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

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

12 files changed, 6853 insertions, 0 deletions

diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
new file mode 100644
index 000000000..20864c657
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -0,0 +1,3121 @@
+//! Error Reporting Code for the inference engine
+//!
+//! Because of the way inference, and in particular region inference,
+//! works, it often happens that errors are not detected until far after
+//! the relevant line of code has been type-checked. Therefore, there is
+//! an elaborate system to track why a particular constraint in the
+//! inference graph arose so that we can explain to the user what gave
+//! rise to a particular error.
+//!
+//! The system is based around a set of "origin" types. An "origin" is the
+//! reason that a constraint or inference variable arose. There are
+//! different "origin" enums for different kinds of constraints/variables
+//! (e.g., `TypeOrigin`, `RegionVariableOrigin`). An origin always has
+//! a span, but also more information so that we can generate a meaningful
+//! error message.
+//!
+//! Having a catalog of all the different reasons an error can arise is
+//! also useful for other reasons, like cross-referencing FAQs etc, though
+//! we are not really taking advantage of this yet.
+//!
+//! # Region Inference
+//!
+//! Region inference is particularly tricky because it always succeeds "in
+//! the moment" and simply registers a constraint. Then, at the end, we
+//! can compute the full graph and report errors, so we need to be able to
+//! store and later report what gave rise to the conflicting constraints.
+//!
+//! # Subtype Trace
+//!
+//! Determining whether `T1 <: T2` often involves a number of subtypes and
+//! subconstraints along the way. A "TypeTrace" is an extended version
+//! of an origin that traces the types and other values that were being
+//! compared. It is not necessarily comprehensive (in fact, at the time of
+//! this writing it only tracks the root values being compared) but I'd
+//! like to extend it to include significant "waypoints". For example, if
+//! you are comparing `(T1, T2) <: (T3, T4)`, and the problem is that `T2
+//! <: T4` fails, I'd like the trace to include enough information to say
+//! "in the 2nd element of the tuple". Similarly, failures when comparing
+//! arguments or return types in fn types should be able to cite the
+//! specific position, etc.
+//!
+//! # Reality vs plan
+//!
+//! Of course, there is still a LOT of code in typeck that has yet to be
+//! ported to this system, and which relies on string concatenation at the
+//! time of error detection.
+
+use super::lexical_region_resolve::RegionResolutionError;
+use super::region_constraints::GenericKind;
+use super::{InferCtxt, RegionVariableOrigin, SubregionOrigin, TypeTrace, ValuePairs};
+
+use crate::infer;
+use crate::infer::error_reporting::nice_region_error::find_anon_type::find_anon_type;
+use crate::traits::error_reporting::report_object_safety_error;
+use crate::traits::{
+    IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
+    StatementAsExpression,
+};
+
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_errors::{pluralize, struct_span_err, Diagnostic, ErrorGuaranteed};
+use rustc_errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString, MultiSpan};
+use rustc_hir as hir;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::lang_items::LangItem;
+use rustc_hir::Node;
+use rustc_middle::dep_graph::DepContext;
+use rustc_middle::ty::print::with_no_trimmed_paths;
+use rustc_middle::ty::{
+    self, error::TypeError, Binder, List, Region, Subst, Ty, TyCtxt, TypeFoldable,
+    TypeSuperVisitable, TypeVisitable,
+};
+use rustc_span::{sym, symbol::kw, BytePos, DesugaringKind, Pos, Span};
+use rustc_target::spec::abi;
+use std::ops::ControlFlow;
+use std::{cmp, fmt, iter};
+
+mod note;
+
+mod need_type_info;
+pub use need_type_info::TypeAnnotationNeeded;
+
+pub mod nice_region_error;
+
+pub(super) fn note_and_explain_region<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    err: &mut Diagnostic,
+    prefix: &str,
+    region: ty::Region<'tcx>,
+    suffix: &str,
+    alt_span: Option<Span>,
+) {
+    let (description, span) = match *region {
+        ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic => {
+            msg_span_from_free_region(tcx, region, alt_span)
+        }
+
+        ty::ReEmpty(ty::UniverseIndex::ROOT) => ("the empty lifetime".to_owned(), alt_span),
+
+        // uh oh, hope no user ever sees THIS
+        ty::ReEmpty(ui) => (format!("the empty lifetime in universe {:?}", ui), alt_span),
+
+        ty::RePlaceholder(_) => return,
+
+        // FIXME(#13998) RePlaceholder should probably print like
+        // ReFree rather than dumping Debug output on the user.
+        //
+        // We shouldn't really be having unification failures with ReVar
+        // and ReLateBound though.
+        ty::ReVar(_) | ty::ReLateBound(..) | ty::ReErased => {
+            (format!("lifetime {:?}", region), alt_span)
+        }
+    };
+
+    emit_msg_span(err, prefix, description, span, suffix);
+}
+
+fn explain_free_region<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    err: &mut Diagnostic,
+    prefix: &str,
+    region: ty::Region<'tcx>,
+    suffix: &str,
+) {
+    let (description, span) = msg_span_from_free_region(tcx, region, None);
+
+    label_msg_span(err, prefix, description, span, suffix);
+}
+
+fn msg_span_from_free_region<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    region: ty::Region<'tcx>,
+    alt_span: Option<Span>,
+) -> (String, Option<Span>) {
+    match *region {
+        ty::ReEarlyBound(_) | ty::ReFree(_) => {
+            let (msg, span) = msg_span_from_early_bound_and_free_regions(tcx, region);
+            (msg, Some(span))
+        }
+        ty::ReStatic => ("the static lifetime".to_owned(), alt_span),
+        ty::ReEmpty(ty::UniverseIndex::ROOT) => ("an empty lifetime".to_owned(), alt_span),
+        ty::ReEmpty(ui) => (format!("an empty lifetime in universe {:?}", ui), alt_span),
+        _ => bug!("{:?}", region),
+    }
+}
+
+fn msg_span_from_early_bound_and_free_regions<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    region: ty::Region<'tcx>,
+) -> (String, Span) {
+    let scope = region.free_region_binding_scope(tcx).expect_local();
+    match *region {
+        ty::ReEarlyBound(ref br) => {
+            let mut sp = tcx.def_span(scope);
+            if let Some(param) =
+                tcx.hir().get_generics(scope).and_then(|generics| generics.get_named(br.name))
+            {
+                sp = param.span;
+            }
+            let text = if br.has_name() {
+                format!("the lifetime `{}` as defined here", br.name)
+            } else {
+                format!("the anonymous lifetime as defined here")
+            };
+            (text, sp)
+        }
+        ty::ReFree(ref fr) => {
+            if !fr.bound_region.is_named()
+                && let Some((ty, _)) = find_anon_type(tcx, region, &fr.bound_region)
+            {
+                ("the anonymous lifetime defined here".to_string(), ty.span)
+            } else {
+                match fr.bound_region {
+                    ty::BoundRegionKind::BrNamed(_, name) => {
+                        let mut sp = tcx.def_span(scope);
+                        if let Some(param) =
+                            tcx.hir().get_generics(scope).and_then(|generics| generics.get_named(name))
+                        {
+                            sp = param.span;
+                        }
+                        let text = if name == kw::UnderscoreLifetime {
+                            format!("the anonymous lifetime as defined here")
+                        } else {
+                            format!("the lifetime `{}` as defined here", name)
+                        };
+                        (text, sp)
+                    }
+                    ty::BrAnon(idx) => (
+                        format!("the anonymous lifetime #{} defined here", idx + 1),
+                        tcx.def_span(scope)
+                    ),
+                    _ => (
+                        format!("the lifetime `{}` as defined here", region),
+                        tcx.def_span(scope),
+                    ),
+                }
+            }
+        }
+        _ => bug!(),
+    }
+}
+
+fn emit_msg_span(
+    err: &mut Diagnostic,
+    prefix: &str,
+    description: String,
+    span: Option<Span>,
+    suffix: &str,
+) {
+    let message = format!("{}{}{}", prefix, description, suffix);
+
+    if let Some(span) = span {
+        err.span_note(span, &message);
+    } else {
+        err.note(&message);
+    }
+}
+
+fn label_msg_span(
+    err: &mut Diagnostic,
+    prefix: &str,
+    description: String,
+    span: Option<Span>,
+    suffix: &str,
+) {
+    let message = format!("{}{}{}", prefix, description, suffix);
+
+    if let Some(span) = span {
+        err.span_label(span, &message);
+    } else {
+        err.note(&message);
+    }
+}
+
+pub fn unexpected_hidden_region_diagnostic<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    span: Span,
+    hidden_ty: Ty<'tcx>,
+    hidden_region: ty::Region<'tcx>,
+    opaque_ty: ty::OpaqueTypeKey<'tcx>,
+) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+    let opaque_ty = tcx.mk_opaque(opaque_ty.def_id.to_def_id(), opaque_ty.substs);
+    let mut err = struct_span_err!(
+        tcx.sess,
+        span,
+        E0700,
+        "hidden type for `{opaque_ty}` captures lifetime that does not appear in bounds",
+    );
+
+    // Explain the region we are capturing.
+    match *hidden_region {
+        ty::ReEmpty(ty::UniverseIndex::ROOT) => {
+            // All lifetimes shorter than the function body are `empty` in
+            // lexical region resolution. The default explanation of "an empty
+            // lifetime" isn't really accurate here.
+            let message = format!(
+                "hidden type `{}` captures lifetime smaller than the function body",
+                hidden_ty
+            );
+            err.span_note(span, &message);
+        }
+        ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic | ty::ReEmpty(_) => {
+            // Assuming regionck succeeded (*), we ought to always be
+            // capturing *some* region from the fn header, and hence it
+            // ought to be free. So under normal circumstances, we will go
+            // down this path which gives a decent human readable
+            // explanation.
+            //
+            // (*) if not, the `tainted_by_errors` field would be set to
+            // `Some(ErrorGuaranteed)` in any case, so we wouldn't be here at all.
+            explain_free_region(
+                tcx,
+                &mut err,
+                &format!("hidden type `{}` captures ", hidden_ty),
+                hidden_region,
+                "",
+            );
+            if let Some(reg_info) = tcx.is_suitable_region(hidden_region) {
+                let fn_returns = tcx.return_type_impl_or_dyn_traits(reg_info.def_id);
+                nice_region_error::suggest_new_region_bound(
+                    tcx,
+                    &mut err,
+                    fn_returns,
+                    hidden_region.to_string(),
+                    None,
+                    format!("captures `{}`", hidden_region),
+                    None,
+                )
+            }
+        }
+        _ => {
+            // Ugh. This is a painful case: the hidden region is not one
+            // that we can easily summarize or explain. This can happen
+            // in a case like
+            // `src/test/ui/multiple-lifetimes/ordinary-bounds-unsuited.rs`:
+            //
+            // ```
+            // fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> {
+            //   if condition() { a } else { b }
+            // }
+            // ```
+            //
+            // Here the captured lifetime is the intersection of `'a` and
+            // `'b`, which we can't quite express.
+
+            // We can at least report a really cryptic error for now.
+            note_and_explain_region(
+                tcx,
+                &mut err,
+                &format!("hidden type `{}` captures ", hidden_ty),
+                hidden_region,
+                "",
+                None,
+            );
+        }
+    }
+
+    err
+}
+
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    pub fn report_region_errors(
+        &self,
+        generic_param_scope: LocalDefId,
+        errors: &[RegionResolutionError<'tcx>],
+    ) {
+        debug!("report_region_errors(): {} errors to start", errors.len());
+
+        // try to pre-process the errors, which will group some of them
+        // together into a `ProcessedErrors` group:
+        let errors = self.process_errors(errors);
+
+        debug!("report_region_errors: {} errors after preprocessing", errors.len());
+
+        for error in errors {
+            debug!("report_region_errors: error = {:?}", error);
+
+            if !self.try_report_nice_region_error(&error) {
+                match error.clone() {
+                    // These errors could indicate all manner of different
+                    // problems with many different solutions. Rather
+                    // than generate a "one size fits all" error, what we
+                    // attempt to do is go through a number of specific
+                    // scenarios and try to find the best way to present
+                    // the error. If all of these fails, we fall back to a rather
+                    // general bit of code that displays the error information
+                    RegionResolutionError::ConcreteFailure(origin, sub, sup) => {
+                        if sub.is_placeholder() || sup.is_placeholder() {
+                            self.report_placeholder_failure(origin, sub, sup).emit();
+                        } else {
+                            self.report_concrete_failure(origin, sub, sup).emit();
+                        }
+                    }
+
+                    RegionResolutionError::GenericBoundFailure(origin, param_ty, sub) => {
+                        self.report_generic_bound_failure(
+                            generic_param_scope,
+                            origin.span(),
+                            Some(origin),
+                            param_ty,
+                            sub,
+                        );
+                    }
+
+                    RegionResolutionError::SubSupConflict(
+                        _,
+                        var_origin,
+                        sub_origin,
+                        sub_r,
+                        sup_origin,
+                        sup_r,
+                        _,
+                    ) => {
+                        if sub_r.is_placeholder() {
+                            self.report_placeholder_failure(sub_origin, sub_r, sup_r).emit();
+                        } else if sup_r.is_placeholder() {
+                            self.report_placeholder_failure(sup_origin, sub_r, sup_r).emit();
+                        } else {
+                            self.report_sub_sup_conflict(
+                                var_origin, sub_origin, sub_r, sup_origin, sup_r,
+                            );
+                        }
+                    }
+
+                    RegionResolutionError::UpperBoundUniverseConflict(
+                        _,
+                        _,
+                        var_universe,
+                        sup_origin,
+                        sup_r,
+                    ) => {
+                        assert!(sup_r.is_placeholder());
+
+                        // Make a dummy value for the "sub region" --
+                        // this is the initial value of the
+                        // placeholder. In practice, we expect more
+                        // tailored errors that don't really use this
+                        // value.
+                        let sub_r = self.tcx.mk_region(ty::ReEmpty(var_universe));
+
+                        self.report_placeholder_failure(sup_origin, sub_r, sup_r).emit();
+                    }
+                }
+            }
+        }
+    }
+
+    // This method goes through all the errors and try to group certain types
+    // of error together, for the purpose of suggesting explicit lifetime
+    // parameters to the user. This is done so that we can have a more
+    // complete view of what lifetimes should be the same.
+    // If the return value is an empty vector, it means that processing
+    // failed (so the return value of this method should not be used).
+    //
+    // The method also attempts to weed out messages that seem like
+    // duplicates that will be unhelpful to the end-user. But
+    // obviously it never weeds out ALL errors.
+    fn process_errors(
+        &self,
+        errors: &[RegionResolutionError<'tcx>],
+    ) -> Vec<RegionResolutionError<'tcx>> {
+        debug!("process_errors()");
+
+        // We want to avoid reporting generic-bound failures if we can
+        // avoid it: these have a very high rate of being unhelpful in
+        // practice. This is because they are basically secondary
+        // checks that test the state of the region graph after the
+        // rest of inference is done, and the other kinds of errors
+        // indicate that the region constraint graph is internally
+        // inconsistent, so these test results are likely to be
+        // meaningless.
+        //
+        // Therefore, we filter them out of the list unless they are
+        // the only thing in the list.
+
+        let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
+            RegionResolutionError::GenericBoundFailure(..) => true,
+            RegionResolutionError::ConcreteFailure(..)
+            | RegionResolutionError::SubSupConflict(..)
+            | RegionResolutionError::UpperBoundUniverseConflict(..) => false,
+        };
+
+        let mut errors = if errors.iter().all(|e| is_bound_failure(e)) {
+            errors.to_owned()
+        } else {
+            errors.iter().filter(|&e| !is_bound_failure(e)).cloned().collect()
+        };
+
+        // sort the errors by span, for better error message stability.
+        errors.sort_by_key(|u| match *u {
+            RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
+            RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
+            RegionResolutionError::SubSupConflict(_, ref rvo, _, _, _, _, _) => rvo.span(),
+            RegionResolutionError::UpperBoundUniverseConflict(_, ref rvo, _, _, _) => rvo.span(),
+        });
+        errors
+    }
+
+    /// Adds a note if the types come from similarly named crates
+    fn check_and_note_conflicting_crates(&self, err: &mut Diagnostic, terr: &TypeError<'tcx>) {
+        use hir::def_id::CrateNum;
+        use rustc_hir::definitions::DisambiguatedDefPathData;
+        use ty::print::Printer;
+        use ty::subst::GenericArg;
+
+        struct AbsolutePathPrinter<'tcx> {
+            tcx: TyCtxt<'tcx>,
+        }
+
+        struct NonTrivialPath;
+
+        impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
+            type Error = NonTrivialPath;
+
+            type Path = Vec<String>;
+            type Region = !;
+            type Type = !;
+            type DynExistential = !;
+            type Const = !;
+
+            fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+                self.tcx
+            }
+
+            fn print_region(self, _region: ty::Region<'_>) -> Result<Self::Region, Self::Error> {
+                Err(NonTrivialPath)
+            }
+
+            fn print_type(self, _ty: Ty<'tcx>) -> Result<Self::Type, Self::Error> {
+                Err(NonTrivialPath)
+            }
+
+            fn print_dyn_existential(
+                self,
+                _predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
+            ) -> Result<Self::DynExistential, Self::Error> {
+                Err(NonTrivialPath)
+            }
+
+            fn print_const(self, _ct: ty::Const<'tcx>) -> Result<Self::Const, Self::Error> {
+                Err(NonTrivialPath)
+            }
+
+            fn path_crate(self, cnum: CrateNum) -> Result<Self::Path, Self::Error> {
+                Ok(vec![self.tcx.crate_name(cnum).to_string()])
+            }
+            fn path_qualified(
+                self,
+                _self_ty: Ty<'tcx>,
+                _trait_ref: Option<ty::TraitRef<'tcx>>,
+            ) -> Result<Self::Path, Self::Error> {
+                Err(NonTrivialPath)
+            }
+
+            fn path_append_impl(
+                self,
+                _print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+                _disambiguated_data: &DisambiguatedDefPathData,
+                _self_ty: Ty<'tcx>,
+                _trait_ref: Option<ty::TraitRef<'tcx>>,
+            ) -> Result<Self::Path, Self::Error> {
+                Err(NonTrivialPath)
+            }
+            fn path_append(
+                self,
+                print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+                disambiguated_data: &DisambiguatedDefPathData,
+            ) -> Result<Self::Path, Self::Error> {
+                let mut path = print_prefix(self)?;
+                path.push(disambiguated_data.to_string());
+                Ok(path)
+            }
+            fn path_generic_args(
+                self,
+                print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+                _args: &[GenericArg<'tcx>],
+            ) -> Result<Self::Path, Self::Error> {
+                print_prefix(self)
+            }
+        }
+
+        let report_path_match = |err: &mut Diagnostic, did1: DefId, did2: DefId| {
+            // Only external crates, if either is from a local
+            // module we could have false positives
+            if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
+                let abs_path =
+                    |def_id| AbsolutePathPrinter { tcx: self.tcx }.print_def_path(def_id, &[]);
+
+                // We compare strings because DefPath can be different
+                // for imported and non-imported crates
+                let same_path = || -> Result<_, NonTrivialPath> {
+                    Ok(self.tcx.def_path_str(did1) == self.tcx.def_path_str(did2)
+                        || abs_path(did1)? == abs_path(did2)?)
+                };
+                if same_path().unwrap_or(false) {
+                    let crate_name = self.tcx.crate_name(did1.krate);
+                    err.note(&format!(
+                        "perhaps two different versions of crate `{}` are being used?",
+                        crate_name
+                    ));
+                }
+            }
+        };
+        match *terr {
+            TypeError::Sorts(ref exp_found) => {
+                // if they are both "path types", there's a chance of ambiguity
+                // due to different versions of the same crate
+                if let (&ty::Adt(exp_adt, _), &ty::Adt(found_adt, _)) =
+                    (exp_found.expected.kind(), exp_found.found.kind())
+                {
+                    report_path_match(err, exp_adt.did(), found_adt.did());
+                }
+            }
+            TypeError::Traits(ref exp_found) => {
+                report_path_match(err, exp_found.expected, exp_found.found);
+            }
+            _ => (), // FIXME(#22750) handle traits and stuff
+        }
+    }
+
+    fn note_error_origin(
+        &self,
+        err: &mut Diagnostic,
+        cause: &ObligationCause<'tcx>,
+        exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
+        terr: &TypeError<'tcx>,
+    ) {
+        match *cause.code() {
+            ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
+                let ty = self.resolve_vars_if_possible(root_ty);
+                if !matches!(ty.kind(), ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)))
+                {
+                    // don't show type `_`
+                    if span.desugaring_kind() == Some(DesugaringKind::ForLoop)
+                    && let ty::Adt(def, substs) = ty.kind()
+                    && Some(def.did()) == self.tcx.get_diagnostic_item(sym::Option)
+                    {
+                        err.span_label(span, format!("this is an iterator with items of type `{}`", substs.type_at(0)));
+                    } else {
+                        err.span_label(span, format!("this expression has type `{}`", ty));
+                    }
+                }
+                if let Some(ty::error::ExpectedFound { found, .. }) = exp_found
+                    && ty.is_box() && ty.boxed_ty() == found
+                    && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
+                {
+                    err.span_suggestion(
+                        span,
+                        "consider dereferencing the boxed value",
+                        format!("*{}", snippet),
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
+            ObligationCauseCode::Pattern { origin_expr: false, span: Some(span), .. } => {
+                err.span_label(span, "expected due to this");
+            }
+            ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
+                arm_block_id,
+                arm_span,
+                arm_ty,
+                prior_arm_block_id,
+                prior_arm_span,
+                prior_arm_ty,
+                source,
+                ref prior_arms,
+                scrut_hir_id,
+                opt_suggest_box_span,
+                scrut_span,
+                ..
+            }) => match source {
+                hir::MatchSource::TryDesugar => {
+                    if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
+                        let scrut_expr = self.tcx.hir().expect_expr(scrut_hir_id);
+                        let scrut_ty = if let hir::ExprKind::Call(_, args) = &scrut_expr.kind {
+                            let arg_expr = args.first().expect("try desugaring call w/out arg");
+                            self.in_progress_typeck_results.and_then(|typeck_results| {
+                                typeck_results.borrow().expr_ty_opt(arg_expr)
+                            })
+                        } else {
+                            bug!("try desugaring w/out call expr as scrutinee");
+                        };
+
+                        match scrut_ty {
+                            Some(ty) if expected == ty => {
+                                let source_map = self.tcx.sess.source_map();
+                                err.span_suggestion(
+                                    source_map.end_point(cause.span),
+                                    "try removing this `?`",
+                                    "",
+                                    Applicability::MachineApplicable,
+                                );
+                            }
+                            _ => {}
+                        }
+                    }
+                }
+                _ => {
+                    // `prior_arm_ty` can be `!`, `expected` will have better info when present.
+                    let t = self.resolve_vars_if_possible(match exp_found {
+                        Some(ty::error::ExpectedFound { expected, .. }) => expected,
+                        _ => prior_arm_ty,
+                    });
+                    let source_map = self.tcx.sess.source_map();
+                    let mut any_multiline_arm = source_map.is_multiline(arm_span);
+                    if prior_arms.len() <= 4 {
+                        for sp in prior_arms {
+                            any_multiline_arm |= source_map.is_multiline(*sp);
+                            err.span_label(*sp, format!("this is found to be of type `{}`", t));
+                        }
+                    } else if let Some(sp) = prior_arms.last() {
+                        any_multiline_arm |= source_map.is_multiline(*sp);
+                        err.span_label(
+                            *sp,
+                            format!("this and all prior arms are found to be of type `{}`", t),
+                        );
+                    }
+                    let outer_error_span = if any_multiline_arm {
+                        // Cover just `match` and the scrutinee expression, not
+                        // the entire match body, to reduce diagram noise.
+                        cause.span.shrink_to_lo().to(scrut_span)
+                    } else {
+                        cause.span
+                    };
+                    let msg = "`match` arms have incompatible types";
+                    err.span_label(outer_error_span, msg);
+                    self.suggest_remove_semi_or_return_binding(
+                        err,
+                        prior_arm_block_id,
+                        prior_arm_ty,
+                        prior_arm_span,
+                        arm_block_id,
+                        arm_ty,
+                        arm_span,
+                    );
+                    if let Some(ret_sp) = opt_suggest_box_span {
+                        // Get return type span and point to it.
+                        self.suggest_boxing_for_return_impl_trait(
+                            err,
+                            ret_sp,
+                            prior_arms.iter().chain(std::iter::once(&arm_span)).map(|s| *s),
+                        );
+                    }
+                }
+            },
+            ObligationCauseCode::IfExpression(box IfExpressionCause {
+                then_id,
+                else_id,
+                then_ty,
+                else_ty,
+                outer_span,
+                opt_suggest_box_span,
+            }) => {
+                let then_span = self.find_block_span_from_hir_id(then_id);
+                let else_span = self.find_block_span_from_hir_id(else_id);
+                err.span_label(then_span, "expected because of this");
+                if let Some(sp) = outer_span {
+                    err.span_label(sp, "`if` and `else` have incompatible types");
+                }
+                self.suggest_remove_semi_or_return_binding(
+                    err,
+                    Some(then_id),
+                    then_ty,
+                    then_span,
+                    Some(else_id),
+                    else_ty,
+                    else_span,
+                );
+                if let Some(ret_sp) = opt_suggest_box_span {
+                    self.suggest_boxing_for_return_impl_trait(
+                        err,
+                        ret_sp,
+                        [then_span, else_span].into_iter(),
+                    );
+                }
+            }
+            ObligationCauseCode::LetElse => {
+                err.help("try adding a diverging expression, such as `return` or `panic!(..)`");
+                err.help("...or use `match` instead of `let...else`");
+            }
+            _ => {
+                if let ObligationCauseCode::BindingObligation(_, binding_span) =
+                    cause.code().peel_derives()
+                {
+                    if matches!(terr, TypeError::RegionsPlaceholderMismatch) {
+                        err.span_note(*binding_span, "the lifetime requirement is introduced here");
+                    }
+                }
+            }
+        }
+    }
+
+    fn suggest_remove_semi_or_return_binding(
+        &self,
+        err: &mut Diagnostic,
+        first_id: Option<hir::HirId>,
+        first_ty: Ty<'tcx>,
+        first_span: Span,
+        second_id: Option<hir::HirId>,
+        second_ty: Ty<'tcx>,
+        second_span: Span,
+    ) {
+        let remove_semicolon = [
+            (first_id, self.resolve_vars_if_possible(second_ty)),
+            (second_id, self.resolve_vars_if_possible(first_ty)),
+        ]
+        .into_iter()
+        .find_map(|(id, ty)| {
+            let hir::Node::Block(blk) = self.tcx.hir().get(id?) else { return None };
+            self.could_remove_semicolon(blk, ty)
+        });
+        match remove_semicolon {
+            Some((sp, StatementAsExpression::NeedsBoxing)) => {
+                err.multipart_suggestion(
+                    "consider removing this semicolon and boxing the expressions",
+                    vec![
+                        (first_span.shrink_to_lo(), "Box::new(".to_string()),
+                        (first_span.shrink_to_hi(), ")".to_string()),
+                        (second_span.shrink_to_lo(), "Box::new(".to_string()),
+                        (second_span.shrink_to_hi(), ")".to_string()),
+                        (sp, String::new()),
+                    ],
+                    Applicability::MachineApplicable,
+                );
+            }
+            Some((sp, StatementAsExpression::CorrectType)) => {
+                err.span_suggestion_short(
+                    sp,
+                    "consider removing this semicolon",
+                    "",
+                    Applicability::MachineApplicable,
+                );
+            }
+            None => {
+                for (id, ty) in [(first_id, second_ty), (second_id, first_ty)] {
+                    if let Some(id) = id
+                        && let hir::Node::Block(blk) = self.tcx.hir().get(id)
+                        && self.consider_returning_binding(blk, ty, err)
+                    {
+                        break;
+                    }
+                }
+            }
+        }
+    }
+
+    fn suggest_boxing_for_return_impl_trait(
+        &self,
+        err: &mut Diagnostic,
+        return_sp: Span,
+        arm_spans: impl Iterator<Item = Span>,
+    ) {
+        err.multipart_suggestion(
+            "you could change the return type to be a boxed trait object",
+            vec![
+                (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
+                (return_sp.shrink_to_hi(), ">".to_string()),
+            ],
+            Applicability::MaybeIncorrect,
+        );
+        let sugg = arm_spans
+            .flat_map(|sp| {
+                [(sp.shrink_to_lo(), "Box::new(".to_string()), (sp.shrink_to_hi(), ")".to_string())]
+                    .into_iter()
+            })
+            .collect::<Vec<_>>();
+        err.multipart_suggestion(
+            "if you change the return type to expect trait objects, box the returned expressions",
+            sugg,
+            Applicability::MaybeIncorrect,
+        );
+    }
+
+    /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
+    /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
+    /// populate `other_value` with `other_ty`.
+    ///
+    /// ```text
+    /// Foo<Bar<Qux>>
+    /// ^^^^--------^ this is highlighted
+    /// |   |
+    /// |   this type argument is exactly the same as the other type, not highlighted
+    /// this is highlighted
+    /// Bar<Qux>
+    /// -------- this type is the same as a type argument in the other type, not highlighted
+    /// ```
+    fn highlight_outer(
+        &self,
+        value: &mut DiagnosticStyledString,
+        other_value: &mut DiagnosticStyledString,
+        name: String,
+        sub: ty::subst::SubstsRef<'tcx>,
+        pos: usize,
+        other_ty: Ty<'tcx>,
+    ) {
+        // `value` and `other_value` hold two incomplete type representation for display.
+        // `name` is the path of both types being compared. `sub`
+        value.push_highlighted(name);
+        let len = sub.len();
+        if len > 0 {
+            value.push_highlighted("<");
+        }
+
+        // Output the lifetimes for the first type
+        let lifetimes = sub
+            .regions()
+            .map(|lifetime| {
+                let s = lifetime.to_string();
+                if s.is_empty() { "'_".to_string() } else { s }
+            })
+            .collect::<Vec<_>>()
+            .join(", ");
+        if !lifetimes.is_empty() {
+            if sub.regions().count() < len {
+                value.push_normal(lifetimes + ", ");
+            } else {
+                value.push_normal(lifetimes);
+            }
+        }
+
+        // Highlight all the type arguments that aren't at `pos` and compare the type argument at
+        // `pos` and `other_ty`.
+        for (i, type_arg) in sub.types().enumerate() {
+            if i == pos {
+                let values = self.cmp(type_arg, other_ty);
+                value.0.extend((values.0).0);
+                other_value.0.extend((values.1).0);
+            } else {
+                value.push_highlighted(type_arg.to_string());
+            }
+
+            if len > 0 && i != len - 1 {
+                value.push_normal(", ");
+            }
+        }
+        if len > 0 {
+            value.push_highlighted(">");
+        }
+    }
+
+    /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
+    /// as that is the difference to the other type.
+    ///
+    /// For the following code:
+    ///
+    /// ```ignore (illustrative)
+    /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
+    /// ```
+    ///
+    /// The type error output will behave in the following way:
+    ///
+    /// ```text
+    /// Foo<Bar<Qux>>
+    /// ^^^^--------^ this is highlighted
+    /// |   |
+    /// |   this type argument is exactly the same as the other type, not highlighted
+    /// this is highlighted
+    /// Bar<Qux>
+    /// -------- this type is the same as a type argument in the other type, not highlighted
+    /// ```
+    fn cmp_type_arg(
+        &self,
+        mut t1_out: &mut DiagnosticStyledString,
+        mut t2_out: &mut DiagnosticStyledString,
+        path: String,
+        sub: &'tcx [ty::GenericArg<'tcx>],
+        other_path: String,
+        other_ty: Ty<'tcx>,
+    ) -> Option<()> {
+        // FIXME/HACK: Go back to `SubstsRef` to use its inherent methods,
+        // ideally that shouldn't be necessary.
+        let sub = self.tcx.intern_substs(sub);
+        for (i, ta) in sub.types().enumerate() {
+            if ta == other_ty {
+                self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, other_ty);
+                return Some(());
+            }
+            if let ty::Adt(def, _) = ta.kind() {
+                let path_ = self.tcx.def_path_str(def.did());
+                if path_ == other_path {
+                    self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, other_ty);
+                    return Some(());
+                }
+            }
+        }
+        None
+    }
+
+    /// Adds a `,` to the type representation only if it is appropriate.
+    fn push_comma(
+        &self,
+        value: &mut DiagnosticStyledString,
+        other_value: &mut DiagnosticStyledString,
+        len: usize,
+        pos: usize,
+    ) {
+        if len > 0 && pos != len - 1 {
+            value.push_normal(", ");
+            other_value.push_normal(", ");
+        }
+    }
+
+    /// Given two `fn` signatures highlight only sub-parts that are different.
+    fn cmp_fn_sig(
+        &self,
+        sig1: &ty::PolyFnSig<'tcx>,
+        sig2: &ty::PolyFnSig<'tcx>,
+    ) -> (DiagnosticStyledString, DiagnosticStyledString) {
+        let get_lifetimes = |sig| {
+            use rustc_hir::def::Namespace;
+            let (_, sig, reg) = ty::print::FmtPrinter::new(self.tcx, Namespace::TypeNS)
+                .name_all_regions(sig)
+                .unwrap();
+            let lts: Vec<String> = reg.into_iter().map(|(_, kind)| kind.to_string()).collect();
+            (if lts.is_empty() { String::new() } else { format!("for<{}> ", lts.join(", ")) }, sig)
+        };
+
+        let (lt1, sig1) = get_lifetimes(sig1);
+        let (lt2, sig2) = get_lifetimes(sig2);
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        let mut values = (
+            DiagnosticStyledString::normal("".to_string()),
+            DiagnosticStyledString::normal("".to_string()),
+        );
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        // ^^^^^^
+        values.0.push(sig1.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);
+        values.1.push(sig2.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        //        ^^^^^^^^^^
+        if sig1.abi != abi::Abi::Rust {
+            values.0.push(format!("extern {} ", sig1.abi), sig1.abi != sig2.abi);
+        }
+        if sig2.abi != abi::Abi::Rust {
+            values.1.push(format!("extern {} ", sig2.abi), sig1.abi != sig2.abi);
+        }
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        //                   ^^^^^^^^
+        let lifetime_diff = lt1 != lt2;
+        values.0.push(lt1, lifetime_diff);
+        values.1.push(lt2, lifetime_diff);
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        //                           ^^^
+        values.0.push_normal("fn(");
+        values.1.push_normal("fn(");
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        //                              ^^^^^
+        let len1 = sig1.inputs().len();
+        let len2 = sig2.inputs().len();
+        if len1 == len2 {
+            for (i, (l, r)) in iter::zip(sig1.inputs(), sig2.inputs()).enumerate() {
+                let (x1, x2) = self.cmp(*l, *r);
+                (values.0).0.extend(x1.0);
+                (values.1).0.extend(x2.0);
+                self.push_comma(&mut values.0, &mut values.1, len1, i);
+            }
+        } else {
+            for (i, l) in sig1.inputs().iter().enumerate() {
+                values.0.push_highlighted(l.to_string());
+                if i != len1 - 1 {
+                    values.0.push_highlighted(", ");
+                }
+            }
+            for (i, r) in sig2.inputs().iter().enumerate() {
+                values.1.push_highlighted(r.to_string());
+                if i != len2 - 1 {
+                    values.1.push_highlighted(", ");
+                }
+            }
+        }
+
+        if sig1.c_variadic {
+            if len1 > 0 {
+                values.0.push_normal(", ");
+            }
+            values.0.push("...", !sig2.c_variadic);
+        }
+        if sig2.c_variadic {
+            if len2 > 0 {
+                values.1.push_normal(", ");
+            }
+            values.1.push("...", !sig1.c_variadic);
+        }
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        //                                   ^
+        values.0.push_normal(")");
+        values.1.push_normal(")");
+
+        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
+        //                                     ^^^^^^^^
+        let output1 = sig1.output();
+        let output2 = sig2.output();
+        let (x1, x2) = self.cmp(output1, output2);
+        if !output1.is_unit() {
+            values.0.push_normal(" -> ");
+            (values.0).0.extend(x1.0);
+        }
+        if !output2.is_unit() {
+            values.1.push_normal(" -> ");
+            (values.1).0.extend(x2.0);
+        }
+        values
+    }
+
+    /// Compares two given types, eliding parts that are the same between them and highlighting
+    /// relevant differences, and return two representation of those types for highlighted printing.
+    pub fn cmp(
+        &self,
+        t1: Ty<'tcx>,
+        t2: Ty<'tcx>,
+    ) -> (DiagnosticStyledString, DiagnosticStyledString) {
+        debug!("cmp(t1={}, t1.kind={:?}, t2={}, t2.kind={:?})", t1, t1.kind(), t2, t2.kind());
+
+        // helper functions
+        fn equals<'tcx>(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
+            match (a.kind(), b.kind()) {
+                (a, b) if *a == *b => true,
+                (&ty::Int(_), &ty::Infer(ty::InferTy::IntVar(_)))
+                | (
+                    &ty::Infer(ty::InferTy::IntVar(_)),
+                    &ty::Int(_) | &ty::Infer(ty::InferTy::IntVar(_)),
+                )
+                | (&ty::Float(_), &ty::Infer(ty::InferTy::FloatVar(_)))
+                | (
+                    &ty::Infer(ty::InferTy::FloatVar(_)),
+                    &ty::Float(_) | &ty::Infer(ty::InferTy::FloatVar(_)),
+                ) => true,
+                _ => false,
+            }
+        }
+
+        fn push_ty_ref<'tcx>(
+            region: ty::Region<'tcx>,
+            ty: Ty<'tcx>,
+            mutbl: hir::Mutability,
+            s: &mut DiagnosticStyledString,
+        ) {
+            let mut r = region.to_string();
+            if r == "'_" {
+                r.clear();
+            } else {
+                r.push(' ');
+            }
+            s.push_highlighted(format!("&{}{}", r, mutbl.prefix_str()));
+            s.push_normal(ty.to_string());
+        }
+
+        // process starts here
+        match (t1.kind(), t2.kind()) {
+            (&ty::Adt(def1, sub1), &ty::Adt(def2, sub2)) => {
+                let did1 = def1.did();
+                let did2 = def2.did();
+                let sub_no_defaults_1 =
+                    self.tcx.generics_of(did1).own_substs_no_defaults(self.tcx, sub1);
+                let sub_no_defaults_2 =
+                    self.tcx.generics_of(did2).own_substs_no_defaults(self.tcx, sub2);
+                let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
+                let path1 = self.tcx.def_path_str(did1);
+                let path2 = self.tcx.def_path_str(did2);
+                if did1 == did2 {
+                    // Easy case. Replace same types with `_` to shorten the output and highlight
+                    // the differing ones.
+                    //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
+                    //     Foo<Bar, _>
+                    //     Foo<Quz, _>
+                    //         ---  ^ type argument elided
+                    //         |
+                    //         highlighted in output
+                    values.0.push_normal(path1);
+                    values.1.push_normal(path2);
+
+                    // Avoid printing out default generic parameters that are common to both
+                    // types.
+                    let len1 = sub_no_defaults_1.len();
+                    let len2 = sub_no_defaults_2.len();
+                    let common_len = cmp::min(len1, len2);
+                    let remainder1: Vec<_> = sub1.types().skip(common_len).collect();
+                    let remainder2: Vec<_> = sub2.types().skip(common_len).collect();
+                    let common_default_params =
+                        iter::zip(remainder1.iter().rev(), remainder2.iter().rev())
+                            .filter(|(a, b)| a == b)
+                            .count();
+                    let len = sub1.len() - common_default_params;
+                    let consts_offset = len - sub1.consts().count();
+
+                    // Only draw `<...>` if there are lifetime/type arguments.
+                    if len > 0 {
+                        values.0.push_normal("<");
+                        values.1.push_normal("<");
+                    }
+
+                    fn lifetime_display(lifetime: Region<'_>) -> String {
+                        let s = lifetime.to_string();
+                        if s.is_empty() { "'_".to_string() } else { s }
+                    }
+                    // At one point we'd like to elide all lifetimes here, they are irrelevant for
+                    // all diagnostics that use this output
+                    //
+                    //     Foo<'x, '_, Bar>
+                    //     Foo<'y, '_, Qux>
+                    //         ^^  ^^  --- type arguments are not elided
+                    //         |   |
+                    //         |   elided as they were the same
+                    //         not elided, they were different, but irrelevant
+                    //
+                    // For bound lifetimes, keep the names of the lifetimes,
+                    // even if they are the same so that it's clear what's happening
+                    // if we have something like
+                    //
+                    // for<'r, 's> fn(Inv<'r>, Inv<'s>)
+                    // for<'r> fn(Inv<'r>, Inv<'r>)
+                    let lifetimes = sub1.regions().zip(sub2.regions());
+                    for (i, lifetimes) in lifetimes.enumerate() {
+                        let l1 = lifetime_display(lifetimes.0);
+                        let l2 = lifetime_display(lifetimes.1);
+                        if lifetimes.0 != lifetimes.1 {
+                            values.0.push_highlighted(l1);
+                            values.1.push_highlighted(l2);
+                        } else if lifetimes.0.is_late_bound() {
+                            values.0.push_normal(l1);
+                            values.1.push_normal(l2);
+                        } else {
+                            values.0.push_normal("'_");
+                            values.1.push_normal("'_");
+                        }
+                        self.push_comma(&mut values.0, &mut values.1, len, i);
+                    }
+
+                    // We're comparing two types with the same path, so we compare the type
+                    // arguments for both. If they are the same, do not highlight and elide from the
+                    // output.
+                    //     Foo<_, Bar>
+                    //     Foo<_, Qux>
+                    //         ^ elided type as this type argument was the same in both sides
+                    let type_arguments = sub1.types().zip(sub2.types());
+                    let regions_len = sub1.regions().count();
+                    let num_display_types = consts_offset - regions_len;
+                    for (i, (ta1, ta2)) in type_arguments.take(num_display_types).enumerate() {
+                        let i = i + regions_len;
+                        if ta1 == ta2 {
+                            values.0.push_normal("_");
+                            values.1.push_normal("_");
+                        } else {
+                            let (x1, x2) = self.cmp(ta1, ta2);
+                            (values.0).0.extend(x1.0);
+                            (values.1).0.extend(x2.0);
+                        }
+                        self.push_comma(&mut values.0, &mut values.1, len, i);
+                    }
+
+                    // Do the same for const arguments, if they are equal, do not highlight and
+                    // elide them from the output.
+                    let const_arguments = sub1.consts().zip(sub2.consts());
+                    for (i, (ca1, ca2)) in const_arguments.enumerate() {
+                        let i = i + consts_offset;
+                        if ca1 == ca2 {
+                            values.0.push_normal("_");
+                            values.1.push_normal("_");
+                        } else {
+                            values.0.push_highlighted(ca1.to_string());
+                            values.1.push_highlighted(ca2.to_string());
+                        }
+                        self.push_comma(&mut values.0, &mut values.1, len, i);
+                    }
+
+                    // Close the type argument bracket.
+                    // Only draw `<...>` if there are lifetime/type arguments.
+                    if len > 0 {
+                        values.0.push_normal(">");
+                        values.1.push_normal(">");
+                    }
+                    values
+                } else {
+                    // Check for case:
+                    //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
+                    //     Foo<Bar<Qux>
+                    //         ------- this type argument is exactly the same as the other type
+                    //     Bar<Qux>
+                    if self
+                        .cmp_type_arg(
+                            &mut values.0,
+                            &mut values.1,
+                            path1.clone(),
+                            sub_no_defaults_1,
+                            path2.clone(),
+                            t2,
+                        )
+                        .is_some()
+                    {
+                        return values;
+                    }
+                    // Check for case:
+                    //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
+                    //     Bar<Qux>
+                    //     Foo<Bar<Qux>>
+                    //         ------- this type argument is exactly the same as the other type
+                    if self
+                        .cmp_type_arg(
+                            &mut values.1,
+                            &mut values.0,
+                            path2,
+                            sub_no_defaults_2,
+                            path1,
+                            t1,
+                        )
+                        .is_some()
+                    {
+                        return values;
+                    }
+
+                    // We can't find anything in common, highlight relevant part of type path.
+                    //     let x: foo::bar::Baz<Qux> = y:<foo::bar::Bar<Zar>>();
+                    //     foo::bar::Baz<Qux>
+                    //     foo::bar::Bar<Zar>
+                    //               -------- this part of the path is different
+
+                    let t1_str = t1.to_string();
+                    let t2_str = t2.to_string();
+                    let min_len = t1_str.len().min(t2_str.len());
+
+                    const SEPARATOR: &str = "::";
+                    let separator_len = SEPARATOR.len();
+                    let split_idx: usize =
+                        iter::zip(t1_str.split(SEPARATOR), t2_str.split(SEPARATOR))
+                            .take_while(|(mod1_str, mod2_str)| mod1_str == mod2_str)
+                            .map(|(mod_str, _)| mod_str.len() + separator_len)
+                            .sum();
+
+                    debug!(
+                        "cmp: separator_len={}, split_idx={}, min_len={}",
+                        separator_len, split_idx, min_len
+                    );
+
+                    if split_idx >= min_len {
+                        // paths are identical, highlight everything
+                        (
+                            DiagnosticStyledString::highlighted(t1_str),
+                            DiagnosticStyledString::highlighted(t2_str),
+                        )
+                    } else {
+                        let (common, uniq1) = t1_str.split_at(split_idx);
+                        let (_, uniq2) = t2_str.split_at(split_idx);
+                        debug!("cmp: common={}, uniq1={}, uniq2={}", common, uniq1, uniq2);
+
+                        values.0.push_normal(common);
+                        values.0.push_highlighted(uniq1);
+                        values.1.push_normal(common);
+                        values.1.push_highlighted(uniq2);
+
+                        values
+                    }
+                }
+            }
+
+            // When finding T != &T, highlight only the borrow
+            (&ty::Ref(r1, ref_ty1, mutbl1), _) if equals(ref_ty1, t2) => {
+                let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
+                push_ty_ref(r1, ref_ty1, mutbl1, &mut values.0);
+                values.1.push_normal(t2.to_string());
+                values
+            }
+            (_, &ty::Ref(r2, ref_ty2, mutbl2)) if equals(t1, ref_ty2) => {
+                let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
+                values.0.push_normal(t1.to_string());
+                push_ty_ref(r2, ref_ty2, mutbl2, &mut values.1);
+                values
+            }
+
+            // When encountering &T != &mut T, highlight only the borrow
+            (&ty::Ref(r1, ref_ty1, mutbl1), &ty::Ref(r2, ref_ty2, mutbl2))
+                if equals(ref_ty1, ref_ty2) =>
+            {
+                let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
+                push_ty_ref(r1, ref_ty1, mutbl1, &mut values.0);
+                push_ty_ref(r2, ref_ty2, mutbl2, &mut values.1);
+                values
+            }
+
+            // When encountering tuples of the same size, highlight only the differing types
+            (&ty::Tuple(substs1), &ty::Tuple(substs2)) if substs1.len() == substs2.len() => {
+                let mut values =
+                    (DiagnosticStyledString::normal("("), DiagnosticStyledString::normal("("));
+                let len = substs1.len();
+                for (i, (left, right)) in substs1.iter().zip(substs2).enumerate() {
+                    let (x1, x2) = self.cmp(left, right);
+                    (values.0).0.extend(x1.0);
+                    (values.1).0.extend(x2.0);
+                    self.push_comma(&mut values.0, &mut values.1, len, i);
+                }
+                if len == 1 {
+                    // Keep the output for single element tuples as `(ty,)`.
+                    values.0.push_normal(",");
+                    values.1.push_normal(",");
+                }
+                values.0.push_normal(")");
+                values.1.push_normal(")");
+                values
+            }
+
+            (ty::FnDef(did1, substs1), ty::FnDef(did2, substs2)) => {
+                let sig1 = self.tcx.bound_fn_sig(*did1).subst(self.tcx, substs1);
+                let sig2 = self.tcx.bound_fn_sig(*did2).subst(self.tcx, substs2);
+                let mut values = self.cmp_fn_sig(&sig1, &sig2);
+                let path1 = format!(" {{{}}}", self.tcx.def_path_str_with_substs(*did1, substs1));
+                let path2 = format!(" {{{}}}", self.tcx.def_path_str_with_substs(*did2, substs2));
+                let same_path = path1 == path2;
+                values.0.push(path1, !same_path);
+                values.1.push(path2, !same_path);
+                values
+            }
+
+            (ty::FnDef(did1, substs1), ty::FnPtr(sig2)) => {
+                let sig1 = self.tcx.bound_fn_sig(*did1).subst(self.tcx, substs1);
+                let mut values = self.cmp_fn_sig(&sig1, sig2);
+                values.0.push_highlighted(format!(
+                    " {{{}}}",
+                    self.tcx.def_path_str_with_substs(*did1, substs1)
+                ));
+                values
+            }
+
+            (ty::FnPtr(sig1), ty::FnDef(did2, substs2)) => {
+                let sig2 = self.tcx.bound_fn_sig(*did2).subst(self.tcx, substs2);
+                let mut values = self.cmp_fn_sig(sig1, &sig2);
+                values.1.push_normal(format!(
+                    " {{{}}}",
+                    self.tcx.def_path_str_with_substs(*did2, substs2)
+                ));
+                values
+            }
+
+            (ty::FnPtr(sig1), ty::FnPtr(sig2)) => self.cmp_fn_sig(sig1, sig2),
+
+            _ => {
+                if t1 == t2 {
+                    // The two types are the same, elide and don't highlight.
+                    (DiagnosticStyledString::normal("_"), DiagnosticStyledString::normal("_"))
+                } else {
+                    // We couldn't find anything in common, highlight everything.
+                    (
+                        DiagnosticStyledString::highlighted(t1.to_string()),
+                        DiagnosticStyledString::highlighted(t2.to_string()),
+                    )
+                }
+            }
+        }
+    }
+
+    /// Extend a type error with extra labels pointing at "non-trivial" types, like closures and
+    /// the return type of `async fn`s.
+    ///
+    /// `secondary_span` gives the caller the opportunity to expand `diag` with a `span_label`.
+    ///
+    /// `swap_secondary_and_primary` is used to make projection errors in particular nicer by using
+    /// the message in `secondary_span` as the primary label, and apply the message that would
+    /// otherwise be used for the primary label on the `secondary_span` `Span`. This applies on
+    /// E0271, like `src/test/ui/issues/issue-39970.stderr`.
+    #[tracing::instrument(
+        level = "debug",
+        skip(self, diag, secondary_span, swap_secondary_and_primary, force_label)
+    )]
+    pub fn note_type_err(
+        &self,
+        diag: &mut Diagnostic,
+        cause: &ObligationCause<'tcx>,
+        secondary_span: Option<(Span, String)>,
+        mut values: Option<ValuePairs<'tcx>>,
+        terr: &TypeError<'tcx>,
+        swap_secondary_and_primary: bool,
+        force_label: bool,
+    ) {
+        let span = cause.span();
+
+        // For some types of errors, expected-found does not make
+        // sense, so just ignore the values we were given.
+        if let TypeError::CyclicTy(_) = terr {
+            values = None;
+        }
+        struct OpaqueTypesVisitor<'tcx> {
+            types: FxHashMap<TyCategory, FxHashSet<Span>>,
+            expected: FxHashMap<TyCategory, FxHashSet<Span>>,
+            found: FxHashMap<TyCategory, FxHashSet<Span>>,
+            ignore_span: Span,
+            tcx: TyCtxt<'tcx>,
+        }
+
+        impl<'tcx> OpaqueTypesVisitor<'tcx> {
+            fn visit_expected_found(
+                tcx: TyCtxt<'tcx>,
+                expected: Ty<'tcx>,
+                found: Ty<'tcx>,
+                ignore_span: Span,
+            ) -> Self {
+                let mut types_visitor = OpaqueTypesVisitor {
+                    types: Default::default(),
+                    expected: Default::default(),
+                    found: Default::default(),
+                    ignore_span,
+                    tcx,
+                };
+                // The visitor puts all the relevant encountered types in `self.types`, but in
+                // here we want to visit two separate types with no relation to each other, so we
+                // move the results from `types` to `expected` or `found` as appropriate.
+                expected.visit_with(&mut types_visitor);
+                std::mem::swap(&mut types_visitor.expected, &mut types_visitor.types);
+                found.visit_with(&mut types_visitor);
+                std::mem::swap(&mut types_visitor.found, &mut types_visitor.types);
+                types_visitor
+            }
+
+            fn report(&self, err: &mut Diagnostic) {
+                self.add_labels_for_types(err, "expected", &self.expected);
+                self.add_labels_for_types(err, "found", &self.found);
+            }
+
+            fn add_labels_for_types(
+                &self,
+                err: &mut Diagnostic,
+                target: &str,
+                types: &FxHashMap<TyCategory, FxHashSet<Span>>,
+            ) {
+                for (key, values) in types.iter() {
+                    let count = values.len();
+                    let kind = key.descr();
+                    let mut returned_async_output_error = false;
+                    for &sp in values {
+                        if sp.is_desugaring(DesugaringKind::Async) && !returned_async_output_error {
+                            if [sp] != err.span.primary_spans() {
+                                let mut span: MultiSpan = sp.into();
+                                span.push_span_label(
+                                    sp,
+                                    format!(
+                                        "checked the `Output` of this `async fn`, {}{} {}{}",
+                                        if count > 1 { "one of the " } else { "" },
+                                        target,
+                                        kind,
+                                        pluralize!(count),
+                                    ),
+                                );
+                                err.span_note(
+                                    span,
+                                    "while checking the return type of the `async fn`",
+                                );
+                            } else {
+                                err.span_label(
+                                    sp,
+                                    format!(
+                                        "checked the `Output` of this `async fn`, {}{} {}{}",
+                                        if count > 1 { "one of the " } else { "" },
+                                        target,
+                                        kind,
+                                        pluralize!(count),
+                                    ),
+                                );
+                                err.note("while checking the return type of the `async fn`");
+                            }
+                            returned_async_output_error = true;
+                        } else {
+                            err.span_label(
+                                sp,
+                                format!(
+                                    "{}{} {}{}",
+                                    if count == 1 { "the " } else { "one of the " },
+                                    target,
+                                    kind,
+                                    pluralize!(count),
+                                ),
+                            );
+                        }
+                    }
+                }
+            }
+        }
+
+        impl<'tcx> ty::visit::TypeVisitor<'tcx> for OpaqueTypesVisitor<'tcx> {
+            fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+                if let Some((kind, def_id)) = TyCategory::from_ty(self.tcx, t) {
+                    let span = self.tcx.def_span(def_id);
+                    // Avoid cluttering the output when the "found" and error span overlap:
+                    //
+                    // error[E0308]: mismatched types
+                    //   --> $DIR/issue-20862.rs:2:5
+                    //    |
+                    // LL |     |y| x + y
+                    //    |     ^^^^^^^^^
+                    //    |     |
+                    //    |     the found closure
+                    //    |     expected `()`, found closure
+                    //    |
+                    //    = note: expected unit type `()`
+                    //                 found closure `[closure@$DIR/issue-20862.rs:2:5: 2:14 x:_]`
+                    if !self.ignore_span.overlaps(span) {
+                        self.types.entry(kind).or_default().insert(span);
+                    }
+                }
+                t.super_visit_with(self)
+            }
+        }
+
+        debug!("note_type_err(diag={:?})", diag);
+        enum Mismatch<'a> {
+            Variable(ty::error::ExpectedFound<Ty<'a>>),
+            Fixed(&'static str),
+        }
+        let (expected_found, exp_found, is_simple_error, values) = match values {
+            None => (None, Mismatch::Fixed("type"), false, None),
+            Some(values) => {
+                let values = self.resolve_vars_if_possible(values);
+                let (is_simple_error, exp_found) = match values {
+                    ValuePairs::Terms(infer::ExpectedFound {
+                        expected: ty::Term::Ty(expected),
+                        found: ty::Term::Ty(found),
+                    }) => {
+                        let is_simple_err = expected.is_simple_text() && found.is_simple_text();
+                        OpaqueTypesVisitor::visit_expected_found(self.tcx, expected, found, span)
+                            .report(diag);
+
+                        (
+                            is_simple_err,
+                            Mismatch::Variable(infer::ExpectedFound { expected, found }),
+                        )
+                    }
+                    ValuePairs::TraitRefs(_) | ValuePairs::PolyTraitRefs(_) => {
+                        (false, Mismatch::Fixed("trait"))
+                    }
+                    _ => (false, Mismatch::Fixed("type")),
+                };
+                let vals = match self.values_str(values) {
+                    Some((expected, found)) => Some((expected, found)),
+                    None => {
+                        // Derived error. Cancel the emitter.
+                        // NOTE(eddyb) this was `.cancel()`, but `diag`
+                        // is borrowed, so we can't fully defuse it.
+                        diag.downgrade_to_delayed_bug();
+                        return;
+                    }
+                };
+                (vals, exp_found, is_simple_error, Some(values))
+            }
+        };
+
+        match terr {
+            // Ignore msg for object safe coercion
+            // since E0038 message will be printed
+            TypeError::ObjectUnsafeCoercion(_) => {}
+            _ => {
+                let mut label_or_note = |span: Span, msg: &str| {
+                    if force_label || &[span] == diag.span.primary_spans() {
+                        diag.span_label(span, msg);
+                    } else {
+                        diag.span_note(span, msg);
+                    }
+                };
+                if let Some((sp, msg)) = secondary_span {
+                    if swap_secondary_and_primary {
+                        let terr = if let Some(infer::ValuePairs::Terms(infer::ExpectedFound {
+                            expected,
+                            ..
+                        })) = values
+                        {
+                            format!("expected this to be `{}`", expected)
+                        } else {
+                            terr.to_string()
+                        };
+                        label_or_note(sp, &terr);
+                        label_or_note(span, &msg);
+                    } else {
+                        label_or_note(span, &terr.to_string());
+                        label_or_note(sp, &msg);
+                    }
+                } else {
+                    label_or_note(span, &terr.to_string());
+                }
+            }
+        };
+        if let Some((expected, found)) = expected_found {
+            let (expected_label, found_label, exp_found) = match exp_found {
+                Mismatch::Variable(ef) => (
+                    ef.expected.prefix_string(self.tcx),
+                    ef.found.prefix_string(self.tcx),
+                    Some(ef),
+                ),
+                Mismatch::Fixed(s) => (s.into(), s.into(), None),
+            };
+            match (&terr, expected == found) {
+                (TypeError::Sorts(values), extra) => {
+                    let sort_string = |ty: Ty<'tcx>| match (extra, ty.kind()) {
+                        (true, ty::Opaque(def_id, _)) => {
+                            let sm = self.tcx.sess.source_map();
+                            let pos = sm.lookup_char_pos(self.tcx.def_span(*def_id).lo());
+                            format!(
+                                " (opaque type at <{}:{}:{}>)",
+                                sm.filename_for_diagnostics(&pos.file.name),
+                                pos.line,
+                                pos.col.to_usize() + 1,
+                            )
+                        }
+                        (true, _) => format!(" ({})", ty.sort_string(self.tcx)),
+                        (false, _) => "".to_string(),
+                    };
+                    if !(values.expected.is_simple_text() && values.found.is_simple_text())
+                        || (exp_found.map_or(false, |ef| {
+                            // This happens when the type error is a subset of the expectation,
+                            // like when you have two references but one is `usize` and the other
+                            // is `f32`. In those cases we still want to show the `note`. If the
+                            // value from `ef` is `Infer(_)`, then we ignore it.
+                            if !ef.expected.is_ty_infer() {
+                                ef.expected != values.expected
+                            } else if !ef.found.is_ty_infer() {
+                                ef.found != values.found
+                            } else {
+                                false
+                            }
+                        }))
+                    {
+                        diag.note_expected_found_extra(
+                            &expected_label,
+                            expected,
+                            &found_label,
+                            found,
+                            &sort_string(values.expected),
+                            &sort_string(values.found),
+                        );
+                    }
+                }
+                (TypeError::ObjectUnsafeCoercion(_), _) => {
+                    diag.note_unsuccessful_coercion(found, expected);
+                }
+                (_, _) => {
+                    debug!(
+                        "note_type_err: exp_found={:?}, expected={:?} found={:?}",
+                        exp_found, expected, found
+                    );
+                    if !is_simple_error || terr.must_include_note() {
+                        diag.note_expected_found(&expected_label, expected, &found_label, found);
+                    }
+                }
+            }
+        }
+        let exp_found = match exp_found {
+            Mismatch::Variable(exp_found) => Some(exp_found),
+            Mismatch::Fixed(_) => None,
+        };
+        let exp_found = match terr {
+            // `terr` has more accurate type information than `exp_found` in match expressions.
+            ty::error::TypeError::Sorts(terr)
+                if exp_found.map_or(false, |ef| terr.found == ef.found) =>
+            {
+                Some(*terr)
+            }
+            _ => exp_found,
+        };
+        debug!("exp_found {:?} terr {:?} cause.code {:?}", exp_found, terr, cause.code());
+        if let Some(exp_found) = exp_found {
+            let should_suggest_fixes =
+                if let ObligationCauseCode::Pattern { root_ty, .. } = cause.code() {
+                    // Skip if the root_ty of the pattern is not the same as the expected_ty.
+                    // If these types aren't equal then we've probably peeled off a layer of arrays.
+                    self.same_type_modulo_infer(*root_ty, exp_found.expected)
+                } else {
+                    true
+                };
+
+            if should_suggest_fixes {
+                self.suggest_tuple_pattern(cause, &exp_found, diag);
+                self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
+                self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
+                self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
+            }
+        }
+
+        // In some (most?) cases cause.body_id points to actual body, but in some cases
+        // it's an actual definition. According to the comments (e.g. in
+        // librustc_typeck/check/compare_method.rs:compare_predicate_entailment) the latter
+        // is relied upon by some other code. This might (or might not) need cleanup.
+        let body_owner_def_id =
+            self.tcx.hir().opt_local_def_id(cause.body_id).unwrap_or_else(|| {
+                self.tcx.hir().body_owner_def_id(hir::BodyId { hir_id: cause.body_id })
+            });
+        self.check_and_note_conflicting_crates(diag, terr);
+        self.tcx.note_and_explain_type_err(diag, terr, cause, span, body_owner_def_id.to_def_id());
+
+        if let Some(ValuePairs::PolyTraitRefs(exp_found)) = values
+            && let ty::Closure(def_id, _) = exp_found.expected.skip_binder().self_ty().kind()
+            && let Some(def_id) = def_id.as_local()
+        {
+            let span = self.tcx.def_span(def_id);
+            diag.span_note(span, "this closure does not fulfill the lifetime requirements");
+        }
+
+        // It reads better to have the error origin as the final
+        // thing.
+        self.note_error_origin(diag, cause, exp_found, terr);
+
+        debug!(?diag);
+    }
+
+    fn suggest_tuple_pattern(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut Diagnostic,
+    ) {
+        // Heavily inspired by `FnCtxt::suggest_compatible_variants`, with
+        // some modifications due to that being in typeck and this being in infer.
+        if let ObligationCauseCode::Pattern { .. } = cause.code() {
+            if let ty::Adt(expected_adt, substs) = exp_found.expected.kind() {
+                let compatible_variants: Vec<_> = expected_adt
+                    .variants()
+                    .iter()
+                    .filter(|variant| {
+                        variant.fields.len() == 1 && variant.ctor_kind == hir::def::CtorKind::Fn
+                    })
+                    .filter_map(|variant| {
+                        let sole_field = &variant.fields[0];
+                        let sole_field_ty = sole_field.ty(self.tcx, substs);
+                        if self.same_type_modulo_infer(sole_field_ty, exp_found.found) {
+                            let variant_path =
+                                with_no_trimmed_paths!(self.tcx.def_path_str(variant.def_id));
+                            // FIXME #56861: DRYer prelude filtering
+                            if let Some(path) = variant_path.strip_prefix("std::prelude::") {
+                                if let Some((_, path)) = path.split_once("::") {
+                                    return Some(path.to_string());
+                                }
+                            }
+                            Some(variant_path)
+                        } else {
+                            None
+                        }
+                    })
+                    .collect();
+                match &compatible_variants[..] {
+                    [] => {}
+                    [variant] => {
+                        diag.multipart_suggestion_verbose(
+                            &format!("try wrapping the pattern in `{}`", variant),
+                            vec![
+                                (cause.span.shrink_to_lo(), format!("{}(", variant)),
+                                (cause.span.shrink_to_hi(), ")".to_string()),
+                            ],
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                    _ => {
+                        // More than one matching variant.
+                        diag.multipart_suggestions(
+                            &format!(
+                                "try wrapping the pattern in a variant of `{}`",
+                                self.tcx.def_path_str(expected_adt.did())
+                            ),
+                            compatible_variants.into_iter().map(|variant| {
+                                vec![
+                                    (cause.span.shrink_to_lo(), format!("{}(", variant)),
+                                    (cause.span.shrink_to_hi(), ")".to_string()),
+                                ]
+                            }),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+            }
+        }
+    }
+
+    pub fn get_impl_future_output_ty(&self, ty: Ty<'tcx>) -> Option<Binder<'tcx, Ty<'tcx>>> {
+        if let ty::Opaque(def_id, substs) = ty.kind() {
+            let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
+            // Future::Output
+            let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
+
+            let bounds = self.tcx.bound_explicit_item_bounds(*def_id);
+
+            for predicate in bounds.transpose_iter().map(|e| e.map_bound(|(p, _)| *p)) {
+                let predicate = predicate.subst(self.tcx, substs);
+                let output = predicate
+                    .kind()
+                    .map_bound(|kind| match kind {
+                        ty::PredicateKind::Projection(projection_predicate)
+                            if projection_predicate.projection_ty.item_def_id == item_def_id =>
+                        {
+                            projection_predicate.term.ty()
+                        }
+                        _ => None,
+                    })
+                    .transpose();
+                if output.is_some() {
+                    // We don't account for multiple `Future::Output = Ty` constraints.
+                    return output;
+                }
+            }
+        }
+        None
+    }
+
+    /// A possible error is to forget to add `.await` when using futures:
+    ///
+    /// ```compile_fail,E0308
+    /// async fn make_u32() -> u32 {
+    ///     22
+    /// }
+    ///
+    /// fn take_u32(x: u32) {}
+    ///
+    /// async fn foo() {
+    ///     let x = make_u32();
+    ///     take_u32(x);
+    /// }
+    /// ```
+    ///
+    /// This routine checks if the found type `T` implements `Future<Output=U>` where `U` is the
+    /// expected type. If this is the case, and we are inside of an async body, it suggests adding
+    /// `.await` to the tail of the expression.
+    fn suggest_await_on_expect_found(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        exp_span: Span,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut Diagnostic,
+    ) {
+        debug!(
+            "suggest_await_on_expect_found: exp_span={:?}, expected_ty={:?}, found_ty={:?}",
+            exp_span, exp_found.expected, exp_found.found,
+        );
+
+        if let ObligationCauseCode::CompareImplItemObligation { .. } = cause.code() {
+            return;
+        }
+
+        match (
+            self.get_impl_future_output_ty(exp_found.expected).map(Binder::skip_binder),
+            self.get_impl_future_output_ty(exp_found.found).map(Binder::skip_binder),
+        ) {
+            (Some(exp), Some(found)) if self.same_type_modulo_infer(exp, found) => match cause
+                .code()
+            {
+                ObligationCauseCode::IfExpression(box IfExpressionCause { then_id, .. }) => {
+                    let then_span = self.find_block_span_from_hir_id(*then_id);
+                    diag.multipart_suggestion(
+                        "consider `await`ing on both `Future`s",
+                        vec![
+                            (then_span.shrink_to_hi(), ".await".to_string()),
+                            (exp_span.shrink_to_hi(), ".await".to_string()),
+                        ],
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
+                    prior_arms,
+                    ..
+                }) => {
+                    if let [.., arm_span] = &prior_arms[..] {
+                        diag.multipart_suggestion(
+                            "consider `await`ing on both `Future`s",
+                            vec![
+                                (arm_span.shrink_to_hi(), ".await".to_string()),
+                                (exp_span.shrink_to_hi(), ".await".to_string()),
+                            ],
+                            Applicability::MaybeIncorrect,
+                        );
+                    } else {
+                        diag.help("consider `await`ing on both `Future`s");
+                    }
+                }
+                _ => {
+                    diag.help("consider `await`ing on both `Future`s");
+                }
+            },
+            (_, Some(ty)) if self.same_type_modulo_infer(exp_found.expected, ty) => {
+                diag.span_suggestion_verbose(
+                    exp_span.shrink_to_hi(),
+                    "consider `await`ing on the `Future`",
+                    ".await",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            (Some(ty), _) if self.same_type_modulo_infer(ty, exp_found.found) => match cause.code()
+            {
+                ObligationCauseCode::Pattern { span: Some(then_span), .. } => {
+                    diag.span_suggestion_verbose(
+                        then_span.shrink_to_hi(),
+                        "consider `await`ing on the `Future`",
+                        ".await",
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                ObligationCauseCode::IfExpression(box IfExpressionCause { then_id, .. }) => {
+                    let then_span = self.find_block_span_from_hir_id(*then_id);
+                    diag.span_suggestion_verbose(
+                        then_span.shrink_to_hi(),
+                        "consider `await`ing on the `Future`",
+                        ".await",
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
+                    ref prior_arms,
+                    ..
+                }) => {
+                    diag.multipart_suggestion_verbose(
+                        "consider `await`ing on the `Future`",
+                        prior_arms
+                            .iter()
+                            .map(|arm| (arm.shrink_to_hi(), ".await".to_string()))
+                            .collect(),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                _ => {}
+            },
+            _ => {}
+        }
+    }
+
+    fn suggest_accessing_field_where_appropriate(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut Diagnostic,
+    ) {
+        debug!(
+            "suggest_accessing_field_where_appropriate(cause={:?}, exp_found={:?})",
+            cause, exp_found
+        );
+        if let ty::Adt(expected_def, expected_substs) = exp_found.expected.kind() {
+            if expected_def.is_enum() {
+                return;
+            }
+
+            if let Some((name, ty)) = expected_def
+                .non_enum_variant()
+                .fields
+                .iter()
+                .filter(|field| field.vis.is_accessible_from(field.did, self.tcx))
+                .map(|field| (field.name, field.ty(self.tcx, expected_substs)))
+                .find(|(_, ty)| self.same_type_modulo_infer(*ty, exp_found.found))
+            {
+                if let ObligationCauseCode::Pattern { span: Some(span), .. } = *cause.code() {
+                    if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                        let suggestion = if expected_def.is_struct() {
+                            format!("{}.{}", snippet, name)
+                        } else if expected_def.is_union() {
+                            format!("unsafe {{ {}.{} }}", snippet, name)
+                        } else {
+                            return;
+                        };
+                        diag.span_suggestion(
+                            span,
+                            &format!(
+                                "you might have meant to use field `{}` whose type is `{}`",
+                                name, ty
+                            ),
+                            suggestion,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+            }
+        }
+    }
+
+    /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
+    /// suggests it.
+    fn suggest_as_ref_where_appropriate(
+        &self,
+        span: Span,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut Diagnostic,
+    ) {
+        if let (ty::Adt(exp_def, exp_substs), ty::Ref(_, found_ty, _)) =
+            (exp_found.expected.kind(), exp_found.found.kind())
+        {
+            if let ty::Adt(found_def, found_substs) = *found_ty.kind() {
+                let path_str = format!("{:?}", exp_def);
+                if exp_def == &found_def {
+                    let opt_msg = "you can convert from `&Option<T>` to `Option<&T>` using \
+                                       `.as_ref()`";
+                    let result_msg = "you can convert from `&Result<T, E>` to \
+                                          `Result<&T, &E>` using `.as_ref()`";
+                    let have_as_ref = &[
+                        ("std::option::Option", opt_msg),
+                        ("core::option::Option", opt_msg),
+                        ("std::result::Result", result_msg),
+                        ("core::result::Result", result_msg),
+                    ];
+                    if let Some(msg) = have_as_ref
+                        .iter()
+                        .find_map(|(path, msg)| (&path_str == path).then_some(msg))
+                    {
+                        let mut show_suggestion = true;
+                        for (exp_ty, found_ty) in
+                            iter::zip(exp_substs.types(), found_substs.types())
+                        {
+                            match *exp_ty.kind() {
+                                ty::Ref(_, exp_ty, _) => {
+                                    match (exp_ty.kind(), found_ty.kind()) {
+                                        (_, ty::Param(_))
+                                        | (_, ty::Infer(_))
+                                        | (ty::Param(_), _)
+                                        | (ty::Infer(_), _) => {}
+                                        _ if self.same_type_modulo_infer(exp_ty, found_ty) => {}
+                                        _ => show_suggestion = false,
+                                    };
+                                }
+                                ty::Param(_) | ty::Infer(_) => {}
+                                _ => show_suggestion = false,
+                            }
+                        }
+                        if let (Ok(snippet), true) =
+                            (self.tcx.sess.source_map().span_to_snippet(span), show_suggestion)
+                        {
+                            diag.span_suggestion(
+                                span,
+                                *msg,
+                                format!("{}.as_ref()", snippet),
+                                Applicability::MachineApplicable,
+                            );
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    pub fn report_and_explain_type_error(
+        &self,
+        trace: TypeTrace<'tcx>,
+        terr: &TypeError<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        use crate::traits::ObligationCauseCode::MatchExpressionArm;
+
+        debug!("report_and_explain_type_error(trace={:?}, terr={:?})", trace, terr);
+
+        let span = trace.cause.span();
+        let failure_code = trace.cause.as_failure_code(terr);
+        let mut diag = match failure_code {
+            FailureCode::Error0038(did) => {
+                let violations = self.tcx.object_safety_violations(did);
+                report_object_safety_error(self.tcx, span, did, violations)
+            }
+            FailureCode::Error0317(failure_str) => {
+                struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
+            }
+            FailureCode::Error0580(failure_str) => {
+                struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
+            }
+            FailureCode::Error0308(failure_str) => {
+                let mut err = struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str);
+                if let Some((expected, found)) = trace.values.ty() {
+                    match (expected.kind(), found.kind()) {
+                        (ty::Tuple(_), ty::Tuple(_)) => {}
+                        // If a tuple of length one was expected and the found expression has
+                        // parentheses around it, perhaps the user meant to write `(expr,)` to
+                        // build a tuple (issue #86100)
+                        (ty::Tuple(fields), _) => {
+                            self.emit_tuple_wrap_err(&mut err, span, found, fields)
+                        }
+                        // If a character was expected and the found expression is a string literal
+                        // containing a single character, perhaps the user meant to write `'c'` to
+                        // specify a character literal (issue #92479)
+                        (ty::Char, ty::Ref(_, r, _)) if r.is_str() => {
+                            if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
+                                && let Some(code) = code.strip_prefix('"').and_then(|s| s.strip_suffix('"'))
+                                && code.chars().count() == 1
+                            {
+                                err.span_suggestion(
+                                    span,
+                                    "if you meant to write a `char` literal, use single quotes",
+                                    format!("'{}'", code),
+                                    Applicability::MachineApplicable,
+                                );
+                            }
+                        }
+                        // If a string was expected and the found expression is a character literal,
+                        // perhaps the user meant to write `"s"` to specify a string literal.
+                        (ty::Ref(_, r, _), ty::Char) if r.is_str() => {
+                            if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
+                                if let Some(code) =
+                                    code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
+                                {
+                                    err.span_suggestion(
+                                        span,
+                                        "if you meant to write a `str` literal, use double quotes",
+                                        format!("\"{}\"", code),
+                                        Applicability::MachineApplicable,
+                                    );
+                                }
+                            }
+                        }
+                        _ => {}
+                    }
+                }
+                let code = trace.cause.code();
+                if let &MatchExpressionArm(box MatchExpressionArmCause { source, .. }) = code
+                    && let hir::MatchSource::TryDesugar = source
+                    && let Some((expected_ty, found_ty)) = self.values_str(trace.values)
+                {
+                    err.note(&format!(
+                        "`?` operator cannot convert from `{}` to `{}`",
+                        found_ty.content(),
+                        expected_ty.content(),
+                    ));
+                }
+                err
+            }
+            FailureCode::Error0644(failure_str) => {
+                struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
+            }
+        };
+        self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr, false, false);
+        diag
+    }
+
+    fn emit_tuple_wrap_err(
+        &self,
+        err: &mut Diagnostic,
+        span: Span,
+        found: Ty<'tcx>,
+        expected_fields: &List<Ty<'tcx>>,
+    ) {
+        let [expected_tup_elem] = expected_fields[..] else { return };
+
+        if !self.same_type_modulo_infer(expected_tup_elem, found) {
+            return;
+        }
+
+        let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
+            else { return };
+
+        let msg = "use a trailing comma to create a tuple with one element";
+        if code.starts_with('(') && code.ends_with(')') {
+            let before_close = span.hi() - BytePos::from_u32(1);
+            err.span_suggestion(
+                span.with_hi(before_close).shrink_to_hi(),
+                msg,
+                ",",
+                Applicability::MachineApplicable,
+            );
+        } else {
+            err.multipart_suggestion(
+                msg,
+                vec![(span.shrink_to_lo(), "(".into()), (span.shrink_to_hi(), ",)".into())],
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+
+    fn values_str(
+        &self,
+        values: ValuePairs<'tcx>,
+    ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
+        match values {
+            infer::Regions(exp_found) => self.expected_found_str(exp_found),
+            infer::Terms(exp_found) => self.expected_found_str_term(exp_found),
+            infer::TraitRefs(exp_found) => {
+                let pretty_exp_found = ty::error::ExpectedFound {
+                    expected: exp_found.expected.print_only_trait_path(),
+                    found: exp_found.found.print_only_trait_path(),
+                };
+                match self.expected_found_str(pretty_exp_found) {
+                    Some((expected, found)) if expected == found => {
+                        self.expected_found_str(exp_found)
+                    }
+                    ret => ret,
+                }
+            }
+            infer::PolyTraitRefs(exp_found) => {
+                let pretty_exp_found = ty::error::ExpectedFound {
+                    expected: exp_found.expected.print_only_trait_path(),
+                    found: exp_found.found.print_only_trait_path(),
+                };
+                match self.expected_found_str(pretty_exp_found) {
+                    Some((expected, found)) if expected == found => {
+                        self.expected_found_str(exp_found)
+                    }
+                    ret => ret,
+                }
+            }
+        }
+    }
+
+    fn expected_found_str_term(
+        &self,
+        exp_found: ty::error::ExpectedFound<ty::Term<'tcx>>,
+    ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
+        let exp_found = self.resolve_vars_if_possible(exp_found);
+        if exp_found.references_error() {
+            return None;
+        }
+
+        Some(match (exp_found.expected, exp_found.found) {
+            (ty::Term::Ty(expected), ty::Term::Ty(found)) => self.cmp(expected, found),
+            (expected, found) => (
+                DiagnosticStyledString::highlighted(expected.to_string()),
+                DiagnosticStyledString::highlighted(found.to_string()),
+            ),
+        })
+    }
+
+    /// Returns a string of the form "expected `{}`, found `{}`".
+    fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
+        &self,
+        exp_found: ty::error::ExpectedFound<T>,
+    ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
+        let exp_found = self.resolve_vars_if_possible(exp_found);
+        if exp_found.references_error() {
+            return None;
+        }
+
+        Some((
+            DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
+            DiagnosticStyledString::highlighted(exp_found.found.to_string()),
+        ))
+    }
+
+    pub fn report_generic_bound_failure(
+        &self,
+        generic_param_scope: LocalDefId,
+        span: Span,
+        origin: Option<SubregionOrigin<'tcx>>,
+        bound_kind: GenericKind<'tcx>,
+        sub: Region<'tcx>,
+    ) {
+        self.construct_generic_bound_failure(generic_param_scope, span, origin, bound_kind, sub)
+            .emit();
+    }
+
+    pub fn construct_generic_bound_failure(
+        &self,
+        generic_param_scope: LocalDefId,
+        span: Span,
+        origin: Option<SubregionOrigin<'tcx>>,
+        bound_kind: GenericKind<'tcx>,
+        sub: Region<'tcx>,
+    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+        // Attempt to obtain the span of the parameter so we can
+        // suggest adding an explicit lifetime bound to it.
+        let generics = self.tcx.generics_of(generic_param_scope);
+        // type_param_span is (span, has_bounds)
+        let type_param_span = match bound_kind {
+            GenericKind::Param(ref param) => {
+                // Account for the case where `param` corresponds to `Self`,
+                // which doesn't have the expected type argument.
+                if !(generics.has_self && param.index == 0) {
+                    let type_param = generics.type_param(param, self.tcx);
+                    type_param.def_id.as_local().map(|def_id| {
+                        // Get the `hir::Param` to verify whether it already has any bounds.
+                        // We do this to avoid suggesting code that ends up as `T: 'a'b`,
+                        // instead we suggest `T: 'a + 'b` in that case.
+                        let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
+                        let ast_generics = self.tcx.hir().get_generics(hir_id.owner);
+                        let bounds =
+                            ast_generics.and_then(|g| g.bounds_span_for_suggestions(def_id));
+                        // `sp` only covers `T`, change it so that it covers
+                        // `T:` when appropriate
+                        if let Some(span) = bounds {
+                            (span, true)
+                        } else {
+                            let sp = self.tcx.def_span(def_id);
+                            (sp.shrink_to_hi(), false)
+                        }
+                    })
+                } else {
+                    None
+                }
+            }
+            _ => None,
+        };
+
+        let new_lt = {
+            let mut possible = (b'a'..=b'z').map(|c| format!("'{}", c as char));
+            let lts_names =
+                iter::successors(Some(generics), |g| g.parent.map(|p| self.tcx.generics_of(p)))
+                    .flat_map(|g| &g.params)
+                    .filter(|p| matches!(p.kind, ty::GenericParamDefKind::Lifetime))
+                    .map(|p| p.name.as_str())
+                    .collect::<Vec<_>>();
+            possible
+                .find(|candidate| !lts_names.contains(&&candidate[..]))
+                .unwrap_or("'lt".to_string())
+        };
+
+        let add_lt_sugg = generics
+            .params
+            .first()
+            .and_then(|param| param.def_id.as_local())
+            .map(|def_id| (self.tcx.def_span(def_id).shrink_to_lo(), format!("{}, ", new_lt)));
+
+        let labeled_user_string = match bound_kind {
+            GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
+            GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
+        };
+
+        if let Some(SubregionOrigin::CompareImplItemObligation {
+            span,
+            impl_item_def_id,
+            trait_item_def_id,
+        }) = origin
+        {
+            return self.report_extra_impl_obligation(
+                span,
+                impl_item_def_id,
+                trait_item_def_id,
+                &format!("`{}: {}`", bound_kind, sub),
+            );
+        }
+
+        fn binding_suggestion<'tcx, S: fmt::Display>(
+            err: &mut Diagnostic,
+            type_param_span: Option<(Span, bool)>,
+            bound_kind: GenericKind<'tcx>,
+            sub: S,
+        ) {
+            let msg = "consider adding an explicit lifetime bound";
+            if let Some((sp, has_lifetimes)) = type_param_span {
+                let suggestion =
+                    if has_lifetimes { format!(" + {}", sub) } else { format!(": {}", sub) };
+                err.span_suggestion_verbose(
+                    sp,
+                    &format!("{}...", msg),
+                    suggestion,
+                    Applicability::MaybeIncorrect, // Issue #41966
+                );
+            } else {
+                let consider = format!("{} `{}: {}`...", msg, bound_kind, sub,);
+                err.help(&consider);
+            }
+        }
+
+        let new_binding_suggestion =
+            |err: &mut Diagnostic, type_param_span: Option<(Span, bool)>| {
+                let msg = "consider introducing an explicit lifetime bound";
+                if let Some((sp, has_lifetimes)) = type_param_span {
+                    let suggestion = if has_lifetimes {
+                        format!(" + {}", new_lt)
+                    } else {
+                        format!(": {}", new_lt)
+                    };
+                    let mut sugg =
+                        vec![(sp, suggestion), (span.shrink_to_hi(), format!(" + {}", new_lt))];
+                    if let Some(lt) = add_lt_sugg {
+                        sugg.push(lt);
+                        sugg.rotate_right(1);
+                    }
+                    // `MaybeIncorrect` due to issue #41966.
+                    err.multipart_suggestion(msg, sugg, Applicability::MaybeIncorrect);
+                }
+            };
+
+        #[derive(Debug)]
+        enum SubOrigin<'hir> {
+            GAT(&'hir hir::Generics<'hir>),
+            Impl,
+            Trait,
+            Fn,
+            Unknown,
+        }
+        let sub_origin = 'origin: {
+            match *sub {
+                ty::ReEarlyBound(ty::EarlyBoundRegion { def_id, .. }) => {
+                    let node = self.tcx.hir().get_if_local(def_id).unwrap();
+                    match node {
+                        Node::GenericParam(param) => {
+                            for h in self.tcx.hir().parent_iter(param.hir_id) {
+                                break 'origin match h.1 {
+                                    Node::ImplItem(hir::ImplItem {
+                                        kind: hir::ImplItemKind::TyAlias(..),
+                                        generics,
+                                        ..
+                                    })
+                                    | Node::TraitItem(hir::TraitItem {
+                                        kind: hir::TraitItemKind::Type(..),
+                                        generics,
+                                        ..
+                                    }) => SubOrigin::GAT(generics),
+                                    Node::ImplItem(hir::ImplItem {
+                                        kind: hir::ImplItemKind::Fn(..),
+                                        ..
+                                    })
+                                    | Node::TraitItem(hir::TraitItem {
+                                        kind: hir::TraitItemKind::Fn(..),
+                                        ..
+                                    })
+                                    | Node::Item(hir::Item {
+                                        kind: hir::ItemKind::Fn(..), ..
+                                    }) => SubOrigin::Fn,
+                                    Node::Item(hir::Item {
+                                        kind: hir::ItemKind::Trait(..),
+                                        ..
+                                    }) => SubOrigin::Trait,
+                                    Node::Item(hir::Item {
+                                        kind: hir::ItemKind::Impl(..), ..
+                                    }) => SubOrigin::Impl,
+                                    _ => continue,
+                                };
+                            }
+                        }
+                        _ => {}
+                    }
+                }
+                _ => {}
+            }
+            SubOrigin::Unknown
+        };
+        debug!(?sub_origin);
+
+        let mut err = match (*sub, sub_origin) {
+            // In the case of GATs, we have to be careful. If we a type parameter `T` on an impl,
+            // but a lifetime `'a` on an associated type, then we might need to suggest adding
+            // `where T: 'a`. Importantly, this is on the GAT span, not on the `T` declaration.
+            (ty::ReEarlyBound(ty::EarlyBoundRegion { name: _, .. }), SubOrigin::GAT(generics)) => {
+                // Does the required lifetime have a nice name we can print?
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0309,
+                    "{} may not live long enough",
+                    labeled_user_string
+                );
+                let pred = format!("{}: {}", bound_kind, sub);
+                let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred,);
+                err.span_suggestion(
+                    generics.tail_span_for_predicate_suggestion(),
+                    "consider adding a where clause",
+                    suggestion,
+                    Applicability::MaybeIncorrect,
+                );
+                err
+            }
+            (
+                ty::ReEarlyBound(ty::EarlyBoundRegion { name, .. })
+                | ty::ReFree(ty::FreeRegion { bound_region: ty::BrNamed(_, name), .. }),
+                _,
+            ) if name != kw::UnderscoreLifetime => {
+                // Does the required lifetime have a nice name we can print?
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0309,
+                    "{} may not live long enough",
+                    labeled_user_string
+                );
+                // Explicitly use the name instead of `sub`'s `Display` impl. The `Display` impl
+                // for the bound is not suitable for suggestions when `-Zverbose` is set because it
+                // uses `Debug` output, so we handle it specially here so that suggestions are
+                // always correct.
+                binding_suggestion(&mut err, type_param_span, bound_kind, name);
+                err
+            }
+
+            (ty::ReStatic, _) => {
+                // Does the required lifetime have a nice name we can print?
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0310,
+                    "{} may not live long enough",
+                    labeled_user_string
+                );
+                binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
+                err
+            }
+
+            _ => {
+                // If not, be less specific.
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0311,
+                    "{} may not live long enough",
+                    labeled_user_string
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    &format!("{} must be valid for ", labeled_user_string),
+                    sub,
+                    "...",
+                    None,
+                );
+                if let Some(infer::RelateParamBound(_, t, _)) = origin {
+                    let return_impl_trait =
+                        self.tcx.return_type_impl_trait(generic_param_scope).is_some();
+                    let t = self.resolve_vars_if_possible(t);
+                    match t.kind() {
+                        // We've got:
+                        // fn get_later<G, T>(g: G, dest: &mut T) -> impl FnOnce() + '_
+                        // suggest:
+                        // fn get_later<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a
+                        ty::Closure(_, _substs) | ty::Opaque(_, _substs) if return_impl_trait => {
+                            new_binding_suggestion(&mut err, type_param_span);
+                        }
+                        _ => {
+                            binding_suggestion(&mut err, type_param_span, bound_kind, new_lt);
+                        }
+                    }
+                }
+                err
+            }
+        };
+
+        if let Some(origin) = origin {
+            self.note_region_origin(&mut err, &origin);
+        }
+        err
+    }
+
+    fn report_sub_sup_conflict(
+        &self,
+        var_origin: RegionVariableOrigin,
+        sub_origin: SubregionOrigin<'tcx>,
+        sub_region: Region<'tcx>,
+        sup_origin: SubregionOrigin<'tcx>,
+        sup_region: Region<'tcx>,
+    ) {
+        let mut err = self.report_inference_failure(var_origin);
+
+        note_and_explain_region(
+            self.tcx,
+            &mut err,
+            "first, the lifetime cannot outlive ",
+            sup_region,
+            "...",
+            None,
+        );
+
+        debug!("report_sub_sup_conflict: var_origin={:?}", var_origin);
+        debug!("report_sub_sup_conflict: sub_region={:?}", sub_region);
+        debug!("report_sub_sup_conflict: sub_origin={:?}", sub_origin);
+        debug!("report_sub_sup_conflict: sup_region={:?}", sup_region);
+        debug!("report_sub_sup_conflict: sup_origin={:?}", sup_origin);
+
+        if let (&infer::Subtype(ref sup_trace), &infer::Subtype(ref sub_trace)) =
+            (&sup_origin, &sub_origin)
+        {
+            debug!("report_sub_sup_conflict: sup_trace={:?}", sup_trace);
+            debug!("report_sub_sup_conflict: sub_trace={:?}", sub_trace);
+            debug!("report_sub_sup_conflict: sup_trace.values={:?}", sup_trace.values);
+            debug!("report_sub_sup_conflict: sub_trace.values={:?}", sub_trace.values);
+
+            if let (Some((sup_expected, sup_found)), Some((sub_expected, sub_found))) =
+                (self.values_str(sup_trace.values), self.values_str(sub_trace.values))
+            {
+                if sub_expected == sup_expected && sub_found == sup_found {
+                    note_and_explain_region(
+                        self.tcx,
+                        &mut err,
+                        "...but the lifetime must also be valid for ",
+                        sub_region,
+                        "...",
+                        None,
+                    );
+                    err.span_note(
+                        sup_trace.cause.span,
+                        &format!("...so that the {}", sup_trace.cause.as_requirement_str()),
+                    );
+
+                    err.note_expected_found(&"", sup_expected, &"", sup_found);
+                    err.emit();
+                    return;
+                }
+            }
+        }
+
+        self.note_region_origin(&mut err, &sup_origin);
+
+        note_and_explain_region(
+            self.tcx,
+            &mut err,
+            "but, the lifetime must be valid for ",
+            sub_region,
+            "...",
+            None,
+        );
+
+        self.note_region_origin(&mut err, &sub_origin);
+        err.emit();
+    }
+
+    /// Determine whether an error associated with the given span and definition
+    /// should be treated as being caused by the implicit `From` conversion
+    /// within `?` desugaring.
+    pub fn is_try_conversion(&self, span: Span, trait_def_id: DefId) -> bool {
+        span.is_desugaring(DesugaringKind::QuestionMark)
+            && self.tcx.is_diagnostic_item(sym::From, trait_def_id)
+    }
+
+    /// Structurally compares two types, modulo any inference variables.
+    ///
+    /// Returns `true` if two types are equal, or if one type is an inference variable compatible
+    /// with the other type. A TyVar inference type is compatible with any type, and an IntVar or
+    /// FloatVar inference type are compatible with themselves or their concrete types (Int and
+    /// Float types, respectively). When comparing two ADTs, these rules apply recursively.
+    pub fn same_type_modulo_infer(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
+        let (a, b) = self.resolve_vars_if_possible((a, b));
+        match (a.kind(), b.kind()) {
+            (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b)) => {
+                if def_a != def_b {
+                    return false;
+                }
+
+                substs_a
+                    .types()
+                    .zip(substs_b.types())
+                    .all(|(a, b)| self.same_type_modulo_infer(a, b))
+            }
+            (&ty::FnDef(did_a, substs_a), &ty::FnDef(did_b, substs_b)) => {
+                if did_a != did_b {
+                    return false;
+                }
+
+                substs_a
+                    .types()
+                    .zip(substs_b.types())
+                    .all(|(a, b)| self.same_type_modulo_infer(a, b))
+            }
+            (&ty::Int(_) | &ty::Uint(_), &ty::Infer(ty::InferTy::IntVar(_)))
+            | (
+                &ty::Infer(ty::InferTy::IntVar(_)),
+                &ty::Int(_) | &ty::Uint(_) | &ty::Infer(ty::InferTy::IntVar(_)),
+            )
+            | (&ty::Float(_), &ty::Infer(ty::InferTy::FloatVar(_)))
+            | (
+                &ty::Infer(ty::InferTy::FloatVar(_)),
+                &ty::Float(_) | &ty::Infer(ty::InferTy::FloatVar(_)),
+            )
+            | (&ty::Infer(ty::InferTy::TyVar(_)), _)
+            | (_, &ty::Infer(ty::InferTy::TyVar(_))) => true,
+            (&ty::Ref(_, ty_a, mut_a), &ty::Ref(_, ty_b, mut_b)) => {
+                mut_a == mut_b && self.same_type_modulo_infer(ty_a, ty_b)
+            }
+            (&ty::RawPtr(a), &ty::RawPtr(b)) => {
+                a.mutbl == b.mutbl && self.same_type_modulo_infer(a.ty, b.ty)
+            }
+            (&ty::Slice(a), &ty::Slice(b)) => self.same_type_modulo_infer(a, b),
+            (&ty::Array(a_ty, a_ct), &ty::Array(b_ty, b_ct)) => {
+                self.same_type_modulo_infer(a_ty, b_ty) && a_ct == b_ct
+            }
+            (&ty::Tuple(a), &ty::Tuple(b)) => {
+                if a.len() != b.len() {
+                    return false;
+                }
+                std::iter::zip(a.iter(), b.iter()).all(|(a, b)| self.same_type_modulo_infer(a, b))
+            }
+            (&ty::FnPtr(a), &ty::FnPtr(b)) => {
+                let a = a.skip_binder().inputs_and_output;
+                let b = b.skip_binder().inputs_and_output;
+                if a.len() != b.len() {
+                    return false;
+                }
+                std::iter::zip(a.iter(), b.iter()).all(|(a, b)| self.same_type_modulo_infer(a, b))
+            }
+            // FIXME(compiler-errors): This needs to be generalized more
+            _ => a == b,
+        }
+    }
+}
+
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    fn report_inference_failure(
+        &self,
+        var_origin: RegionVariableOrigin,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let br_string = |br: ty::BoundRegionKind| {
+            let mut s = match br {
+                ty::BrNamed(_, name) => name.to_string(),
+                _ => String::new(),
+            };
+            if !s.is_empty() {
+                s.push(' ');
+            }
+            s
+        };
+        let var_description = match var_origin {
+            infer::MiscVariable(_) => String::new(),
+            infer::PatternRegion(_) => " for pattern".to_string(),
+            infer::AddrOfRegion(_) => " for borrow expression".to_string(),
+            infer::Autoref(_) => " for autoref".to_string(),
+            infer::Coercion(_) => " for automatic coercion".to_string(),
+            infer::LateBoundRegion(_, br, infer::FnCall) => {
+                format!(" for lifetime parameter {}in function call", br_string(br))
+            }
+            infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
+                format!(" for lifetime parameter {}in generic type", br_string(br))
+            }
+            infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
+                " for lifetime parameter {}in trait containing associated type `{}`",
+                br_string(br),
+                self.tcx.associated_item(def_id).name
+            ),
+            infer::EarlyBoundRegion(_, name) => format!(" for lifetime parameter `{}`", name),
+            infer::UpvarRegion(ref upvar_id, _) => {
+                let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
+                format!(" for capture of `{}` by closure", var_name)
+            }
+            infer::Nll(..) => bug!("NLL variable found in lexical phase"),
+        };
+
+        struct_span_err!(
+            self.tcx.sess,
+            var_origin.span(),
+            E0495,
+            "cannot infer an appropriate lifetime{} due to conflicting requirements",
+            var_description
+        )
+    }
+}
+
+pub enum FailureCode {
+    Error0038(DefId),
+    Error0317(&'static str),
+    Error0580(&'static str),
+    Error0308(&'static str),
+    Error0644(&'static str),
+}
+
+pub trait ObligationCauseExt<'tcx> {
+    fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode;
+    fn as_requirement_str(&self) -> &'static str;
+}
+
+impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
+    fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
+        use self::FailureCode::*;
+        use crate::traits::ObligationCauseCode::*;
+        match self.code() {
+            CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => {
+                Error0308("method not compatible with trait")
+            }
+            CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => {
+                Error0308("type not compatible with trait")
+            }
+            CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => {
+                Error0308("const not compatible with trait")
+            }
+            MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => {
+                Error0308(match source {
+                    hir::MatchSource::TryDesugar => "`?` operator has incompatible types",
+                    _ => "`match` arms have incompatible types",
+                })
+            }
+            IfExpression { .. } => Error0308("`if` and `else` have incompatible types"),
+            IfExpressionWithNoElse => Error0317("`if` may be missing an `else` clause"),
+            LetElse => Error0308("`else` clause of `let...else` does not diverge"),
+            MainFunctionType => Error0580("`main` function has wrong type"),
+            StartFunctionType => Error0308("`#[start]` function has wrong type"),
+            IntrinsicType => Error0308("intrinsic has wrong type"),
+            MethodReceiver => Error0308("mismatched `self` parameter type"),
+
+            // In the case where we have no more specific thing to
+            // say, also take a look at the error code, maybe we can
+            // tailor to that.
+            _ => match terr {
+                TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
+                    Error0644("closure/generator type that references itself")
+                }
+                TypeError::IntrinsicCast => {
+                    Error0308("cannot coerce intrinsics to function pointers")
+                }
+                TypeError::ObjectUnsafeCoercion(did) => Error0038(*did),
+                _ => Error0308("mismatched types"),
+            },
+        }
+    }
+
+    fn as_requirement_str(&self) -> &'static str {
+        use crate::traits::ObligationCauseCode::*;
+        match self.code() {
+            CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => {
+                "method type is compatible with trait"
+            }
+            CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => {
+                "associated type is compatible with trait"
+            }
+            CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => {
+                "const is compatible with trait"
+            }
+            ExprAssignable => "expression is assignable",
+            IfExpression { .. } => "`if` and `else` have incompatible types",
+            IfExpressionWithNoElse => "`if` missing an `else` returns `()`",
+            MainFunctionType => "`main` function has the correct type",
+            StartFunctionType => "`#[start]` function has the correct type",
+            IntrinsicType => "intrinsic has the correct type",
+            MethodReceiver => "method receiver has the correct type",
+            _ => "types are compatible",
+        }
+    }
+}
+
+/// This is a bare signal of what kind of type we're dealing with. `ty::TyKind` tracks
+/// extra information about each type, but we only care about the category.
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
+pub enum TyCategory {
+    Closure,
+    Opaque,
+    Generator(hir::GeneratorKind),
+    Foreign,
+}
+
+impl TyCategory {
+    fn descr(&self) -> &'static str {
+        match self {
+            Self::Closure => "closure",
+            Self::Opaque => "opaque type",
+            Self::Generator(gk) => gk.descr(),
+            Self::Foreign => "foreign type",
+        }
+    }
+
+    pub fn from_ty(tcx: TyCtxt<'_>, ty: Ty<'_>) -> Option<(Self, DefId)> {
+        match *ty.kind() {
+            ty::Closure(def_id, _) => Some((Self::Closure, def_id)),
+            ty::Opaque(def_id, _) => Some((Self::Opaque, def_id)),
+            ty::Generator(def_id, ..) => {
+                Some((Self::Generator(tcx.generator_kind(def_id).unwrap()), def_id))
+            }
+            ty::Foreign(def_id) => Some((Self::Foreign, def_id)),
+            _ => None,
+        }
+    }
+}
+
+impl<'tcx> InferCtxt<'_, 'tcx> {
+    /// Given a [`hir::Block`], get the span of its last expression or
+    /// statement, peeling off any inner blocks.
+    pub fn find_block_span(&self, block: &'tcx hir::Block<'tcx>) -> Span {
+        let block = block.innermost_block();
+        if let Some(expr) = &block.expr {
+            expr.span
+        } else if let Some(stmt) = block.stmts.last() {
+            // possibly incorrect trailing `;` in the else arm
+            stmt.span
+        } else {
+            // empty block; point at its entirety
+            block.span
+        }
+    }
+
+    /// Given a [`hir::HirId`] for a block, get the span of its last expression
+    /// or statement, peeling off any inner blocks.
+    pub fn find_block_span_from_hir_id(&self, hir_id: hir::HirId) -> Span {
+        match self.tcx.hir().get(hir_id) {
+            hir::Node::Block(blk) => self.find_block_span(blk),
+            // The parser was in a weird state if either of these happen, but
+            // it's better not to panic.
+            hir::Node::Expr(e) => e.span,
+            _ => rustc_span::DUMMY_SP,
+        }
+    }
+
+    /// Be helpful when the user wrote `{... expr; }` and taking the `;` off
+    /// is enough to fix the error.
+    pub fn could_remove_semicolon(
+        &self,
+        blk: &'tcx hir::Block<'tcx>,
+        expected_ty: Ty<'tcx>,
+    ) -> Option<(Span, StatementAsExpression)> {
+        let blk = blk.innermost_block();
+        // Do not suggest if we have a tail expr.
+        if blk.expr.is_some() {
+            return None;
+        }
+        let last_stmt = blk.stmts.last()?;
+        let hir::StmtKind::Semi(ref last_expr) = last_stmt.kind else {
+            return None;
+        };
+        let last_expr_ty = self.in_progress_typeck_results?.borrow().expr_ty_opt(*last_expr)?;
+        let needs_box = match (last_expr_ty.kind(), expected_ty.kind()) {
+            _ if last_expr_ty.references_error() => return None,
+            _ if self.same_type_modulo_infer(last_expr_ty, expected_ty) => {
+                StatementAsExpression::CorrectType
+            }
+            (ty::Opaque(last_def_id, _), ty::Opaque(exp_def_id, _))
+                if last_def_id == exp_def_id =>
+            {
+                StatementAsExpression::CorrectType
+            }
+            (ty::Opaque(last_def_id, last_bounds), ty::Opaque(exp_def_id, exp_bounds)) => {
+                debug!(
+                    "both opaque, likely future {:?} {:?} {:?} {:?}",
+                    last_def_id, last_bounds, exp_def_id, exp_bounds
+                );
+
+                let last_local_id = last_def_id.as_local()?;
+                let exp_local_id = exp_def_id.as_local()?;
+
+                match (
+                    &self.tcx.hir().expect_item(last_local_id).kind,
+                    &self.tcx.hir().expect_item(exp_local_id).kind,
+                ) {
+                    (
+                        hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds: last_bounds, .. }),
+                        hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds: exp_bounds, .. }),
+                    ) if iter::zip(*last_bounds, *exp_bounds).all(|(left, right)| {
+                        match (left, right) {
+                            (
+                                hir::GenericBound::Trait(tl, ml),
+                                hir::GenericBound::Trait(tr, mr),
+                            ) if tl.trait_ref.trait_def_id() == tr.trait_ref.trait_def_id()
+                                && ml == mr =>
+                            {
+                                true
+                            }
+                            (
+                                hir::GenericBound::LangItemTrait(langl, _, _, argsl),
+                                hir::GenericBound::LangItemTrait(langr, _, _, argsr),
+                            ) if langl == langr => {
+                                // FIXME: consider the bounds!
+                                debug!("{:?} {:?}", argsl, argsr);
+                                true
+                            }
+                            _ => false,
+                        }
+                    }) =>
+                    {
+                        StatementAsExpression::NeedsBoxing
+                    }
+                    _ => StatementAsExpression::CorrectType,
+                }
+            }
+            _ => return None,
+        };
+        let span = if last_stmt.span.from_expansion() {
+            let mac_call = rustc_span::source_map::original_sp(last_stmt.span, blk.span);
+            self.tcx.sess.source_map().mac_call_stmt_semi_span(mac_call)?
+        } else {
+            last_stmt.span.with_lo(last_stmt.span.hi() - BytePos(1))
+        };
+        Some((span, needs_box))
+    }
+
+    /// Suggest returning a local binding with a compatible type if the block
+    /// has no return expression.
+    pub fn consider_returning_binding(
+        &self,
+        blk: &'tcx hir::Block<'tcx>,
+        expected_ty: Ty<'tcx>,
+        err: &mut Diagnostic,
+    ) -> bool {
+        let blk = blk.innermost_block();
+        // Do not suggest if we have a tail expr.
+        if blk.expr.is_some() {
+            return false;
+        }
+        let mut shadowed = FxHashSet::default();
+        let mut candidate_idents = vec![];
+        let mut find_compatible_candidates = |pat: &hir::Pat<'_>| {
+            if let hir::PatKind::Binding(_, hir_id, ident, _) = &pat.kind
+                && let Some(pat_ty) = self
+                    .in_progress_typeck_results
+                    .and_then(|typeck_results| typeck_results.borrow().node_type_opt(*hir_id))
+            {
+                let pat_ty = self.resolve_vars_if_possible(pat_ty);
+                if self.same_type_modulo_infer(pat_ty, expected_ty)
+                    && !(pat_ty, expected_ty).references_error()
+                    && shadowed.insert(ident.name)
+                {
+                    candidate_idents.push((*ident, pat_ty));
+                }
+            }
+            true
+        };
+
+        let hir = self.tcx.hir();
+        for stmt in blk.stmts.iter().rev() {
+            let hir::StmtKind::Local(local) = &stmt.kind else { continue; };
+            local.pat.walk(&mut find_compatible_candidates);
+        }
+        match hir.find(hir.get_parent_node(blk.hir_id)) {
+            Some(hir::Node::Expr(hir::Expr { hir_id, .. })) => {
+                match hir.find(hir.get_parent_node(*hir_id)) {
+                    Some(hir::Node::Arm(hir::Arm { pat, .. })) => {
+                        pat.walk(&mut find_compatible_candidates);
+                    }
+                    Some(
+                        hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body), .. })
+                        | hir::Node::ImplItem(hir::ImplItem {
+                            kind: hir::ImplItemKind::Fn(_, body),
+                            ..
+                        })
+                        | hir::Node::TraitItem(hir::TraitItem {
+                            kind: hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body)),
+                            ..
+                        })
+                        | hir::Node::Expr(hir::Expr {
+                            kind: hir::ExprKind::Closure(hir::Closure { body, .. }),
+                            ..
+                        }),
+                    ) => {
+                        for param in hir.body(*body).params {
+                            param.pat.walk(&mut find_compatible_candidates);
+                        }
+                    }
+                    Some(hir::Node::Expr(hir::Expr {
+                        kind:
+                            hir::ExprKind::If(
+                                hir::Expr { kind: hir::ExprKind::Let(let_), .. },
+                                then_block,
+                                _,
+                            ),
+                        ..
+                    })) if then_block.hir_id == *hir_id => {
+                        let_.pat.walk(&mut find_compatible_candidates);
+                    }
+                    _ => {}
+                }
+            }
+            _ => {}
+        }
+
+        match &candidate_idents[..] {
+            [(ident, _ty)] => {
+                let sm = self.tcx.sess.source_map();
+                if let Some(stmt) = blk.stmts.last() {
+                    let stmt_span = sm.stmt_span(stmt.span, blk.span);
+                    let sugg = if sm.is_multiline(blk.span)
+                        && let Some(spacing) = sm.indentation_before(stmt_span)
+                    {
+                        format!("\n{spacing}{ident}")
+                    } else {
+                        format!(" {ident}")
+                    };
+                    err.span_suggestion_verbose(
+                        stmt_span.shrink_to_hi(),
+                        format!("consider returning the local binding `{ident}`"),
+                        sugg,
+                        Applicability::MaybeIncorrect,
+                    );
+                } else {
+                    let sugg = if sm.is_multiline(blk.span)
+                        && let Some(spacing) = sm.indentation_before(blk.span.shrink_to_lo())
+                    {
+                        format!("\n{spacing}    {ident}\n{spacing}")
+                    } else {
+                        format!(" {ident} ")
+                    };
+                    let left_span = sm.span_through_char(blk.span, '{').shrink_to_hi();
+                    err.span_suggestion_verbose(
+                        sm.span_extend_while(left_span, |c| c.is_whitespace()).unwrap_or(left_span),
+                        format!("consider returning the local binding `{ident}`"),
+                        sugg,
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                true
+            }
+            values if (1..3).contains(&values.len()) => {
+                let spans = values.iter().map(|(ident, _)| ident.span).collect::<Vec<_>>();
+                err.span_note(spans, "consider returning one of these bindings");
+                true
+            }
+            _ => false,
+        }
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
new file mode 100644
index 000000000..561d1354e
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
@@ -0,0 +1,1134 @@
+use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use crate::infer::InferCtxt;
+use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_hir as hir;
+use rustc_hir::def::Res;
+use rustc_hir::def::{CtorOf, DefKind, Namespace};
+use rustc_hir::def_id::DefId;
+use rustc_hir::intravisit::{self, Visitor};
+use rustc_hir::{Body, Closure, Expr, ExprKind, FnRetTy, HirId, Local, LocalSource};
+use rustc_middle::hir::nested_filter;
+use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
+use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
+use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter, Print, Printer};
+use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
+use rustc_middle::ty::{self, DefIdTree, InferConst};
+use rustc_middle::ty::{IsSuggestable, Ty, TyCtxt, TypeckResults};
+use rustc_span::symbol::{kw, Ident};
+use rustc_span::{BytePos, Span};
+use std::borrow::Cow;
+use std::iter;
+
+pub enum TypeAnnotationNeeded {
+    /// ```compile_fail,E0282
+    /// let x = "hello".chars().rev().collect();
+    /// ```
+    E0282,
+    /// An implementation cannot be chosen unambiguously because of lack of information.
+    /// ```compile_fail,E0283
+    /// let _ = Default::default();
+    /// ```
+    E0283,
+    /// ```compile_fail,E0284
+    /// let mut d: u64 = 2;
+    /// d = d % 1u32.into();
+    /// ```
+    E0284,
+}
+
+impl Into<rustc_errors::DiagnosticId> for TypeAnnotationNeeded {
+    fn into(self) -> rustc_errors::DiagnosticId {
+        match self {
+            Self::E0282 => rustc_errors::error_code!(E0282),
+            Self::E0283 => rustc_errors::error_code!(E0283),
+            Self::E0284 => rustc_errors::error_code!(E0284),
+        }
+    }
+}
+
+/// Information about a constant or a type containing inference variables.
+pub struct InferenceDiagnosticsData {
+    pub name: String,
+    pub span: Option<Span>,
+    pub kind: UnderspecifiedArgKind,
+    pub parent: Option<InferenceDiagnosticsParentData>,
+}
+
+/// Data on the parent definition where a generic argument was declared.
+pub struct InferenceDiagnosticsParentData {
+    prefix: &'static str,
+    name: String,
+}
+
+pub enum UnderspecifiedArgKind {
+    Type { prefix: Cow<'static, str> },
+    Const { is_parameter: bool },
+}
+
+impl InferenceDiagnosticsData {
+    /// Generate a label for a generic argument which can't be inferred. When not
+    /// much is known about the argument, `use_diag` may be used to describe the
+    /// labeled value.
+    fn cannot_infer_msg(&self) -> String {
+        if self.name == "_" && matches!(self.kind, UnderspecifiedArgKind::Type { .. }) {
+            return "cannot infer type".to_string();
+        }
+
+        let suffix = match &self.parent {
+            Some(parent) => parent.suffix_string(),
+            None => String::new(),
+        };
+
+        // For example: "cannot infer type for type parameter `T`"
+        format!("cannot infer {} `{}`{}", self.kind.prefix_string(), self.name, suffix)
+    }
+
+    fn where_x_is_specified(&self, in_type: Ty<'_>) -> String {
+        if in_type.is_ty_infer() {
+            String::new()
+        } else if self.name == "_" {
+            // FIXME: Consider specializing this message if there is a single `_`
+            // in the type.
+            ", where the placeholders `_` are specified".to_string()
+        } else {
+            format!(", where the {} `{}` is specified", self.kind.prefix_string(), self.name)
+        }
+    }
+}
+
+impl InferenceDiagnosticsParentData {
+    fn for_parent_def_id(
+        tcx: TyCtxt<'_>,
+        parent_def_id: DefId,
+    ) -> Option<InferenceDiagnosticsParentData> {
+        let parent_name =
+            tcx.def_key(parent_def_id).disambiguated_data.data.get_opt_name()?.to_string();
+
+        Some(InferenceDiagnosticsParentData {
+            prefix: tcx.def_kind(parent_def_id).descr(parent_def_id),
+            name: parent_name,
+        })
+    }
+
+    fn for_def_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<InferenceDiagnosticsParentData> {
+        Self::for_parent_def_id(tcx, tcx.parent(def_id))
+    }
+
+    fn suffix_string(&self) -> String {
+        format!(" declared on the {} `{}`", self.prefix, self.name)
+    }
+}
+
+impl UnderspecifiedArgKind {
+    fn prefix_string(&self) -> Cow<'static, str> {
+        match self {
+            Self::Type { prefix } => format!("type for {}", prefix).into(),
+            Self::Const { is_parameter: true } => "the value of const parameter".into(),
+            Self::Const { is_parameter: false } => "the value of the constant".into(),
+        }
+    }
+}
+
+fn fmt_printer<'a, 'tcx>(infcx: &'a InferCtxt<'_, 'tcx>, ns: Namespace) -> FmtPrinter<'a, 'tcx> {
+    let mut printer = FmtPrinter::new(infcx.tcx, ns);
+    let ty_getter = move |ty_vid| {
+        if infcx.probe_ty_var(ty_vid).is_ok() {
+            warn!("resolved ty var in error message");
+        }
+        if let TypeVariableOriginKind::TypeParameterDefinition(name, _) =
+            infcx.inner.borrow_mut().type_variables().var_origin(ty_vid).kind
+        {
+            Some(name)
+        } else {
+            None
+        }
+    };
+    printer.ty_infer_name_resolver = Some(Box::new(ty_getter));
+    let const_getter = move |ct_vid| {
+        if infcx.probe_const_var(ct_vid).is_ok() {
+            warn!("resolved const var in error message");
+        }
+        if let ConstVariableOriginKind::ConstParameterDefinition(name, _) =
+            infcx.inner.borrow_mut().const_unification_table().probe_value(ct_vid).origin.kind
+        {
+            return Some(name);
+        } else {
+            None
+        }
+    };
+    printer.const_infer_name_resolver = Some(Box::new(const_getter));
+    printer
+}
+
+fn ty_to_string<'tcx>(infcx: &InferCtxt<'_, 'tcx>, ty: Ty<'tcx>) -> String {
+    let printer = fmt_printer(infcx, Namespace::TypeNS);
+    let ty = infcx.resolve_vars_if_possible(ty);
+    match ty.kind() {
+        // We don't want the regular output for `fn`s because it includes its path in
+        // invalid pseudo-syntax, we want the `fn`-pointer output instead.
+        ty::FnDef(..) => ty.fn_sig(infcx.tcx).print(printer).unwrap().into_buffer(),
+        // FIXME: The same thing for closures, but this only works when the closure
+        // does not capture anything.
+        //
+        // We do have to hide the `extern "rust-call"` ABI in that case though,
+        // which is too much of a bother for now.
+        _ => ty.print(printer).unwrap().into_buffer(),
+    }
+}
+
+/// We don't want to directly use `ty_to_string` for closures as their type isn't really
+/// something users are familar with. Directly printing the `fn_sig` of closures also
+/// doesn't work as they actually use the "rust-call" API.
+fn closure_as_fn_str<'tcx>(infcx: &InferCtxt<'_, 'tcx>, ty: Ty<'tcx>) -> String {
+    let ty::Closure(_, substs) = ty.kind() else { unreachable!() };
+    let fn_sig = substs.as_closure().sig();
+    let args = fn_sig
+        .inputs()
+        .skip_binder()
+        .iter()
+        .next()
+        .map(|args| {
+            args.tuple_fields()
+                .iter()
+                .map(|arg| ty_to_string(infcx, arg))
+                .collect::<Vec<_>>()
+                .join(", ")
+        })
+        .unwrap_or_default();
+    let ret = if fn_sig.output().skip_binder().is_unit() {
+        String::new()
+    } else {
+        format!(" -> {}", ty_to_string(infcx, fn_sig.output().skip_binder()))
+    };
+    format!("fn({}){}", args, ret)
+}
+
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    /// Extracts data used by diagnostic for either types or constants
+    /// which were stuck during inference.
+    pub fn extract_inference_diagnostics_data(
+        &self,
+        arg: GenericArg<'tcx>,
+        highlight: Option<ty::print::RegionHighlightMode<'tcx>>,
+    ) -> InferenceDiagnosticsData {
+        match arg.unpack() {
+            GenericArgKind::Type(ty) => {
+                if let ty::Infer(ty::TyVar(ty_vid)) = *ty.kind() {
+                    let mut inner = self.inner.borrow_mut();
+                    let ty_vars = &inner.type_variables();
+                    let var_origin = ty_vars.var_origin(ty_vid);
+                    if let TypeVariableOriginKind::TypeParameterDefinition(name, def_id) =
+                        var_origin.kind
+                    {
+                        if name != kw::SelfUpper {
+                            return InferenceDiagnosticsData {
+                                name: name.to_string(),
+                                span: Some(var_origin.span),
+                                kind: UnderspecifiedArgKind::Type {
+                                    prefix: "type parameter".into(),
+                                },
+                                parent: def_id.and_then(|def_id| {
+                                    InferenceDiagnosticsParentData::for_def_id(self.tcx, def_id)
+                                }),
+                            };
+                        }
+                    }
+                }
+
+                let mut printer = ty::print::FmtPrinter::new(self.tcx, Namespace::TypeNS);
+                if let Some(highlight) = highlight {
+                    printer.region_highlight_mode = highlight;
+                }
+                InferenceDiagnosticsData {
+                    name: ty.print(printer).unwrap().into_buffer(),
+                    span: None,
+                    kind: UnderspecifiedArgKind::Type { prefix: ty.prefix_string(self.tcx) },
+                    parent: None,
+                }
+            }
+            GenericArgKind::Const(ct) => {
+                if let ty::ConstKind::Infer(InferConst::Var(vid)) = ct.kind() {
+                    let origin =
+                        self.inner.borrow_mut().const_unification_table().probe_value(vid).origin;
+                    if let ConstVariableOriginKind::ConstParameterDefinition(name, def_id) =
+                        origin.kind
+                    {
+                        return InferenceDiagnosticsData {
+                            name: name.to_string(),
+                            span: Some(origin.span),
+                            kind: UnderspecifiedArgKind::Const { is_parameter: true },
+                            parent: InferenceDiagnosticsParentData::for_def_id(self.tcx, def_id),
+                        };
+                    }
+
+                    debug_assert!(!origin.span.is_dummy());
+                    let mut printer = ty::print::FmtPrinter::new(self.tcx, Namespace::ValueNS);
+                    if let Some(highlight) = highlight {
+                        printer.region_highlight_mode = highlight;
+                    }
+                    InferenceDiagnosticsData {
+                        name: ct.print(printer).unwrap().into_buffer(),
+                        span: Some(origin.span),
+                        kind: UnderspecifiedArgKind::Const { is_parameter: false },
+                        parent: None,
+                    }
+                } else {
+                    // If we end up here the `FindInferSourceVisitor`
+                    // won't work, as its expected argument isn't an inference variable.
+                    //
+                    // FIXME: Ideally we should look into the generic constant
+                    // to figure out which inference var is actually unresolved so that
+                    // this path is unreachable.
+                    let mut printer = ty::print::FmtPrinter::new(self.tcx, Namespace::ValueNS);
+                    if let Some(highlight) = highlight {
+                        printer.region_highlight_mode = highlight;
+                    }
+                    InferenceDiagnosticsData {
+                        name: ct.print(printer).unwrap().into_buffer(),
+                        span: None,
+                        kind: UnderspecifiedArgKind::Const { is_parameter: false },
+                        parent: None,
+                    }
+                }
+            }
+            GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),
+        }
+    }
+
+    /// Used as a fallback in [InferCtxt::emit_inference_failure_err]
+    /// in case we weren't able to get a better error.
+    fn bad_inference_failure_err(
+        &self,
+        span: Span,
+        arg_data: InferenceDiagnosticsData,
+        error_code: TypeAnnotationNeeded,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let error_code = error_code.into();
+        let mut err =
+            self.tcx.sess.struct_span_err_with_code(span, "type annotations needed", error_code);
+        err.span_label(span, arg_data.cannot_infer_msg());
+        err
+    }
+
+    pub fn emit_inference_failure_err(
+        &self,
+        body_id: Option<hir::BodyId>,
+        failure_span: Span,
+        arg: GenericArg<'tcx>,
+        error_code: TypeAnnotationNeeded,
+        should_label_span: bool,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let arg = self.resolve_vars_if_possible(arg);
+        let arg_data = self.extract_inference_diagnostics_data(arg, None);
+
+        let Some(typeck_results) = self.in_progress_typeck_results else {
+            // If we don't have any typeck results we're outside
+            // of a body, so we won't be able to get better info
+            // here.
+            return self.bad_inference_failure_err(failure_span, arg_data, error_code);
+        };
+        let typeck_results = typeck_results.borrow();
+        let typeck_results = &typeck_results;
+
+        let mut local_visitor = FindInferSourceVisitor::new(&self, typeck_results, arg);
+        if let Some(body_id) = body_id {
+            let expr = self.tcx.hir().expect_expr(body_id.hir_id);
+            local_visitor.visit_expr(expr);
+        }
+
+        let Some(InferSource { span, kind }) = local_visitor.infer_source else {
+            return self.bad_inference_failure_err(failure_span, arg_data, error_code)
+        };
+
+        let error_code = error_code.into();
+        let mut err = self.tcx.sess.struct_span_err_with_code(
+            span,
+            &format!("type annotations needed{}", kind.ty_msg(self)),
+            error_code,
+        );
+
+        if should_label_span && !failure_span.overlaps(span) {
+            err.span_label(failure_span, "type must be known at this point");
+        }
+
+        match kind {
+            InferSourceKind::LetBinding { insert_span, pattern_name, ty } => {
+                let suggestion_msg = if let Some(name) = pattern_name {
+                    format!(
+                        "consider giving `{}` an explicit type{}",
+                        name,
+                        arg_data.where_x_is_specified(ty)
+                    )
+                } else {
+                    format!(
+                        "consider giving this pattern a type{}",
+                        arg_data.where_x_is_specified(ty)
+                    )
+                };
+                err.span_suggestion_verbose(
+                    insert_span,
+                    &suggestion_msg,
+                    format!(": {}", ty_to_string(self, ty)),
+                    Applicability::HasPlaceholders,
+                );
+            }
+            InferSourceKind::ClosureArg { insert_span, ty } => {
+                err.span_suggestion_verbose(
+                    insert_span,
+                    &format!(
+                        "consider giving this closure parameter an explicit type{}",
+                        arg_data.where_x_is_specified(ty)
+                    ),
+                    format!(": {}", ty_to_string(self, ty)),
+                    Applicability::HasPlaceholders,
+                );
+            }
+            InferSourceKind::GenericArg {
+                insert_span,
+                argument_index,
+                generics_def_id,
+                def_id: _,
+                generic_args,
+            } => {
+                let generics = self.tcx.generics_of(generics_def_id);
+                let is_type = matches!(arg.unpack(), GenericArgKind::Type(_));
+
+                let cannot_infer_msg = format!(
+                    "cannot infer {} of the {} parameter `{}`{}",
+                    if is_type { "type" } else { "the value" },
+                    if is_type { "type" } else { "const" },
+                    generics.params[argument_index].name,
+                    // We use the `generics_def_id` here, as even when suggesting `None::<T>`,
+                    // the type parameter `T` was still declared on the enum, not on the
+                    // variant.
+                    InferenceDiagnosticsParentData::for_parent_def_id(self.tcx, generics_def_id)
+                        .map_or(String::new(), |parent| parent.suffix_string()),
+                );
+
+                err.span_label(span, cannot_infer_msg);
+
+                let args = fmt_printer(self, Namespace::TypeNS)
+                    .comma_sep(generic_args.iter().copied().map(|arg| {
+                        if arg.is_suggestable(self.tcx, true) {
+                            return arg;
+                        }
+
+                        match arg.unpack() {
+                            GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),
+                            GenericArgKind::Type(_) => self
+                                .next_ty_var(TypeVariableOrigin {
+                                    span: rustc_span::DUMMY_SP,
+                                    kind: TypeVariableOriginKind::MiscVariable,
+                                })
+                                .into(),
+                            GenericArgKind::Const(arg) => self
+                                .next_const_var(
+                                    arg.ty(),
+                                    ConstVariableOrigin {
+                                        span: rustc_span::DUMMY_SP,
+                                        kind: ConstVariableOriginKind::MiscVariable,
+                                    },
+                                )
+                                .into(),
+                        }
+                    }))
+                    .unwrap()
+                    .into_buffer();
+
+                err.span_suggestion_verbose(
+                    insert_span,
+                    &format!(
+                        "consider specifying the generic argument{}",
+                        pluralize!(generic_args.len()),
+                    ),
+                    format!("::<{}>", args),
+                    Applicability::HasPlaceholders,
+                );
+            }
+            InferSourceKind::FullyQualifiedMethodCall { receiver, successor, substs, def_id } => {
+                let printer = fmt_printer(self, Namespace::ValueNS);
+                let def_path = printer.print_def_path(def_id, substs).unwrap().into_buffer();
+
+                // We only care about whether we have to add `&` or `&mut ` for now.
+                // This is the case if the last adjustment is a borrow and the
+                // first adjustment was not a builtin deref.
+                let adjustment = match typeck_results.expr_adjustments(receiver) {
+                    [
+                        Adjustment { kind: Adjust::Deref(None), target: _ },
+                        ..,
+                        Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(..)), target: _ },
+                    ] => "",
+                    [
+                        ..,
+                        Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mut_)), target: _ },
+                    ] => match mut_ {
+                        AutoBorrowMutability::Mut { .. } => "&mut ",
+                        AutoBorrowMutability::Not => "&",
+                    },
+                    _ => "",
+                };
+
+                let suggestion = vec![
+                    (receiver.span.shrink_to_lo(), format!("{def_path}({adjustment}")),
+                    (receiver.span.shrink_to_hi().with_hi(successor.1), successor.0.to_string()),
+                ];
+                err.multipart_suggestion_verbose(
+                    "try using a fully qualified path to specify the expected types",
+                    suggestion,
+                    Applicability::HasPlaceholders,
+                );
+            }
+            InferSourceKind::ClosureReturn { ty, data, should_wrap_expr } => {
+                let ret = ty_to_string(self, ty);
+                let (arrow, post) = match data {
+                    FnRetTy::DefaultReturn(_) => ("-> ", " "),
+                    _ => ("", ""),
+                };
+                let suggestion = match should_wrap_expr {
+                    Some(end_span) => vec![
+                        (data.span(), format!("{}{}{}{{ ", arrow, ret, post)),
+                        (end_span, " }".to_string()),
+                    ],
+                    None => vec![(data.span(), format!("{}{}{}", arrow, ret, post))],
+                };
+                err.multipart_suggestion_verbose(
+                    "try giving this closure an explicit return type",
+                    suggestion,
+                    Applicability::HasPlaceholders,
+                );
+            }
+        }
+        err
+    }
+
+    pub fn need_type_info_err_in_generator(
+        &self,
+        kind: hir::GeneratorKind,
+        span: Span,
+        ty: Ty<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let ty = self.resolve_vars_if_possible(ty);
+        let data = self.extract_inference_diagnostics_data(ty.into(), None);
+
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            span,
+            E0698,
+            "type inside {} must be known in this context",
+            kind,
+        );
+        err.span_label(span, data.cannot_infer_msg());
+        err
+    }
+}
+
+#[derive(Debug)]
+struct InferSource<'tcx> {
+    span: Span,
+    kind: InferSourceKind<'tcx>,
+}
+
+#[derive(Debug)]
+enum InferSourceKind<'tcx> {
+    LetBinding {
+        insert_span: Span,
+        pattern_name: Option<Ident>,
+        ty: Ty<'tcx>,
+    },
+    ClosureArg {
+        insert_span: Span,
+        ty: Ty<'tcx>,
+    },
+    GenericArg {
+        insert_span: Span,
+        argument_index: usize,
+        generics_def_id: DefId,
+        def_id: DefId,
+        generic_args: &'tcx [GenericArg<'tcx>],
+    },
+    FullyQualifiedMethodCall {
+        receiver: &'tcx Expr<'tcx>,
+        /// If the method has other arguments, this is ", " and the start of the first argument,
+        /// while for methods without arguments this is ")" and the end of the method call.
+        successor: (&'static str, BytePos),
+        substs: SubstsRef<'tcx>,
+        def_id: DefId,
+    },
+    ClosureReturn {
+        ty: Ty<'tcx>,
+        data: &'tcx FnRetTy<'tcx>,
+        should_wrap_expr: Option<Span>,
+    },
+}
+
+impl<'tcx> InferSource<'tcx> {
+    fn from_expansion(&self) -> bool {
+        let source_from_expansion = match self.kind {
+            InferSourceKind::LetBinding { insert_span, .. }
+            | InferSourceKind::ClosureArg { insert_span, .. }
+            | InferSourceKind::GenericArg { insert_span, .. } => insert_span.from_expansion(),
+            InferSourceKind::FullyQualifiedMethodCall { receiver, .. } => {
+                receiver.span.from_expansion()
+            }
+            InferSourceKind::ClosureReturn { data, should_wrap_expr, .. } => {
+                data.span().from_expansion() || should_wrap_expr.map_or(false, Span::from_expansion)
+            }
+        };
+        source_from_expansion || self.span.from_expansion()
+    }
+}
+
+impl<'tcx> InferSourceKind<'tcx> {
+    fn ty_msg(&self, infcx: &InferCtxt<'_, 'tcx>) -> String {
+        match *self {
+            InferSourceKind::LetBinding { ty, .. }
+            | InferSourceKind::ClosureArg { ty, .. }
+            | InferSourceKind::ClosureReturn { ty, .. } => {
+                if ty.is_closure() {
+                    format!(" for the closure `{}`", closure_as_fn_str(infcx, ty))
+                } else if !ty.is_ty_infer() {
+                    format!(" for `{}`", ty_to_string(infcx, ty))
+                } else {
+                    String::new()
+                }
+            }
+            // FIXME: We should be able to add some additional info here.
+            InferSourceKind::GenericArg { .. }
+            | InferSourceKind::FullyQualifiedMethodCall { .. } => String::new(),
+        }
+    }
+}
+
+#[derive(Debug)]
+struct InsertableGenericArgs<'tcx> {
+    insert_span: Span,
+    substs: SubstsRef<'tcx>,
+    generics_def_id: DefId,
+    def_id: DefId,
+}
+
+/// A visitor which searches for the "best" spot to use in the inference error.
+///
+/// For this it walks over the hir body and tries to check all places where
+/// inference variables could be bound.
+///
+/// While doing so, the currently best spot is stored in `infer_source`.
+/// For details on how we rank spots, see [Self::source_cost]
+struct FindInferSourceVisitor<'a, 'tcx> {
+    infcx: &'a InferCtxt<'a, 'tcx>,
+    typeck_results: &'a TypeckResults<'tcx>,
+
+    target: GenericArg<'tcx>,
+
+    attempt: usize,
+    infer_source_cost: usize,
+    infer_source: Option<InferSource<'tcx>>,
+}
+
+impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
+    fn new(
+        infcx: &'a InferCtxt<'a, 'tcx>,
+        typeck_results: &'a TypeckResults<'tcx>,
+        target: GenericArg<'tcx>,
+    ) -> Self {
+        FindInferSourceVisitor {
+            infcx,
+            typeck_results,
+
+            target,
+
+            attempt: 0,
+            infer_source_cost: usize::MAX,
+            infer_source: None,
+        }
+    }
+
+    /// Computes cost for the given source.
+    ///
+    /// Sources with a small cost are prefer and should result
+    /// in a clearer and idiomatic suggestion.
+    fn source_cost(&self, source: &InferSource<'tcx>) -> usize {
+        #[derive(Clone, Copy)]
+        struct CostCtxt<'tcx> {
+            tcx: TyCtxt<'tcx>,
+        }
+        impl<'tcx> CostCtxt<'tcx> {
+            fn arg_cost(self, arg: GenericArg<'tcx>) -> usize {
+                match arg.unpack() {
+                    GenericArgKind::Lifetime(_) => 0, // erased
+                    GenericArgKind::Type(ty) => self.ty_cost(ty),
+                    GenericArgKind::Const(_) => 3, // some non-zero value
+                }
+            }
+            fn ty_cost(self, ty: Ty<'tcx>) -> usize {
+                match *ty.kind() {
+                    ty::Closure(..) => 1000,
+                    ty::FnDef(..) => 150,
+                    ty::FnPtr(..) => 30,
+                    ty::Adt(def, substs) => {
+                        5 + self
+                            .tcx
+                            .generics_of(def.did())
+                            .own_substs_no_defaults(self.tcx, substs)
+                            .iter()
+                            .map(|&arg| self.arg_cost(arg))
+                            .sum::<usize>()
+                    }
+                    ty::Tuple(args) => 5 + args.iter().map(|arg| self.ty_cost(arg)).sum::<usize>(),
+                    ty::Ref(_, ty, _) => 2 + self.ty_cost(ty),
+                    ty::Infer(..) => 0,
+                    _ => 1,
+                }
+            }
+        }
+
+        // The sources are listed in order of preference here.
+        let tcx = self.infcx.tcx;
+        let ctx = CostCtxt { tcx };
+        let base_cost = match source.kind {
+            InferSourceKind::LetBinding { ty, .. } => ctx.ty_cost(ty),
+            InferSourceKind::ClosureArg { ty, .. } => ctx.ty_cost(ty),
+            InferSourceKind::GenericArg { def_id, generic_args, .. } => {
+                let variant_cost = match tcx.def_kind(def_id) {
+                    // `None::<u32>` and friends are ugly.
+                    DefKind::Variant | DefKind::Ctor(CtorOf::Variant, _) => 15,
+                    _ => 10,
+                };
+                variant_cost + generic_args.iter().map(|&arg| ctx.arg_cost(arg)).sum::<usize>()
+            }
+            InferSourceKind::FullyQualifiedMethodCall { substs, .. } => {
+                20 + substs.iter().map(|arg| ctx.arg_cost(arg)).sum::<usize>()
+            }
+            InferSourceKind::ClosureReturn { ty, should_wrap_expr, .. } => {
+                30 + ctx.ty_cost(ty) + if should_wrap_expr.is_some() { 10 } else { 0 }
+            }
+        };
+
+        let suggestion_may_apply = if source.from_expansion() { 10000 } else { 0 };
+
+        base_cost + suggestion_may_apply
+    }
+
+    /// Uses `fn source_cost` to determine whether this inference source is preferable to
+    /// previous sources. We generally prefer earlier sources.
+    #[instrument(level = "debug", skip(self))]
+    fn update_infer_source(&mut self, new_source: InferSource<'tcx>) {
+        let cost = self.source_cost(&new_source) + self.attempt;
+        debug!(?cost);
+        self.attempt += 1;
+        if cost < self.infer_source_cost {
+            self.infer_source_cost = cost;
+            self.infer_source = Some(new_source);
+        }
+    }
+
+    fn node_substs_opt(&self, hir_id: HirId) -> Option<SubstsRef<'tcx>> {
+        let substs = self.typeck_results.node_substs_opt(hir_id);
+        self.infcx.resolve_vars_if_possible(substs)
+    }
+
+    fn opt_node_type(&self, hir_id: HirId) -> Option<Ty<'tcx>> {
+        let ty = self.typeck_results.node_type_opt(hir_id);
+        self.infcx.resolve_vars_if_possible(ty)
+    }
+
+    // Check whether this generic argument is the inference variable we
+    // are looking for.
+    fn generic_arg_is_target(&self, arg: GenericArg<'tcx>) -> bool {
+        if arg == self.target {
+            return true;
+        }
+
+        match (arg.unpack(), self.target.unpack()) {
+            (GenericArgKind::Type(inner_ty), GenericArgKind::Type(target_ty)) => {
+                use ty::{Infer, TyVar};
+                match (inner_ty.kind(), target_ty.kind()) {
+                    (&Infer(TyVar(a_vid)), &Infer(TyVar(b_vid))) => {
+                        self.infcx.inner.borrow_mut().type_variables().sub_unified(a_vid, b_vid)
+                    }
+                    _ => false,
+                }
+            }
+            (GenericArgKind::Const(inner_ct), GenericArgKind::Const(target_ct)) => {
+                use ty::InferConst::*;
+                match (inner_ct.kind(), target_ct.kind()) {
+                    (ty::ConstKind::Infer(Var(a_vid)), ty::ConstKind::Infer(Var(b_vid))) => self
+                        .infcx
+                        .inner
+                        .borrow_mut()
+                        .const_unification_table()
+                        .unioned(a_vid, b_vid),
+                    _ => false,
+                }
+            }
+            _ => false,
+        }
+    }
+
+    /// Does this generic argument contain our target inference variable
+    /// in a way which can be written by the user.
+    fn generic_arg_contains_target(&self, arg: GenericArg<'tcx>) -> bool {
+        let mut walker = arg.walk();
+        while let Some(inner) = walker.next() {
+            if self.generic_arg_is_target(inner) {
+                return true;
+            }
+            match inner.unpack() {
+                GenericArgKind::Lifetime(_) => {}
+                GenericArgKind::Type(ty) => {
+                    if matches!(ty.kind(), ty::Opaque(..) | ty::Closure(..) | ty::Generator(..)) {
+                        // Opaque types can't be named by the user right now.
+                        //
+                        // Both the generic arguments of closures and generators can
+                        // also not be named. We may want to only look into the closure
+                        // signature in case it has no captures, as that can be represented
+                        // using `fn(T) -> R`.
+
+                        // FIXME(type_alias_impl_trait): These opaque types
+                        // can actually be named, so it would make sense to
+                        // adjust this case and add a test for it.
+                        walker.skip_current_subtree();
+                    }
+                }
+                GenericArgKind::Const(ct) => {
+                    if matches!(ct.kind(), ty::ConstKind::Unevaluated(..)) {
+                        // You can't write the generic arguments for
+                        // unevaluated constants.
+                        walker.skip_current_subtree();
+                    }
+                }
+            }
+        }
+        false
+    }
+
+    fn expr_inferred_subst_iter(
+        &self,
+        expr: &'tcx hir::Expr<'tcx>,
+    ) -> Box<dyn Iterator<Item = InsertableGenericArgs<'tcx>> + 'a> {
+        let tcx = self.infcx.tcx;
+        match expr.kind {
+            hir::ExprKind::Path(ref path) => {
+                if let Some(substs) = self.node_substs_opt(expr.hir_id) {
+                    return self.path_inferred_subst_iter(expr.hir_id, substs, path);
+                }
+            }
+            // FIXME(#98711): Ideally we would also deal with type relative
+            // paths here, even if that is quite rare.
+            //
+            // See the `need_type_info/expr-struct-type-relative-gat.rs` test
+            // for an example where that would be needed.
+            //
+            // However, the `type_dependent_def_id` for `Self::Output` in an
+            // impl is currently the `DefId` of `Output` in the trait definition
+            // which makes this somewhat difficult and prevents us from just
+            // using `self.path_inferred_subst_iter` here.
+            hir::ExprKind::Struct(&hir::QPath::Resolved(_self_ty, path), _, _) => {
+                if let Some(ty) = self.opt_node_type(expr.hir_id) {
+                    if let ty::Adt(_, substs) = ty.kind() {
+                        return Box::new(self.resolved_path_inferred_subst_iter(path, substs));
+                    }
+                }
+            }
+            hir::ExprKind::MethodCall(segment, _, _) => {
+                if let Some(def_id) = self.typeck_results.type_dependent_def_id(expr.hir_id) {
+                    let generics = tcx.generics_of(def_id);
+                    let insertable: Option<_> = try {
+                        if generics.has_impl_trait() {
+                            None?
+                        }
+                        let substs = self.node_substs_opt(expr.hir_id)?;
+                        let span = tcx.hir().span(segment.hir_id?);
+                        let insert_span = segment.ident.span.shrink_to_hi().with_hi(span.hi());
+                        InsertableGenericArgs {
+                            insert_span,
+                            substs,
+                            generics_def_id: def_id,
+                            def_id,
+                        }
+                    };
+                    return Box::new(insertable.into_iter());
+                }
+            }
+            _ => {}
+        }
+
+        Box::new(iter::empty())
+    }
+
+    fn resolved_path_inferred_subst_iter(
+        &self,
+        path: &'tcx hir::Path<'tcx>,
+        substs: SubstsRef<'tcx>,
+    ) -> impl Iterator<Item = InsertableGenericArgs<'tcx>> + 'a {
+        let tcx = self.infcx.tcx;
+        // The last segment of a path often has `Res::Err` and the
+        // correct `Res` is the one of the whole path.
+        //
+        // FIXME: We deal with that one separately for now,
+        // would be good to remove this special case.
+        let last_segment_using_path_data: Option<_> = try {
+            let generics_def_id = tcx.res_generics_def_id(path.res)?;
+            let generics = tcx.generics_of(generics_def_id);
+            if generics.has_impl_trait() {
+                None?
+            }
+            let insert_span =
+                path.segments.last().unwrap().ident.span.shrink_to_hi().with_hi(path.span.hi());
+            InsertableGenericArgs {
+                insert_span,
+                substs,
+                generics_def_id,
+                def_id: path.res.def_id(),
+            }
+        };
+
+        path.segments
+            .iter()
+            .filter_map(move |segment| {
+                let res = segment.res?;
+                let generics_def_id = tcx.res_generics_def_id(res)?;
+                let generics = tcx.generics_of(generics_def_id);
+                if generics.has_impl_trait() {
+                    return None;
+                }
+                let span = tcx.hir().span(segment.hir_id?);
+                let insert_span = segment.ident.span.shrink_to_hi().with_hi(span.hi());
+                Some(InsertableGenericArgs {
+                    insert_span,
+                    substs,
+                    generics_def_id,
+                    def_id: res.def_id(),
+                })
+            })
+            .chain(last_segment_using_path_data)
+    }
+
+    fn path_inferred_subst_iter(
+        &self,
+        hir_id: HirId,
+        substs: SubstsRef<'tcx>,
+        qpath: &'tcx hir::QPath<'tcx>,
+    ) -> Box<dyn Iterator<Item = InsertableGenericArgs<'tcx>> + 'a> {
+        let tcx = self.infcx.tcx;
+        match qpath {
+            hir::QPath::Resolved(_self_ty, path) => {
+                Box::new(self.resolved_path_inferred_subst_iter(path, substs))
+            }
+            hir::QPath::TypeRelative(ty, segment) => {
+                let Some(def_id) = self.typeck_results.type_dependent_def_id(hir_id) else {
+                    return Box::new(iter::empty());
+                };
+
+                let generics = tcx.generics_of(def_id);
+                let segment: Option<_> = try {
+                    if !segment.infer_args || generics.has_impl_trait() {
+                        None?;
+                    }
+                    let span = tcx.hir().span(segment.hir_id?);
+                    let insert_span = segment.ident.span.shrink_to_hi().with_hi(span.hi());
+                    InsertableGenericArgs { insert_span, substs, generics_def_id: def_id, def_id }
+                };
+
+                let parent_def_id = generics.parent.unwrap();
+                if tcx.def_kind(parent_def_id) == DefKind::Impl {
+                    let parent_ty = tcx.bound_type_of(parent_def_id).subst(tcx, substs);
+                    match (parent_ty.kind(), &ty.kind) {
+                        (
+                            ty::Adt(def, substs),
+                            hir::TyKind::Path(hir::QPath::Resolved(_self_ty, path)),
+                        ) => {
+                            if tcx.res_generics_def_id(path.res) != Some(def.did()) {
+                                match path.res {
+                                    Res::Def(DefKind::TyAlias, _) => {
+                                        // FIXME: Ideally we should support this. For that
+                                        // we have to map back from the self type to the
+                                        // type alias though. That's difficult.
+                                        //
+                                        // See the `need_type_info/type-alias.rs` test for
+                                        // some examples.
+                                    }
+                                    // There cannot be inference variables in the self type,
+                                    // so there's nothing for us to do here.
+                                    Res::SelfTy { .. } => {}
+                                    _ => warn!(
+                                        "unexpected path: def={:?} substs={:?} path={:?}",
+                                        def, substs, path,
+                                    ),
+                                }
+                            } else {
+                                return Box::new(
+                                    self.resolved_path_inferred_subst_iter(path, substs)
+                                        .chain(segment),
+                                );
+                            }
+                        }
+                        _ => (),
+                    }
+                }
+
+                Box::new(segment.into_iter())
+            }
+            hir::QPath::LangItem(_, _, _) => Box::new(iter::empty()),
+        }
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.infcx.tcx.hir()
+    }
+
+    fn visit_local(&mut self, local: &'tcx Local<'tcx>) {
+        intravisit::walk_local(self, local);
+
+        if let Some(ty) = self.opt_node_type(local.hir_id) {
+            if self.generic_arg_contains_target(ty.into()) {
+                match local.source {
+                    LocalSource::Normal if local.ty.is_none() => {
+                        self.update_infer_source(InferSource {
+                            span: local.pat.span,
+                            kind: InferSourceKind::LetBinding {
+                                insert_span: local.pat.span.shrink_to_hi(),
+                                pattern_name: local.pat.simple_ident(),
+                                ty,
+                            },
+                        })
+                    }
+                    _ => {}
+                }
+            }
+        }
+    }
+
+    /// For closures, we first visit the parameters and then the content,
+    /// as we prefer those.
+    fn visit_body(&mut self, body: &'tcx Body<'tcx>) {
+        for param in body.params {
+            debug!(
+                "param: span {:?}, ty_span {:?}, pat.span {:?}",
+                param.span, param.ty_span, param.pat.span
+            );
+            if param.ty_span != param.pat.span {
+                debug!("skipping param: has explicit type");
+                continue;
+            }
+
+            let Some(param_ty) = self.opt_node_type(param.hir_id) else {
+                continue
+            };
+
+            if self.generic_arg_contains_target(param_ty.into()) {
+                self.update_infer_source(InferSource {
+                    span: param.pat.span,
+                    kind: InferSourceKind::ClosureArg {
+                        insert_span: param.pat.span.shrink_to_hi(),
+                        ty: param_ty,
+                    },
+                })
+            }
+        }
+        intravisit::walk_body(self, body);
+    }
+
+    #[instrument(level = "debug", skip(self))]
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        let tcx = self.infcx.tcx;
+        match expr.kind {
+            // When encountering `func(arg)` first look into `arg` and then `func`,
+            // as `arg` is "more specific".
+            ExprKind::Call(func, args) => {
+                for arg in args {
+                    self.visit_expr(arg);
+                }
+                self.visit_expr(func);
+            }
+            _ => intravisit::walk_expr(self, expr),
+        }
+
+        for args in self.expr_inferred_subst_iter(expr) {
+            debug!(?args);
+            let InsertableGenericArgs { insert_span, substs, generics_def_id, def_id } = args;
+            let generics = tcx.generics_of(generics_def_id);
+            if let Some(argument_index) = generics
+                .own_substs(substs)
+                .iter()
+                .position(|&arg| self.generic_arg_contains_target(arg))
+            {
+                let substs = self.infcx.resolve_vars_if_possible(substs);
+                let generic_args = &generics.own_substs_no_defaults(tcx, substs)
+                    [generics.own_counts().lifetimes..];
+                let span = match expr.kind {
+                    ExprKind::MethodCall(path, _, _) => path.ident.span,
+                    _ => expr.span,
+                };
+
+                self.update_infer_source(InferSource {
+                    span,
+                    kind: InferSourceKind::GenericArg {
+                        insert_span,
+                        argument_index,
+                        generics_def_id,
+                        def_id,
+                        generic_args,
+                    },
+                });
+            }
+        }
+
+        if let Some(node_ty) = self.opt_node_type(expr.hir_id) {
+            if let (
+                &ExprKind::Closure(&Closure { fn_decl, body, fn_decl_span, .. }),
+                ty::Closure(_, substs),
+            ) = (&expr.kind, node_ty.kind())
+            {
+                let output = substs.as_closure().sig().output().skip_binder();
+                if self.generic_arg_contains_target(output.into()) {
+                    let body = self.infcx.tcx.hir().body(body);
+                    let should_wrap_expr = if matches!(body.value.kind, ExprKind::Block(..)) {
+                        None
+                    } else {
+                        Some(body.value.span.shrink_to_hi())
+                    };
+                    self.update_infer_source(InferSource {
+                        span: fn_decl_span,
+                        kind: InferSourceKind::ClosureReturn {
+                            ty: output,
+                            data: &fn_decl.output,
+                            should_wrap_expr,
+                        },
+                    })
+                }
+            }
+        }
+
+        let has_impl_trait = |def_id| {
+            iter::successors(Some(tcx.generics_of(def_id)), |generics| {
+                generics.parent.map(|def_id| tcx.generics_of(def_id))
+            })
+            .any(|generics| generics.has_impl_trait())
+        };
+        if let ExprKind::MethodCall(path, args, span) = expr.kind
+            && let Some(substs) = self.node_substs_opt(expr.hir_id)
+            && substs.iter().any(|arg| self.generic_arg_contains_target(arg))
+            && let Some(def_id) = self.typeck_results.type_dependent_def_id(expr.hir_id)
+            && self.infcx.tcx.trait_of_item(def_id).is_some()
+            && !has_impl_trait(def_id)
+        {
+            let successor =
+                args.get(1).map_or_else(|| (")", span.hi()), |arg| (", ", arg.span.lo()));
+            let substs = self.infcx.resolve_vars_if_possible(substs);
+            self.update_infer_source(InferSource {
+                span: path.ident.span,
+                kind: InferSourceKind::FullyQualifiedMethodCall {
+                    receiver: args.first().unwrap(),
+                    successor,
+                    substs,
+                    def_id,
+                }
+            })
+        }
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs
new file mode 100644
index 000000000..9a2ab3e32
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs
@@ -0,0 +1,234 @@
+//! Error Reporting for Anonymous Region Lifetime Errors
+//! where both the regions are anonymous.
+
+use crate::infer::error_reporting::nice_region_error::find_anon_type::find_anon_type;
+use crate::infer::error_reporting::nice_region_error::util::AnonymousParamInfo;
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::infer::SubregionOrigin;
+use crate::infer::TyCtxt;
+
+use rustc_errors::{struct_span_err, Applicability, Diagnostic, ErrorGuaranteed};
+use rustc_hir as hir;
+use rustc_hir::{GenericParamKind, Ty};
+use rustc_middle::ty::Region;
+use rustc_span::symbol::kw;
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    /// Print the error message for lifetime errors when both the concerned regions are anonymous.
+    ///
+    /// Consider a case where we have
+    ///
+    /// ```compile_fail,E0623
+    /// fn foo(x: &mut Vec<&u8>, y: &u8) {
+    ///     x.push(y);
+    /// }
+    /// ```
+    ///
+    /// The example gives
+    ///
+    /// ```text
+    /// fn foo(x: &mut Vec<&u8>, y: &u8) {
+    ///                    ---      --- these references are declared with different lifetimes...
+    ///     x.push(y);
+    ///     ^ ...but data from `y` flows into `x` here
+    /// ```
+    ///
+    /// It has been extended for the case of structs too.
+    ///
+    /// Consider the example
+    ///
+    /// ```no_run
+    /// struct Ref<'a> { x: &'a u32 }
+    /// ```
+    ///
+    /// ```text
+    /// fn foo(mut x: Vec<Ref>, y: Ref) {
+    ///                   ---      --- these structs are declared with different lifetimes...
+    ///     x.push(y);
+    ///     ^ ...but data from `y` flows into `x` here
+    /// }
+    /// ```
+    ///
+    /// It will later be extended to trait objects.
+    pub(super) fn try_report_anon_anon_conflict(&self) -> Option<ErrorGuaranteed> {
+        let (span, sub, sup) = self.regions()?;
+
+        if let Some(RegionResolutionError::ConcreteFailure(
+            SubregionOrigin::ReferenceOutlivesReferent(..),
+            ..,
+        )) = self.error
+        {
+            // This error doesn't make much sense in this case.
+            return None;
+        }
+
+        // Determine whether the sub and sup consist of both anonymous (elided) regions.
+        let anon_reg_sup = self.tcx().is_suitable_region(sup)?;
+
+        let anon_reg_sub = self.tcx().is_suitable_region(sub)?;
+        let scope_def_id_sup = anon_reg_sup.def_id;
+        let bregion_sup = anon_reg_sup.boundregion;
+        let scope_def_id_sub = anon_reg_sub.def_id;
+        let bregion_sub = anon_reg_sub.boundregion;
+
+        let ty_sup = find_anon_type(self.tcx(), sup, &bregion_sup)?;
+
+        let ty_sub = find_anon_type(self.tcx(), sub, &bregion_sub)?;
+
+        debug!(
+            "try_report_anon_anon_conflict: found_param1={:?} sup={:?} br1={:?}",
+            ty_sub, sup, bregion_sup
+        );
+        debug!(
+            "try_report_anon_anon_conflict: found_param2={:?} sub={:?} br2={:?}",
+            ty_sup, sub, bregion_sub
+        );
+
+        let (ty_sup, ty_fndecl_sup) = ty_sup;
+        let (ty_sub, ty_fndecl_sub) = ty_sub;
+
+        let AnonymousParamInfo { param: anon_param_sup, .. } =
+            self.find_param_with_region(sup, sup)?;
+        let AnonymousParamInfo { param: anon_param_sub, .. } =
+            self.find_param_with_region(sub, sub)?;
+
+        let sup_is_ret_type =
+            self.is_return_type_anon(scope_def_id_sup, bregion_sup, ty_fndecl_sup);
+        let sub_is_ret_type =
+            self.is_return_type_anon(scope_def_id_sub, bregion_sub, ty_fndecl_sub);
+
+        let span_label_var1 = match anon_param_sup.pat.simple_ident() {
+            Some(simple_ident) => format!(" from `{}`", simple_ident),
+            None => String::new(),
+        };
+
+        let span_label_var2 = match anon_param_sub.pat.simple_ident() {
+            Some(simple_ident) => format!(" into `{}`", simple_ident),
+            None => String::new(),
+        };
+
+        debug!(
+            "try_report_anon_anon_conflict: sub_is_ret_type={:?} sup_is_ret_type={:?}",
+            sub_is_ret_type, sup_is_ret_type
+        );
+
+        let mut err = struct_span_err!(self.tcx().sess, span, E0623, "lifetime mismatch");
+
+        match (sup_is_ret_type, sub_is_ret_type) {
+            (ret_capture @ Some(ret_span), _) | (_, ret_capture @ Some(ret_span)) => {
+                let param_span =
+                    if sup_is_ret_type == ret_capture { ty_sub.span } else { ty_sup.span };
+
+                err.span_label(
+                    param_span,
+                    "this parameter and the return type are declared with different lifetimes...",
+                );
+                err.span_label(ret_span, "");
+                err.span_label(span, format!("...but data{} is returned here", span_label_var1));
+            }
+
+            (None, None) => {
+                if ty_sup.hir_id == ty_sub.hir_id {
+                    err.span_label(ty_sup.span, "this type is declared with multiple lifetimes...");
+                    err.span_label(ty_sub.span, "");
+                    err.span_label(span, "...but data with one lifetime flows into the other here");
+                } else {
+                    err.span_label(
+                        ty_sup.span,
+                        "these two types are declared with different lifetimes...",
+                    );
+                    err.span_label(ty_sub.span, "");
+                    err.span_label(
+                        span,
+                        format!("...but data{} flows{} here", span_label_var1, span_label_var2),
+                    );
+                }
+            }
+        }
+
+        if suggest_adding_lifetime_params(self.tcx(), sub, ty_sup, ty_sub, &mut err) {
+            err.note("each elided lifetime in input position becomes a distinct lifetime");
+        }
+
+        let reported = err.emit();
+        Some(reported)
+    }
+}
+
+pub fn suggest_adding_lifetime_params<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    sub: Region<'tcx>,
+    ty_sup: &Ty<'_>,
+    ty_sub: &Ty<'_>,
+    err: &mut Diagnostic,
+) -> bool {
+    let (
+        hir::Ty { kind: hir::TyKind::Rptr(lifetime_sub, _), .. },
+        hir::Ty { kind: hir::TyKind::Rptr(lifetime_sup, _), .. },
+    ) = (ty_sub, ty_sup) else {
+        return false;
+    };
+
+    if !lifetime_sub.name.is_anonymous() || !lifetime_sup.name.is_anonymous() {
+        return false;
+    };
+
+    let Some(anon_reg) = tcx.is_suitable_region(sub) else {
+        return false;
+    };
+
+    let hir_id = tcx.hir().local_def_id_to_hir_id(anon_reg.def_id);
+
+    let node = tcx.hir().get(hir_id);
+    let is_impl = matches!(&node, hir::Node::ImplItem(_));
+    let generics = match node {
+        hir::Node::Item(&hir::Item { kind: hir::ItemKind::Fn(_, ref generics, ..), .. })
+        | hir::Node::TraitItem(&hir::TraitItem { ref generics, .. })
+        | hir::Node::ImplItem(&hir::ImplItem { ref generics, .. }) => generics,
+        _ => return false,
+    };
+
+    let suggestion_param_name = generics
+        .params
+        .iter()
+        .filter(|p| matches!(p.kind, GenericParamKind::Lifetime { .. }))
+        .map(|p| p.name.ident().name)
+        .find(|i| *i != kw::UnderscoreLifetime);
+    let introduce_new = suggestion_param_name.is_none();
+    let suggestion_param_name =
+        suggestion_param_name.map(|n| n.to_string()).unwrap_or_else(|| "'a".to_owned());
+
+    debug!(?lifetime_sup.span);
+    debug!(?lifetime_sub.span);
+    let make_suggestion = |span: rustc_span::Span| {
+        if span.is_empty() {
+            (span, format!("{}, ", suggestion_param_name))
+        } else if let Ok("&") = tcx.sess.source_map().span_to_snippet(span).as_deref() {
+            (span.shrink_to_hi(), format!("{} ", suggestion_param_name))
+        } else {
+            (span, suggestion_param_name.clone())
+        }
+    };
+    let mut suggestions =
+        vec![make_suggestion(lifetime_sub.span), make_suggestion(lifetime_sup.span)];
+
+    if introduce_new {
+        let new_param_suggestion =
+            if let Some(first) = generics.params.iter().find(|p| !p.name.ident().span.is_empty()) {
+                (first.span.shrink_to_lo(), format!("{}, ", suggestion_param_name))
+            } else {
+                (generics.span, format!("<{}>", suggestion_param_name))
+            };
+
+        suggestions.push(new_param_suggestion);
+    }
+
+    let mut sugg = String::from("consider introducing a named lifetime parameter");
+    if is_impl {
+        sugg.push_str(" and update trait if needed");
+    }
+    err.multipart_suggestion(sugg, suggestions, Applicability::MaybeIncorrect);
+
+    true
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs
new file mode 100644
index 000000000..c1b201da6
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs
@@ -0,0 +1,234 @@
+use rustc_hir as hir;
+use rustc_hir::intravisit::{self, Visitor};
+use rustc_middle::hir::map::Map;
+use rustc_middle::hir::nested_filter;
+use rustc_middle::middle::resolve_lifetime as rl;
+use rustc_middle::ty::{self, Region, TyCtxt};
+
+/// This function calls the `visit_ty` method for the parameters
+/// corresponding to the anonymous regions. The `nested_visitor.found_type`
+/// contains the anonymous type.
+///
+/// # Arguments
+/// region - the anonymous region corresponding to the anon_anon conflict
+/// br - the bound region corresponding to the above region which is of type `BrAnon(_)`
+///
+/// # Example
+/// ```compile_fail,E0623
+/// fn foo(x: &mut Vec<&u8>, y: &u8)
+///    { x.push(y); }
+/// ```
+/// The function returns the nested type corresponding to the anonymous region
+/// for e.g., `&u8` and `Vec<&u8>`.
+pub fn find_anon_type<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    region: Region<'tcx>,
+    br: &ty::BoundRegionKind,
+) -> Option<(&'tcx hir::Ty<'tcx>, &'tcx hir::FnSig<'tcx>)> {
+    let anon_reg = tcx.is_suitable_region(region)?;
+    let hir_id = tcx.hir().local_def_id_to_hir_id(anon_reg.def_id);
+    let fn_sig = tcx.hir().get(hir_id).fn_sig()?;
+
+    fn_sig
+        .decl
+        .inputs
+        .iter()
+        .find_map(|arg| find_component_for_bound_region(tcx, arg, br))
+        .map(|ty| (ty, fn_sig))
+}
+
+// This method creates a FindNestedTypeVisitor which returns the type corresponding
+// to the anonymous region.
+fn find_component_for_bound_region<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    arg: &'tcx hir::Ty<'tcx>,
+    br: &ty::BoundRegionKind,
+) -> Option<&'tcx hir::Ty<'tcx>> {
+    let mut nested_visitor = FindNestedTypeVisitor {
+        tcx,
+        bound_region: *br,
+        found_type: None,
+        current_index: ty::INNERMOST,
+    };
+    nested_visitor.visit_ty(arg);
+    nested_visitor.found_type
+}
+
+// The FindNestedTypeVisitor captures the corresponding `hir::Ty` of the
+// anonymous region. The example above would lead to a conflict between
+// the two anonymous lifetimes for &u8 in x and y respectively. This visitor
+// would be invoked twice, once for each lifetime, and would
+// walk the types like &mut Vec<&u8> and &u8 looking for the HIR
+// where that lifetime appears. This allows us to highlight the
+// specific part of the type in the error message.
+struct FindNestedTypeVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    // The bound_region corresponding to the Refree(freeregion)
+    // associated with the anonymous region we are looking for.
+    bound_region: ty::BoundRegionKind,
+    // The type where the anonymous lifetime appears
+    // for e.g., Vec<`&u8`> and <`&u8`>
+    found_type: Option<&'tcx hir::Ty<'tcx>>,
+    current_index: ty::DebruijnIndex,
+}
+
+impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.tcx.hir()
+    }
+
+    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
+        match arg.kind {
+            hir::TyKind::BareFn(_) => {
+                self.current_index.shift_in(1);
+                intravisit::walk_ty(self, arg);
+                self.current_index.shift_out(1);
+                return;
+            }
+
+            hir::TyKind::TraitObject(bounds, ..) => {
+                for bound in bounds {
+                    self.current_index.shift_in(1);
+                    self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
+                    self.current_index.shift_out(1);
+                }
+            }
+
+            hir::TyKind::Rptr(ref lifetime, _) => {
+                // the lifetime of the TyRptr
+                let hir_id = lifetime.hir_id;
+                match (self.tcx.named_region(hir_id), self.bound_region) {
+                    // Find the index of the named region that was part of the
+                    // error. We will then search the function parameters for a bound
+                    // region at the right depth with the same index
+                    (Some(rl::Region::EarlyBound(_, id)), ty::BrNamed(def_id, _)) => {
+                        debug!("EarlyBound id={:?} def_id={:?}", id, def_id);
+                        if id == def_id {
+                            self.found_type = Some(arg);
+                            return; // we can stop visiting now
+                        }
+                    }
+
+                    // Find the index of the named region that was part of the
+                    // error. We will then search the function parameters for a bound
+                    // region at the right depth with the same index
+                    (
+                        Some(rl::Region::LateBound(debruijn_index, _, id)),
+                        ty::BrNamed(def_id, _),
+                    ) => {
+                        debug!(
+                            "FindNestedTypeVisitor::visit_ty: LateBound depth = {:?}",
+                            debruijn_index
+                        );
+                        debug!("LateBound id={:?} def_id={:?}", id, def_id);
+                        if debruijn_index == self.current_index && id == def_id {
+                            self.found_type = Some(arg);
+                            return; // we can stop visiting now
+                        }
+                    }
+
+                    (
+                        Some(
+                            rl::Region::Static
+                            | rl::Region::Free(_, _)
+                            | rl::Region::EarlyBound(_, _)
+                            | rl::Region::LateBound(_, _, _),
+                        )
+                        | None,
+                        _,
+                    ) => {
+                        debug!("no arg found");
+                    }
+                }
+            }
+            // Checks if it is of type `hir::TyKind::Path` which corresponds to a struct.
+            hir::TyKind::Path(_) => {
+                let subvisitor = &mut TyPathVisitor {
+                    tcx: self.tcx,
+                    found_it: false,
+                    bound_region: self.bound_region,
+                    current_index: self.current_index,
+                };
+                intravisit::walk_ty(subvisitor, arg); // call walk_ty; as visit_ty is empty,
+                // this will visit only outermost type
+                if subvisitor.found_it {
+                    self.found_type = Some(arg);
+                }
+            }
+            _ => {}
+        }
+        // walk the embedded contents: e.g., if we are visiting `Vec<&Foo>`,
+        // go on to visit `&Foo`
+        intravisit::walk_ty(self, arg);
+    }
+}
+
+// The visitor captures the corresponding `hir::Ty` of the anonymous region
+// in the case of structs ie. `hir::TyKind::Path`.
+// This visitor would be invoked for each lifetime corresponding to a struct,
+// and would walk the types like Vec<Ref> in the above example and Ref looking for the HIR
+// where that lifetime appears. This allows us to highlight the
+// specific part of the type in the error message.
+struct TyPathVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    found_it: bool,
+    bound_region: ty::BoundRegionKind,
+    current_index: ty::DebruijnIndex,
+}
+
+impl<'tcx> Visitor<'tcx> for TyPathVisitor<'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+
+    fn nested_visit_map(&mut self) -> Map<'tcx> {
+        self.tcx.hir()
+    }
+
+    fn visit_lifetime(&mut self, lifetime: &hir::Lifetime) {
+        match (self.tcx.named_region(lifetime.hir_id), self.bound_region) {
+            // the lifetime of the TyPath!
+            (Some(rl::Region::EarlyBound(_, id)), ty::BrNamed(def_id, _)) => {
+                debug!("EarlyBound id={:?} def_id={:?}", id, def_id);
+                if id == def_id {
+                    self.found_it = true;
+                    return; // we can stop visiting now
+                }
+            }
+
+            (Some(rl::Region::LateBound(debruijn_index, _, id)), ty::BrNamed(def_id, _)) => {
+                debug!("FindNestedTypeVisitor::visit_ty: LateBound depth = {:?}", debruijn_index,);
+                debug!("id={:?}", id);
+                debug!("def_id={:?}", def_id);
+                if debruijn_index == self.current_index && id == def_id {
+                    self.found_it = true;
+                    return; // we can stop visiting now
+                }
+            }
+
+            (
+                Some(
+                    rl::Region::Static
+                    | rl::Region::EarlyBound(_, _)
+                    | rl::Region::LateBound(_, _, _)
+                    | rl::Region::Free(_, _),
+                )
+                | None,
+                _,
+            ) => {
+                debug!("no arg found");
+            }
+        }
+    }
+
+    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
+        // ignore nested types
+        //
+        // If you have a type like `Foo<'a, &Ty>` we
+        // are only interested in the immediate lifetimes ('a).
+        //
+        // Making `visit_ty` empty will ignore the `&Ty` embedded
+        // inside, it will get reached by the outer visitor.
+        debug!("`Ty` corresponding to a struct is {:?}", arg);
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs
new file mode 100644
index 000000000..893ca3cf7
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs
@@ -0,0 +1,102 @@
+//! Error Reporting for when the lifetime for a type doesn't match the `impl` selected for a predicate
+//! to hold.
+
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::error_reporting::note_and_explain_region;
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::infer::{SubregionOrigin, TypeTrace};
+use crate::traits::ObligationCauseCode;
+use rustc_data_structures::fx::FxHashSet;
+use rustc_errors::{Applicability, ErrorGuaranteed, MultiSpan};
+use rustc_hir as hir;
+use rustc_hir::intravisit::Visitor;
+use rustc_middle::ty::TypeVisitor;
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    pub(super) fn try_report_mismatched_static_lifetime(&self) -> Option<ErrorGuaranteed> {
+        let error = self.error.as_ref()?;
+        debug!("try_report_mismatched_static_lifetime {:?}", error);
+
+        let RegionResolutionError::ConcreteFailure(origin, sub, sup) = error.clone() else {
+            return None;
+        };
+        if !sub.is_static() {
+            return None;
+        }
+        let SubregionOrigin::Subtype(box TypeTrace { ref cause, .. }) = origin else {
+            return None;
+        };
+        // If we added a "points at argument expression" obligation, we remove it here, we care
+        // about the original obligation only.
+        let code = match cause.code() {
+            ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => &*parent_code,
+            code => code,
+        };
+        let ObligationCauseCode::MatchImpl(parent, impl_def_id) = code else {
+            return None;
+        };
+        let ObligationCauseCode::BindingObligation(_def_id, binding_span) = *parent.code() else {
+            return None;
+        };
+        let mut err = self.tcx().sess.struct_span_err(cause.span, "incompatible lifetime on type");
+        // FIXME: we should point at the lifetime
+        let mut multi_span: MultiSpan = vec![binding_span].into();
+        multi_span.push_span_label(binding_span, "introduces a `'static` lifetime requirement");
+        err.span_note(multi_span, "because this has an unmet lifetime requirement");
+        note_and_explain_region(self.tcx(), &mut err, "", sup, "...", Some(binding_span));
+        if let Some(impl_node) = self.tcx().hir().get_if_local(*impl_def_id) {
+            // If an impl is local, then maybe this isn't what they want. Try to
+            // be as helpful as possible with implicit lifetimes.
+
+            // First, let's get the hir self type of the impl
+            let hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Impl(hir::Impl { self_ty: impl_self_ty, .. }),
+                ..
+            }) = impl_node else {
+                bug!("Node not an impl.");
+            };
+
+            // Next, let's figure out the set of trait objects with implicit static bounds
+            let ty = self.tcx().type_of(*impl_def_id);
+            let mut v = super::static_impl_trait::TraitObjectVisitor(FxHashSet::default());
+            v.visit_ty(ty);
+            let mut traits = vec![];
+            for matching_def_id in v.0 {
+                let mut hir_v =
+                    super::static_impl_trait::HirTraitObjectVisitor(&mut traits, matching_def_id);
+                hir_v.visit_ty(&impl_self_ty);
+            }
+
+            if traits.is_empty() {
+                // If there are no trait object traits to point at, either because
+                // there aren't trait objects or because none are implicit, then just
+                // write a single note on the impl itself.
+
+                let impl_span = self.tcx().def_span(*impl_def_id);
+                err.span_note(impl_span, "...does not necessarily outlive the static lifetime introduced by the compatible `impl`");
+            } else {
+                // Otherwise, point at all implicit static lifetimes
+
+                err.note("...does not necessarily outlive the static lifetime introduced by the compatible `impl`");
+                for span in &traits {
+                    err.span_note(*span, "this has an implicit `'static` lifetime requirement");
+                    // It would be nice to put this immediately under the above note, but they get
+                    // pushed to the end.
+                    err.span_suggestion_verbose(
+                        span.shrink_to_hi(),
+                        "consider relaxing the implicit `'static` requirement",
+                        " + '_",
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+            }
+        } else {
+            // Otherwise just point out the impl.
+
+            let impl_span = self.tcx().def_span(*impl_def_id);
+            err.span_note(impl_span, "...does not necessarily outlive the static lifetime introduced by the compatible `impl`");
+        }
+        let reported = err.emit();
+        Some(reported)
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs
new file mode 100644
index 000000000..53d9acf7d
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs
@@ -0,0 +1,77 @@
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::infer::lexical_region_resolve::RegionResolutionError::*;
+use crate::infer::InferCtxt;
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_span::source_map::Span;
+
+mod different_lifetimes;
+pub mod find_anon_type;
+mod mismatched_static_lifetime;
+mod named_anon_conflict;
+mod placeholder_error;
+mod static_impl_trait;
+mod trait_impl_difference;
+mod util;
+
+pub use different_lifetimes::suggest_adding_lifetime_params;
+pub use find_anon_type::find_anon_type;
+pub use static_impl_trait::{suggest_new_region_bound, HirTraitObjectVisitor, TraitObjectVisitor};
+pub use util::find_param_with_region;
+
+impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
+    pub fn try_report_nice_region_error(&self, error: &RegionResolutionError<'tcx>) -> bool {
+        NiceRegionError::new(self, error.clone()).try_report().is_some()
+    }
+}
+
+pub struct NiceRegionError<'cx, 'tcx> {
+    infcx: &'cx InferCtxt<'cx, 'tcx>,
+    error: Option<RegionResolutionError<'tcx>>,
+    regions: Option<(Span, ty::Region<'tcx>, ty::Region<'tcx>)>,
+}
+
+impl<'cx, 'tcx> NiceRegionError<'cx, 'tcx> {
+    pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>, error: RegionResolutionError<'tcx>) -> Self {
+        Self { infcx, error: Some(error), regions: None }
+    }
+
+    pub fn new_from_span(
+        infcx: &'cx InferCtxt<'cx, 'tcx>,
+        span: Span,
+        sub: ty::Region<'tcx>,
+        sup: ty::Region<'tcx>,
+    ) -> Self {
+        Self { infcx, error: None, regions: Some((span, sub, sup)) }
+    }
+
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    pub fn try_report_from_nll(&self) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
+        // Due to the improved diagnostics returned by the MIR borrow checker, only a subset of
+        // the nice region errors are required when running under the MIR borrow checker.
+        self.try_report_named_anon_conflict().or_else(|| self.try_report_placeholder_conflict())
+    }
+
+    pub fn try_report(&self) -> Option<ErrorGuaranteed> {
+        self.try_report_from_nll()
+            .map(|mut diag| diag.emit())
+            .or_else(|| self.try_report_impl_not_conforming_to_trait())
+            .or_else(|| self.try_report_anon_anon_conflict())
+            .or_else(|| self.try_report_static_impl_trait())
+            .or_else(|| self.try_report_mismatched_static_lifetime())
+    }
+
+    pub(super) fn regions(&self) -> Option<(Span, ty::Region<'tcx>, ty::Region<'tcx>)> {
+        match (&self.error, self.regions) {
+            (Some(ConcreteFailure(origin, sub, sup)), None) => Some((origin.span(), *sub, *sup)),
+            (Some(SubSupConflict(_, _, origin, sub, _, sup, _)), None) => {
+                Some((origin.span(), *sub, *sup))
+            }
+            (None, Some((span, sub, sup))) => Some((span, sub, sup)),
+            _ => None,
+        }
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs
new file mode 100644
index 000000000..76cb76d9f
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs
@@ -0,0 +1,116 @@
+//! Error Reporting for Anonymous Region Lifetime Errors
+//! where one region is named and the other is anonymous.
+use crate::infer::error_reporting::nice_region_error::find_anon_type::find_anon_type;
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_middle::ty;
+use rustc_span::symbol::kw;
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    /// When given a `ConcreteFailure` for a function with parameters containing a named region and
+    /// an anonymous region, emit an descriptive diagnostic error.
+    pub(super) fn try_report_named_anon_conflict(
+        &self,
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
+        let (span, sub, sup) = self.regions()?;
+
+        debug!(
+            "try_report_named_anon_conflict(sub={:?}, sup={:?}, error={:?})",
+            sub, sup, self.error,
+        );
+
+        // Determine whether the sub and sup consist of one named region ('a)
+        // and one anonymous (elided) region. If so, find the parameter arg
+        // where the anonymous region appears (there must always be one; we
+        // only introduced anonymous regions in parameters) as well as a
+        // version new_ty of its type where the anonymous region is replaced
+        // with the named one.
+        let (named, anon, anon_param_info, region_info) = if sub.has_name()
+            && self.tcx().is_suitable_region(sup).is_some()
+            && self.find_param_with_region(sup, sub).is_some()
+        {
+            (
+                sub,
+                sup,
+                self.find_param_with_region(sup, sub).unwrap(),
+                self.tcx().is_suitable_region(sup).unwrap(),
+            )
+        } else if sup.has_name()
+            && self.tcx().is_suitable_region(sub).is_some()
+            && self.find_param_with_region(sub, sup).is_some()
+        {
+            (
+                sup,
+                sub,
+                self.find_param_with_region(sub, sup).unwrap(),
+                self.tcx().is_suitable_region(sub).unwrap(),
+            )
+        } else {
+            return None; // inapplicable
+        };
+
+        // Suggesting to add a `'static` lifetime to a parameter is nearly always incorrect,
+        // and can steer users down the wrong path.
+        if named.is_static() {
+            return None;
+        }
+
+        debug!("try_report_named_anon_conflict: named = {:?}", named);
+        debug!("try_report_named_anon_conflict: anon_param_info = {:?}", anon_param_info);
+        debug!("try_report_named_anon_conflict: region_info = {:?}", region_info);
+
+        let param = anon_param_info.param;
+        let new_ty = anon_param_info.param_ty;
+        let new_ty_span = anon_param_info.param_ty_span;
+        let br = anon_param_info.bound_region;
+        let is_first = anon_param_info.is_first;
+        let scope_def_id = region_info.def_id;
+        let is_impl_item = region_info.is_impl_item;
+
+        match br {
+            ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) => {}
+            _ => {
+                /* not an anonymous region */
+                debug!("try_report_named_anon_conflict: not an anonymous region");
+                return None;
+            }
+        }
+
+        if is_impl_item {
+            debug!("try_report_named_anon_conflict: impl item, bail out");
+            return None;
+        }
+
+        if find_anon_type(self.tcx(), anon, &br).is_some()
+            && self.is_self_anon(is_first, scope_def_id)
+        {
+            return None;
+        }
+
+        let (error_var, span_label_var) = match param.pat.simple_ident() {
+            Some(simple_ident) => (
+                format!("the type of `{}`", simple_ident),
+                format!("the type of `{}`", simple_ident),
+            ),
+            None => ("parameter type".to_owned(), "type".to_owned()),
+        };
+
+        let mut diag = struct_span_err!(
+            self.tcx().sess,
+            span,
+            E0621,
+            "explicit lifetime required in {}",
+            error_var
+        );
+
+        diag.span_label(span, format!("lifetime `{}` required", named));
+        diag.span_suggestion(
+            new_ty_span,
+            &format!("add explicit lifetime `{}` to {}", named, span_label_var),
+            new_ty,
+            Applicability::Unspecified,
+        );
+
+        Some(diag)
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs
new file mode 100644
index 000000000..998699158
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs
@@ -0,0 +1,501 @@
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::infer::ValuePairs;
+use crate::infer::{SubregionOrigin, TypeTrace};
+use crate::traits::{ObligationCause, ObligationCauseCode};
+use rustc_data_structures::intern::Interned;
+use rustc_errors::{Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_hir::def::Namespace;
+use rustc_hir::def_id::DefId;
+use rustc_middle::ty::error::ExpectedFound;
+use rustc_middle::ty::print::{FmtPrinter, Print, RegionHighlightMode};
+use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::{self, RePlaceholder, ReVar, Region, TyCtxt};
+
+use std::fmt::{self, Write};
+
+impl<'tcx> NiceRegionError<'_, 'tcx> {
+    /// When given a `ConcreteFailure` for a function with arguments containing a named region and
+    /// an anonymous region, emit a descriptive diagnostic error.
+    pub(super) fn try_report_placeholder_conflict(
+        &self,
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
+        match &self.error {
+            ///////////////////////////////////////////////////////////////////////////
+            // NB. The ordering of cases in this match is very
+            // sensitive, because we are often matching against
+            // specific cases and then using an `_` to match all
+            // others.
+
+            ///////////////////////////////////////////////////////////////////////////
+            // Check for errors from comparing trait failures -- first
+            // with two placeholders, then with one.
+            Some(RegionResolutionError::SubSupConflict(
+                vid,
+                _,
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                sub_placeholder @ Region(Interned(RePlaceholder(_), _)),
+                _,
+                sup_placeholder @ Region(Interned(RePlaceholder(_), _)),
+                _,
+            )) => self.try_report_trait_placeholder_mismatch(
+                Some(self.tcx().mk_region(ReVar(*vid))),
+                cause,
+                Some(*sub_placeholder),
+                Some(*sup_placeholder),
+                values,
+            ),
+
+            Some(RegionResolutionError::SubSupConflict(
+                vid,
+                _,
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                sub_placeholder @ Region(Interned(RePlaceholder(_), _)),
+                _,
+                _,
+                _,
+            )) => self.try_report_trait_placeholder_mismatch(
+                Some(self.tcx().mk_region(ReVar(*vid))),
+                cause,
+                Some(*sub_placeholder),
+                None,
+                values,
+            ),
+
+            Some(RegionResolutionError::SubSupConflict(
+                vid,
+                _,
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                _,
+                _,
+                sup_placeholder @ Region(Interned(RePlaceholder(_), _)),
+                _,
+            )) => self.try_report_trait_placeholder_mismatch(
+                Some(self.tcx().mk_region(ReVar(*vid))),
+                cause,
+                None,
+                Some(*sup_placeholder),
+                values,
+            ),
+
+            Some(RegionResolutionError::SubSupConflict(
+                vid,
+                _,
+                _,
+                _,
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                sup_placeholder @ Region(Interned(RePlaceholder(_), _)),
+                _,
+            )) => self.try_report_trait_placeholder_mismatch(
+                Some(self.tcx().mk_region(ReVar(*vid))),
+                cause,
+                None,
+                Some(*sup_placeholder),
+                values,
+            ),
+
+            Some(RegionResolutionError::UpperBoundUniverseConflict(
+                vid,
+                _,
+                _,
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                sup_placeholder @ Region(Interned(RePlaceholder(_), _)),
+            )) => self.try_report_trait_placeholder_mismatch(
+                Some(self.tcx().mk_region(ReVar(*vid))),
+                cause,
+                None,
+                Some(*sup_placeholder),
+                values,
+            ),
+
+            Some(RegionResolutionError::ConcreteFailure(
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                sub_region @ Region(Interned(RePlaceholder(_), _)),
+                sup_region @ Region(Interned(RePlaceholder(_), _)),
+            )) => self.try_report_trait_placeholder_mismatch(
+                None,
+                cause,
+                Some(*sub_region),
+                Some(*sup_region),
+                values,
+            ),
+
+            Some(RegionResolutionError::ConcreteFailure(
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                sub_region @ Region(Interned(RePlaceholder(_), _)),
+                sup_region,
+            )) => self.try_report_trait_placeholder_mismatch(
+                (!sup_region.has_name()).then_some(*sup_region),
+                cause,
+                Some(*sub_region),
+                None,
+                values,
+            ),
+
+            Some(RegionResolutionError::ConcreteFailure(
+                SubregionOrigin::Subtype(box TypeTrace { cause, values }),
+                sub_region,
+                sup_region @ Region(Interned(RePlaceholder(_), _)),
+            )) => self.try_report_trait_placeholder_mismatch(
+                (!sub_region.has_name()).then_some(*sub_region),
+                cause,
+                None,
+                Some(*sup_region),
+                values,
+            ),
+
+            _ => None,
+        }
+    }
+
+    fn try_report_trait_placeholder_mismatch(
+        &self,
+        vid: Option<Region<'tcx>>,
+        cause: &ObligationCause<'tcx>,
+        sub_placeholder: Option<Region<'tcx>>,
+        sup_placeholder: Option<Region<'tcx>>,
+        value_pairs: &ValuePairs<'tcx>,
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
+        let (expected_substs, found_substs, trait_def_id) = match value_pairs {
+            ValuePairs::TraitRefs(ExpectedFound { expected, found })
+                if expected.def_id == found.def_id =>
+            {
+                (expected.substs, found.substs, expected.def_id)
+            }
+            ValuePairs::PolyTraitRefs(ExpectedFound { expected, found })
+                if expected.def_id() == found.def_id() =>
+            {
+                // It's possible that the placeholders come from a binder
+                // outside of this value pair. Use `no_bound_vars` as a
+                // simple heuristic for that.
+                (expected.no_bound_vars()?.substs, found.no_bound_vars()?.substs, expected.def_id())
+            }
+            _ => return None,
+        };
+
+        Some(self.report_trait_placeholder_mismatch(
+            vid,
+            cause,
+            sub_placeholder,
+            sup_placeholder,
+            trait_def_id,
+            expected_substs,
+            found_substs,
+        ))
+    }
+
+    // error[E0308]: implementation of `Foo` does not apply to enough lifetimes
+    //   --> /home/nmatsakis/tmp/foo.rs:12:5
+    //    |
+    // 12 |     all::<&'static u32>();
+    //    |     ^^^^^^^^^^^^^^^^^^^ lifetime mismatch
+    //    |
+    //    = note: Due to a where-clause on the function `all`,
+    //    = note: `T` must implement `...` for any two lifetimes `'1` and `'2`.
+    //    = note: However, the type `T` only implements `...` for some specific lifetime `'2`.
+    #[instrument(level = "debug", skip(self))]
+    fn report_trait_placeholder_mismatch(
+        &self,
+        vid: Option<Region<'tcx>>,
+        cause: &ObligationCause<'tcx>,
+        sub_placeholder: Option<Region<'tcx>>,
+        sup_placeholder: Option<Region<'tcx>>,
+        trait_def_id: DefId,
+        expected_substs: SubstsRef<'tcx>,
+        actual_substs: SubstsRef<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let span = cause.span();
+        let msg = format!(
+            "implementation of `{}` is not general enough",
+            self.tcx().def_path_str(trait_def_id),
+        );
+        let mut err = self.tcx().sess.struct_span_err(span, &msg);
+
+        let leading_ellipsis = if let ObligationCauseCode::ItemObligation(def_id) = *cause.code() {
+            err.span_label(span, "doesn't satisfy where-clause");
+            err.span_label(
+                self.tcx().def_span(def_id),
+                &format!("due to a where-clause on `{}`...", self.tcx().def_path_str(def_id)),
+            );
+            true
+        } else {
+            err.span_label(span, &msg);
+            false
+        };
+
+        let expected_trait_ref = self.infcx.resolve_vars_if_possible(ty::TraitRef {
+            def_id: trait_def_id,
+            substs: expected_substs,
+        });
+        let actual_trait_ref = self
+            .infcx
+            .resolve_vars_if_possible(ty::TraitRef { def_id: trait_def_id, substs: actual_substs });
+
+        // Search the expected and actual trait references to see (a)
+        // whether the sub/sup placeholders appear in them (sometimes
+        // you have a trait ref like `T: Foo<fn(&u8)>`, where the
+        // placeholder was created as part of an inner type) and (b)
+        // whether the inference variable appears. In each case,
+        // assign a counter value in each case if so.
+        let mut counter = 0;
+        let mut has_sub = None;
+        let mut has_sup = None;
+
+        let mut actual_has_vid = None;
+        let mut expected_has_vid = None;
+
+        self.tcx().for_each_free_region(&expected_trait_ref, |r| {
+            if Some(r) == sub_placeholder && has_sub.is_none() {
+                has_sub = Some(counter);
+                counter += 1;
+            } else if Some(r) == sup_placeholder && has_sup.is_none() {
+                has_sup = Some(counter);
+                counter += 1;
+            }
+
+            if Some(r) == vid && expected_has_vid.is_none() {
+                expected_has_vid = Some(counter);
+                counter += 1;
+            }
+        });
+
+        self.tcx().for_each_free_region(&actual_trait_ref, |r| {
+            if Some(r) == vid && actual_has_vid.is_none() {
+                actual_has_vid = Some(counter);
+                counter += 1;
+            }
+        });
+
+        let actual_self_ty_has_vid =
+            self.tcx().any_free_region_meets(&actual_trait_ref.self_ty(), |r| Some(r) == vid);
+
+        let expected_self_ty_has_vid =
+            self.tcx().any_free_region_meets(&expected_trait_ref.self_ty(), |r| Some(r) == vid);
+
+        let any_self_ty_has_vid = actual_self_ty_has_vid || expected_self_ty_has_vid;
+
+        debug!(
+            ?actual_has_vid,
+            ?expected_has_vid,
+            ?has_sub,
+            ?has_sup,
+            ?actual_self_ty_has_vid,
+            ?expected_self_ty_has_vid,
+        );
+
+        self.explain_actual_impl_that_was_found(
+            &mut err,
+            sub_placeholder,
+            sup_placeholder,
+            has_sub,
+            has_sup,
+            expected_trait_ref,
+            actual_trait_ref,
+            vid,
+            expected_has_vid,
+            actual_has_vid,
+            any_self_ty_has_vid,
+            leading_ellipsis,
+        );
+
+        err
+    }
+
+    /// Add notes with details about the expected and actual trait refs, with attention to cases
+    /// when placeholder regions are involved: either the trait or the self type containing
+    /// them needs to be mentioned the closest to the placeholders.
+    /// This makes the error messages read better, however at the cost of some complexity
+    /// due to the number of combinations we have to deal with.
+    fn explain_actual_impl_that_was_found(
+        &self,
+        err: &mut Diagnostic,
+        sub_placeholder: Option<Region<'tcx>>,
+        sup_placeholder: Option<Region<'tcx>>,
+        has_sub: Option<usize>,
+        has_sup: Option<usize>,
+        expected_trait_ref: ty::TraitRef<'tcx>,
+        actual_trait_ref: ty::TraitRef<'tcx>,
+        vid: Option<Region<'tcx>>,
+        expected_has_vid: Option<usize>,
+        actual_has_vid: Option<usize>,
+        any_self_ty_has_vid: bool,
+        leading_ellipsis: bool,
+    ) {
+        // HACK(eddyb) maybe move this in a more central location.
+        #[derive(Copy, Clone)]
+        struct Highlighted<'tcx, T> {
+            tcx: TyCtxt<'tcx>,
+            highlight: RegionHighlightMode<'tcx>,
+            value: T,
+        }
+
+        impl<'tcx, T> Highlighted<'tcx, T> {
+            fn map<U>(self, f: impl FnOnce(T) -> U) -> Highlighted<'tcx, U> {
+                Highlighted { tcx: self.tcx, highlight: self.highlight, value: f(self.value) }
+            }
+        }
+
+        impl<'tcx, T> fmt::Display for Highlighted<'tcx, T>
+        where
+            T: for<'a> Print<
+                'tcx,
+                FmtPrinter<'a, 'tcx>,
+                Error = fmt::Error,
+                Output = FmtPrinter<'a, 'tcx>,
+            >,
+        {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                let mut printer = ty::print::FmtPrinter::new(self.tcx, Namespace::TypeNS);
+                printer.region_highlight_mode = self.highlight;
+
+                let s = self.value.print(printer)?.into_buffer();
+                f.write_str(&s)
+            }
+        }
+
+        // The weird thing here with the `maybe_highlighting_region` calls and the
+        // the match inside is meant to be like this:
+        //
+        // - The match checks whether the given things (placeholders, etc) appear
+        //   in the types are about to print
+        // - Meanwhile, the `maybe_highlighting_region` calls set up
+        //   highlights so that, if they do appear, we will replace
+        //   them `'0` and whatever.  (This replacement takes place
+        //   inside the closure given to `maybe_highlighting_region`.)
+        //
+        // There is some duplication between the calls -- i.e., the
+        // `maybe_highlighting_region` checks if (e.g.) `has_sub` is
+        // None, an then we check again inside the closure, but this
+        // setup sort of minimized the number of calls and so form.
+
+        let highlight_trait_ref = |trait_ref| Highlighted {
+            tcx: self.tcx(),
+            highlight: RegionHighlightMode::new(self.tcx()),
+            value: trait_ref,
+        };
+
+        let same_self_type = actual_trait_ref.self_ty() == expected_trait_ref.self_ty();
+
+        let mut expected_trait_ref = highlight_trait_ref(expected_trait_ref);
+        expected_trait_ref.highlight.maybe_highlighting_region(sub_placeholder, has_sub);
+        expected_trait_ref.highlight.maybe_highlighting_region(sup_placeholder, has_sup);
+        err.note(&{
+            let passive_voice = match (has_sub, has_sup) {
+                (Some(_), _) | (_, Some(_)) => any_self_ty_has_vid,
+                (None, None) => {
+                    expected_trait_ref.highlight.maybe_highlighting_region(vid, expected_has_vid);
+                    match expected_has_vid {
+                        Some(_) => true,
+                        None => any_self_ty_has_vid,
+                    }
+                }
+            };
+
+            let mut note = if same_self_type {
+                let mut self_ty = expected_trait_ref.map(|tr| tr.self_ty());
+                self_ty.highlight.maybe_highlighting_region(vid, actual_has_vid);
+
+                if self_ty.value.is_closure()
+                    && self
+                        .tcx()
+                        .fn_trait_kind_from_lang_item(expected_trait_ref.value.def_id)
+                        .is_some()
+                {
+                    let closure_sig = self_ty.map(|closure| {
+                        if let ty::Closure(_, substs) = closure.kind() {
+                            self.tcx().signature_unclosure(
+                                substs.as_closure().sig(),
+                                rustc_hir::Unsafety::Normal,
+                            )
+                        } else {
+                            bug!("type is not longer closure");
+                        }
+                    });
+
+                    format!(
+                        "{}closure with signature `{}` must implement `{}`",
+                        if leading_ellipsis { "..." } else { "" },
+                        closure_sig,
+                        expected_trait_ref.map(|tr| tr.print_only_trait_path()),
+                    )
+                } else {
+                    format!(
+                        "{}`{}` must implement `{}`",
+                        if leading_ellipsis { "..." } else { "" },
+                        self_ty,
+                        expected_trait_ref.map(|tr| tr.print_only_trait_path()),
+                    )
+                }
+            } else if passive_voice {
+                format!(
+                    "{}`{}` would have to be implemented for the type `{}`",
+                    if leading_ellipsis { "..." } else { "" },
+                    expected_trait_ref.map(|tr| tr.print_only_trait_path()),
+                    expected_trait_ref.map(|tr| tr.self_ty()),
+                )
+            } else {
+                format!(
+                    "{}`{}` must implement `{}`",
+                    if leading_ellipsis { "..." } else { "" },
+                    expected_trait_ref.map(|tr| tr.self_ty()),
+                    expected_trait_ref.map(|tr| tr.print_only_trait_path()),
+                )
+            };
+
+            match (has_sub, has_sup) {
+                (Some(n1), Some(n2)) => {
+                    let _ = write!(
+                        note,
+                        ", for any two lifetimes `'{}` and `'{}`...",
+                        std::cmp::min(n1, n2),
+                        std::cmp::max(n1, n2),
+                    );
+                }
+                (Some(n), _) | (_, Some(n)) => {
+                    let _ = write!(note, ", for any lifetime `'{}`...", n,);
+                }
+                (None, None) => {
+                    if let Some(n) = expected_has_vid {
+                        let _ = write!(note, ", for some specific lifetime `'{}`...", n,);
+                    }
+                }
+            }
+
+            note
+        });
+
+        let mut actual_trait_ref = highlight_trait_ref(actual_trait_ref);
+        actual_trait_ref.highlight.maybe_highlighting_region(vid, actual_has_vid);
+        err.note(&{
+            let passive_voice = match actual_has_vid {
+                Some(_) => any_self_ty_has_vid,
+                None => true,
+            };
+
+            let mut note = if same_self_type {
+                format!(
+                    "...but it actually implements `{}`",
+                    actual_trait_ref.map(|tr| tr.print_only_trait_path()),
+                )
+            } else if passive_voice {
+                format!(
+                    "...but `{}` is actually implemented for the type `{}`",
+                    actual_trait_ref.map(|tr| tr.print_only_trait_path()),
+                    actual_trait_ref.map(|tr| tr.self_ty()),
+                )
+            } else {
+                format!(
+                    "...but `{}` actually implements `{}`",
+                    actual_trait_ref.map(|tr| tr.self_ty()),
+                    actual_trait_ref.map(|tr| tr.print_only_trait_path()),
+                )
+            };
+
+            if let Some(n) = actual_has_vid {
+                let _ = write!(note, ", for some specific lifetime `'{}`", n);
+            }
+
+            note
+        });
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs
new file mode 100644
index 000000000..9886c572a
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs
@@ -0,0 +1,577 @@
+//! Error Reporting for static impl Traits.
+
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::infer::{SubregionOrigin, TypeTrace};
+use crate::traits::{ObligationCauseCode, UnifyReceiverContext};
+use rustc_data_structures::fx::FxHashSet;
+use rustc_errors::{struct_span_err, Applicability, Diagnostic, ErrorGuaranteed, MultiSpan};
+use rustc_hir::def_id::DefId;
+use rustc_hir::intravisit::{walk_ty, Visitor};
+use rustc_hir::{self as hir, GenericBound, Item, ItemKind, Lifetime, LifetimeName, Node, TyKind};
+use rustc_middle::ty::{
+    self, AssocItemContainer, StaticLifetimeVisitor, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor,
+};
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+use std::ops::ControlFlow;
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    /// Print the error message for lifetime errors when the return type is a static `impl Trait`,
+    /// `dyn Trait` or if a method call on a trait object introduces a static requirement.
+    pub(super) fn try_report_static_impl_trait(&self) -> Option<ErrorGuaranteed> {
+        debug!("try_report_static_impl_trait(error={:?})", self.error);
+        let tcx = self.tcx();
+        let (var_origin, sub_origin, sub_r, sup_origin, sup_r, spans) = match self.error.as_ref()? {
+            RegionResolutionError::SubSupConflict(
+                _,
+                var_origin,
+                sub_origin,
+                sub_r,
+                sup_origin,
+                sup_r,
+                spans,
+            ) if sub_r.is_static() => (var_origin, sub_origin, sub_r, sup_origin, sup_r, spans),
+            RegionResolutionError::ConcreteFailure(
+                SubregionOrigin::Subtype(box TypeTrace { cause, .. }),
+                sub_r,
+                sup_r,
+            ) if sub_r.is_static() => {
+                // This is for an implicit `'static` requirement coming from `impl dyn Trait {}`.
+                if let ObligationCauseCode::UnifyReceiver(ctxt) = cause.code() {
+                    // This may have a closure and it would cause ICE
+                    // through `find_param_with_region` (#78262).
+                    let anon_reg_sup = tcx.is_suitable_region(*sup_r)?;
+                    let fn_returns = tcx.return_type_impl_or_dyn_traits(anon_reg_sup.def_id);
+                    if fn_returns.is_empty() {
+                        return None;
+                    }
+
+                    let param = self.find_param_with_region(*sup_r, *sub_r)?;
+                    let lifetime = if sup_r.has_name() {
+                        format!("lifetime `{}`", sup_r)
+                    } else {
+                        "an anonymous lifetime `'_`".to_string()
+                    };
+                    let mut err = struct_span_err!(
+                        tcx.sess,
+                        cause.span,
+                        E0772,
+                        "{} has {} but calling `{}` introduces an implicit `'static` lifetime \
+                         requirement",
+                        param
+                            .param
+                            .pat
+                            .simple_ident()
+                            .map(|s| format!("`{}`", s))
+                            .unwrap_or_else(|| "`fn` parameter".to_string()),
+                        lifetime,
+                        ctxt.assoc_item.name,
+                    );
+                    err.span_label(param.param_ty_span, &format!("this data with {}...", lifetime));
+                    err.span_label(
+                        cause.span,
+                        &format!(
+                            "...is used and required to live as long as `'static` here \
+                             because of an implicit lifetime bound on the {}",
+                            match ctxt.assoc_item.container {
+                                AssocItemContainer::TraitContainer => {
+                                    let id = ctxt.assoc_item.container_id(tcx);
+                                    format!("`impl` of `{}`", tcx.def_path_str(id))
+                                }
+                                AssocItemContainer::ImplContainer => "inherent `impl`".to_string(),
+                            },
+                        ),
+                    );
+                    if self.find_impl_on_dyn_trait(&mut err, param.param_ty, &ctxt) {
+                        let reported = err.emit();
+                        return Some(reported);
+                    } else {
+                        err.cancel();
+                    }
+                }
+                return None;
+            }
+            _ => return None,
+        };
+        debug!(
+            "try_report_static_impl_trait(var={:?}, sub={:?} {:?} sup={:?} {:?})",
+            var_origin, sub_origin, sub_r, sup_origin, sup_r
+        );
+        let anon_reg_sup = tcx.is_suitable_region(*sup_r)?;
+        debug!("try_report_static_impl_trait: anon_reg_sup={:?}", anon_reg_sup);
+        let sp = var_origin.span();
+        let return_sp = sub_origin.span();
+        let param = self.find_param_with_region(*sup_r, *sub_r)?;
+        let (lifetime_name, lifetime) = if sup_r.has_name() {
+            (sup_r.to_string(), format!("lifetime `{}`", sup_r))
+        } else {
+            ("'_".to_owned(), "an anonymous lifetime `'_`".to_string())
+        };
+        let param_name = param
+            .param
+            .pat
+            .simple_ident()
+            .map(|s| format!("`{}`", s))
+            .unwrap_or_else(|| "`fn` parameter".to_string());
+        let mut err = struct_span_err!(
+            tcx.sess,
+            sp,
+            E0759,
+            "{} has {} but it needs to satisfy a `'static` lifetime requirement",
+            param_name,
+            lifetime,
+        );
+
+        let (mention_influencer, influencer_point) =
+            if sup_origin.span().overlaps(param.param_ty_span) {
+                // Account for `async fn` like in `async-await/issues/issue-62097.rs`.
+                // The desugaring of `async `fn`s causes `sup_origin` and `param` to point at the same
+                // place (but with different `ctxt`, hence `overlaps` instead of `==` above).
+                //
+                // This avoids the following:
+                //
+                // LL |     pub async fn run_dummy_fn(&self) {
+                //    |                               ^^^^^
+                //    |                               |
+                //    |                               this data with an anonymous lifetime `'_`...
+                //    |                               ...is captured here...
+                (false, sup_origin.span())
+            } else {
+                (!sup_origin.span().overlaps(return_sp), param.param_ty_span)
+            };
+        err.span_label(influencer_point, &format!("this data with {}...", lifetime));
+
+        debug!("try_report_static_impl_trait: param_info={:?}", param);
+
+        let mut spans = spans.clone();
+
+        if mention_influencer {
+            spans.push(sup_origin.span());
+        }
+        // We dedup the spans *ignoring* expansion context.
+        spans.sort();
+        spans.dedup_by_key(|span| (span.lo(), span.hi()));
+
+        // We try to make the output have fewer overlapping spans if possible.
+        let require_msg = if spans.is_empty() {
+            "...is used and required to live as long as `'static` here"
+        } else {
+            "...and is required to live as long as `'static` here"
+        };
+        let require_span =
+            if sup_origin.span().overlaps(return_sp) { sup_origin.span() } else { return_sp };
+
+        for span in &spans {
+            err.span_label(*span, "...is used here...");
+        }
+
+        if spans.iter().any(|sp| sp.overlaps(return_sp) || *sp > return_sp) {
+            // If any of the "captured here" labels appears on the same line or after
+            // `require_span`, we put it on a note to ensure the text flows by appearing
+            // always at the end.
+            err.span_note(require_span, require_msg);
+        } else {
+            // We don't need a note, it's already at the end, it can be shown as a `span_label`.
+            err.span_label(require_span, require_msg);
+        }
+
+        if let SubregionOrigin::RelateParamBound(_, _, Some(bound)) = sub_origin {
+            err.span_note(*bound, "`'static` lifetime requirement introduced by this bound");
+        }
+        if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = sub_origin {
+            if let ObligationCauseCode::ReturnValue(hir_id)
+            | ObligationCauseCode::BlockTailExpression(hir_id) = cause.code()
+            {
+                let parent_id = tcx.hir().get_parent_item(*hir_id);
+                let parent_id = tcx.hir().local_def_id_to_hir_id(parent_id);
+                if let Some(fn_decl) = tcx.hir().fn_decl_by_hir_id(parent_id) {
+                    let mut span: MultiSpan = fn_decl.output.span().into();
+                    let mut add_label = true;
+                    if let hir::FnRetTy::Return(ty) = fn_decl.output {
+                        let mut v = StaticLifetimeVisitor(vec![], tcx.hir());
+                        v.visit_ty(ty);
+                        if !v.0.is_empty() {
+                            span = v.0.clone().into();
+                            for sp in v.0 {
+                                span.push_span_label(sp, "`'static` requirement introduced here");
+                            }
+                            add_label = false;
+                        }
+                    }
+                    if add_label {
+                        span.push_span_label(
+                            fn_decl.output.span(),
+                            "requirement introduced by this return type",
+                        );
+                    }
+                    span.push_span_label(cause.span, "because of this returned expression");
+                    err.span_note(
+                        span,
+                        "`'static` lifetime requirement introduced by the return type",
+                    );
+                }
+            }
+        }
+
+        let fn_returns = tcx.return_type_impl_or_dyn_traits(anon_reg_sup.def_id);
+
+        let mut override_error_code = None;
+        if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sup_origin
+            && let ObligationCauseCode::UnifyReceiver(ctxt) = cause.code()
+            // Handle case of `impl Foo for dyn Bar { fn qux(&self) {} }` introducing a
+            // `'static` lifetime when called as a method on a binding: `bar.qux()`.
+            && self.find_impl_on_dyn_trait(&mut err, param.param_ty, &ctxt)
+        {
+            override_error_code = Some(ctxt.assoc_item.name);
+        }
+
+        if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sub_origin
+            && let code = match cause.code() {
+                ObligationCauseCode::MatchImpl(parent, ..) => parent.code(),
+                _ => cause.code(),
+            }
+            && let (&ObligationCauseCode::ItemObligation(item_def_id), None) = (code, override_error_code)
+        {
+            // Same case of `impl Foo for dyn Bar { fn qux(&self) {} }` introducing a `'static`
+            // lifetime as above, but called using a fully-qualified path to the method:
+            // `Foo::qux(bar)`.
+            let mut v = TraitObjectVisitor(FxHashSet::default());
+            v.visit_ty(param.param_ty);
+            if let Some((ident, self_ty)) =
+                self.get_impl_ident_and_self_ty_from_trait(item_def_id, &v.0)
+                && self.suggest_constrain_dyn_trait_in_impl(&mut err, &v.0, ident, self_ty)
+            {
+                override_error_code = Some(ident.name);
+            }
+        }
+        if let (Some(ident), true) = (override_error_code, fn_returns.is_empty()) {
+            // Provide a more targeted error code and description.
+            err.code(rustc_errors::error_code!(E0772));
+            err.set_primary_message(&format!(
+                "{} has {} but calling `{}` introduces an implicit `'static` lifetime \
+                requirement",
+                param_name, lifetime, ident,
+            ));
+        }
+
+        let arg = match param.param.pat.simple_ident() {
+            Some(simple_ident) => format!("argument `{}`", simple_ident),
+            None => "the argument".to_string(),
+        };
+        let captures = format!("captures data from {}", arg);
+        suggest_new_region_bound(
+            tcx,
+            &mut err,
+            fn_returns,
+            lifetime_name,
+            Some(arg),
+            captures,
+            Some((param.param_ty_span, param.param_ty.to_string())),
+        );
+
+        let reported = err.emit();
+        Some(reported)
+    }
+}
+
+pub fn suggest_new_region_bound(
+    tcx: TyCtxt<'_>,
+    err: &mut Diagnostic,
+    fn_returns: Vec<&rustc_hir::Ty<'_>>,
+    lifetime_name: String,
+    arg: Option<String>,
+    captures: String,
+    param: Option<(Span, String)>,
+) {
+    debug!("try_report_static_impl_trait: fn_return={:?}", fn_returns);
+    // FIXME: account for the need of parens in `&(dyn Trait + '_)`
+    let consider = "consider changing the";
+    let declare = "to declare that the";
+    let explicit = format!("you can add an explicit `{}` lifetime bound", lifetime_name);
+    let explicit_static =
+        arg.map(|arg| format!("explicit `'static` bound to the lifetime of {}", arg));
+    let add_static_bound = "alternatively, add an explicit `'static` bound to this reference";
+    let plus_lt = format!(" + {}", lifetime_name);
+    for fn_return in fn_returns {
+        if fn_return.span.desugaring_kind().is_some() {
+            // Skip `async` desugaring `impl Future`.
+            continue;
+        }
+        match fn_return.kind {
+            TyKind::OpaqueDef(item_id, _) => {
+                let item = tcx.hir().item(item_id);
+                let ItemKind::OpaqueTy(opaque) = &item.kind else {
+                    return;
+                };
+
+                if let Some(span) = opaque
+                    .bounds
+                    .iter()
+                    .filter_map(|arg| match arg {
+                        GenericBound::Outlives(Lifetime {
+                            name: LifetimeName::Static,
+                            span,
+                            ..
+                        }) => Some(*span),
+                        _ => None,
+                    })
+                    .next()
+                {
+                    if let Some(explicit_static) = &explicit_static {
+                        err.span_suggestion_verbose(
+                            span,
+                            &format!("{} `impl Trait`'s {}", consider, explicit_static),
+                            &lifetime_name,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                    if let Some((param_span, param_ty)) = param.clone() {
+                        err.span_suggestion_verbose(
+                            param_span,
+                            add_static_bound,
+                            param_ty,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                } else if opaque
+                    .bounds
+                    .iter()
+                    .filter_map(|arg| match arg {
+                        GenericBound::Outlives(Lifetime { name, span, .. })
+                            if name.ident().to_string() == lifetime_name =>
+                        {
+                            Some(*span)
+                        }
+                        _ => None,
+                    })
+                    .next()
+                    .is_some()
+                {
+                } else {
+                    err.span_suggestion_verbose(
+                        fn_return.span.shrink_to_hi(),
+                        &format!(
+                            "{declare} `impl Trait` {captures}, {explicit}",
+                            declare = declare,
+                            captures = captures,
+                            explicit = explicit,
+                        ),
+                        &plus_lt,
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+            }
+            TyKind::TraitObject(_, lt, _) => match lt.name {
+                LifetimeName::ImplicitObjectLifetimeDefault => {
+                    err.span_suggestion_verbose(
+                        fn_return.span.shrink_to_hi(),
+                        &format!(
+                            "{declare} trait object {captures}, {explicit}",
+                            declare = declare,
+                            captures = captures,
+                            explicit = explicit,
+                        ),
+                        &plus_lt,
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                name if name.ident().to_string() != lifetime_name => {
+                    // With this check we avoid suggesting redundant bounds. This
+                    // would happen if there are nested impl/dyn traits and only
+                    // one of them has the bound we'd suggest already there, like
+                    // in `impl Foo<X = dyn Bar> + '_`.
+                    if let Some(explicit_static) = &explicit_static {
+                        err.span_suggestion_verbose(
+                            lt.span,
+                            &format!("{} trait object's {}", consider, explicit_static),
+                            &lifetime_name,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                    if let Some((param_span, param_ty)) = param.clone() {
+                        err.span_suggestion_verbose(
+                            param_span,
+                            add_static_bound,
+                            param_ty,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+                _ => {}
+            },
+            _ => {}
+        }
+    }
+}
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    fn get_impl_ident_and_self_ty_from_trait(
+        &self,
+        def_id: DefId,
+        trait_objects: &FxHashSet<DefId>,
+    ) -> Option<(Ident, &'tcx hir::Ty<'tcx>)> {
+        let tcx = self.tcx();
+        match tcx.hir().get_if_local(def_id) {
+            Some(Node::ImplItem(impl_item)) => {
+                match tcx.hir().find_by_def_id(tcx.hir().get_parent_item(impl_item.hir_id())) {
+                    Some(Node::Item(Item {
+                        kind: ItemKind::Impl(hir::Impl { self_ty, .. }),
+                        ..
+                    })) => Some((impl_item.ident, self_ty)),
+                    _ => None,
+                }
+            }
+            Some(Node::TraitItem(trait_item)) => {
+                let trait_did = tcx.hir().get_parent_item(trait_item.hir_id());
+                match tcx.hir().find_by_def_id(trait_did) {
+                    Some(Node::Item(Item { kind: ItemKind::Trait(..), .. })) => {
+                        // The method being called is defined in the `trait`, but the `'static`
+                        // obligation comes from the `impl`. Find that `impl` so that we can point
+                        // at it in the suggestion.
+                        let trait_did = trait_did.to_def_id();
+                        match tcx
+                            .hir()
+                            .trait_impls(trait_did)
+                            .iter()
+                            .filter_map(|&impl_did| {
+                                match tcx.hir().get_if_local(impl_did.to_def_id()) {
+                                    Some(Node::Item(Item {
+                                        kind: ItemKind::Impl(hir::Impl { self_ty, .. }),
+                                        ..
+                                    })) if trait_objects.iter().all(|did| {
+                                        // FIXME: we should check `self_ty` against the receiver
+                                        // type in the `UnifyReceiver` context, but for now, use
+                                        // this imperfect proxy. This will fail if there are
+                                        // multiple `impl`s for the same trait like
+                                        // `impl Foo for Box<dyn Bar>` and `impl Foo for dyn Bar`.
+                                        // In that case, only the first one will get suggestions.
+                                        let mut traits = vec![];
+                                        let mut hir_v = HirTraitObjectVisitor(&mut traits, *did);
+                                        hir_v.visit_ty(self_ty);
+                                        !traits.is_empty()
+                                    }) =>
+                                    {
+                                        Some(self_ty)
+                                    }
+                                    _ => None,
+                                }
+                            })
+                            .next()
+                        {
+                            Some(self_ty) => Some((trait_item.ident, self_ty)),
+                            _ => None,
+                        }
+                    }
+                    _ => None,
+                }
+            }
+            _ => None,
+        }
+    }
+
+    /// When we call a method coming from an `impl Foo for dyn Bar`, `dyn Bar` introduces a default
+    /// `'static` obligation. Suggest relaxing that implicit bound.
+    fn find_impl_on_dyn_trait(
+        &self,
+        err: &mut Diagnostic,
+        ty: Ty<'_>,
+        ctxt: &UnifyReceiverContext<'tcx>,
+    ) -> bool {
+        let tcx = self.tcx();
+
+        // Find the method being called.
+        let Ok(Some(instance)) = ty::Instance::resolve(
+            tcx,
+            ctxt.param_env,
+            ctxt.assoc_item.def_id,
+            self.infcx.resolve_vars_if_possible(ctxt.substs),
+        ) else {
+            return false;
+        };
+
+        let mut v = TraitObjectVisitor(FxHashSet::default());
+        v.visit_ty(ty);
+
+        // Get the `Ident` of the method being called and the corresponding `impl` (to point at
+        // `Bar` in `impl Foo for dyn Bar {}` and the definition of the method being called).
+        let Some((ident, self_ty)) = self.get_impl_ident_and_self_ty_from_trait(instance.def_id(), &v.0) else {
+            return false;
+        };
+
+        // Find the trait object types in the argument, so we point at *only* the trait object.
+        self.suggest_constrain_dyn_trait_in_impl(err, &v.0, ident, self_ty)
+    }
+
+    fn suggest_constrain_dyn_trait_in_impl(
+        &self,
+        err: &mut Diagnostic,
+        found_dids: &FxHashSet<DefId>,
+        ident: Ident,
+        self_ty: &hir::Ty<'_>,
+    ) -> bool {
+        let mut suggested = false;
+        for found_did in found_dids {
+            let mut traits = vec![];
+            let mut hir_v = HirTraitObjectVisitor(&mut traits, *found_did);
+            hir_v.visit_ty(&self_ty);
+            for span in &traits {
+                let mut multi_span: MultiSpan = vec![*span].into();
+                multi_span
+                    .push_span_label(*span, "this has an implicit `'static` lifetime requirement");
+                multi_span.push_span_label(
+                    ident.span,
+                    "calling this method introduces the `impl`'s 'static` requirement",
+                );
+                err.span_note(multi_span, "the used `impl` has a `'static` requirement");
+                err.span_suggestion_verbose(
+                    span.shrink_to_hi(),
+                    "consider relaxing the implicit `'static` requirement",
+                    " + '_",
+                    Applicability::MaybeIncorrect,
+                );
+                suggested = true;
+            }
+        }
+        suggested
+    }
+}
+
+/// Collect all the trait objects in a type that could have received an implicit `'static` lifetime.
+pub struct TraitObjectVisitor(pub FxHashSet<DefId>);
+
+impl<'tcx> TypeVisitor<'tcx> for TraitObjectVisitor {
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        match t.kind() {
+            ty::Dynamic(preds, re) if re.is_static() => {
+                if let Some(def_id) = preds.principal_def_id() {
+                    self.0.insert(def_id);
+                }
+                ControlFlow::CONTINUE
+            }
+            _ => t.super_visit_with(self),
+        }
+    }
+}
+
+/// Collect all `hir::Ty<'_>` `Span`s for trait objects with an implicit lifetime.
+pub struct HirTraitObjectVisitor<'a>(pub &'a mut Vec<Span>, pub DefId);
+
+impl<'a, 'tcx> Visitor<'tcx> for HirTraitObjectVisitor<'a> {
+    fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx>) {
+        if let TyKind::TraitObject(
+            poly_trait_refs,
+            Lifetime { name: LifetimeName::ImplicitObjectLifetimeDefault, .. },
+            _,
+        ) = t.kind
+        {
+            for ptr in poly_trait_refs {
+                if Some(self.1) == ptr.trait_ref.trait_def_id() {
+                    self.0.push(ptr.span);
+                }
+            }
+        }
+        walk_ty(self, t);
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs
new file mode 100644
index 000000000..da465a764
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs
@@ -0,0 +1,176 @@
+//! Error Reporting for `impl` items that do not match the obligations from their `trait`.
+
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::infer::Subtype;
+use crate::traits::ObligationCauseCode::CompareImplItemObligation;
+use rustc_errors::{ErrorGuaranteed, MultiSpan};
+use rustc_hir as hir;
+use rustc_hir::def::Res;
+use rustc_hir::def_id::DefId;
+use rustc_hir::intravisit::Visitor;
+use rustc_middle::hir::nested_filter;
+use rustc_middle::ty::print::RegionHighlightMode;
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
+use rustc_span::Span;
+
+use std::ops::ControlFlow;
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    /// Print the error message for lifetime errors when the `impl` doesn't conform to the `trait`.
+    pub(super) fn try_report_impl_not_conforming_to_trait(&self) -> Option<ErrorGuaranteed> {
+        let error = self.error.as_ref()?;
+        debug!("try_report_impl_not_conforming_to_trait {:?}", error);
+        if let RegionResolutionError::SubSupConflict(
+                _,
+                var_origin,
+                sub_origin,
+                _sub,
+                sup_origin,
+                _sup,
+                _,
+            ) = error.clone()
+            && let (Subtype(sup_trace), Subtype(sub_trace)) = (&sup_origin, &sub_origin)
+            && let sub_expected_found @ Some((sub_expected, sub_found)) = sub_trace.values.ty()
+            && let sup_expected_found @ Some(_) = sup_trace.values.ty()
+            && let CompareImplItemObligation { trait_item_def_id, .. } = sub_trace.cause.code()
+            && sup_expected_found == sub_expected_found
+        {
+            let guar =
+                self.emit_err(var_origin.span(), sub_expected, sub_found, *trait_item_def_id);
+            return Some(guar);
+        }
+        None
+    }
+
+    fn emit_err(
+        &self,
+        sp: Span,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+        trait_def_id: DefId,
+    ) -> ErrorGuaranteed {
+        let trait_sp = self.tcx().def_span(trait_def_id);
+        let mut err = self
+            .tcx()
+            .sess
+            .struct_span_err(sp, "`impl` item signature doesn't match `trait` item signature");
+
+        // Mark all unnamed regions in the type with a number.
+        // This diagnostic is called in response to lifetime errors, so be informative.
+        struct HighlightBuilder<'tcx> {
+            highlight: RegionHighlightMode<'tcx>,
+            counter: usize,
+        }
+
+        impl<'tcx> HighlightBuilder<'tcx> {
+            fn build(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> RegionHighlightMode<'tcx> {
+                let mut builder =
+                    HighlightBuilder { highlight: RegionHighlightMode::new(tcx), counter: 1 };
+                builder.visit_ty(ty);
+                builder.highlight
+            }
+        }
+
+        impl<'tcx> ty::visit::TypeVisitor<'tcx> for HighlightBuilder<'tcx> {
+            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+                if !r.has_name() && self.counter <= 3 {
+                    self.highlight.highlighting_region(r, self.counter);
+                    self.counter += 1;
+                }
+                r.super_visit_with(self)
+            }
+        }
+
+        let expected_highlight = HighlightBuilder::build(self.tcx(), expected);
+        let expected = self
+            .infcx
+            .extract_inference_diagnostics_data(expected.into(), Some(expected_highlight))
+            .name;
+        let found_highlight = HighlightBuilder::build(self.tcx(), found);
+        let found =
+            self.infcx.extract_inference_diagnostics_data(found.into(), Some(found_highlight)).name;
+
+        err.span_label(sp, &format!("found `{}`", found));
+        err.span_label(trait_sp, &format!("expected `{}`", expected));
+
+        // Get the span of all the used type parameters in the method.
+        let assoc_item = self.tcx().associated_item(trait_def_id);
+        let mut visitor = TypeParamSpanVisitor { tcx: self.tcx(), types: vec![] };
+        match assoc_item.kind {
+            ty::AssocKind::Fn => {
+                let hir = self.tcx().hir();
+                if let Some(hir_id) =
+                    assoc_item.def_id.as_local().map(|id| hir.local_def_id_to_hir_id(id))
+                {
+                    if let Some(decl) = hir.fn_decl_by_hir_id(hir_id) {
+                        visitor.visit_fn_decl(decl);
+                    }
+                }
+            }
+            _ => {}
+        }
+        let mut type_param_span: MultiSpan = visitor.types.to_vec().into();
+        for &span in &visitor.types {
+            type_param_span
+                .push_span_label(span, "consider borrowing this type parameter in the trait");
+        }
+
+        err.note(&format!("expected `{}`\n   found `{}`", expected, found));
+
+        err.span_help(
+            type_param_span,
+            "the lifetime requirements from the `impl` do not correspond to the requirements in \
+             the `trait`",
+        );
+        if visitor.types.is_empty() {
+            err.help(
+                "verify the lifetime relationships in the `trait` and `impl` between the `self` \
+                 argument, the other inputs and its output",
+            );
+        }
+        err.emit()
+    }
+}
+
+struct TypeParamSpanVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    types: Vec<Span>,
+}
+
+impl<'tcx> Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.tcx.hir()
+    }
+
+    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
+        match arg.kind {
+            hir::TyKind::Rptr(_, ref mut_ty) => {
+                // We don't want to suggest looking into borrowing `&T` or `&Self`.
+                hir::intravisit::walk_ty(self, mut_ty.ty);
+                return;
+            }
+            hir::TyKind::Path(hir::QPath::Resolved(None, path)) => match &path.segments {
+                [segment]
+                    if segment
+                        .res
+                        .map(|res| {
+                            matches!(
+                                res,
+                                Res::SelfTy { trait_: _, alias_to: _ }
+                                    | Res::Def(hir::def::DefKind::TyParam, _)
+                            )
+                        })
+                        .unwrap_or(false) =>
+                {
+                    self.types.push(path.span);
+                }
+                _ => {}
+            },
+            _ => {}
+        }
+        hir::intravisit::walk_ty(self, arg);
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
new file mode 100644
index 000000000..3e9d491af
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
@@ -0,0 +1,167 @@
+//! Helper functions corresponding to lifetime errors due to
+//! anonymous regions.
+
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::TyCtxt;
+use rustc_hir as hir;
+use rustc_hir::def_id::LocalDefId;
+use rustc_middle::ty::{self, Binder, DefIdTree, Region, Ty, TypeVisitable};
+use rustc_span::Span;
+
+/// Information about the anonymous region we are searching for.
+#[derive(Debug)]
+pub struct AnonymousParamInfo<'tcx> {
+    /// The parameter corresponding to the anonymous region.
+    pub param: &'tcx hir::Param<'tcx>,
+    /// The type corresponding to the anonymous region parameter.
+    pub param_ty: Ty<'tcx>,
+    /// The ty::BoundRegionKind corresponding to the anonymous region.
+    pub bound_region: ty::BoundRegionKind,
+    /// The `Span` of the parameter type.
+    pub param_ty_span: Span,
+    /// Signals that the argument is the first parameter in the declaration.
+    pub is_first: bool,
+}
+
+// This method walks the Type of the function body parameters using
+// `fold_regions()` function and returns the
+// &hir::Param of the function parameter corresponding to the anonymous
+// region and the Ty corresponding to the named region.
+// Currently only the case where the function declaration consists of
+// one named region and one anonymous region is handled.
+// Consider the example `fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32`
+// Here, we would return the hir::Param for y, we return the type &'a
+// i32, which is the type of y but with the anonymous region replaced
+// with 'a, the corresponding bound region and is_first which is true if
+// the hir::Param is the first parameter in the function declaration.
+#[instrument(skip(tcx), level = "debug")]
+pub fn find_param_with_region<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    anon_region: Region<'tcx>,
+    replace_region: Region<'tcx>,
+) -> Option<AnonymousParamInfo<'tcx>> {
+    let (id, bound_region) = match *anon_region {
+        ty::ReFree(ref free_region) => (free_region.scope, free_region.bound_region),
+        ty::ReEarlyBound(ebr) => {
+            (tcx.parent(ebr.def_id), ty::BoundRegionKind::BrNamed(ebr.def_id, ebr.name))
+        }
+        _ => return None, // not a free region
+    };
+
+    let hir = &tcx.hir();
+    let def_id = id.as_local()?;
+    let hir_id = hir.local_def_id_to_hir_id(def_id);
+
+    // FIXME: use def_kind
+    // Don't perform this on closures
+    match hir.get(hir_id) {
+        hir::Node::Expr(&hir::Expr { kind: hir::ExprKind::Closure { .. }, .. }) => {
+            return None;
+        }
+        _ => {}
+    }
+
+    let body_id = hir.maybe_body_owned_by(def_id)?;
+
+    let owner_id = hir.body_owner(body_id);
+    let fn_decl = hir.fn_decl_by_hir_id(owner_id).unwrap();
+    let poly_fn_sig = tcx.fn_sig(id);
+
+    let fn_sig = tcx.liberate_late_bound_regions(id, poly_fn_sig);
+    let body = hir.body(body_id);
+    body.params
+        .iter()
+        .take(if fn_sig.c_variadic {
+            fn_sig.inputs().len()
+        } else {
+            assert_eq!(fn_sig.inputs().len(), body.params.len());
+            body.params.len()
+        })
+        .enumerate()
+        .find_map(|(index, param)| {
+            // May return None; sometimes the tables are not yet populated.
+            let ty = fn_sig.inputs()[index];
+            let mut found_anon_region = false;
+            let new_param_ty = tcx.fold_regions(ty, |r, _| {
+                if r == anon_region {
+                    found_anon_region = true;
+                    replace_region
+                } else {
+                    r
+                }
+            });
+            if found_anon_region {
+                let ty_hir_id = fn_decl.inputs[index].hir_id;
+                let param_ty_span = hir.span(ty_hir_id);
+                let is_first = index == 0;
+                Some(AnonymousParamInfo {
+                    param,
+                    param_ty: new_param_ty,
+                    param_ty_span,
+                    bound_region,
+                    is_first,
+                })
+            } else {
+                None
+            }
+        })
+}
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    pub(super) fn find_param_with_region(
+        &self,
+        anon_region: Region<'tcx>,
+        replace_region: Region<'tcx>,
+    ) -> Option<AnonymousParamInfo<'tcx>> {
+        find_param_with_region(self.tcx(), anon_region, replace_region)
+    }
+
+    // Here, we check for the case where the anonymous region
+    // is in the return type as written by the user.
+    // FIXME(#42703) - Need to handle certain cases here.
+    pub(super) fn is_return_type_anon(
+        &self,
+        scope_def_id: LocalDefId,
+        br: ty::BoundRegionKind,
+        hir_sig: &hir::FnSig<'_>,
+    ) -> Option<Span> {
+        let fn_ty = self.tcx().type_of(scope_def_id);
+        if let ty::FnDef(_, _) = fn_ty.kind() {
+            let ret_ty = fn_ty.fn_sig(self.tcx()).output();
+            let span = hir_sig.decl.output.span();
+            let future_output = if hir_sig.header.is_async() {
+                ret_ty.map_bound(|ty| self.infcx.get_impl_future_output_ty(ty)).transpose()
+            } else {
+                None
+            };
+            return match future_output {
+                Some(output) if self.includes_region(output, br) => Some(span),
+                None if self.includes_region(ret_ty, br) => Some(span),
+                _ => None,
+            };
+        }
+        None
+    }
+
+    fn includes_region(
+        &self,
+        ty: Binder<'tcx, impl TypeVisitable<'tcx>>,
+        region: ty::BoundRegionKind,
+    ) -> bool {
+        let late_bound_regions = self.tcx().collect_referenced_late_bound_regions(&ty);
+        late_bound_regions.iter().any(|r| *r == region)
+    }
+
+    // Here we check for the case where anonymous region
+    // corresponds to self and if yes, we display E0312.
+    // FIXME(#42700) - Need to format self properly to
+    // enable E0621 for it.
+    pub(super) fn is_self_anon(&self, is_first: bool, scope_def_id: LocalDefId) -> bool {
+        is_first
+            && self
+                .tcx()
+                .opt_associated_item(scope_def_id.to_def_id())
+                .map(|i| i.fn_has_self_parameter)
+                == Some(true)
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/note.rs b/compiler/rustc_infer/src/infer/error_reporting/note.rs
new file mode 100644
index 000000000..c1940c5c0
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/note.rs
@@ -0,0 +1,414 @@
+use crate::infer::error_reporting::{note_and_explain_region, ObligationCauseExt};
+use crate::infer::{self, InferCtxt, SubregionOrigin};
+use rustc_errors::{struct_span_err, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_middle::traits::ObligationCauseCode;
+use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::{self, Region};
+
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    pub(super) fn note_region_origin(&self, err: &mut Diagnostic, origin: &SubregionOrigin<'tcx>) {
+        let mut label_or_note = |span, msg: &str| {
+            let sub_count = err.children.iter().filter(|d| d.span.is_dummy()).count();
+            let expanded_sub_count = err.children.iter().filter(|d| !d.span.is_dummy()).count();
+            let span_is_primary = err.span.primary_spans().iter().all(|&sp| sp == span);
+            if span_is_primary && sub_count == 0 && expanded_sub_count == 0 {
+                err.span_label(span, msg);
+            } else if span_is_primary && expanded_sub_count == 0 {
+                err.note(msg);
+            } else {
+                err.span_note(span, msg);
+            }
+        };
+        match *origin {
+            infer::Subtype(ref trace) => {
+                if let Some((expected, found)) = self.values_str(trace.values) {
+                    label_or_note(
+                        trace.cause.span,
+                        &format!("...so that the {}", trace.cause.as_requirement_str()),
+                    );
+
+                    err.note_expected_found(&"", expected, &"", found);
+                } else {
+                    // FIXME: this really should be handled at some earlier stage. Our
+                    // handling of region checking when type errors are present is
+                    // *terrible*.
+
+                    label_or_note(
+                        trace.cause.span,
+                        &format!("...so that {}", trace.cause.as_requirement_str()),
+                    );
+                }
+            }
+            infer::Reborrow(span) => {
+                label_or_note(span, "...so that reference does not outlive borrowed content");
+            }
+            infer::ReborrowUpvar(span, ref upvar_id) => {
+                let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
+                label_or_note(span, &format!("...so that closure can access `{}`", var_name));
+            }
+            infer::RelateObjectBound(span) => {
+                label_or_note(span, "...so that it can be closed over into an object");
+            }
+            infer::DataBorrowed(ty, span) => {
+                label_or_note(
+                    span,
+                    &format!(
+                        "...so that the type `{}` is not borrowed for too long",
+                        self.ty_to_string(ty)
+                    ),
+                );
+            }
+            infer::ReferenceOutlivesReferent(ty, span) => {
+                label_or_note(
+                    span,
+                    &format!(
+                        "...so that the reference type `{}` does not outlive the data it points at",
+                        self.ty_to_string(ty)
+                    ),
+                );
+            }
+            infer::RelateParamBound(span, t, opt_span) => {
+                label_or_note(
+                    span,
+                    &format!(
+                        "...so that the type `{}` will meet its required lifetime bounds{}",
+                        self.ty_to_string(t),
+                        if opt_span.is_some() { "..." } else { "" },
+                    ),
+                );
+                if let Some(span) = opt_span {
+                    err.span_note(span, "...that is required by this bound");
+                }
+            }
+            infer::RelateRegionParamBound(span) => {
+                label_or_note(
+                    span,
+                    "...so that the declared lifetime parameter bounds are satisfied",
+                );
+            }
+            infer::CompareImplItemObligation { span, .. } => {
+                label_or_note(
+                    span,
+                    "...so that the definition in impl matches the definition from the trait",
+                );
+            }
+            infer::CheckAssociatedTypeBounds { ref parent, .. } => {
+                self.note_region_origin(err, &parent);
+            }
+        }
+    }
+
+    pub(super) fn report_concrete_failure(
+        &self,
+        origin: SubregionOrigin<'tcx>,
+        sub: Region<'tcx>,
+        sup: Region<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        match origin {
+            infer::Subtype(box trace) => {
+                let terr = TypeError::RegionsDoesNotOutlive(sup, sub);
+                let mut err = self.report_and_explain_type_error(trace, &terr);
+                match (*sub, *sup) {
+                    (ty::RePlaceholder(_), ty::RePlaceholder(_)) => {}
+                    (ty::RePlaceholder(_), _) => {
+                        note_and_explain_region(
+                            self.tcx,
+                            &mut err,
+                            "",
+                            sup,
+                            " doesn't meet the lifetime requirements",
+                            None,
+                        );
+                    }
+                    (_, ty::RePlaceholder(_)) => {
+                        note_and_explain_region(
+                            self.tcx,
+                            &mut err,
+                            "the required lifetime does not necessarily outlive ",
+                            sub,
+                            "",
+                            None,
+                        );
+                    }
+                    _ => {
+                        note_and_explain_region(self.tcx, &mut err, "", sup, "...", None);
+                        note_and_explain_region(
+                            self.tcx,
+                            &mut err,
+                            "...does not necessarily outlive ",
+                            sub,
+                            "",
+                            None,
+                        );
+                    }
+                }
+                err
+            }
+            infer::Reborrow(span) => {
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0312,
+                    "lifetime of reference outlives lifetime of borrowed content..."
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "...the reference is valid for ",
+                    sub,
+                    "...",
+                    None,
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "...but the borrowed content is only valid for ",
+                    sup,
+                    "",
+                    None,
+                );
+                err
+            }
+            infer::ReborrowUpvar(span, ref upvar_id) => {
+                let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0313,
+                    "lifetime of borrowed pointer outlives lifetime of captured variable `{}`...",
+                    var_name
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "...the borrowed pointer is valid for ",
+                    sub,
+                    "...",
+                    None,
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    &format!("...but `{}` is only valid for ", var_name),
+                    sup,
+                    "",
+                    None,
+                );
+                err
+            }
+            infer::RelateObjectBound(span) => {
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0476,
+                    "lifetime of the source pointer does not outlive lifetime bound of the \
+                     object type"
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "object type is valid for ",
+                    sub,
+                    "",
+                    None,
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "source pointer is only valid for ",
+                    sup,
+                    "",
+                    None,
+                );
+                err
+            }
+            infer::RelateParamBound(span, ty, opt_span) => {
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0477,
+                    "the type `{}` does not fulfill the required lifetime",
+                    self.ty_to_string(ty)
+                );
+                match *sub {
+                    ty::ReStatic => note_and_explain_region(
+                        self.tcx,
+                        &mut err,
+                        "type must satisfy ",
+                        sub,
+                        if opt_span.is_some() { " as required by this binding" } else { "" },
+                        opt_span,
+                    ),
+                    _ => note_and_explain_region(
+                        self.tcx,
+                        &mut err,
+                        "type must outlive ",
+                        sub,
+                        if opt_span.is_some() { " as required by this binding" } else { "" },
+                        opt_span,
+                    ),
+                }
+                err
+            }
+            infer::RelateRegionParamBound(span) => {
+                let mut err =
+                    struct_span_err!(self.tcx.sess, span, E0478, "lifetime bound not satisfied");
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "lifetime parameter instantiated with ",
+                    sup,
+                    "",
+                    None,
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "but lifetime parameter must outlive ",
+                    sub,
+                    "",
+                    None,
+                );
+                err
+            }
+            infer::DataBorrowed(ty, span) => {
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0490,
+                    "a value of type `{}` is borrowed for too long",
+                    self.ty_to_string(ty)
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "the type is valid for ",
+                    sub,
+                    "",
+                    None,
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "but the borrow lasts for ",
+                    sup,
+                    "",
+                    None,
+                );
+                err
+            }
+            infer::ReferenceOutlivesReferent(ty, span) => {
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0491,
+                    "in type `{}`, reference has a longer lifetime than the data it references",
+                    self.ty_to_string(ty)
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "the pointer is valid for ",
+                    sub,
+                    "",
+                    None,
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "but the referenced data is only valid for ",
+                    sup,
+                    "",
+                    None,
+                );
+                err
+            }
+            infer::CompareImplItemObligation { span, impl_item_def_id, trait_item_def_id } => self
+                .report_extra_impl_obligation(
+                    span,
+                    impl_item_def_id,
+                    trait_item_def_id,
+                    &format!("`{}: {}`", sup, sub),
+                ),
+            infer::CheckAssociatedTypeBounds { impl_item_def_id, trait_item_def_id, parent } => {
+                let mut err = self.report_concrete_failure(*parent, sub, sup);
+
+                let trait_item_span = self.tcx.def_span(trait_item_def_id);
+                let item_name = self.tcx.item_name(impl_item_def_id.to_def_id());
+                err.span_label(
+                    trait_item_span,
+                    format!("definition of `{}` from trait", item_name),
+                );
+
+                let trait_predicates = self.tcx.explicit_predicates_of(trait_item_def_id);
+                let impl_predicates = self.tcx.explicit_predicates_of(impl_item_def_id);
+
+                let impl_predicates: rustc_data_structures::fx::FxHashSet<_> =
+                    impl_predicates.predicates.into_iter().map(|(pred, _)| pred).collect();
+                let clauses: Vec<_> = trait_predicates
+                    .predicates
+                    .into_iter()
+                    .filter(|&(pred, _)| !impl_predicates.contains(pred))
+                    .map(|(pred, _)| format!("{}", pred))
+                    .collect();
+
+                if !clauses.is_empty() {
+                    let generics = self.tcx.hir().get_generics(impl_item_def_id).unwrap();
+                    let where_clause_span = generics.tail_span_for_predicate_suggestion();
+
+                    let suggestion = format!(
+                        "{} {}",
+                        generics.add_where_or_trailing_comma(),
+                        clauses.join(", "),
+                    );
+                    err.span_suggestion(
+                        where_clause_span,
+                        &format!(
+                            "try copying {} from the trait",
+                            if clauses.len() > 1 { "these clauses" } else { "this clause" }
+                        ),
+                        suggestion,
+                        rustc_errors::Applicability::MaybeIncorrect,
+                    );
+                }
+
+                err
+            }
+        }
+    }
+
+    pub(super) fn report_placeholder_failure(
+        &self,
+        placeholder_origin: SubregionOrigin<'tcx>,
+        sub: Region<'tcx>,
+        sup: Region<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        // I can't think how to do better than this right now. -nikomatsakis
+        debug!(?placeholder_origin, ?sub, ?sup, "report_placeholder_failure");
+        match placeholder_origin {
+            infer::Subtype(box ref trace)
+                if matches!(
+                    &trace.cause.code().peel_derives(),
+                    ObligationCauseCode::BindingObligation(..)
+                ) =>
+            {
+                // Hack to get around the borrow checker because trace.cause has an `Rc`.
+                if let ObligationCauseCode::BindingObligation(_, span) =
+                    &trace.cause.code().peel_derives()
+                {
+                    let span = *span;
+                    let mut err = self.report_concrete_failure(placeholder_origin, sub, sup);
+                    err.span_note(span, "the lifetime requirement is introduced here");
+                    err
+                } else {
+                    unreachable!()
+                }
+            }
+            infer::Subtype(box trace) => {
+                let terr = TypeError::RegionsPlaceholderMismatch;
+                return self.report_and_explain_type_error(trace, &terr);
+            }
+            _ => return self.report_concrete_failure(placeholder_origin, sub, sup),
+        }
+    }
+}