ObligationCauseCode::WellFormed(..)
+            | ObligationCauseCode::MatchImpl(..)
+            | ObligationCauseCode::ReturnType
+            | ObligationCauseCode::ReturnValue(_)
+            | ObligationCauseCode::BlockTailExpression(_)
+            | ObligationCauseCode::AwaitableExpr(_)
+            | ObligationCauseCode::ForLoopIterator
+            | ObligationCauseCode::QuestionMark
+            | ObligationCauseCode::CheckAssociatedTypeBounds { .. }
+            | ObligationCauseCode::LetElse
+            | ObligationCauseCode::BinOp { .. } => {}
+            ObligationCauseCode::SliceOrArrayElem => {
+                err.note("slice and array elements must have `Sized` type");
+            }
+            ObligationCauseCode::TupleElem => {
+                err.note("only the last element of a tuple may have a dynamically sized type");
+            }
+            ObligationCauseCode::ProjectionWf(data) => {
+                err.note(&format!("required so that the projection `{}` is well-formed", data,));
+            }
+            ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
+                err.note(&format!(
+                    "required so that reference `{}` does not outlive its referent",
+                    ref_ty,
+                ));
+            }
+            ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
+                err.note(&format!(
+                    "required so that the lifetime bound of `{}` for `{}` is satisfied",
+                    region, object_ty,
+                ));
+            }
+            ObligationCauseCode::ItemObligation(_item_def_id) => {
+                // We hold the `DefId` of the item introducing the obligation, but displaying it
+                // doesn't add user usable information. It always point at an associated item.
+            }
+            ObligationCauseCode::BindingObligation(item_def_id, span) => {
+                let item_name = tcx.def_path_str(item_def_id);
+                let mut multispan = MultiSpan::from(span);
+                if let Some(ident) = tcx.opt_item_ident(item_def_id) {
+                    let sm = tcx.sess.source_map();
+                    let same_line =
+                        match (sm.lookup_line(ident.span.hi()), sm.lookup_line(span.lo())) {
+                            (Ok(l), Ok(r)) => l.line == r.line,
+                            _ => true,
+                        };
+                    if !ident.span.overlaps(span) && !same_line {
+                        multispan.push_span_label(ident.span, "required by a bound in this");
+                    }
+                }
+                let descr = format!("required by a bound in `{}`", item_name);
+                if span != DUMMY_SP {
+                    let msg = format!("required by this bound in `{}`", item_name);
+                    multispan.push_span_label(span, msg);
+                    err.span_note(multispan, &descr);
+                } else {
+                    err.span_note(tcx.def_span(item_def_id), &descr);
+                }
+            }
+            ObligationCauseCode::ObjectCastObligation(concrete_ty, object_ty) => {
+                err.note(&format!(
+                    "required for the cast from `{}` to the object type `{}`",
+                    self.ty_to_string(concrete_ty),
+                    self.ty_to_string(object_ty)
+                ));
+            }
+            ObligationCauseCode::Coercion { source: _, target } => {
+                err.note(&format!("required by cast to type `{}`", self.ty_to_string(target)));
+            }
+            ObligationCauseCode::RepeatElementCopy { is_const_fn } => {
+                err.note(
+                    "the `Copy` trait is required because this value will be copied for each element of the array",
+                );
+
+                if is_const_fn {
+                    err.help(
+                        "consider creating a new `const` item and initializing it with the result \
+                        of the function call to be used in the repeat position, like \
+                        `const VAL: Type = const_fn();` and `let x = [VAL; 42];`",
+                    );
+                }
+
+                if self.tcx.sess.is_nightly_build() && is_const_fn {
+                    err.help(
+                        "create an inline `const` block, see RFC #2920 \
+                         <https://github.com/rust-lang/rfcs/pull/2920> for more information",
+                    );
+                }
+            }
+            ObligationCauseCode::VariableType(hir_id) => {
+                let parent_node = self.tcx.hir().get_parent_node(hir_id);
+                match self.tcx.hir().find(parent_node) {
+                    Some(Node::Local(hir::Local {
+                        init: Some(hir::Expr { kind: hir::ExprKind::Index(_, _), span, .. }),
+                        ..
+                    })) => {
+                        // When encountering an assignment of an unsized trait, like
+                        // `let x = ""[..];`, provide a suggestion to borrow the initializer in
+                        // order to use have a slice instead.
+                        err.span_suggestion_verbose(
+                            span.shrink_to_lo(),
+                            "consider borrowing here",
+                            "&",
+                            Applicability::MachineApplicable,
+                        );
+                        err.note("all local variables must have a statically known size");
+                    }
+                    Some(Node::Param(param)) => {
+                        err.span_suggestion_verbose(
+                            param.ty_span.shrink_to_lo(),
+                            "function arguments must have a statically known size, borrowed types \
+                            always have a known size",
+                            "&",
+                            Applicability::MachineApplicable,
+                        );
+                    }
+                    _ => {
+                        err.note("all local variables must have a statically known size");
+                    }
+                }
+                if !self.tcx.features().unsized_locals {
+                    err.help("unsized locals are gated as an unstable feature");
+                }
+            }
+            ObligationCauseCode::SizedArgumentType(sp) => {
+                if let Some(span) = sp {
+                    err.span_suggestion_verbose(
+                        span.shrink_to_lo(),
+                        "function arguments must have a statically known size, borrowed types \
+                         always have a known size",
+                        "&",
+                        Applicability::MachineApplicable,
+                    );
+                } else {
+                    err.note("all function arguments must have a statically known size");
+                }
+                if tcx.sess.opts.unstable_features.is_nightly_build()
+                    && !self.tcx.features().unsized_fn_params
+                {
+                    err.help("unsized fn params are gated as an unstable feature");
+                }
+            }
+            ObligationCauseCode::SizedReturnType => {
+                err.note("the return type of a function must have a statically known size");
+            }
+            ObligationCauseCode::SizedYieldType => {
+                err.note("the yield type of a generator must have a statically known size");
+            }
+            ObligationCauseCode::SizedBoxType => {
+                err.note("the type of a box expression must have a statically known size");
+            }
+            ObligationCauseCode::AssignmentLhsSized => {
+                err.note("the left-hand-side of an assignment must have a statically known size");
+            }
+            ObligationCauseCode::TupleInitializerSized => {
+                err.note("tuples must have a statically known size to be initialized");
+            }
+            ObligationCauseCode::StructInitializerSized => {
+                err.note("structs must have a statically known size to be initialized");
+            }
+            ObligationCauseCode::FieldSized { adt_kind: ref item, last, span } => {
+                match *item {
+                    AdtKind::Struct => {
+                        if last {
+                            err.note(
+                                "the last field of a packed struct may only have a \
+                                dynamically sized type if it does not need drop to be run",
+                            );
+                        } else {
+                            err.note(
+                                "only the last field of a struct may have a dynamically sized type",
+                            );
+                        }
+                    }
+                    AdtKind::Union => {
+                        err.note("no field of a union may have a dynamically sized type");
+                    }
+                    AdtKind::Enum => {
+                        err.note("no field of an enum variant may have a dynamically sized type");
+                    }
+                }
+                err.help("change the field's type to have a statically known size");
+                err.span_suggestion(
+                    span.shrink_to_lo(),
+                    "borrowed types always have a statically known size",
+                    "&",
+                    Applicability::MachineApplicable,
+                );
+                err.multipart_suggestion(
+                    "the `Box` type always has a statically known size and allocates its contents \
+                     in the heap",
+                    vec![
+                        (span.shrink_to_lo(), "Box<".to_string()),
+                        (span.shrink_to_hi(), ">".to_string()),
+                    ],
+                    Applicability::MachineApplicable,
+                );
+            }
+            ObligationCauseCode::ConstSized => {
+                err.note("constant expressions must have a statically known size");
+            }
+            ObligationCauseCode::InlineAsmSized => {
+                err.note("all inline asm arguments must have a statically known size");
+            }
+            ObligationCauseCode::ConstPatternStructural => {
+                err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
+            }
+            ObligationCauseCode::SharedStatic => {
+                err.note("shared static variables must have a type that implements `Sync`");
+            }
+            ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
+                let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
+                let ty = parent_trait_ref.skip_binder().self_ty();
+                if parent_trait_ref.references_error() {
+                    // NOTE(eddyb) this was `.cancel()`, but `err`
+                    // is borrowed, so we can't fully defuse it.
+                    err.downgrade_to_delayed_bug();
+                    return;
+                }
+
+                // If the obligation for a tuple is set directly by a Generator or Closure,
+                // then the tuple must be the one containing capture types.
+                let is_upvar_tys_infer_tuple = if !matches!(ty.kind(), ty::Tuple(..)) {
+                    false
+                } else {
+                    if let ObligationCauseCode::BuiltinDerivedObligation(data) = &*data.parent_code
+                    {
+                        let parent_trait_ref =
+                            self.resolve_vars_if_possible(data.parent_trait_pred);
+                        let nested_ty = parent_trait_ref.skip_binder().self_ty();
+                        matches!(nested_ty.kind(), ty::Generator(..))
+                            || matches!(nested_ty.kind(), ty::Closure(..))
+                    } else {
+                        false
+                    }
+                };
+
+                let from_generator = tcx.lang_items().from_generator_fn().unwrap();
+
+                // Don't print the tuple of capture types
+                'print: {
+                    if !is_upvar_tys_infer_tuple {
+                        let msg = format!("required because it appears within the type `{}`", ty);
+                        match ty.kind() {
+                            ty::Adt(def, _) => {
+                                // `gen_future` is used in all async functions; it doesn't add any additional info.
+                                if self.tcx.is_diagnostic_item(sym::gen_future, def.did()) {
+                                    break 'print;
+                                }
+                                match self.tcx.opt_item_ident(def.did()) {
+                                    Some(ident) => err.span_note(ident.span, &msg),
+                                    None => err.note(&msg),
+                                }
+                            }
+                            ty::Opaque(def_id, _) => {
+                                // Avoid printing the future from `core::future::from_generator`, it's not helpful
+                                if tcx.parent(*def_id) == from_generator {
+                                    break 'print;
+                                }
+
+                                // If the previous type is `from_generator`, this is the future generated by the body of an async function.
+                                // Avoid printing it twice (it was already printed in the `ty::Generator` arm below).
+                                let is_future = tcx.ty_is_opaque_future(ty);
+                                debug!(
+                                    ?obligated_types,
+                                    ?is_future,
+                                    "note_obligation_cause_code: check for async fn"
+                                );
+                                if is_future
+                                    && obligated_types.last().map_or(false, |ty| match ty.kind() {
+                                        ty::Opaque(last_def_id, _) => {
+                                            tcx.parent(*last_def_id) == from_generator
+                                        }
+                                        _ => false,
+                                    })
+                                {
+                                    break 'print;
+                                }
+                                err.span_note(self.tcx.def_span(def_id), &msg)
+                            }
+                            ty::GeneratorWitness(bound_tys) => {
+                                use std::fmt::Write;
+
+                                // FIXME: this is kind of an unusual format for rustc, can we make it more clear?
+                                // Maybe we should just remove this note altogether?
+                                // FIXME: only print types which don't meet the trait requirement
+                                let mut msg =
+                                    "required because it captures the following types: ".to_owned();
+                                for ty in bound_tys.skip_binder() {
+                                    write!(msg, "`{}`, ", ty).unwrap();
+                                }
+                                err.note(msg.trim_end_matches(", "))
+                            }
+                            ty::Generator(def_id, _, _) => {
+                                let sp = self.tcx.def_span(def_id);
+
+                                // Special-case this to say "async block" instead of `[static generator]`.
+                                let kind = tcx.generator_kind(def_id).unwrap();
+                                err.span_note(
+                                    sp,
+                                    &format!("required because it's used within this {}", kind),
+                                )
+                            }
+                            ty::Closure(def_id, _) => err.span_note(
+                                self.tcx.def_span(def_id),
+                                &format!("required because it's used within this closure"),
+                            ),
+                            _ => err.note(&msg),
+                        };
+                    }
+                }
+
+                obligated_types.push(ty);
+
+                let parent_predicate = parent_trait_ref.to_predicate(tcx);
+                if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
+                    // #74711: avoid a stack overflow
+                    ensure_sufficient_stack(|| {
+                        self.note_obligation_cause_code(
+                            err,
+                            &parent_predicate,
+                            param_env,
+                            &data.parent_code,
+                            obligated_types,
+                            seen_requirements,
+                        )
+                    });
+                } else {
+                    ensure_sufficient_stack(|| {
+                        self.note_obligation_cause_code(
+                            err,
+                            &parent_predicate,
+                            param_env,
+                            cause_code.peel_derives(),
+                            obligated_types,
+                            seen_requirements,
+                        )
+                    });
+                }
+            }
+            ObligationCauseCode::ImplDerivedObligation(ref data) => {
+                let mut parent_trait_pred =
+                    self.resolve_vars_if_possible(data.derived.parent_trait_pred);
+                parent_trait_pred.remap_constness_diag(param_env);
+                let parent_def_id = parent_trait_pred.def_id();
+                let msg = format!(
+                    "required because of the requirements on the impl of `{}` for `{}`",
+                    parent_trait_pred.print_modifiers_and_trait_path(),
+                    parent_trait_pred.skip_binder().self_ty()
+                );
+                let mut is_auto_trait = false;
+                match self.tcx.hir().get_if_local(data.impl_def_id) {
+                    Some(Node::Item(hir::Item {
+                        kind: hir::ItemKind::Trait(is_auto, ..),
+                        ident,
+                        ..
+                    })) => {
+                        // FIXME: we should do something else so that it works even on crate foreign
+                        // auto traits.
+                        is_auto_trait = matches!(is_auto, hir::IsAuto::Yes);
+                        err.span_note(ident.span, &msg)
+                    }
+                    Some(Node::Item(hir::Item {
+                        kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, .. }),
+                        ..
+                    })) => {
+                        let mut spans = Vec::with_capacity(2);
+                        if let Some(trait_ref) = of_trait {
+                            spans.push(trait_ref.path.span);
+                        }
+                        spans.push(self_ty.span);
+                        err.span_note(spans, &msg)
+                    }
+                    _ => err.note(&msg),
+                };
+
+                let mut parent_predicate = parent_trait_pred.to_predicate(tcx);
+                let mut data = &data.derived;
+                let mut count = 0;
+                seen_requirements.insert(parent_def_id);
+                if is_auto_trait {
+                    // We don't want to point at the ADT saying "required because it appears within
+                    // the type `X`", like we would otherwise do in test `supertrait-auto-trait.rs`.
+                    while let ObligationCauseCode::BuiltinDerivedObligation(derived) =
+                        &*data.parent_code
+                    {
+                        let child_trait_ref =
+                            self.resolve_vars_if_possible(derived.parent_trait_pred);
+                        let child_def_id = child_trait_ref.def_id();
+                        if seen_requirements.insert(child_def_id) {
+                            break;
+                        }
+                        data = derived;
+                        parent_predicate = child_trait_ref.to_predicate(tcx);
+                        parent_trait_pred = child_trait_ref;
+                    }
+                }
+                while let ObligationCauseCode::ImplDerivedObligation(child) = &*data.parent_code {
+                    // Skip redundant recursive obligation notes. See `ui/issue-20413.rs`.
+                    let child_trait_pred =
+                        self.resolve_vars_if_possible(child.derived.parent_trait_pred);
+                    let child_def_id = child_trait_pred.def_id();
+                    if seen_requirements.insert(child_def_id) {
+                        break;
+                    }
+                    count += 1;
+                    data = &child.derived;
+                    parent_predicate = child_trait_pred.to_predicate(tcx);
+                    parent_trait_pred = child_trait_pred;
+                }
+                if count > 0 {
+                    err.note(&format!(
+                        "{} redundant requirement{} hidden",
+                        count,
+                        pluralize!(count)
+                    ));
+                    err.note(&format!(
+                        "required because of the requirements on the impl of `{}` for `{}`",
+                        parent_trait_pred.print_modifiers_and_trait_path(),
+                        parent_trait_pred.skip_binder().self_ty()
+                    ));
+                }
+                // #74711: avoid a stack overflow
+                ensure_sufficient_stack(|| {
+                    self.note_obligation_cause_code(
+                        err,
+                        &parent_predicate,
+                        param_env,
+                        &data.parent_code,
+                        obligated_types,
+                        seen_requirements,
+                    )
+                });
+            }
+            ObligationCauseCode::DerivedObligation(ref data) => {
+                let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
+                let parent_predicate = parent_trait_ref.to_predicate(tcx);
+                // #74711: avoid a stack overflow
+                ensure_sufficient_stack(|| {
+                    self.note_obligation_cause_code(
+                        err,
+                        &parent_predicate,
+                        param_env,
+                        &data.parent_code,
+                        obligated_types,
+                        seen_requirements,
+                    )
+                });
+            }
+            ObligationCauseCode::FunctionArgumentObligation {
+                arg_hir_id,
+                call_hir_id,
+                ref parent_code,
+            } => {
+                let hir = self.tcx.hir();
+                if let Some(Node::Expr(expr @ hir::Expr { kind: hir::ExprKind::Block(..), .. })) =
+                    hir.find(arg_hir_id)
+                {
+                    let in_progress_typeck_results =
+                        self.in_progress_typeck_results.map(|t| t.borrow());
+                    let parent_id = hir.get_parent_item(arg_hir_id);
+                    let typeck_results: &TypeckResults<'tcx> = match &in_progress_typeck_results {
+                        Some(t) if t.hir_owner == parent_id => t,
+                        _ => self.tcx.typeck(parent_id),
+                    };
+                    let ty = typeck_results.expr_ty_adjusted(expr);
+                    let span = expr.peel_blocks().span;
+                    if Some(span) != err.span.primary_span() {
+                        err.span_label(
+                            span,
+                            &if ty.references_error() {
+                                String::new()
+                            } else {
+                                format!("this tail expression is of type `{:?}`", ty)
+                            },
+                        );
+                    }
+                }
+                if let Some(Node::Expr(hir::Expr {
+                    kind:
+                        hir::ExprKind::Call(hir::Expr { span, .. }, _)
+                        | hir::ExprKind::MethodCall(
+                            hir::PathSegment { ident: Ident { span, .. }, .. },
+                            ..,
+                        ),
+                    ..
+                })) = hir.find(call_hir_id)
+                {
+                    if Some(*span) != err.span.primary_span() {
+                        err.span_label(*span, "required by a bound introduced by this call");
+                    }
+                }
+                ensure_sufficient_stack(|| {
+                    self.note_obligation_cause_code(
+                        err,
+                        predicate,
+                        param_env,
+                        &parent_code,
+                        obligated_types,
+                        seen_requirements,
+                    )
+                });
+            }
+            ObligationCauseCode::CompareImplItemObligation { trait_item_def_id, kind, .. } => {
+                let item_name = self.tcx.item_name(trait_item_def_id);
+                let msg = format!(
+                    "the requirement `{}` appears on the `impl`'s {kind} `{}` but not on the \
+                     corresponding trait's {kind}",
+                    predicate, item_name,
+                );
+                let sp = self
+                    .tcx
+                    .opt_item_ident(trait_item_def_id)
+                    .map(|i| i.span)
+                    .unwrap_or_else(|| self.tcx.def_span(trait_item_def_id));
+                let mut assoc_span: MultiSpan = sp.into();
+                assoc_span.push_span_label(
+                    sp,
+                    format!("this trait's {kind} doesn't have the requirement `{}`", predicate),
+                );
+                if let Some(ident) = self
+                    .tcx
+                    .opt_associated_item(trait_item_def_id)
+                    .and_then(|i| self.tcx.opt_item_ident(i.container_id(self.tcx)))
+                {
+                    assoc_span.push_span_label(ident.span, "in this trait");
+                }
+                err.span_note(assoc_span, &msg);
+            }
+            ObligationCauseCode::TrivialBound => {
+                err.help("see issue #48214");
+                if tcx.sess.opts.unstable_features.is_nightly_build() {
+                    err.help("add `#![feature(trivial_bounds)]` to the crate attributes to enable");
+                }
+            }
+            ObligationCauseCode::OpaqueReturnType(expr_info) => {
+                if let Some((expr_ty, expr_span)) = expr_info {
+                    let expr_ty = self.resolve_vars_if_possible(expr_ty);
+                    err.span_label(
+                        expr_span,
+                        format!("return type was inferred to be `{expr_ty}` here"),
+                    );
+                }
+            }
+        }
+    }
+
+    fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
+        let suggested_limit = match self.tcx.recursion_limit() {
+            Limit(0) => Limit(2),
+            limit => limit * 2,
+        };
+        err.help(&format!(
+            "consider increasing the recursion limit by adding a \
+             `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
+            suggested_limit,
+            self.tcx.crate_name(LOCAL_CRATE),
+        ));
+    }
+
+    #[instrument(
+        level = "debug", skip(self, err), fields(trait_pred.self_ty = ?trait_pred.self_ty())
+    )]
+    fn suggest_await_before_try(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+        span: Span,
+    ) {
+        let body_hir_id = obligation.cause.body_id;
+        let item_id = self.tcx.hir().get_parent_node(body_hir_id);
+
+        if let Some(body_id) =
+            self.tcx.hir().maybe_body_owned_by(self.tcx.hir().local_def_id(item_id))
+        {
+            let body = self.tcx.hir().body(body_id);
+            if let Some(hir::GeneratorKind::Async(_)) = body.generator_kind {
+                let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
+
+                let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
+                let impls_future = self.type_implements_trait(
+                    future_trait,
+                    self.tcx.erase_late_bound_regions(self_ty),
+                    ty::List::empty(),
+                    obligation.param_env,
+                );
+                if !impls_future.must_apply_modulo_regions() {
+                    return;
+                }
+
+                let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
+                // `<T as Future>::Output`
+                let projection_ty = trait_pred.map_bound(|trait_pred| {
+                    self.tcx.mk_projection(
+                        item_def_id,
+                        // Future::Output has no substs
+                        self.tcx.mk_substs_trait(trait_pred.self_ty(), &[]),
+                    )
+                });
+                let projection_ty = normalize_to(
+                    &mut SelectionContext::new(self),
+                    obligation.param_env,
+                    obligation.cause.clone(),
+                    projection_ty,
+                    &mut vec![],
+                );
+
+                debug!(
+                    normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
+                );
+                let try_obligation = self.mk_trait_obligation_with_new_self_ty(
+                    obligation.param_env,
+                    trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
+                );
+                debug!(try_trait_obligation = ?try_obligation);
+                if self.predicate_may_hold(&try_obligation)
+                    && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
+                    && snippet.ends_with('?')
+                {
+                    err.span_suggestion_verbose(
+                        span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
+                        "consider `await`ing on the `Future`",
+                        ".await",
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+            }
+        }
+    }
+
+    fn suggest_floating_point_literal(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_ref: &ty::PolyTraitRef<'tcx>,
+    ) {
+        let rhs_span = match obligation.cause.code() {
+            ObligationCauseCode::BinOp { rhs_span: Some(span), is_lit, .. } if *is_lit => span,
+            _ => return,
+        };
+        match (
+            trait_ref.skip_binder().self_ty().kind(),
+            trait_ref.skip_binder().substs.type_at(1).kind(),
+        ) {
+            (ty::Float(_), ty::Infer(InferTy::IntVar(_))) => {
+                err.span_suggestion_verbose(
+                    rhs_span.shrink_to_hi(),
+                    "consider using a floating-point literal by writing it with `.0`",
+                    ".0",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            _ => {}
+        }
+    }
+
+    fn suggest_derive(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) {
+        let Some(diagnostic_name) = self.tcx.get_diagnostic_name(trait_pred.def_id()) else {
+            return;
+        };
+        let (adt, substs) = match trait_pred.skip_binder().self_ty().kind() {
+            ty::Adt(adt, substs) if adt.did().is_local() => (adt, substs),
+            _ => return,
+        };
+        let can_derive = {
+            let is_derivable_trait = match diagnostic_name {
+                sym::Default => !adt.is_enum(),
+                sym::PartialEq | sym::PartialOrd => {
+                    let rhs_ty = trait_pred.skip_binder().trait_ref.substs.type_at(1);
+                    trait_pred.skip_binder().self_ty() == rhs_ty
+                }
+                sym::Eq | sym::Ord | sym::Clone | sym::Copy | sym::Hash | sym::Debug => true,
+                _ => false,
+            };
+            is_derivable_trait &&
+                // Ensure all fields impl the trait.
+                adt.all_fields().all(|field| {
+                    let field_ty = field.ty(self.tcx, substs);
+                    let trait_substs = match diagnostic_name {
+                        sym::PartialEq | sym::PartialOrd => {
+                            self.tcx.mk_substs_trait(field_ty, &[field_ty.into()])
+                        }
+                        _ => self.tcx.mk_substs_trait(field_ty, &[]),
+                    };
+                    let trait_pred = trait_pred.map_bound_ref(|tr| ty::TraitPredicate {
+                        trait_ref: ty::TraitRef {
+                            substs: trait_substs,
+                            ..trait_pred.skip_binder().trait_ref
+                        },
+                        ..*tr
+                    });
+                    let field_obl = Obligation::new(
+                        obligation.cause.clone(),
+                        obligation.param_env,
+                        trait_pred.to_predicate(self.tcx),
+                    );
+                    self.predicate_must_hold_modulo_regions(&field_obl)
+                })
+        };
+        if can_derive {
+            err.span_suggestion_verbose(
+                self.tcx.def_span(adt.did()).shrink_to_lo(),
+                &format!(
+                    "consider annotating `{}` with `#[derive({})]`",
+                    trait_pred.skip_binder().self_ty(),
+                    diagnostic_name,
+                ),
+                format!("#[derive({})]\n", diagnostic_name),
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+
+    fn suggest_dereferencing_index(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diagnostic,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) {
+        if let ObligationCauseCode::ImplDerivedObligation(_) = obligation.cause.code()
+            && self.tcx.is_diagnostic_item(sym::SliceIndex, trait_pred.skip_binder().trait_ref.def_id)
+            && let ty::Slice(_) = trait_pred.skip_binder().trait_ref.substs.type_at(1).kind()
+            && let ty::Ref(_, inner_ty, _) = trait_pred.skip_binder().self_ty().kind()
+            && let ty::Uint(ty::UintTy::Usize) = inner_ty.kind()
+        {
+            err.span_suggestion_verbose(
+                obligation.cause.span.shrink_to_lo(),
+            "dereference this index",
+            '*',
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+}
+
+/// Collect all the returned expressions within the input expression.
+/// Used to point at the return spans when we want to suggest some change to them.
+#[derive(Default)]
+pub struct ReturnsVisitor<'v> {
+    pub returns: Vec<&'v hir::Expr<'v>>,
+    in_block_tail: bool,
+}
+
+impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
+    fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
+        // Visit every expression to detect `return` paths, either through the function's tail
+        // expression or `return` statements. We walk all nodes to find `return` statements, but
+        // we only care about tail expressions when `in_block_tail` is `true`, which means that
+        // they're in the return path of the function body.
+        match ex.kind {
+            hir::ExprKind::Ret(Some(ex)) => {
+                self.returns.push(ex);
+            }
+            hir::ExprKind::Block(block, _) if self.in_block_tail => {
+                self.in_block_tail = false;
+                for stmt in block.stmts {
+                    hir::intravisit::walk_stmt(self, stmt);
+                }
+                self.in_block_tail = true;
+                if let Some(expr) = block.expr {
+                    self.visit_expr(expr);
+                }
+            }
+            hir::ExprKind::If(_, then, else_opt) if self.in_block_tail => {
+                self.visit_expr(then);
+                if let Some(el) = else_opt {
+                    self.visit_expr(el);
+                }
+            }
+            hir::ExprKind::Match(_, arms, _) if self.in_block_tail => {
+                for arm in arms {
+                    self.visit_expr(arm.body);
+                }
+            }
+            // We need to walk to find `return`s in the entire body.
+            _ if !self.in_block_tail => hir::intravisit::walk_expr(self, ex),
+            _ => self.returns.push(ex),
+        }
+    }
+
+    fn visit_body(&mut self, body: &'v hir::Body<'v>) {
+        assert!(!self.in_block_tail);
+        if body.generator_kind().is_none() {
+            if let hir::ExprKind::Block(block, None) = body.value.kind {
+                if block.expr.is_some() {
+                    self.in_block_tail = true;
+                }
+            }
+        }
+        hir::intravisit::walk_body(self, body);
+    }
+}
+
+/// Collect all the awaited expressions within the input expression.
+#[derive(Default)]
+struct AwaitsVisitor {
+    awaits: Vec<hir::HirId>,
+}
+
+impl<'v> Visitor<'v> for AwaitsVisitor {
+    fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
+        if let hir::ExprKind::Yield(_, hir::YieldSource::Await { expr: Some(id) }) = ex.kind {
+            self.awaits.push(id)
+        }
+        hir::intravisit::walk_expr(self, ex)
+    }
+}
+
+pub trait NextTypeParamName {
+    fn next_type_param_name(&self, name: Option<&str>) -> String;
+}
+
+impl NextTypeParamName for &[hir::GenericParam<'_>] {
+    fn next_type_param_name(&self, name: Option<&str>) -> String {
+        // This is the list of possible parameter names that we might suggest.
+        let name = name.and_then(|n| n.chars().next()).map(|c| c.to_string().to_uppercase());
+        let name = name.as_deref();
+        let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
+        let used_names = self
+            .iter()
+            .filter_map(|p| match p.name {
+                hir::ParamName::Plain(ident) => Some(ident.name),
+                _ => None,
+            })
+            .collect::<Vec<_>>();
+
+        possible_names
+            .iter()
+            .find(|n| !used_names.contains(&Symbol::intern(n)))
+            .unwrap_or(&"ParamName")
+            .to_string()
+    }
+}
+
+fn suggest_trait_object_return_type_alternatives(
+    err: &mut Diagnostic,
+    ret_ty: Span,
+    trait_obj: &str,
+    is_object_safe: bool,
+) {
+    err.span_suggestion(
+        ret_ty,
+        "use some type `T` that is `T: Sized` as the return type if all return paths have the \
+            same type",
+        "T",
+        Applicability::MaybeIncorrect,
+    );
+    err.span_suggestion(
+        ret_ty,
+        &format!(
+            "use `impl {}` as the return type if all return paths have the same type but you \
+                want to expose only the trait in the signature",
+            trait_obj,
+        ),
+        format!("impl {}", trait_obj),
+        Applicability::MaybeIncorrect,
+    );
+    if is_object_safe {
+        err.multipart_suggestion(
+            &format!(
+                "use a boxed trait object if all return paths implement trait `{}`",
+                trait_obj,
+            ),
+            vec![
+                (ret_ty.shrink_to_lo(), "Box<".to_string()),
+                (ret_ty.shrink_to_hi(), ">".to_string()),
+            ],
+            Applicability::MaybeIncorrect,
+        );
+    }
+}
+
+/// Collect the spans that we see the generic param `param_did`
+struct ReplaceImplTraitVisitor<'a> {
+    ty_spans: &'a mut Vec<Span>,
+    param_did: DefId,
+}
+
+impl<'a, 'hir> hir::intravisit::Visitor<'hir> for ReplaceImplTraitVisitor<'a> {
+    fn visit_ty(&mut self, t: &'hir hir::Ty<'hir>) {
+        if let hir::TyKind::Path(hir::QPath::Resolved(
+            None,
+            hir::Path { res: hir::def::Res::Def(_, segment_did), .. },
+        )) = t.kind
+        {
+            if self.param_did == *segment_did {
+                // `fn foo(t: impl Trait)`
+                //            ^^^^^^^^^^ get this to suggest `T` instead
+
+                // There might be more than one `impl Trait`.
+                self.ty_spans.push(t.span);
+                return;
+            }
+        }
+
+        hir::intravisit::walk_ty(self, t);
+    }
+}
+
+// Replace `param` with `replace_ty`
+struct ReplaceImplTraitFolder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param: &'tcx ty::GenericParamDef,
+    replace_ty: Ty<'tcx>,
+}
+
+impl<'tcx> TypeFolder<'tcx> for ReplaceImplTraitFolder<'tcx> {
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        if let ty::Param(ty::ParamTy { index, .. }) = t.kind() {
+            if self.param.index == *index {
+                return self.replace_ty;
+            }
+        }
+        t.super_fold_with(self)
+    }
+
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+}