Merging debian version 1.65.0+dfsg1-2.

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

34 files changed, 1835 insertions, 808 deletions

diff --git a/compiler/rustc_infer/src/errors/mod.rs b/compiler/rustc_infer/src/errors/mod.rs
new file mode 100644
index 000000000..d232a1864
--- /dev/null
+++ b/compiler/rustc_infer/src/errors/mod.rs
@@ -0,0 +1,499 @@
+use hir::GenericParamKind;
+use rustc_errors::{
+    fluent, AddSubdiagnostic, Applicability, DiagnosticMessage, DiagnosticStyledString, MultiSpan,
+};
+use rustc_hir as hir;
+use rustc_hir::{FnRetTy, Ty};
+use rustc_macros::SessionDiagnostic;
+use rustc_middle::ty::{Region, TyCtxt};
+use rustc_span::symbol::kw;
+use rustc_span::{symbol::Ident, BytePos, Span};
+
+use crate::infer::error_reporting::{
+    need_type_info::{GeneratorKindAsDiagArg, UnderspecifiedArgKind},
+    ObligationCauseAsDiagArg,
+};
+
+pub mod note_and_explain;
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::opaque_hidden_type)]
+pub struct OpaqueHiddenTypeDiag {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[note(infer::opaque_type)]
+    pub opaque_type: Span,
+    #[note(infer::hidden_type)]
+    pub hidden_type: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::type_annotations_needed, code = "E0282")]
+pub struct AnnotationRequired<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub source_kind: &'static str,
+    pub source_name: &'a str,
+    #[label]
+    pub failure_span: Option<Span>,
+    #[subdiagnostic]
+    pub bad_label: Option<InferenceBadError<'a>>,
+    #[subdiagnostic]
+    pub infer_subdiags: Vec<SourceKindSubdiag<'a>>,
+    #[subdiagnostic]
+    pub multi_suggestions: Vec<SourceKindMultiSuggestion<'a>>,
+}
+
+// Copy of `AnnotationRequired` for E0283
+#[derive(SessionDiagnostic)]
+#[diag(infer::type_annotations_needed, code = "E0283")]
+pub struct AmbigousImpl<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub source_kind: &'static str,
+    pub source_name: &'a str,
+    #[label]
+    pub failure_span: Option<Span>,
+    #[subdiagnostic]
+    pub bad_label: Option<InferenceBadError<'a>>,
+    #[subdiagnostic]
+    pub infer_subdiags: Vec<SourceKindSubdiag<'a>>,
+    #[subdiagnostic]
+    pub multi_suggestions: Vec<SourceKindMultiSuggestion<'a>>,
+}
+
+// Copy of `AnnotationRequired` for E0284
+#[derive(SessionDiagnostic)]
+#[diag(infer::type_annotations_needed, code = "E0284")]
+pub struct AmbigousReturn<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub source_kind: &'static str,
+    pub source_name: &'a str,
+    #[label]
+    pub failure_span: Option<Span>,
+    #[subdiagnostic]
+    pub bad_label: Option<InferenceBadError<'a>>,
+    #[subdiagnostic]
+    pub infer_subdiags: Vec<SourceKindSubdiag<'a>>,
+    #[subdiagnostic]
+    pub multi_suggestions: Vec<SourceKindMultiSuggestion<'a>>,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::need_type_info_in_generator, code = "E0698")]
+pub struct NeedTypeInfoInGenerator<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub generator_kind: GeneratorKindAsDiagArg,
+    #[subdiagnostic]
+    pub bad_label: InferenceBadError<'a>,
+}
+
+// Used when a better one isn't available
+#[derive(SessionSubdiagnostic)]
+#[label(infer::label_bad)]
+pub struct InferenceBadError<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub bad_kind: &'static str,
+    pub prefix_kind: UnderspecifiedArgKind,
+    pub has_parent: bool,
+    pub prefix: &'a str,
+    pub parent_prefix: &'a str,
+    pub parent_name: String,
+    pub name: String,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum SourceKindSubdiag<'a> {
+    #[suggestion_verbose(
+        infer::source_kind_subdiag_let,
+        code = ": {type_name}",
+        applicability = "has-placeholders"
+    )]
+    LetLike {
+        #[primary_span]
+        span: Span,
+        name: String,
+        type_name: String,
+        kind: &'static str,
+        x_kind: &'static str,
+        prefix_kind: UnderspecifiedArgKind,
+        prefix: &'a str,
+        arg_name: String,
+    },
+    #[label(infer::source_kind_subdiag_generic_label)]
+    GenericLabel {
+        #[primary_span]
+        span: Span,
+        is_type: bool,
+        param_name: String,
+        parent_exists: bool,
+        parent_prefix: String,
+        parent_name: String,
+    },
+    #[suggestion_verbose(
+        infer::source_kind_subdiag_generic_suggestion,
+        code = "::<{args}>",
+        applicability = "has-placeholders"
+    )]
+    GenericSuggestion {
+        #[primary_span]
+        span: Span,
+        arg_count: usize,
+        args: String,
+    },
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum SourceKindMultiSuggestion<'a> {
+    #[multipart_suggestion_verbose(
+        infer::source_kind_fully_qualified,
+        applicability = "has-placeholders"
+    )]
+    FullyQualified {
+        #[suggestion_part(code = "{def_path}({adjustment}")]
+        span_lo: Span,
+        #[suggestion_part(code = "{successor_pos}")]
+        span_hi: Span,
+        def_path: String,
+        adjustment: &'a str,
+        successor_pos: &'a str,
+    },
+    #[multipart_suggestion_verbose(
+        infer::source_kind_closure_return,
+        applicability = "has-placeholders"
+    )]
+    ClosureReturn {
+        #[suggestion_part(code = "{start_span_code}")]
+        start_span: Span,
+        start_span_code: String,
+        #[suggestion_part(code = " }}")]
+        end_span: Option<Span>,
+    },
+}
+
+impl<'a> SourceKindMultiSuggestion<'a> {
+    pub fn new_fully_qualified(
+        span: Span,
+        def_path: String,
+        adjustment: &'a str,
+        successor: (&'a str, BytePos),
+    ) -> Self {
+        Self::FullyQualified {
+            span_lo: span.shrink_to_lo(),
+            span_hi: span.shrink_to_hi().with_hi(successor.1),
+            def_path,
+            adjustment,
+            successor_pos: successor.0,
+        }
+    }
+
+    pub fn new_closure_return(
+        ty_info: String,
+        data: &'a FnRetTy<'a>,
+        should_wrap_expr: Option<Span>,
+    ) -> Self {
+        let (arrow, post) = match data {
+            FnRetTy::DefaultReturn(_) => ("-> ", " "),
+            _ => ("", ""),
+        };
+        let (start_span, start_span_code, end_span) = match should_wrap_expr {
+            Some(end_span) => {
+                (data.span(), format!("{}{}{}{{ ", arrow, ty_info, post), Some(end_span))
+            }
+            None => (data.span(), format!("{}{}{}", arrow, ty_info, post), None),
+        };
+        Self::ClosureReturn { start_span, start_span_code, end_span }
+    }
+}
+
+pub enum RegionOriginNote<'a> {
+    Plain {
+        span: Span,
+        msg: DiagnosticMessage,
+    },
+    WithName {
+        span: Span,
+        msg: DiagnosticMessage,
+        name: &'a str,
+        continues: bool,
+    },
+    WithRequirement {
+        span: Span,
+        requirement: ObligationCauseAsDiagArg<'a>,
+        expected_found: Option<(DiagnosticStyledString, DiagnosticStyledString)>,
+    },
+}
+
+impl AddSubdiagnostic for RegionOriginNote<'_> {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        let mut label_or_note = |span, msg: DiagnosticMessage| {
+            let sub_count = diag.children.iter().filter(|d| d.span.is_dummy()).count();
+            let expanded_sub_count = diag.children.iter().filter(|d| !d.span.is_dummy()).count();
+            let span_is_primary = diag.span.primary_spans().iter().all(|&sp| sp == span);
+            if span_is_primary && sub_count == 0 && expanded_sub_count == 0 {
+                diag.span_label(span, msg);
+            } else if span_is_primary && expanded_sub_count == 0 {
+                diag.note(msg);
+            } else {
+                diag.span_note(span, msg);
+            }
+        };
+        match self {
+            RegionOriginNote::Plain { span, msg } => {
+                label_or_note(span, msg);
+            }
+            RegionOriginNote::WithName { span, msg, name, continues } => {
+                label_or_note(span, msg);
+                diag.set_arg("name", name);
+                diag.set_arg("continues", continues);
+            }
+            RegionOriginNote::WithRequirement {
+                span,
+                requirement,
+                expected_found: Some((expected, found)),
+            } => {
+                label_or_note(span, fluent::infer::subtype);
+                diag.set_arg("requirement", requirement);
+
+                diag.note_expected_found(&"", expected, &"", found);
+            }
+            RegionOriginNote::WithRequirement { span, requirement, expected_found: None } => {
+                // 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(span, fluent::infer::subtype_2);
+                diag.set_arg("requirement", requirement);
+            }
+        };
+    }
+}
+
+pub enum LifetimeMismatchLabels {
+    InRet {
+        param_span: Span,
+        ret_span: Span,
+        span: Span,
+        label_var1: Option<Ident>,
+    },
+    Normal {
+        hir_equal: bool,
+        ty_sup: Span,
+        ty_sub: Span,
+        span: Span,
+        sup: Option<Ident>,
+        sub: Option<Ident>,
+    },
+}
+
+impl AddSubdiagnostic for LifetimeMismatchLabels {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        match self {
+            LifetimeMismatchLabels::InRet { param_span, ret_span, span, label_var1 } => {
+                diag.span_label(param_span, fluent::infer::declared_different);
+                diag.span_label(ret_span, fluent::infer::nothing);
+                diag.span_label(span, fluent::infer::data_returned);
+                diag.set_arg("label_var1_exists", label_var1.is_some());
+                diag.set_arg("label_var1", label_var1.map(|x| x.to_string()).unwrap_or_default());
+            }
+            LifetimeMismatchLabels::Normal {
+                hir_equal,
+                ty_sup,
+                ty_sub,
+                span,
+                sup: label_var1,
+                sub: label_var2,
+            } => {
+                if hir_equal {
+                    diag.span_label(ty_sup, fluent::infer::declared_multiple);
+                    diag.span_label(ty_sub, fluent::infer::nothing);
+                    diag.span_label(span, fluent::infer::data_lifetime_flow);
+                } else {
+                    diag.span_label(ty_sup, fluent::infer::types_declared_different);
+                    diag.span_label(ty_sub, fluent::infer::nothing);
+                    diag.span_label(span, fluent::infer::data_flows);
+                    diag.set_arg("label_var1_exists", label_var1.is_some());
+                    diag.set_arg(
+                        "label_var1",
+                        label_var1.map(|x| x.to_string()).unwrap_or_default(),
+                    );
+                    diag.set_arg("label_var2_exists", label_var2.is_some());
+                    diag.set_arg(
+                        "label_var2",
+                        label_var2.map(|x| x.to_string()).unwrap_or_default(),
+                    );
+                }
+            }
+        }
+    }
+}
+
+pub struct AddLifetimeParamsSuggestion<'a> {
+    pub tcx: TyCtxt<'a>,
+    pub sub: Region<'a>,
+    pub ty_sup: &'a Ty<'a>,
+    pub ty_sub: &'a Ty<'a>,
+    pub add_note: bool,
+}
+
+impl AddSubdiagnostic for AddLifetimeParamsSuggestion<'_> {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        let mut mk_suggestion = || {
+            let (
+                hir::Ty { kind: hir::TyKind::Rptr(lifetime_sub, _), .. },
+                hir::Ty { kind: hir::TyKind::Rptr(lifetime_sup, _), .. },
+            ) = (self.ty_sub, self.ty_sup) else {
+                return false;
+            };
+
+            if !lifetime_sub.name.is_anonymous() || !lifetime_sup.name.is_anonymous() {
+                return false;
+            };
+
+            let Some(anon_reg) = self.tcx.is_suitable_region(self.sub) else {
+                return false;
+            };
+
+            let hir_id = self.tcx.hir().local_def_id_to_hir_id(anon_reg.def_id);
+
+            let node = self.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("&") = self.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);
+            }
+
+            diag.multipart_suggestion(
+                fluent::infer::lifetime_param_suggestion,
+                suggestions,
+                Applicability::MaybeIncorrect,
+            );
+            diag.set_arg("is_impl", is_impl);
+            true
+        };
+        if mk_suggestion() && self.add_note {
+            diag.note(fluent::infer::lifetime_param_suggestion_elided);
+        }
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::lifetime_mismatch, code = "E0623")]
+pub struct LifetimeMismatch<'a> {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub labels: LifetimeMismatchLabels,
+    #[subdiagnostic]
+    pub suggestion: AddLifetimeParamsSuggestion<'a>,
+}
+
+pub struct IntroducesStaticBecauseUnmetLifetimeReq {
+    pub unmet_requirements: MultiSpan,
+    pub binding_span: Span,
+}
+
+impl AddSubdiagnostic for IntroducesStaticBecauseUnmetLifetimeReq {
+    fn add_to_diagnostic(mut self, diag: &mut rustc_errors::Diagnostic) {
+        self.unmet_requirements
+            .push_span_label(self.binding_span, fluent::infer::msl_introduces_static);
+        diag.span_note(self.unmet_requirements, fluent::infer::msl_unmet_req);
+    }
+}
+
+pub struct ImplNote {
+    pub impl_span: Option<Span>,
+}
+
+impl AddSubdiagnostic for ImplNote {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        match self.impl_span {
+            Some(span) => diag.span_note(span, fluent::infer::msl_impl_note),
+            None => diag.note(fluent::infer::msl_impl_note),
+        };
+    }
+}
+
+pub enum TraitSubdiag {
+    Note { span: Span },
+    Sugg { span: Span },
+}
+
+// FIXME(#100717) used in `Vec<TraitSubdiag>` so requires eager translation/list support
+impl AddSubdiagnostic for TraitSubdiag {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        match self {
+            TraitSubdiag::Note { span } => {
+                diag.span_note(span, "this has an implicit `'static` lifetime requirement");
+            }
+            TraitSubdiag::Sugg { span } => {
+                diag.span_suggestion_verbose(
+                    span,
+                    "consider relaxing the implicit `'static` requirement",
+                    " + '_".to_owned(),
+                    rustc_errors::Applicability::MaybeIncorrect,
+                );
+            }
+        }
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::mismatched_static_lifetime)]
+pub struct MismatchedStaticLifetime<'a> {
+    #[primary_span]
+    pub cause_span: Span,
+    #[subdiagnostic]
+    pub unmet_lifetime_reqs: IntroducesStaticBecauseUnmetLifetimeReq,
+    #[subdiagnostic]
+    pub expl: Option<note_and_explain::RegionExplanation<'a>>,
+    #[subdiagnostic]
+    pub impl_note: ImplNote,
+    #[subdiagnostic]
+    pub trait_subdiags: Vec<TraitSubdiag>,
+}
diff --git a/compiler/rustc_infer/src/errors/note_and_explain.rs b/compiler/rustc_infer/src/errors/note_and_explain.rs
new file mode 100644
index 000000000..7e051835b
--- /dev/null
+++ b/compiler/rustc_infer/src/errors/note_and_explain.rs
@@ -0,0 +1,172 @@
+use crate::infer::error_reporting::nice_region_error::find_anon_type;
+use rustc_errors::{self, fluent, AddSubdiagnostic, IntoDiagnosticArg};
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_span::{symbol::kw, Span};
+
+#[derive(Default)]
+struct DescriptionCtx<'a> {
+    span: Option<Span>,
+    kind: &'a str,
+    arg: String,
+    num_arg: u32,
+}
+
+impl<'a> DescriptionCtx<'a> {
+    fn new<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        region: ty::Region<'tcx>,
+        alt_span: Option<Span>,
+    ) -> Option<Self> {
+        let mut me = DescriptionCtx::default();
+        me.span = alt_span;
+        match *region {
+            ty::ReEarlyBound(_) | ty::ReFree(_) => {
+                return Self::from_early_bound_and_free_regions(tcx, region);
+            }
+            ty::ReStatic => {
+                me.kind = "restatic";
+            }
+
+            ty::RePlaceholder(_) => return None,
+
+            // 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 => {
+                me.kind = "revar";
+                me.arg = format!("{:?}", region);
+            }
+        };
+        Some(me)
+    }
+
+    fn from_early_bound_and_free_regions<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        region: ty::Region<'tcx>,
+    ) -> Option<Self> {
+        let mut me = DescriptionCtx::default();
+        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;
+                }
+                if br.has_name() {
+                    me.kind = "as_defined";
+                    me.arg = br.name.to_string();
+                } else {
+                    me.kind = "as_defined_anon";
+                };
+                me.span = Some(sp)
+            }
+            ty::ReFree(ref fr) => {
+                if !fr.bound_region.is_named()
+                    && let Some((ty, _)) = find_anon_type(tcx, region, &fr.bound_region)
+                {
+                    me.kind = "defined_here";
+                    me.span = Some(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;
+                            }
+                            if name == kw::UnderscoreLifetime {
+                                me.kind = "as_defined_anon";
+                            } else {
+                                me.kind = "as_defined";
+                                me.arg = name.to_string();
+                            };
+                            me.span = Some(sp);
+                        }
+                        ty::BrAnon(idx) => {
+                            me.kind = "anon_num_here";
+                            me.num_arg = idx+1;
+                            me.span = Some(tcx.def_span(scope));
+                        },
+                        _ => {
+                            me.kind = "defined_here_reg";
+                            me.arg = region.to_string();
+                            me.span = Some(tcx.def_span(scope));
+                        },
+                    }
+                }
+            }
+            _ => bug!(),
+        }
+        Some(me)
+    }
+
+    fn add_to(self, diag: &mut rustc_errors::Diagnostic) {
+        diag.set_arg("desc_kind", self.kind);
+        diag.set_arg("desc_arg", self.arg);
+        diag.set_arg("desc_num_arg", self.num_arg);
+    }
+}
+
+pub enum PrefixKind {
+    Empty,
+}
+
+pub enum SuffixKind {
+    Continues,
+}
+
+impl IntoDiagnosticArg for PrefixKind {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        let kind = match self {
+            Self::Empty => "empty",
+        }
+        .into();
+        rustc_errors::DiagnosticArgValue::Str(kind)
+    }
+}
+
+impl IntoDiagnosticArg for SuffixKind {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        let kind = match self {
+            Self::Continues => "continues",
+        }
+        .into();
+        rustc_errors::DiagnosticArgValue::Str(kind)
+    }
+}
+
+pub struct RegionExplanation<'a> {
+    desc: DescriptionCtx<'a>,
+    prefix: PrefixKind,
+    suffix: SuffixKind,
+}
+
+impl RegionExplanation<'_> {
+    pub fn new<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        region: ty::Region<'tcx>,
+        alt_span: Option<Span>,
+        prefix: PrefixKind,
+        suffix: SuffixKind,
+    ) -> Option<Self> {
+        Some(Self { desc: DescriptionCtx::new(tcx, region, alt_span)?, prefix, suffix })
+    }
+}
+
+impl AddSubdiagnostic for RegionExplanation<'_> {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        if let Some(span) = self.desc.span {
+            diag.span_note(span, fluent::infer::region_explanation);
+        } else {
+            diag.note(fluent::infer::region_explanation);
+        }
+        self.desc.add_to(diag);
+        diag.set_arg("pref_kind", self.prefix);
+        diag.set_arg("suff_kind", self.suffix);
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/at.rs b/compiler/rustc_infer/src/infer/at.rs
index 130214a65..00e238648 100644
--- a/compiler/rustc_infer/src/infer/at.rs
+++ b/compiler/rustc_infer/src/infer/at.rs
@@ -74,10 +74,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             evaluation_cache: self.evaluation_cache.clone(),
             reported_trait_errors: self.reported_trait_errors.clone(),
             reported_closure_mismatch: self.reported_closure_mismatch.clone(),
-            tainted_by_errors_flag: self.tainted_by_errors_flag.clone(),
+            tainted_by_errors: self.tainted_by_errors.clone(),
             err_count_on_creation: self.err_count_on_creation,
             in_snapshot: self.in_snapshot.clone(),
             universe: self.universe.clone(),
+            normalize_fn_sig_for_diagnostic: self
+                .normalize_fn_sig_for_diagnostic
+                .as_ref()
+                .map(|f| f.clone()),
         }
     }
 }
diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index ca7862c9d..9488d0a6c 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -180,11 +180,7 @@ impl CanonicalizeMode for CanonicalizeQueryResponse {
         r: ty::Region<'tcx>,
     ) -> ty::Region<'tcx> {
         match *r {
-            ty::ReFree(_)
-            | ty::ReErased
-            | ty::ReStatic
-            | ty::ReEmpty(ty::UniverseIndex::ROOT)
-            | ty::ReEarlyBound(..) => r,
+            ty::ReFree(_) | ty::ReErased | ty::ReStatic | ty::ReEarlyBound(..) => r,
 
             ty::RePlaceholder(placeholder) => canonicalizer.canonical_var_for_region(
                 CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderRegion(placeholder) },
@@ -199,10 +195,6 @@ impl CanonicalizeMode for CanonicalizeQueryResponse {
                 )
             }
 
-            ty::ReEmpty(ui) => {
-                bug!("canonicalizing 'empty in universe {:?}", ui) // FIXME
-            }
-
             _ => {
                 // Other than `'static` or `'empty`, the query
                 // response should be executing in a fully
@@ -372,7 +364,7 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
                 debug!(
                     "canonical: region var found with vid {:?}, \
                      opportunistically resolved to {:?}",
-                    vid, r
+                    vid, resolved_vid
                 );
                 let r = self.tcx.reuse_or_mk_region(r, ty::ReVar(resolved_vid));
                 self.canonicalize_mode.canonicalize_free_region(self, r)
@@ -381,7 +373,6 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
             ty::ReStatic
             | ty::ReEarlyBound(..)
             | ty::ReFree(_)
-            | ty::ReEmpty(_)
             | ty::RePlaceholder(..)
             | ty::ReErased => self.canonicalize_mode.canonicalize_free_region(self, r),
         }
diff --git a/compiler/rustc_infer/src/infer/canonical/query_response.rs b/compiler/rustc_infer/src/infer/canonical/query_response.rs
index 8dc20544f..56e834898 100644
--- a/compiler/rustc_infer/src/infer/canonical/query_response.rs
+++ b/compiler/rustc_infer/src/infer/canonical/query_response.rs
@@ -22,6 +22,7 @@ use rustc_data_structures::captures::Captures;
 use rustc_index::vec::Idx;
 use rustc_index::vec::IndexVec;
 use rustc_middle::arena::ArenaAllocatable;
+use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::relate::TypeRelation;
@@ -63,8 +64,8 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         Canonical<'tcx, QueryResponse<'tcx, T>>: ArenaAllocatable<'tcx>,
     {
         let query_response = self.make_query_response(inference_vars, answer, fulfill_cx)?;
+        debug!("query_response = {:#?}", query_response);
         let canonical_result = self.canonicalize_response(query_response);
-
         debug!("canonical_result = {:#?}", canonical_result);
 
         Ok(self.tcx.arena.alloc(canonical_result))
@@ -125,13 +126,17 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         debug!("ambig_errors = {:#?}", ambig_errors);
 
         let region_obligations = self.take_registered_region_obligations();
+        debug!(?region_obligations);
         let region_constraints = self.with_region_constraints(|region_constraints| {
             make_query_region_constraints(
                 tcx,
-                region_obligations.iter().map(|r_o| (r_o.sup_type, r_o.sub_region)),
+                region_obligations
+                    .iter()
+                    .map(|r_o| (r_o.sup_type, r_o.sub_region, r_o.origin.to_constraint_category())),
                 region_constraints,
             )
         });
+        debug!(?region_constraints);
 
         let certainty =
             if ambig_errors.is_empty() { Certainty::Proven } else { Certainty::Ambiguous };
@@ -246,6 +251,8 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         // the original values `v_o` that was canonicalized into a
         // variable...
 
+        let constraint_category = cause.to_constraint_category();
+
         for (index, original_value) in original_values.var_values.iter().enumerate() {
             // ...with the value `v_r` of that variable from the query.
             let result_value = query_response.substitute_projected(self.tcx, &result_subst, |v| {
@@ -261,12 +268,14 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
                 (GenericArgKind::Lifetime(v_o), GenericArgKind::Lifetime(v_r)) => {
                     // To make `v_o = v_r`, we emit `v_o: v_r` and `v_r: v_o`.
                     if v_o != v_r {
-                        output_query_region_constraints
-                            .outlives
-                            .push(ty::Binder::dummy(ty::OutlivesPredicate(v_o.into(), v_r)));
-                        output_query_region_constraints
-                            .outlives
-                            .push(ty::Binder::dummy(ty::OutlivesPredicate(v_r.into(), v_o)));
+                        output_query_region_constraints.outlives.push((
+                            ty::Binder::dummy(ty::OutlivesPredicate(v_o.into(), v_r)),
+                            constraint_category,
+                        ));
+                        output_query_region_constraints.outlives.push((
+                            ty::Binder::dummy(ty::OutlivesPredicate(v_r.into(), v_o)),
+                            constraint_category,
+                        ));
                     }
                 }
 
@@ -312,7 +321,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
                 // Screen out `'a: 'a` cases -- we skip the binder here but
                 // only compare the inner values to one another, so they are still at
                 // consistent binding levels.
-                let ty::OutlivesPredicate(k1, r2) = r_c.skip_binder();
+                let ty::OutlivesPredicate(k1, r2) = r_c.0.skip_binder();
                 if k1 != r2.into() { Some(r_c) } else { None }
             }),
         );
@@ -557,7 +566,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         cause: ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Obligation<'tcx, ty::Predicate<'tcx>> {
-        let ty::OutlivesPredicate(k1, r2) = predicate.skip_binder();
+        let ty::OutlivesPredicate(k1, r2) = predicate.0.skip_binder();
 
         let atom = match k1.unpack() {
             GenericArgKind::Lifetime(r1) => {
@@ -572,7 +581,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
                 span_bug!(cause.span, "unexpected const outlives {:?}", predicate);
             }
         };
-        let predicate = predicate.rebind(atom).to_predicate(self.tcx);
+        let predicate = predicate.0.rebind(atom).to_predicate(self.tcx);
 
         Obligation::new(cause, param_env, predicate)
     }
@@ -623,7 +632,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
 /// creates query region constraints.
 pub fn make_query_region_constraints<'tcx>(
     tcx: TyCtxt<'tcx>,
-    outlives_obligations: impl Iterator<Item = (Ty<'tcx>, ty::Region<'tcx>)>,
+    outlives_obligations: impl Iterator<Item = (Ty<'tcx>, ty::Region<'tcx>, ConstraintCategory<'tcx>)>,
     region_constraints: &RegionConstraintData<'tcx>,
 ) -> QueryRegionConstraints<'tcx> {
     let RegionConstraintData { constraints, verifys, givens, member_constraints } =
@@ -632,28 +641,35 @@ pub fn make_query_region_constraints<'tcx>(
     assert!(verifys.is_empty());
     assert!(givens.is_empty());
 
+    debug!(?constraints);
+
     let outlives: Vec<_> = constraints
         .iter()
-        .map(|(k, _)| match *k {
-            // Swap regions because we are going from sub (<=) to outlives
-            // (>=).
-            Constraint::VarSubVar(v1, v2) => ty::OutlivesPredicate(
-                tcx.mk_region(ty::ReVar(v2)).into(),
-                tcx.mk_region(ty::ReVar(v1)),
-            ),
-            Constraint::VarSubReg(v1, r2) => {
-                ty::OutlivesPredicate(r2.into(), tcx.mk_region(ty::ReVar(v1)))
-            }
-            Constraint::RegSubVar(r1, v2) => {
-                ty::OutlivesPredicate(tcx.mk_region(ty::ReVar(v2)).into(), r1)
-            }
-            Constraint::RegSubReg(r1, r2) => ty::OutlivesPredicate(r2.into(), r1),
+        .map(|(k, origin)| {
+            // no bound vars in the code above
+            let constraint = ty::Binder::dummy(match *k {
+                // Swap regions because we are going from sub (<=) to outlives
+                // (>=).
+                Constraint::VarSubVar(v1, v2) => ty::OutlivesPredicate(
+                    tcx.mk_region(ty::ReVar(v2)).into(),
+                    tcx.mk_region(ty::ReVar(v1)),
+                ),
+                Constraint::VarSubReg(v1, r2) => {
+                    ty::OutlivesPredicate(r2.into(), tcx.mk_region(ty::ReVar(v1)))
+                }
+                Constraint::RegSubVar(r1, v2) => {
+                    ty::OutlivesPredicate(tcx.mk_region(ty::ReVar(v2)).into(), r1)
+                }
+                Constraint::RegSubReg(r1, r2) => ty::OutlivesPredicate(r2.into(), r1),
+            });
+            (constraint, origin.to_constraint_category())
         })
-        .map(ty::Binder::dummy) // no bound vars in the code above
         .chain(
             outlives_obligations
-                .map(|(ty, r)| ty::OutlivesPredicate(ty.into(), r))
-                .map(ty::Binder::dummy), // no bound vars in the code above
+                // no bound vars in the code above
+                .map(|(ty, r, constraint_category)| {
+                    (ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), r)), constraint_category)
+                }),
         )
         .collect();
 
diff --git a/compiler/rustc_infer/src/infer/canonical/substitute.rs b/compiler/rustc_infer/src/infer/canonical/substitute.rs
index 34b611342..389afe22e 100644
--- a/compiler/rustc_infer/src/infer/canonical/substitute.rs
+++ b/compiler/rustc_infer/src/infer/canonical/substitute.rs
@@ -72,15 +72,16 @@ where
         value
     } else {
         let delegate = FnMutDelegate {
-            regions: |br: ty::BoundRegion| match var_values.var_values[br.var].unpack() {
+            regions: &mut |br: ty::BoundRegion| match var_values.var_values[br.var].unpack() {
                 GenericArgKind::Lifetime(l) => l,
                 r => bug!("{:?} is a region but value is {:?}", br, r),
             },
-            types: |bound_ty: ty::BoundTy| match var_values.var_values[bound_ty.var].unpack() {
+            types: &mut |bound_ty: ty::BoundTy| match var_values.var_values[bound_ty.var].unpack() {
                 GenericArgKind::Type(ty) => ty,
                 r => bug!("{:?} is a type but value is {:?}", bound_ty, r),
             },
-            consts: |bound_ct: ty::BoundVar, _| match var_values.var_values[bound_ct].unpack() {
+            consts: &mut |bound_ct: ty::BoundVar, _| match var_values.var_values[bound_ct].unpack()
+            {
                 GenericArgKind::Const(ct) => ct,
                 c => bug!("{:?} is a const but value is {:?}", bound_ct, c),
             },
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index 8bf1de34a..c406df9e4 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -391,7 +391,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     /// Preconditions:
     ///
     /// - `for_vid` is a "root vid"
-    #[instrument(skip(self), level = "trace")]
+    #[instrument(skip(self), level = "trace", ret)]
     fn generalize(
         &self,
         ty: Ty<'tcx>,
@@ -435,15 +435,8 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             cache: SsoHashMap::new(),
         };
 
-        let ty = match generalize.relate(ty, ty) {
-            Ok(ty) => ty,
-            Err(e) => {
-                debug!(?e, "failure");
-                return Err(e);
-            }
-        };
+        let ty = generalize.relate(ty, ty)?;
         let needs_wf = generalize.needs_wf;
-        trace!(?ty, ?needs_wf, "success");
         Ok(Generalization { ty, needs_wf })
     }
 
@@ -493,12 +486,13 @@ struct Generalizer<'cx, 'tcx> {
 
     param_env: ty::ParamEnv<'tcx>,
 
-    cache: SsoHashMap<Ty<'tcx>, RelateResult<'tcx, Ty<'tcx>>>,
+    cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
 }
 
 /// Result from a generalization operation. This includes
 /// not only the generalized type, but also a bool flag
 /// indicating whether further WF checks are needed.
+#[derive(Debug)]
 struct Generalization<'tcx> {
     ty: Ty<'tcx>,
 
@@ -599,8 +593,8 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 
-        if let Some(result) = self.cache.get(&t) {
-            return result.clone();
+        if let Some(&result) = self.cache.get(&t) {
+            return Ok(result);
         }
         debug!("generalize: t={:?}", t);
 
@@ -670,10 +664,10 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                 Ok(t)
             }
             _ => relate::super_relate_tys(self, t, t),
-        };
+        }?;
 
-        self.cache.insert(t, result.clone());
-        return result;
+        self.cache.insert(t, result);
+        Ok(result)
     }
 
     fn regions(
@@ -694,7 +688,6 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 
             ty::RePlaceholder(..)
             | ty::ReVar(..)
-            | ty::ReEmpty(_)
             | ty::ReStatic
             | ty::ReEarlyBound(..)
             | ty::ReFree(..) => {
@@ -749,10 +742,9 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                     }
                 }
             }
-            ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
-                if self.tcx().lazy_normalization() =>
-            {
-                assert_eq!(promoted, None);
+            ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }) => {
+                assert_eq!(promoted, ());
+
                 let substs = self.relate_with_variance(
                     ty::Variance::Invariant,
                     ty::VarianceDiagInfo::default(),
@@ -856,10 +848,9 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self), ret)]
     fn tys(&mut self, t: Ty<'tcx>, _t: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
         debug_assert_eq!(t, _t);
-        debug!("ConstInferUnifier: t={:?}", t);
 
         match t.kind() {
             &ty::Infer(ty::TyVar(vid)) => {
@@ -883,12 +874,7 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
                             .borrow_mut()
                             .type_variables()
                             .new_var(self.for_universe, origin);
-                        let u = self.tcx().mk_ty_var(new_var_id);
-                        debug!(
-                            "ConstInferUnifier: replacing original vid={:?} with new={:?}",
-                            vid, u
-                        );
-                        Ok(u)
+                        Ok(self.tcx().mk_ty_var(new_var_id))
                     }
                 }
             }
@@ -914,7 +900,6 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
 
             ty::RePlaceholder(..)
             | ty::ReVar(..)
-            | ty::ReEmpty(_)
             | ty::ReStatic
             | ty::ReEarlyBound(..)
             | ty::ReFree(..) => {
@@ -932,14 +917,13 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
         }
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn consts(
         &mut self,
         c: ty::Const<'tcx>,
         _c: ty::Const<'tcx>,
     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
         debug_assert_eq!(c, _c);
-        debug!("ConstInferUnifier: c={:?}", c);
 
         match c.kind() {
             ty::ConstKind::Infer(InferConst::Var(vid)) => {
@@ -980,16 +964,16 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
                     }
                 }
             }
-            ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
-                if self.tcx().lazy_normalization() =>
-            {
-                assert_eq!(promoted, None);
+            ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }) => {
+                assert_eq!(promoted, ());
+
                 let substs = self.relate_with_variance(
                     ty::Variance::Invariant,
                     ty::VarianceDiagInfo::default(),
                     substs,
                     substs,
                 )?;
+
                 Ok(self.tcx().mk_const(ty::ConstS {
                     ty: c.ty(),
                     kind: ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }),
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 20864c657..eb5afe828 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -58,14 +58,16 @@ use crate::traits::{
 };
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_errors::{pluralize, struct_span_err, Diagnostic, ErrorGuaranteed};
+use rustc_errors::{pluralize, struct_span_err, Diagnostic, ErrorGuaranteed, IntoDiagnosticArg};
 use rustc_errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString, MultiSpan};
 use rustc_hir as hir;
+use rustc_hir::def::DefKind;
 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::relate::{self, RelateResult, TypeRelation};
 use rustc_middle::ty::{
     self, error::TypeError, Binder, List, Region, Subst, Ty, TyCtxt, TypeFoldable,
     TypeSuperVisitable, TypeVisitable,
@@ -77,7 +79,7 @@ use std::{cmp, fmt, iter};
 
 mod note;
 
-mod need_type_info;
+pub(crate) mod need_type_info;
 pub use need_type_info::TypeAnnotationNeeded;
 
 pub mod nice_region_error;
@@ -95,11 +97,6 @@ pub(super) fn note_and_explain_region<'tcx>(
             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
@@ -138,8 +135,6 @@ fn msg_span_from_free_region<'tcx>(
             (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),
     }
 }
@@ -249,17 +244,7 @@ pub fn unexpected_hidden_region_diagnostic<'tcx>(
 
     // 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(_) => {
+        ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic => {
             // 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
@@ -385,7 +370,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     RegionResolutionError::UpperBoundUniverseConflict(
                         _,
                         _,
-                        var_universe,
+                        _,
                         sup_origin,
                         sup_r,
                     ) => {
@@ -396,7 +381,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                         // 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));
+                        let sub_r = self.tcx.lifetimes.re_erased;
 
                         self.report_placeholder_failure(sup_origin, sub_r, sup_r).emit();
                     }
@@ -457,7 +442,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 
     /// Adds a note if the types come from similarly named crates
-    fn check_and_note_conflicting_crates(&self, err: &mut Diagnostic, terr: &TypeError<'tcx>) {
+    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;
@@ -561,7 +546,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 }
             }
         };
-        match *terr {
+        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
@@ -583,7 +568,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         err: &mut Diagnostic,
         cause: &ObligationCause<'tcx>,
         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
-        terr: &TypeError<'tcx>,
+        terr: TypeError<'tcx>,
     ) {
         match *cause.code() {
             ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
@@ -739,12 +724,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 err.help("...or use `match` instead of `let...else`");
             }
             _ => {
-                if let ObligationCauseCode::BindingObligation(_, binding_span) =
-                    cause.code().peel_derives()
+                if let ObligationCauseCode::BindingObligation(_, span)
+                | ObligationCauseCode::ExprBindingObligation(_, span, ..)
+                    = cause.code().peel_derives()
+                    && let TypeError::RegionsPlaceholderMismatch = terr
                 {
-                    if matches!(terr, TypeError::RegionsPlaceholderMismatch) {
-                        err.span_note(*binding_span, "the lifetime requirement is introduced here");
-                    }
+                    err.span_note(*span, "the lifetime requirement is introduced here");
                 }
             }
         }
@@ -960,12 +945,23 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
     }
 
+    fn normalize_fn_sig_for_diagnostic(&self, sig: ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx> {
+        if let Some(normalize) = &self.normalize_fn_sig_for_diagnostic {
+            normalize(self, sig)
+        } else {
+            sig
+        }
+    }
+
     /// 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 sig1 = &self.normalize_fn_sig_for_diagnostic(*sig1);
+        let sig2 = &self.normalize_fn_sig_for_diagnostic(*sig2);
+
         let get_lifetimes = |sig| {
             use rustc_hir::def::Namespace;
             let (_, sig, reg) = ty::print::FmtPrinter::new(self.tcx, Namespace::TypeNS)
@@ -1422,9 +1418,9 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// 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(
+    #[instrument(
         level = "debug",
-        skip(self, diag, secondary_span, swap_secondary_and_primary, force_label)
+        skip(self, diag, secondary_span, swap_secondary_and_primary, prefer_label)
     )]
     pub fn note_type_err(
         &self,
@@ -1432,9 +1428,9 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         cause: &ObligationCause<'tcx>,
         secondary_span: Option<(Span, String)>,
         mut values: Option<ValuePairs<'tcx>>,
-        terr: &TypeError<'tcx>,
+        terr: TypeError<'tcx>,
         swap_secondary_and_primary: bool,
-        force_label: bool,
+        prefer_label: bool,
     ) {
         let span = cause.span();
 
@@ -1574,23 +1570,31 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             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::Terms(infer::ExpectedFound { expected, found }) => {
+                        match (expected.unpack(), found.unpack()) {
+                            (ty::TermKind::Ty(expected), ty::TermKind::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 }),
+                                )
+                            }
+                            (ty::TermKind::Const(_), ty::TermKind::Const(_)) => {
+                                (false, Mismatch::Fixed("constant"))
+                            }
+                            _ => (false, Mismatch::Fixed("type")),
+                        }
                     }
                     ValuePairs::TraitRefs(_) | ValuePairs::PolyTraitRefs(_) => {
                         (false, Mismatch::Fixed("trait"))
                     }
-                    _ => (false, Mismatch::Fixed("type")),
+                    ValuePairs::Regions(_) => (false, Mismatch::Fixed("lifetime")),
                 };
                 let vals = match self.values_str(values) {
                     Some((expected, found)) => Some((expected, found)),
@@ -1612,7 +1616,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             TypeError::ObjectUnsafeCoercion(_) => {}
             _ => {
                 let mut label_or_note = |span: Span, msg: &str| {
-                    if force_label || &[span] == diag.span.primary_spans() {
+                    if (prefer_label && is_simple_error) || &[span] == diag.span.primary_spans() {
                         diag.span_label(span, msg);
                     } else {
                         diag.span_note(span, msg);
@@ -1662,6 +1666,19 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 pos.col.to_usize() + 1,
                             )
                         }
+                        (true, ty::Projection(proj))
+                            if self.tcx.def_kind(proj.item_def_id)
+                                == DefKind::ImplTraitPlaceholder =>
+                        {
+                            let sm = self.tcx.sess.source_map();
+                            let pos = sm.lookup_char_pos(self.tcx.def_span(proj.item_def_id).lo());
+                            format!(
+                                " (trait associated 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(),
                     };
@@ -1713,7 +1730,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             ty::error::TypeError::Sorts(terr)
                 if exp_found.map_or(false, |ef| terr.found == ef.found) =>
             {
-                Some(*terr)
+                Some(terr)
             }
             _ => exp_found,
         };
@@ -1750,6 +1767,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         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()
+            && terr.involves_regions()
         {
             let span = self.tcx.def_span(def_id);
             diag.span_note(span, "this closure does not fulfill the lifetime requirements");
@@ -2037,22 +2055,22 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             (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),
+                        (
+                            sym::Option,
+                            "you can convert from `&Option<T>` to `Option<&T>` using \
+                        `.as_ref()`",
+                        ),
+                        (
+                            sym::Result,
+                            "you can convert from `&Result<T, E>` to \
+                        `Result<&T, &E>` using `.as_ref()`",
+                        ),
                     ];
-                    if let Some(msg) = have_as_ref
-                        .iter()
-                        .find_map(|(path, msg)| (&path_str == path).then_some(msg))
-                    {
+                    if let Some(msg) = have_as_ref.iter().find_map(|(name, msg)| {
+                        self.tcx.is_diagnostic_item(*name, exp_def.did()).then_some(msg)
+                    }) {
                         let mut show_suggestion = true;
                         for (exp_ty, found_ty) in
                             iter::zip(exp_substs.types(), found_substs.types())
@@ -2078,7 +2096,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             diag.span_suggestion(
                                 span,
                                 *msg,
-                                format!("{}.as_ref()", snippet),
+                                // HACK: fix issue# 100605, suggesting convert from &Option<T> to Option<&T>, remove the extra `&`
+                                format!("{}.as_ref()", snippet.trim_start_matches('&')),
                                 Applicability::MachineApplicable,
                             );
                         }
@@ -2091,7 +2110,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn report_and_explain_type_error(
         &self,
         trace: TypeTrace<'tcx>,
-        terr: &TypeError<'tcx>,
+        terr: TypeError<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         use crate::traits::ObligationCauseCode::MatchExpressionArm;
 
@@ -2254,11 +2273,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             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()),
+        Some(match (exp_found.expected.unpack(), exp_found.found.unpack()) {
+            (ty::TermKind::Ty(expected), ty::TermKind::Ty(found)) => self.cmp(expected, found),
+            _ => (
+                DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
+                DiagnosticStyledString::highlighted(exp_found.found.to_string()),
             ),
         })
     }
@@ -2376,19 +2395,23 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             type_param_span: Option<(Span, bool)>,
             bound_kind: GenericKind<'tcx>,
             sub: S,
+            add_lt_sugg: Option<(Span, String)>,
         ) {
             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,
+                let mut suggestions = vec![(sp, suggestion)];
+                if let Some(add_lt_sugg) = add_lt_sugg {
+                    suggestions.push(add_lt_sugg);
+                }
+                err.multipart_suggestion_verbose(
+                    format!("{msg}..."),
+                    suggestions,
                     Applicability::MaybeIncorrect, // Issue #41966
                 );
             } else {
-                let consider = format!("{} `{}: {}`...", msg, bound_kind, sub,);
+                let consider = format!("{} `{}: {}`...", msg, bound_kind, sub);
                 err.help(&consider);
             }
         }
@@ -2404,7 +2427,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     };
                     let mut sugg =
                         vec![(sp, suggestion), (span.shrink_to_hi(), format!(" + {}", new_lt))];
-                    if let Some(lt) = add_lt_sugg {
+                    if let Some(lt) = add_lt_sugg.clone() {
                         sugg.push(lt);
                         sugg.rotate_right(1);
                     }
@@ -2510,7 +2533,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // 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);
+                binding_suggestion(&mut err, type_param_span, bound_kind, name, None);
                 err
             }
 
@@ -2523,7 +2546,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     "{} may not live long enough",
                     labeled_user_string
                 );
-                binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
+                binding_suggestion(&mut err, type_param_span, bound_kind, "'static", None);
                 err
             }
 
@@ -2557,7 +2580,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             new_binding_suggestion(&mut err, type_param_span);
                         }
                         _ => {
-                            binding_suggestion(&mut err, type_param_span, bound_kind, new_lt);
+                            binding_suggestion(
+                                &mut err,
+                                type_param_span,
+                                bound_kind,
+                                new_lt,
+                                add_lt_sugg,
+                            );
                         }
                     }
                 }
@@ -2659,67 +2688,95 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// 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;
-                }
+        SameTypeModuloInfer(self).relate(a, b).is_ok()
+    }
+}
 
-                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;
-                }
+struct SameTypeModuloInfer<'a, 'tcx>(&'a InferCtxt<'a, 'tcx>);
 
-                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(_)))
+impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.0.tcx
+    }
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        // Unused, only for consts which we treat as always equal
+        ty::ParamEnv::empty()
+    }
+
+    fn tag(&self) -> &'static str {
+        "SameTypeModuloInfer"
+    }
+
+    fn a_is_expected(&self) -> bool {
+        true
+    }
+
+    fn relate_with_variance<T: relate::Relate<'tcx>>(
+        &mut self,
+        _variance: ty::Variance,
+        _info: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> relate::RelateResult<'tcx, T> {
+        self.relate(a, b)
+    }
+
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        match (a.kind(), b.kind()) {
+            (ty::Int(_) | ty::Uint(_), ty::Infer(ty::InferTy::IntVar(_)))
             | (
-                &ty::Infer(ty::InferTy::IntVar(_)),
-                &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::Float(_), ty::Infer(ty::InferTy::FloatVar(_)))
             | (
-                &ty::Infer(ty::InferTy::FloatVar(_)),
-                &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,
+            | (ty::Infer(ty::InferTy::TyVar(_)), _)
+            | (_, ty::Infer(ty::InferTy::TyVar(_))) => Ok(a),
+            (ty::Infer(_), _) | (_, ty::Infer(_)) => Err(TypeError::Mismatch),
+            _ => relate::super_relate_tys(self, a, b),
+        }
+    }
+
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        if (a.is_var() && b.is_free_or_static())
+            || (b.is_var() && a.is_free_or_static())
+            || (a.is_var() && b.is_var())
+            || a == b
+        {
+            Ok(a)
+        } else {
+            Err(TypeError::Mismatch)
         }
     }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> relate::RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: relate::Relate<'tcx>,
+    {
+        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        _b: ty::Const<'tcx>,
+    ) -> relate::RelateResult<'tcx, ty::Const<'tcx>> {
+        // FIXME(compiler-errors): This could at least do some first-order
+        // relation
+        Ok(a)
+    }
 }
 
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
@@ -2781,12 +2838,12 @@ pub enum FailureCode {
 }
 
 pub trait ObligationCauseExt<'tcx> {
-    fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode;
+    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 {
+    fn as_failure_code(&self, terr: TypeError<'tcx>) -> FailureCode {
         use self::FailureCode::*;
         use crate::traits::ObligationCauseCode::*;
         match self.code() {
@@ -2823,7 +2880,7 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
                 TypeError::IntrinsicCast => {
                     Error0308("cannot coerce intrinsics to function pointers")
                 }
-                TypeError::ObjectUnsafeCoercion(did) => Error0038(*did),
+                TypeError::ObjectUnsafeCoercion(did) => Error0038(did),
                 _ => Error0308("mismatched types"),
             },
         }
@@ -2853,6 +2910,30 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
     }
 }
 
+/// Newtype to allow implementing IntoDiagnosticArg
+pub struct ObligationCauseAsDiagArg<'tcx>(pub ObligationCause<'tcx>);
+
+impl IntoDiagnosticArg for ObligationCauseAsDiagArg<'_> {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        use crate::traits::ObligationCauseCode::*;
+        let kind = match self.0.code() {
+            CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => "method_compat",
+            CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => "type_compat",
+            CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => "const_compat",
+            ExprAssignable => "expr_assignable",
+            IfExpression { .. } => "if_else_different",
+            IfExpressionWithNoElse => "no_else",
+            MainFunctionType => "fn_main_correct_type",
+            StartFunctionType => "fn_start_correct_type",
+            IntrinsicType => "intristic_correct_type",
+            MethodReceiver => "method_correct_type",
+            _ => "other",
+        }
+        .into();
+        rustc_errors::DiagnosticArgValue::Str(kind)
+    }
+}
+
 /// 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)]
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
index 561d1354e..cb2be9358 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
@@ -1,6 +1,10 @@
+use crate::errors::{
+    AmbigousImpl, AmbigousReturn, AnnotationRequired, InferenceBadError, NeedTypeInfoInGenerator,
+    SourceKindMultiSuggestion, SourceKindSubdiag,
+};
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::InferCtxt;
-use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed, IntoDiagnosticArg};
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::def::{CtorOf, DefKind, Namespace};
@@ -14,6 +18,7 @@ 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_session::SessionDiagnostic;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::{BytePos, Span};
 use std::borrow::Cow;
@@ -60,38 +65,49 @@ pub struct InferenceDiagnosticsParentData {
     name: String,
 }
 
+#[derive(Clone)]
 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 can_add_more_info(&self) -> bool {
+        !(self.name == "_" && matches!(self.kind, UnderspecifiedArgKind::Type { .. }))
     }
 
-    fn where_x_is_specified(&self, in_type: Ty<'_>) -> String {
+    fn where_x_is_kind(&self, in_type: Ty<'_>) -> &'static str {
         if in_type.is_ty_infer() {
-            String::new()
+            "empty"
         } else if self.name == "_" {
             // FIXME: Consider specializing this message if there is a single `_`
             // in the type.
-            ", where the placeholders `_` are specified".to_string()
+            "underscore"
         } else {
-            format!(", where the {} `{}` is specified", self.kind.prefix_string(), self.name)
+            "has_name"
+        }
+    }
+
+    /// 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 make_bad_error(&self, span: Span) -> InferenceBadError<'_> {
+        let has_parent = self.parent.is_some();
+        let bad_kind = if self.can_add_more_info() { "more_info" } else { "other" };
+        let (parent_prefix, parent_name) = self
+            .parent
+            .as_ref()
+            .map(|parent| (parent.prefix, parent.name.clone()))
+            .unwrap_or_default();
+        InferenceBadError {
+            span,
+            bad_kind,
+            prefix_kind: self.kind.clone(),
+            prefix: self.kind.try_get_prefix().unwrap_or_default(),
+            name: self.name.clone(),
+            has_parent,
+            parent_prefix,
+            parent_name,
         }
     }
 }
@@ -113,18 +129,24 @@ impl InferenceDiagnosticsParentData {
     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 IntoDiagnosticArg for UnderspecifiedArgKind {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        let kind = match self {
+            Self::Type { .. } => "type",
+            Self::Const { is_parameter: true } => "const_with_param",
+            Self::Const { is_parameter: false } => "const",
+        };
+        rustc_errors::DiagnosticArgValue::Str(kind.into())
     }
 }
 
 impl UnderspecifiedArgKind {
-    fn prefix_string(&self) -> Cow<'static, str> {
+    fn try_get_prefix(&self) -> Option<&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(),
+            Self::Type { prefix } => Some(prefix.as_ref()),
+            Self::Const { .. } => None,
         }
     }
 }
@@ -177,7 +199,7 @@ fn ty_to_string<'tcx>(infcx: &InferCtxt<'_, 'tcx>, ty: Ty<'tcx>) -> String {
 }
 
 /// 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
+/// something users are familiar 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!() };
@@ -303,11 +325,44 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         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
+        let source_kind = "other";
+        let source_name = "";
+        let failure_span = None;
+        let infer_subdiags = Vec::new();
+        let multi_suggestions = Vec::new();
+        let bad_label = Some(arg_data.make_bad_error(span));
+        match error_code {
+            TypeAnnotationNeeded::E0282 => AnnotationRequired {
+                span,
+                source_kind,
+                source_name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
+            TypeAnnotationNeeded::E0283 => AmbigousImpl {
+                span,
+                source_kind,
+                source_name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
+            TypeAnnotationNeeded::E0284 => AmbigousReturn {
+                span,
+                source_kind,
+                source_name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
+        }
     }
 
     pub fn emit_inference_failure_err(
@@ -340,48 +395,39 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             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");
-        }
+        let (source_kind, name) = kind.ty_localized_msg(self);
+        let failure_span = if should_label_span && !failure_span.overlaps(span) {
+            Some(failure_span)
+        } else {
+            None
+        };
 
+        let mut infer_subdiags = Vec::new();
+        let mut multi_suggestions = Vec::new();
         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,
-                );
+                infer_subdiags.push(SourceKindSubdiag::LetLike {
+                    span: insert_span,
+                    name: pattern_name.map(|name| name.to_string()).unwrap_or_else(String::new),
+                    x_kind: arg_data.where_x_is_kind(ty),
+                    prefix_kind: arg_data.kind.clone(),
+                    prefix: arg_data.kind.try_get_prefix().unwrap_or_default(),
+                    arg_name: arg_data.name,
+                    kind: if pattern_name.is_some() { "with_pattern" } else { "other" },
+                    type_name: ty_to_string(self, ty),
+                });
             }
             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,
-                );
+                infer_subdiags.push(SourceKindSubdiag::LetLike {
+                    span: insert_span,
+                    name: String::new(),
+                    x_kind: arg_data.where_x_is_kind(ty),
+                    prefix_kind: arg_data.kind.clone(),
+                    prefix: arg_data.kind.try_get_prefix().unwrap_or_default(),
+                    arg_name: arg_data.name,
+                    kind: "closure",
+                    type_name: ty_to_string(self, ty),
+                });
             }
             InferSourceKind::GenericArg {
                 insert_span,
@@ -393,19 +439,20 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 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.
+                let (parent_exists, parent_prefix, parent_name) =
                     InferenceDiagnosticsParentData::for_parent_def_id(self.tcx, generics_def_id)
-                        .map_or(String::new(), |parent| parent.suffix_string()),
-                );
+                        .map_or((false, String::new(), String::new()), |parent| {
+                            (true, parent.prefix.to_string(), parent.name)
+                        });
 
-                err.span_label(span, cannot_infer_msg);
+                infer_subdiags.push(SourceKindSubdiag::GenericLabel {
+                    span,
+                    is_type,
+                    param_name: generics.params[argument_index].name.to_string(),
+                    parent_exists,
+                    parent_prefix,
+                    parent_name,
+                });
 
                 let args = fmt_printer(self, Namespace::TypeNS)
                     .comma_sep(generic_args.iter().copied().map(|arg| {
@@ -435,15 +482,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     .unwrap()
                     .into_buffer();
 
-                err.span_suggestion_verbose(
-                    insert_span,
-                    &format!(
-                        "consider specifying the generic argument{}",
-                        pluralize!(generic_args.len()),
-                    ),
-                    format!("::<{}>", args),
-                    Applicability::HasPlaceholders,
-                );
+                infer_subdiags.push(SourceKindSubdiag::GenericSuggestion {
+                    span: insert_span,
+                    arg_count: generic_args.len(),
+                    args,
+                });
             }
             InferSourceKind::FullyQualifiedMethodCall { receiver, successor, substs, def_id } => {
                 let printer = fmt_printer(self, Namespace::ValueNS);
@@ -468,37 +511,54 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     _ => "",
                 };
 
-                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,
-                );
+                multi_suggestions.push(SourceKindMultiSuggestion::new_fully_qualified(
+                    receiver.span,
+                    def_path,
+                    adjustment,
+                    successor,
+                ));
             }
             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,
-                );
+                let ty_info = ty_to_string(self, ty);
+                multi_suggestions.push(SourceKindMultiSuggestion::new_closure_return(
+                    ty_info,
+                    data,
+                    should_wrap_expr,
+                ));
+            }
+        }
+        match error_code {
+            TypeAnnotationNeeded::E0282 => AnnotationRequired {
+                span,
+                source_kind,
+                source_name: &name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label: None,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
+            TypeAnnotationNeeded::E0283 => AmbigousImpl {
+                span,
+                source_kind,
+                source_name: &name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label: None,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
+            TypeAnnotationNeeded::E0284 => AmbigousReturn {
+                span,
+                source_kind,
+                source_name: &name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label: None,
             }
+            .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
         }
-        err
     }
 
     pub fn need_type_info_err_in_generator(
@@ -510,15 +570,26 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         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,
+        NeedTypeInfoInGenerator {
+            bad_label: data.make_bad_error(span),
             span,
-            E0698,
-            "type inside {} must be known in this context",
-            kind,
-        );
-        err.span_label(span, data.cannot_infer_msg());
-        err
+            generator_kind: GeneratorKindAsDiagArg(kind),
+        }
+        .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic)
+    }
+}
+
+pub struct GeneratorKindAsDiagArg(pub hir::GeneratorKind);
+
+impl IntoDiagnosticArg for GeneratorKindAsDiagArg {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        let kind = match self.0 {
+            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "async_block",
+            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "async_closure",
+            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "async_fn",
+            hir::GeneratorKind::Gen => "generator",
+        };
+        rustc_errors::DiagnosticArgValue::Str(kind.into())
     }
 }
 
@@ -579,22 +650,22 @@ impl<'tcx> InferSource<'tcx> {
 }
 
 impl<'tcx> InferSourceKind<'tcx> {
-    fn ty_msg(&self, infcx: &InferCtxt<'_, 'tcx>) -> String {
+    fn ty_localized_msg(&self, infcx: &InferCtxt<'_, 'tcx>) -> (&'static str, 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))
+                    ("closure", closure_as_fn_str(infcx, ty))
                 } else if !ty.is_ty_infer() {
-                    format!(" for `{}`", ty_to_string(infcx, ty))
+                    ("normal", ty_to_string(infcx, ty))
                 } else {
-                    String::new()
+                    ("other", String::new())
                 }
             }
             // FIXME: We should be able to add some additional info here.
             InferSourceKind::GenericArg { .. }
-            | InferSourceKind::FullyQualifiedMethodCall { .. } => String::new(),
+            | InferSourceKind::FullyQualifiedMethodCall { .. } => ("other", String::new()),
         }
     }
 }
@@ -830,7 +901,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
                     }
                 }
             }
-            hir::ExprKind::MethodCall(segment, _, _) => {
+            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 {
@@ -838,7 +909,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
                             None?
                         }
                         let substs = self.node_substs_opt(expr.hir_id)?;
-                        let span = tcx.hir().span(segment.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,
@@ -886,13 +957,13 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
         path.segments
             .iter()
             .filter_map(move |segment| {
-                let res = segment.res?;
+                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 span = tcx.hir().span(segment.hir_id);
                 let insert_span = segment.ident.span.shrink_to_hi().with_hi(span.hi());
                 Some(InsertableGenericArgs {
                     insert_span,
@@ -925,7 +996,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
                     if !segment.infer_args || generics.has_impl_trait() {
                         None?;
                     }
-                    let span = tcx.hir().span(segment.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 }
                 };
@@ -1061,7 +1132,7 @@ impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
                 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,
+                    ExprKind::MethodCall(path, ..) => path.ident.span,
                     _ => expr.span,
                 };
 
@@ -1110,7 +1181,7 @@ impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
             })
             .any(|generics| generics.has_impl_trait())
         };
-        if let ExprKind::MethodCall(path, args, span) = expr.kind
+        if let ExprKind::MethodCall(path, receiver, 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)
@@ -1118,12 +1189,12 @@ impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
             && !has_impl_trait(def_id)
         {
             let successor =
-                args.get(1).map_or_else(|| (")", span.hi()), |arg| (", ", arg.span.lo()));
+                args.get(0).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(),
+                    receiver,
                     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
index 9a2ab3e32..3a4320a9a 100644
--- 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
@@ -1,6 +1,9 @@
 //! Error Reporting for Anonymous Region Lifetime Errors
 //! where both the regions are anonymous.
 
+use crate::errors::AddLifetimeParamsSuggestion;
+use crate::errors::LifetimeMismatch;
+use crate::errors::LifetimeMismatchLabels;
 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;
@@ -8,11 +11,10 @@ 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_errors::AddSubdiagnostic;
+use rustc_errors::{Diagnostic, ErrorGuaranteed};
+use rustc_hir::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.
@@ -98,137 +100,50 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         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) {
+        let labels = 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(
+                LifetimeMismatchLabels::InRet {
                     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),
-                    );
+                    ret_span,
+                    span,
+                    label_var1: anon_param_sup.pat.simple_ident(),
                 }
             }
-        }
 
-        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");
-        }
+            (None, None) => LifetimeMismatchLabels::Normal {
+                hir_equal: ty_sup.hir_id == ty_sub.hir_id,
+                ty_sup: ty_sup.span,
+                ty_sub: ty_sub.span,
+                span,
+                sup: anon_param_sup.pat.simple_ident(),
+                sub: anon_param_sub.pat.simple_ident(),
+            },
+        };
 
-        let reported = err.emit();
+        let suggestion =
+            AddLifetimeParamsSuggestion { tcx: self.tcx(), sub, ty_sup, ty_sub, add_note: true };
+        let err = LifetimeMismatch { span, labels, suggestion };
+        let reported = self.tcx().sess.emit_err(err);
         Some(reported)
     }
 }
 
+/// Currently only used in rustc_borrowck, probably should be
+/// removed in favour of public_errors::AddLifetimeParamsSuggestion
 pub fn suggest_adding_lifetime_params<'tcx>(
     tcx: TyCtxt<'tcx>,
     sub: Region<'tcx>,
-    ty_sup: &Ty<'_>,
-    ty_sub: &Ty<'_>,
+    ty_sup: &'tcx Ty<'_>,
+    ty_sub: &'tcx 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
+) {
+    let suggestion = AddLifetimeParamsSuggestion { tcx, sub, ty_sup, ty_sub, add_note: false };
+    suggestion.add_to_diagnostic(err);
 }
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
index c1b201da6..d8f540b74 100644
--- 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
@@ -91,7 +91,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
             hir::TyKind::TraitObject(bounds, ..) => {
                 for bound in bounds {
                     self.current_index.shift_in(1);
-                    self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
+                    self.visit_poly_trait_ref(bound);
                     self.current_index.shift_out(1);
                 }
             }
@@ -103,7 +103,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                     // 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, _)) => {
+                    (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);
@@ -133,7 +133,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                         Some(
                             rl::Region::Static
                             | rl::Region::Free(_, _)
-                            | rl::Region::EarlyBound(_, _)
+                            | rl::Region::EarlyBound(_)
                             | rl::Region::LateBound(_, _, _),
                         )
                         | None,
@@ -188,7 +188,7 @@ impl<'tcx> Visitor<'tcx> for TyPathVisitor<'tcx> {
     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, _)) => {
+            (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;
@@ -209,7 +209,7 @@ impl<'tcx> Visitor<'tcx> for TyPathVisitor<'tcx> {
             (
                 Some(
                     rl::Region::Static
-                    | rl::Region::EarlyBound(_, _)
+                    | rl::Region::EarlyBound(_)
                     | rl::Region::LateBound(_, _, _)
                     | rl::Region::Free(_, _),
                 )
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
index 893ca3cf7..1410e2b63 100644
--- 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
@@ -1,13 +1,14 @@
 //! Error Reporting for when the lifetime for a type doesn't match the `impl` selected for a predicate
 //! to hold.
 
+use crate::errors::{note_and_explain, IntroducesStaticBecauseUnmetLifetimeReq};
+use crate::errors::{ImplNote, MismatchedStaticLifetime, TraitSubdiag};
 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_errors::{ErrorGuaranteed, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_middle::ty::TypeVisitor;
@@ -35,15 +36,27 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         let ObligationCauseCode::MatchImpl(parent, impl_def_id) = code else {
             return None;
         };
-        let ObligationCauseCode::BindingObligation(_def_id, binding_span) = *parent.code() else {
+        let (ObligationCauseCode::BindingObligation(_, binding_span) | ObligationCauseCode::ExprBindingObligation(_, 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));
+        let multi_span: MultiSpan = vec![binding_span].into();
+        let multispan_subdiag = IntroducesStaticBecauseUnmetLifetimeReq {
+            unmet_requirements: multi_span,
+            binding_span,
+        };
+
+        let expl = note_and_explain::RegionExplanation::new(
+            self.tcx(),
+            sup,
+            Some(binding_span),
+            note_and_explain::PrefixKind::Empty,
+            note_and_explain::SuffixKind::Continues,
+        );
+        let mut impl_span = None;
+        let mut trait_subdiags = Vec::new();
         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.
@@ -72,31 +85,30 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                 // 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`");
+                impl_span = Some(self.tcx().def_span(*impl_def_id));
             } 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");
+                    trait_subdiags.push(TraitSubdiag::Note { span: *span });
                     // 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,
-                    );
+                    trait_subdiags.push(TraitSubdiag::Sugg { span: span.shrink_to_hi() });
                 }
             }
         } 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`");
+            impl_span = Some(self.tcx().def_span(*impl_def_id));
         }
-        let reported = err.emit();
+        let err = MismatchedStaticLifetime {
+            cause_span: cause.span,
+            unmet_lifetime_reqs: multispan_subdiag,
+            expl,
+            impl_note: ImplNote { impl_span },
+            trait_subdiags,
+        };
+        let reported = self.tcx().sess.emit_err(err);
         Some(reported)
     }
 }
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
index 998699158..d4db07512 100644
--- 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
@@ -211,7 +211,10 @@ impl<'tcx> NiceRegionError<'_, 'tcx> {
         );
         let mut err = self.tcx().sess.struct_span_err(span, &msg);
 
-        let leading_ellipsis = if let ObligationCauseCode::ItemObligation(def_id) = *cause.code() {
+        let leading_ellipsis = if let ObligationCauseCode::ItemObligation(def_id)
+        | ObligationCauseCode::ExprItemObligation(def_id, ..) =
+            *cause.code()
+        {
             err.span_label(span, "doesn't satisfy where-clause");
             err.span_label(
                 self.tcx().def_span(def_id),
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
index 9886c572a..ae56bea6f 100644
--- 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
@@ -232,7 +232,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                 ObligationCauseCode::MatchImpl(parent, ..) => parent.code(),
                 _ => cause.code(),
             }
-            && let (&ObligationCauseCode::ItemObligation(item_def_id), None) = (code, override_error_code)
+            && let (&ObligationCauseCode::ItemObligation(item_def_id) | &ObligationCauseCode::ExprItemObligation(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:
@@ -300,7 +300,7 @@ pub fn suggest_new_region_bound(
             continue;
         }
         match fn_return.kind {
-            TyKind::OpaqueDef(item_id, _) => {
+            TyKind::OpaqueDef(item_id, _, _) => {
                 let item = tcx.hir().item(item_id);
                 let ItemKind::OpaqueTy(opaque) = &item.kind else {
                     return;
@@ -544,7 +544,7 @@ 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() => {
+            ty::Dynamic(preds, re, _) if re.is_static() => {
                 if let Some(def_id) = preds.principal_def_id() {
                     self.0.insert(def_id);
                 }
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
index da465a764..a6a39d062 100644
--- 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
@@ -154,16 +154,11 @@ impl<'tcx> Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
             }
             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) =>
+                    if matches!(
+                        segment.res,
+                        Res::SelfTy { trait_: _, alias_to: _ }
+                            | Res::Def(hir::def::DefKind::TyParam, _)
+                    ) =>
                 {
                     self.types.push(path.span);
                 }
diff --git a/compiler/rustc_infer/src/infer/error_reporting/note.rs b/compiler/rustc_infer/src/infer/error_reporting/note.rs
index c1940c5c0..adaa47c01 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/note.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/note.rs
@@ -1,100 +1,89 @@
-use crate::infer::error_reporting::{note_and_explain_region, ObligationCauseExt};
+use crate::errors::RegionOriginNote;
+use crate::infer::error_reporting::note_and_explain_region;
 use crate::infer::{self, InferCtxt, SubregionOrigin};
-use rustc_errors::{struct_span_err, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{
+    fluent, struct_span_err, AddSubdiagnostic, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+};
 use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::{self, Region};
 
+use super::ObligationCauseAsDiagArg;
+
 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::Subtype(ref trace) => RegionOriginNote::WithRequirement {
+                span: trace.cause.span,
+                requirement: ObligationCauseAsDiagArg(trace.cause.clone()),
+                expected_found: self.values_str(trace.values),
             }
+            .add_to_diagnostic(err),
+            infer::Reborrow(span) => RegionOriginNote::Plain { span, msg: fluent::infer::reborrow }
+                .add_to_diagnostic(err),
             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));
+                RegionOriginNote::WithName {
+                    span,
+                    msg: fluent::infer::reborrow,
+                    name: &var_name.to_string(),
+                    continues: false,
+                }
+                .add_to_diagnostic(err);
             }
             infer::RelateObjectBound(span) => {
-                label_or_note(span, "...so that it can be closed over into an object");
+                RegionOriginNote::Plain { span, msg: fluent::infer::relate_object_bound }
+                    .add_to_diagnostic(err);
             }
             infer::DataBorrowed(ty, span) => {
-                label_or_note(
+                RegionOriginNote::WithName {
                     span,
-                    &format!(
-                        "...so that the type `{}` is not borrowed for too long",
-                        self.ty_to_string(ty)
-                    ),
-                );
+                    msg: fluent::infer::data_borrowed,
+                    name: &self.ty_to_string(ty),
+                    continues: false,
+                }
+                .add_to_diagnostic(err);
             }
             infer::ReferenceOutlivesReferent(ty, span) => {
-                label_or_note(
+                RegionOriginNote::WithName {
                     span,
-                    &format!(
-                        "...so that the reference type `{}` does not outlive the data it points at",
-                        self.ty_to_string(ty)
-                    ),
-                );
+                    msg: fluent::infer::reference_outlives_referent,
+                    name: &self.ty_to_string(ty),
+                    continues: false,
+                }
+                .add_to_diagnostic(err);
             }
-            infer::RelateParamBound(span, t, opt_span) => {
-                label_or_note(
+            infer::RelateParamBound(span, ty, opt_span) => {
+                RegionOriginNote::WithName {
                     span,
-                    &format!(
-                        "...so that the type `{}` will meet its required lifetime bounds{}",
-                        self.ty_to_string(t),
-                        if opt_span.is_some() { "..." } else { "" },
-                    ),
-                );
+                    msg: fluent::infer::relate_param_bound,
+                    name: &self.ty_to_string(ty),
+                    continues: opt_span.is_some(),
+                }
+                .add_to_diagnostic(err);
                 if let Some(span) = opt_span {
-                    err.span_note(span, "...that is required by this bound");
+                    RegionOriginNote::Plain { span, msg: fluent::infer::relate_param_bound_2 }
+                        .add_to_diagnostic(err);
                 }
             }
             infer::RelateRegionParamBound(span) => {
-                label_or_note(
-                    span,
-                    "...so that the declared lifetime parameter bounds are satisfied",
-                );
+                RegionOriginNote::Plain { span, msg: fluent::infer::relate_region_param_bound }
+                    .add_to_diagnostic(err);
             }
             infer::CompareImplItemObligation { span, .. } => {
-                label_or_note(
-                    span,
-                    "...so that the definition in impl matches the definition from the trait",
-                );
+                RegionOriginNote::Plain { span, msg: fluent::infer::compare_impl_item_obligation }
+                    .add_to_diagnostic(err);
             }
             infer::CheckAssociatedTypeBounds { ref parent, .. } => {
                 self.note_region_origin(err, &parent);
             }
+            infer::AscribeUserTypeProvePredicate(span) => {
+                RegionOriginNote::Plain {
+                    span,
+                    msg: fluent::infer::ascribe_user_type_prove_predicate,
+                }
+                .add_to_diagnostic(err);
+            }
         }
     }
 
@@ -107,7 +96,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         match origin {
             infer::Subtype(box trace) => {
                 let terr = TypeError::RegionsDoesNotOutlive(sup, sub);
-                let mut err = self.report_and_explain_type_error(trace, &terr);
+                let mut err = self.report_and_explain_type_error(trace, terr);
                 match (*sub, *sup) {
                     (ty::RePlaceholder(_), ty::RePlaceholder(_)) => {}
                     (ty::RePlaceholder(_), _) => {
@@ -374,6 +363,27 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
                 err
             }
+            infer::AscribeUserTypeProvePredicate(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 instantiated with ",
+                    sup,
+                    "",
+                    None,
+                );
+                note_and_explain_region(
+                    self.tcx,
+                    &mut err,
+                    "but lifetime must outlive ",
+                    sub,
+                    "",
+                    None,
+                );
+                err
+            }
         }
     }
 
@@ -390,10 +400,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 if matches!(
                     &trace.cause.code().peel_derives(),
                     ObligationCauseCode::BindingObligation(..)
+                        | ObligationCauseCode::ExprBindingObligation(..)
                 ) =>
             {
                 // Hack to get around the borrow checker because trace.cause has an `Rc`.
-                if let ObligationCauseCode::BindingObligation(_, span) =
+                if let ObligationCauseCode::BindingObligation(_, span)
+                | ObligationCauseCode::ExprBindingObligation(_, span, ..) =
                     &trace.cause.code().peel_derives()
                 {
                     let span = *span;
@@ -406,7 +418,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
             infer::Subtype(box trace) => {
                 let terr = TypeError::RegionsPlaceholderMismatch;
-                return self.report_and_explain_type_error(trace, &terr);
+                return self.report_and_explain_type_error(trace, terr);
             }
             _ => return self.report_concrete_failure(placeholder_origin, sub, sup),
         }
diff --git a/compiler/rustc_infer/src/infer/free_regions.rs b/compiler/rustc_infer/src/infer/free_regions.rs
index d566634a4..728d691a2 100644
--- a/compiler/rustc_infer/src/infer/free_regions.rs
+++ b/compiler/rustc_infer/src/infer/free_regions.rs
@@ -27,13 +27,13 @@ impl<'a, 'tcx> RegionRelations<'a, 'tcx> {
     }
 }
 
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug)]
 pub struct FreeRegionMap<'tcx> {
     // Stores the relation `a < b`, where `a` and `b` are regions.
     //
     // Invariant: only free regions like `'x` or `'static` are stored
     // in this relation, not scopes.
-    relation: TransitiveRelation<Region<'tcx>>,
+    pub(crate) relation: TransitiveRelation<Region<'tcx>>,
 }
 
 impl<'tcx> FreeRegionMap<'tcx> {
@@ -45,15 +45,6 @@ impl<'tcx> FreeRegionMap<'tcx> {
         self.relation.is_empty()
     }
 
-    // Record that `'sup:'sub`. Or, put another way, `'sub <= 'sup`.
-    // (with the exception that `'static: 'x` is not notable)
-    pub fn relate_regions(&mut self, sub: Region<'tcx>, sup: Region<'tcx>) {
-        debug!("relate_regions(sub={:?}, sup={:?})", sub, sup);
-        if sub.is_free_or_static() && sup.is_free() {
-            self.relation.add(sub, sup)
-        }
-    }
-
     /// Tests whether `r_a <= r_b`.
     ///
     /// Both regions must meet `is_free_or_static`.
diff --git a/compiler/rustc_infer/src/infer/freshen.rs b/compiler/rustc_infer/src/infer/freshen.rs
index 84004d2b2..fee15afc7 100644
--- a/compiler/rustc_infer/src/infer/freshen.rs
+++ b/compiler/rustc_infer/src/infer/freshen.rs
@@ -126,7 +126,6 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
             | ty::ReFree(_)
             | ty::ReVar(_)
             | ty::RePlaceholder(..)
-            | ty::ReEmpty(_)
             | ty::ReErased => {
                 // replace all free regions with 'erased
                 self.tcx().lifetimes.re_erased
diff --git a/compiler/rustc_infer/src/infer/higher_ranked/mod.rs b/compiler/rustc_infer/src/infer/higher_ranked/mod.rs
index d0d9efe15..0ce271c0e 100644
--- a/compiler/rustc_infer/src/infer/higher_ranked/mod.rs
+++ b/compiler/rustc_infer/src/infer/higher_ranked/mod.rs
@@ -69,7 +69,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// For more details visit the relevant sections of the [rustc dev guide].
     ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
-    #[instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self), ret)]
     pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
     where
         T: TypeFoldable<'tcx> + Copy,
@@ -81,19 +81,19 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let next_universe = self.create_next_universe();
 
         let delegate = FnMutDelegate {
-            regions: |br: ty::BoundRegion| {
+            regions: &mut |br: ty::BoundRegion| {
                 self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
                     universe: next_universe,
                     name: br.kind,
                 }))
             },
-            types: |bound_ty: ty::BoundTy| {
+            types: &mut |bound_ty: ty::BoundTy| {
                 self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
                     universe: next_universe,
                     name: bound_ty.var,
                 }))
             },
-            consts: |bound_var: ty::BoundVar, ty| {
+            consts: &mut |bound_var: ty::BoundVar, ty| {
                 self.tcx.mk_const(ty::ConstS {
                     kind: ty::ConstKind::Placeholder(ty::PlaceholderConst {
                         universe: next_universe,
@@ -104,9 +104,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             },
         };
 
-        let result = self.tcx.replace_bound_vars_uncached(binder, delegate);
-        debug!(?next_universe, ?result);
-        result
+        debug!(?next_universe);
+        self.tcx.replace_bound_vars_uncached(binder, delegate)
     }
 
     /// See [RegionConstraintCollector::leak_check][1].
diff --git a/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs b/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
index 3783cfb4c..5f13b2b3d 100644
--- a/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
+++ b/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
@@ -15,8 +15,9 @@ use rustc_data_structures::graph::implementation::{
 use rustc_data_structures::intern::Interned;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_middle::ty::fold::TypeFoldable;
+use rustc_middle::ty::PlaceholderRegion;
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_middle::ty::{ReEarlyBound, ReEmpty, ReErased, ReFree, ReStatic};
+use rustc_middle::ty::{ReEarlyBound, ReErased, ReFree, ReStatic};
 use rustc_middle::ty::{ReLateBound, RePlaceholder, ReVar};
 use rustc_middle::ty::{Region, RegionVid};
 use rustc_span::Span;
@@ -51,6 +52,13 @@ pub struct LexicalRegionResolutions<'tcx> {
 
 #[derive(Copy, Clone, Debug)]
 pub(crate) enum VarValue<'tcx> {
+    /// Empty lifetime is for data that is never accessed.  We tag the
+    /// empty lifetime with a universe -- the idea is that we don't
+    /// want `exists<'a> { forall<'b> { 'b: 'a } }` to be satisfiable.
+    /// Therefore, the `'empty` in a universe `U` is less than all
+    /// regions visible from `U`, but not less than regions not visible
+    /// from `U`.
+    Empty(ty::UniverseIndex),
     Value(Region<'tcx>),
     ErrorValue,
 }
@@ -117,7 +125,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         &mut self,
         errors: &mut Vec<RegionResolutionError<'tcx>>,
     ) -> LexicalRegionResolutions<'tcx> {
-        let mut var_data = self.construct_var_data(self.tcx());
+        let mut var_data = self.construct_var_data();
 
         if cfg!(debug_assertions) {
             self.dump_constraints();
@@ -137,13 +145,12 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
 
     /// Initially, the value for all variables is set to `'empty`, the
     /// empty region. The `expansion` phase will grow this larger.
-    fn construct_var_data(&self, tcx: TyCtxt<'tcx>) -> LexicalRegionResolutions<'tcx> {
+    fn construct_var_data(&self) -> LexicalRegionResolutions<'tcx> {
         LexicalRegionResolutions {
             values: IndexVec::from_fn_n(
                 |vid| {
                     let vid_universe = self.var_infos[vid].universe;
-                    let re_empty = tcx.mk_region(ty::ReEmpty(vid_universe));
-                    VarValue::Value(re_empty)
+                    VarValue::Empty(vid_universe)
                 },
                 self.num_vars(),
             ),
@@ -189,20 +196,131 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         }
     }
 
+    /// Gets the LUb of a given region and the empty region
+    fn lub_empty(&self, a_region: Region<'tcx>) -> Result<Region<'tcx>, PlaceholderRegion> {
+        match *a_region {
+            ReLateBound(..) | ReErased => {
+                bug!("cannot relate region: {:?}", a_region);
+            }
+
+            ReVar(v_id) => {
+                span_bug!(
+                    self.var_infos[v_id].origin.span(),
+                    "lub invoked with non-concrete regions: {:?}",
+                    a_region,
+                );
+            }
+
+            ReStatic => {
+                // nothing lives longer than `'static`
+                Ok(self.tcx().lifetimes.re_static)
+            }
+
+            ReEarlyBound(_) | ReFree(_) => {
+                // All empty regions are less than early-bound, free,
+                // and scope regions.
+                Ok(a_region)
+            }
+
+            RePlaceholder(placeholder) => Err(placeholder),
+        }
+    }
+
     fn expansion(&self, var_values: &mut LexicalRegionResolutions<'tcx>) {
+        // In the first pass, we expand region vids according to constraints we
+        // have previously found. In the second pass, we loop through the region
+        // vids we expanded and expand *across* region vids (effectively
+        // "expanding" new `RegSubVar` constraints).
+
+        // Tracks the `VarSubVar` constraints generated for each region vid. We
+        // later use this to expand across vids.
         let mut constraints = IndexVec::from_elem_n(Vec::new(), var_values.values.len());
+        // Tracks the changed region vids.
         let mut changes = Vec::new();
         for constraint in self.data.constraints.keys() {
-            let (a_vid, a_region, b_vid, b_data) = match *constraint {
+            match *constraint {
                 Constraint::RegSubVar(a_region, b_vid) => {
                     let b_data = var_values.value_mut(b_vid);
-                    (None, a_region, b_vid, b_data)
+
+                    if self.expand_node(a_region, b_vid, b_data) {
+                        changes.push(b_vid);
+                    }
                 }
                 Constraint::VarSubVar(a_vid, b_vid) => match *var_values.value(a_vid) {
                     VarValue::ErrorValue => continue,
+                    VarValue::Empty(a_universe) => {
+                        let b_data = var_values.value_mut(b_vid);
+
+                        let changed = (|| match *b_data {
+                            VarValue::Empty(b_universe) => {
+                                // Empty regions are ordered according to the universe
+                                // they are associated with.
+                                let ui = a_universe.min(b_universe);
+
+                                debug!(
+                                    "Expanding value of {:?} \
+                                    from empty lifetime with universe {:?} \
+                                    to empty lifetime with universe {:?}",
+                                    b_vid, b_universe, ui
+                                );
+
+                                *b_data = VarValue::Empty(ui);
+                                true
+                            }
+                            VarValue::Value(cur_region) => {
+                                let lub = match self.lub_empty(cur_region) {
+                                    Ok(r) => r,
+                                    // If the empty and placeholder regions are in the same universe,
+                                    // then the LUB is the Placeholder region (which is the cur_region).
+                                    // If they are not in the same universe, the LUB is the Static lifetime.
+                                    Err(placeholder) if a_universe == placeholder.universe => {
+                                        cur_region
+                                    }
+                                    Err(_) => self.tcx().lifetimes.re_static,
+                                };
+
+                                if lub == cur_region {
+                                    return false;
+                                }
+
+                                debug!(
+                                    "Expanding value of {:?} from {:?} to {:?}",
+                                    b_vid, cur_region, lub
+                                );
+
+                                *b_data = VarValue::Value(lub);
+                                true
+                            }
+
+                            VarValue::ErrorValue => false,
+                        })();
+
+                        if changed {
+                            changes.push(b_vid);
+                        }
+                        match b_data {
+                            VarValue::Value(Region(Interned(ReStatic, _)))
+                            | VarValue::ErrorValue => (),
+                            _ => {
+                                constraints[a_vid].push((a_vid, b_vid));
+                                constraints[b_vid].push((a_vid, b_vid));
+                            }
+                        }
+                    }
                     VarValue::Value(a_region) => {
                         let b_data = var_values.value_mut(b_vid);
-                        (Some(a_vid), a_region, b_vid, b_data)
+
+                        if self.expand_node(a_region, b_vid, b_data) {
+                            changes.push(b_vid);
+                        }
+                        match b_data {
+                            VarValue::Value(Region(Interned(ReStatic, _)))
+                            | VarValue::ErrorValue => (),
+                            _ => {
+                                constraints[a_vid].push((a_vid, b_vid));
+                                constraints[b_vid].push((a_vid, b_vid));
+                            }
+                        }
                     }
                 },
                 Constraint::RegSubReg(..) | Constraint::VarSubReg(..) => {
@@ -210,18 +328,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                     // is done, in `collect_errors`.
                     continue;
                 }
-            };
-            if self.expand_node(a_region, b_vid, b_data) {
-                changes.push(b_vid);
-            }
-            if let Some(a_vid) = a_vid {
-                match b_data {
-                    VarValue::Value(Region(Interned(ReStatic, _))) | VarValue::ErrorValue => (),
-                    _ => {
-                        constraints[a_vid].push((a_vid, b_vid));
-                        constraints[b_vid].push((a_vid, b_vid));
-                    }
-                }
             }
         }
 
@@ -242,6 +348,10 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         }
     }
 
+    /// Expands the value of the region represented with `b_vid` with current
+    /// value `b_data` to the lub of `b_data` and `a_region`. The corresponds
+    /// with the constraint `'?b: 'a` (`'a <: '?b`), where `'a` is some known
+    /// region and `'?b` is some region variable.
     fn expand_node(
         &self,
         a_region: Region<'tcx>,
@@ -263,14 +373,28 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         }
 
         match *b_data {
+            VarValue::Empty(empty_ui) => {
+                let lub = match self.lub_empty(a_region) {
+                    Ok(r) => r,
+                    // If this empty region is from a universe that can
+                    // name the placeholder, then the placeholder is
+                    // larger; otherwise, the only ancestor is `'static`.
+                    Err(placeholder) if empty_ui.can_name(placeholder.universe) => {
+                        self.tcx().mk_region(RePlaceholder(placeholder))
+                    }
+                    Err(_) => self.tcx().lifetimes.re_static,
+                };
+
+                debug!("Expanding value of {:?} from empty lifetime to {:?}", b_vid, lub);
+
+                *b_data = VarValue::Value(lub);
+                true
+            }
             VarValue::Value(cur_region) => {
                 // This is a specialized version of the `lub_concrete_regions`
                 // check below for a common case, here purely as an
                 // optimization.
                 let b_universe = self.var_infos[b_vid].universe;
-                if let ReEmpty(a_universe) = *a_region && a_universe == b_universe {
-                    return false;
-                }
 
                 let mut lub = self.lub_concrete_regions(a_region, cur_region);
                 if lub == cur_region {
@@ -300,6 +424,78 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         }
     }
 
+    /// True if `a <= b`.
+    fn sub_region_values(&self, a: VarValue<'tcx>, b: VarValue<'tcx>) -> bool {
+        match (a, b) {
+            // Error region is `'static`
+            (VarValue::ErrorValue, _) | (_, VarValue::ErrorValue) => return true,
+            (VarValue::Empty(a_ui), VarValue::Empty(b_ui)) => {
+                // Empty regions are ordered according to the universe
+                // they are associated with.
+                a_ui.min(b_ui) == b_ui
+            }
+            (VarValue::Value(a), VarValue::Empty(_)) => {
+                match *a {
+                    ReLateBound(..) | ReErased => {
+                        bug!("cannot relate region: {:?}", a);
+                    }
+
+                    ReVar(v_id) => {
+                        span_bug!(
+                            self.var_infos[v_id].origin.span(),
+                            "lub_concrete_regions invoked with non-concrete region: {:?}",
+                            a
+                        );
+                    }
+
+                    ReStatic | ReEarlyBound(_) | ReFree(_) => {
+                        // nothing lives longer than `'static`
+
+                        // All empty regions are less than early-bound, free,
+                        // and scope regions.
+
+                        false
+                    }
+
+                    RePlaceholder(_) => {
+                        // The LUB is either `a` or `'static`
+                        false
+                    }
+                }
+            }
+            (VarValue::Empty(a_ui), VarValue::Value(b)) => {
+                match *b {
+                    ReLateBound(..) | ReErased => {
+                        bug!("cannot relate region: {:?}", b);
+                    }
+
+                    ReVar(v_id) => {
+                        span_bug!(
+                            self.var_infos[v_id].origin.span(),
+                            "lub_concrete_regions invoked with non-concrete regions: {:?}",
+                            b
+                        );
+                    }
+
+                    ReStatic | ReEarlyBound(_) | ReFree(_) => {
+                        // nothing lives longer than `'static`
+                        // All empty regions are less than early-bound, free,
+                        // and scope regions.
+                        true
+                    }
+
+                    RePlaceholder(placeholder) => {
+                        // If this empty region is from a universe that can
+                        // name the placeholder, then the placeholder is
+                        // larger; otherwise, the only ancestor is `'static`.
+                        if a_ui.can_name(placeholder.universe) { true } else { false }
+                    }
+                }
+            }
+            (VarValue::Value(a), VarValue::Value(b)) => self.sub_concrete_regions(a, b),
+        }
+    }
+
     /// True if `a <= b`, but not defined over inference variables.
     #[instrument(level = "trace", skip(self))]
     fn sub_concrete_regions(&self, a: Region<'tcx>, b: Region<'tcx>) -> bool {
@@ -333,9 +529,9 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
     ///
     /// Neither `a` nor `b` may be an inference variable (hence the
     /// term "concrete regions").
-    #[instrument(level = "trace", skip(self))]
+    #[instrument(level = "trace", skip(self), ret)]
     fn lub_concrete_regions(&self, a: Region<'tcx>, b: Region<'tcx>) -> Region<'tcx> {
-        let r = match (*a, *b) {
+        match (*a, *b) {
             (ReLateBound(..), _) | (_, ReLateBound(..)) | (ReErased, _) | (_, ReErased) => {
                 bug!("cannot relate region: LUB({:?}, {:?})", a, b);
             }
@@ -355,37 +551,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                 self.tcx().lifetimes.re_static
             }
 
-            (ReEmpty(_), ReEarlyBound(_) | ReFree(_)) => {
-                // All empty regions are less than early-bound, free,
-                // and scope regions.
-                b
-            }
-
-            (ReEarlyBound(_) | ReFree(_), ReEmpty(_)) => {
-                // All empty regions are less than early-bound, free,
-                // and scope regions.
-                a
-            }
-
-            (ReEmpty(a_ui), ReEmpty(b_ui)) => {
-                // Empty regions are ordered according to the universe
-                // they are associated with.
-                let ui = a_ui.min(b_ui);
-                self.tcx().mk_region(ReEmpty(ui))
-            }
-
-            (ReEmpty(empty_ui), RePlaceholder(placeholder))
-            | (RePlaceholder(placeholder), ReEmpty(empty_ui)) => {
-                // If this empty region is from a universe that can
-                // name the placeholder, then the placeholder is
-                // larger; otherwise, the only ancestor is `'static`.
-                if empty_ui.can_name(placeholder.universe) {
-                    self.tcx().mk_region(RePlaceholder(placeholder))
-                } else {
-                    self.tcx().lifetimes.re_static
-                }
-            }
-
             (ReEarlyBound(_) | ReFree(_), ReEarlyBound(_) | ReFree(_)) => {
                 self.region_rels.lub_free_regions(a, b)
             }
@@ -399,11 +564,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                     self.tcx().lifetimes.re_static
                 }
             }
-        };
-
-        debug!("lub_concrete_regions({:?}, {:?}) = {:?}", a, b, r);
-
-        r
+        }
     }
 
     /// After expansion is complete, go and check upper bounds (i.e.,
@@ -512,7 +673,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
 
         for (node_vid, value) in var_data.values.iter_enumerated() {
             match *value {
-                VarValue::Value(_) => { /* Inference successful */ }
+                VarValue::Empty(_) | VarValue::Value(_) => { /* Inference successful */ }
                 VarValue::ErrorValue => {
                     // Inference impossible: this value contains
                     // inconsistent constraints.
@@ -833,12 +994,25 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
             }
 
             VerifyBound::OutlivedBy(r) => {
-                self.sub_concrete_regions(min, var_values.normalize(self.tcx(), *r))
+                let a = match *min {
+                    ty::ReVar(rid) => var_values.values[rid],
+                    _ => VarValue::Value(min),
+                };
+                let b = match **r {
+                    ty::ReVar(rid) => var_values.values[rid],
+                    _ => VarValue::Value(*r),
+                };
+                self.sub_region_values(a, b)
             }
 
-            VerifyBound::IsEmpty => {
-                matches!(*min, ty::ReEmpty(_))
-            }
+            VerifyBound::IsEmpty => match *min {
+                ty::ReVar(rid) => match var_values.values[rid] {
+                    VarValue::ErrorValue => false,
+                    VarValue::Empty(_) => true,
+                    VarValue::Value(_) => false,
+                },
+                _ => false,
+            },
 
             VerifyBound::AnyBound(bs) => {
                 bs.iter().any(|b| self.bound_is_met(b, var_values, generic_ty, min))
@@ -880,6 +1054,7 @@ impl<'tcx> LexicalRegionResolutions<'tcx> {
     ) -> ty::Region<'tcx> {
         let result = match *r {
             ty::ReVar(rid) => match self.values[rid] {
+                VarValue::Empty(_) => r,
                 VarValue::Value(r) => r,
                 VarValue::ErrorValue => tcx.lifetimes.re_static,
             },
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index d7d1b5fa2..3abed1221 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -20,6 +20,7 @@ use rustc_middle::infer::canonical::{Canonical, CanonicalVarValues};
 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind, ToType};
 use rustc_middle::mir::interpret::{ErrorHandled, EvalToValTreeResult};
+use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::traits::select;
 use rustc_middle::ty::abstract_const::{AbstractConst, FailureKind};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
@@ -32,7 +33,7 @@ pub use rustc_middle::ty::IntVarValue;
 use rustc_middle::ty::{self, GenericParamDefKind, InferConst, Ty, TyCtxt};
 use rustc_middle::ty::{ConstVid, FloatVid, IntVid, TyVid};
 use rustc_span::symbol::Symbol;
-use rustc_span::Span;
+use rustc_span::{Span, DUMMY_SP};
 
 use std::cell::{Cell, Ref, RefCell};
 use std::fmt;
@@ -316,12 +317,12 @@ pub struct InferCtxt<'a, 'tcx> {
     ///
     /// Don't read this flag directly, call `is_tainted_by_errors()`
     /// and `set_tainted_by_errors()`.
-    tainted_by_errors_flag: Cell<bool>,
+    tainted_by_errors: Cell<Option<ErrorGuaranteed>>,
 
     /// Track how many errors were reported when this infcx is created.
     /// If the number of errors increases, that's also a sign (line
     /// `tainted_by_errors`) to avoid reporting certain kinds of errors.
-    // FIXME(matthewjasper) Merge into `tainted_by_errors_flag`
+    // FIXME(matthewjasper) Merge into `tainted_by_errors`
     err_count_on_creation: usize,
 
     /// This flag is true while there is an active snapshot.
@@ -337,6 +338,9 @@ pub struct InferCtxt<'a, 'tcx> {
     /// when we enter into a higher-ranked (`for<..>`) type or trait
     /// bound.
     universe: Cell<ty::UniverseIndex>,
+
+    normalize_fn_sig_for_diagnostic:
+        Option<Lrc<dyn Fn(&InferCtxt<'_, 'tcx>, ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx>>>,
 }
 
 /// See the `error_reporting` module for more details.
@@ -350,12 +354,11 @@ pub enum ValuePairs<'tcx> {
 
 impl<'tcx> ValuePairs<'tcx> {
     pub fn ty(&self) -> Option<(Ty<'tcx>, Ty<'tcx>)> {
-        if let ValuePairs::Terms(ExpectedFound {
-            expected: ty::Term::Ty(expected),
-            found: ty::Term::Ty(found),
-        }) = self
+        if let ValuePairs::Terms(ExpectedFound { expected, found }) = self
+            && let Some(expected) = expected.ty()
+            && let Some(found) = found.ty()
         {
-            Some((*expected, *found))
+            Some((expected, found))
         } else {
             None
         }
@@ -406,7 +409,11 @@ pub enum SubregionOrigin<'tcx> {
 
     /// Comparing the signature and requirements of an impl method against
     /// the containing trait.
-    CompareImplItemObligation { span: Span, impl_item_def_id: LocalDefId, trait_item_def_id: DefId },
+    CompareImplItemObligation {
+        span: Span,
+        impl_item_def_id: LocalDefId,
+        trait_item_def_id: DefId,
+    },
 
     /// Checking that the bounds of a trait's associated type hold for a given impl
     CheckAssociatedTypeBounds {
@@ -414,12 +421,24 @@ pub enum SubregionOrigin<'tcx> {
         impl_item_def_id: LocalDefId,
         trait_item_def_id: DefId,
     },
+
+    AscribeUserTypeProvePredicate(Span),
 }
 
 // `SubregionOrigin` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 static_assert_size!(SubregionOrigin<'_>, 32);
 
+impl<'tcx> SubregionOrigin<'tcx> {
+    pub fn to_constraint_category(&self) -> ConstraintCategory<'tcx> {
+        match self {
+            Self::Subtype(type_trace) => type_trace.cause.to_constraint_category(),
+            Self::AscribeUserTypeProvePredicate(span) => ConstraintCategory::Predicate(*span),
+            _ => ConstraintCategory::BoringNoLocation,
+        }
+    }
+}
+
 /// Times when we replace late-bound regions with variables:
 #[derive(Clone, Copy, Debug)]
 pub enum LateBoundRegionConversionTime {
@@ -504,7 +523,7 @@ pub enum FixupError<'tcx> {
 }
 
 /// See the `region_obligations` field for more information.
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct RegionObligation<'tcx> {
     pub sub_region: ty::Region<'tcx>,
     pub sup_type: Ty<'tcx>,
@@ -540,6 +559,8 @@ pub struct InferCtxtBuilder<'tcx> {
     defining_use_anchor: DefiningAnchor,
     considering_regions: bool,
     fresh_typeck_results: Option<RefCell<ty::TypeckResults<'tcx>>>,
+    normalize_fn_sig_for_diagnostic:
+        Option<Lrc<dyn Fn(&InferCtxt<'_, 'tcx>, ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx>>>,
 }
 
 pub trait TyCtxtInferExt<'tcx> {
@@ -553,6 +574,7 @@ impl<'tcx> TyCtxtInferExt<'tcx> for TyCtxt<'tcx> {
             defining_use_anchor: DefiningAnchor::Error,
             considering_regions: true,
             fresh_typeck_results: None,
+            normalize_fn_sig_for_diagnostic: None,
         }
     }
 }
@@ -582,6 +604,14 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
         self
     }
 
+    pub fn with_normalize_fn_sig_for_diagnostic(
+        mut self,
+        fun: Lrc<dyn Fn(&InferCtxt<'_, 'tcx>, ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx>>,
+    ) -> Self {
+        self.normalize_fn_sig_for_diagnostic = Some(fun);
+        self
+    }
+
     /// Given a canonical value `C` as a starting point, create an
     /// inference context that contains each of the bound values
     /// within instantiated as a fresh variable. The `f` closure is
@@ -611,6 +641,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             defining_use_anchor,
             considering_regions,
             ref fresh_typeck_results,
+            ref normalize_fn_sig_for_diagnostic,
         } = *self;
         let in_progress_typeck_results = fresh_typeck_results.as_ref();
         f(InferCtxt {
@@ -624,11 +655,14 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             evaluation_cache: Default::default(),
             reported_trait_errors: Default::default(),
             reported_closure_mismatch: Default::default(),
-            tainted_by_errors_flag: Cell::new(false),
+            tainted_by_errors: Cell::new(None),
             err_count_on_creation: tcx.sess.err_count(),
             in_snapshot: Cell::new(false),
             skip_leak_check: Cell::new(false),
             universe: Cell::new(ty::UniverseIndex::ROOT),
+            normalize_fn_sig_for_diagnostic: normalize_fn_sig_for_diagnostic
+                .as_ref()
+                .map(|f| f.clone()),
         })
     }
 }
@@ -988,7 +1022,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         cause: &ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         predicate: ty::PolyCoercePredicate<'tcx>,
-    ) -> Option<InferResult<'tcx, ()>> {
+    ) -> Result<InferResult<'tcx, ()>, (TyVid, TyVid)> {
         let subtype_predicate = predicate.map_bound(|p| ty::SubtypePredicate {
             a_is_expected: false, // when coercing from `a` to `b`, `b` is expected
             a: p.a,
@@ -1002,7 +1036,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         cause: &ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         predicate: ty::PolySubtypePredicate<'tcx>,
-    ) -> Option<InferResult<'tcx, ()>> {
+    ) -> Result<InferResult<'tcx, ()>, (TyVid, TyVid)> {
         // Check for two unresolved inference variables, in which case we can
         // make no progress. This is partly a micro-optimization, but it's
         // also an opportunity to "sub-unify" the variables. This isn't
@@ -1021,12 +1055,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         match (r_a.kind(), r_b.kind()) {
             (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
                 self.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
-                return None;
+                return Err((a_vid, b_vid));
             }
             _ => {}
         }
 
-        Some(self.commit_if_ok(|_snapshot| {
+        Ok(self.commit_if_ok(|_snapshot| {
             let ty::SubtypePredicate { a_is_expected, a, b } =
                 self.replace_bound_vars_with_placeholders(predicate);
 
@@ -1227,23 +1261,25 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn is_tainted_by_errors(&self) -> bool {
         debug!(
             "is_tainted_by_errors(err_count={}, err_count_on_creation={}, \
-             tainted_by_errors_flag={})",
+             tainted_by_errors={})",
             self.tcx.sess.err_count(),
             self.err_count_on_creation,
-            self.tainted_by_errors_flag.get()
+            self.tainted_by_errors.get().is_some()
         );
 
         if self.tcx.sess.err_count() > self.err_count_on_creation {
             return true; // errors reported since this infcx was made
         }
-        self.tainted_by_errors_flag.get()
+        self.tainted_by_errors.get().is_some()
     }
 
     /// Set the "tainted by errors" flag to true. We call this when we
     /// observe an error from a prior pass.
     pub fn set_tainted_by_errors(&self) {
         debug!("set_tainted_by_errors()");
-        self.tainted_by_errors_flag.set(true)
+        self.tainted_by_errors.set(Some(
+            self.tcx.sess.delay_span_bug(DUMMY_SP, "`InferCtxt` incorrectly tainted by errors"),
+        ));
     }
 
     pub fn skip_region_resolution(&self) {
@@ -1313,7 +1349,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// `resolve_vars_if_possible` as well as `fully_resolve`.
     ///
     /// Make sure to call [`InferCtxt::process_registered_region_obligations`]
-    /// first, or preferrably use [`InferCtxt::check_region_obligations_and_report_errors`]
+    /// first, or preferably use [`InferCtxt::check_region_obligations_and_report_errors`]
     /// to do both of these operations together.
     pub fn resolve_regions_and_report_errors(
         &self,
@@ -1527,8 +1563,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         actual: Ty<'tcx>,
         err: TypeError<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
-        let trace = TypeTrace::types(cause, true, expected, actual);
-        self.report_and_explain_type_error(trace, &err)
+        self.report_and_explain_type_error(TypeTrace::types(cause, true, expected, actual), err)
     }
 
     pub fn report_mismatched_consts(
@@ -1538,8 +1573,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         actual: ty::Const<'tcx>,
         err: TypeError<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
-        let trace = TypeTrace::consts(cause, true, expected, actual);
-        self.report_and_explain_type_error(trace, &err)
+        self.report_and_explain_type_error(TypeTrace::consts(cause, true, expected, actual), err)
     }
 
     pub fn replace_bound_vars_with_fresh_vars<T>(
@@ -1656,7 +1690,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn try_const_eval_resolve(
         &self,
         param_env: ty::ParamEnv<'tcx>,
-        unevaluated: ty::Unevaluated<'tcx>,
+        unevaluated: ty::Unevaluated<'tcx, ()>,
         ty: Ty<'tcx>,
         span: Option<Span>,
     ) -> Result<ty::Const<'tcx>, ErrorHandled> {
@@ -1691,7 +1725,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn const_eval_resolve(
         &self,
         mut param_env: ty::ParamEnv<'tcx>,
-        unevaluated: ty::Unevaluated<'tcx>,
+        unevaluated: ty::Unevaluated<'tcx, ()>,
         span: Option<Span>,
     ) -> EvalToValTreeResult<'tcx> {
         let mut substs = self.resolve_vars_if_possible(unevaluated.substs);
@@ -1700,7 +1734,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         // Postpone the evaluation of constants whose substs depend on inference
         // variables
         if substs.has_infer_types_or_consts() {
-            let ac = AbstractConst::new(self.tcx, unevaluated.shrink());
+            let ac = AbstractConst::new(self.tcx, unevaluated);
             match ac {
                 Ok(None) => {
                     substs = InternalSubsts::identity_for_item(self.tcx, unevaluated.def.did);
@@ -1722,11 +1756,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         debug!(?param_env_erased);
         debug!(?substs_erased);
 
-        let unevaluated = ty::Unevaluated {
-            def: unevaluated.def,
-            substs: substs_erased,
-            promoted: unevaluated.promoted,
-        };
+        let unevaluated =
+            ty::Unevaluated { def: unevaluated.def, substs: substs_erased, promoted: () };
 
         // The return value is the evaluated value which doesn't contain any reference to inference
         // variables, thus we don't need to substitute back the original values.
@@ -1814,7 +1845,7 @@ impl<'tcx> TyOrConstInferVar<'tcx> {
 
     /// Tries to extract an inference variable from a type, returns `None`
     /// for types other than `ty::Infer(_)` (or `InferTy::Fresh*`).
-    pub fn maybe_from_ty(ty: Ty<'tcx>) -> Option<Self> {
+    fn maybe_from_ty(ty: Ty<'tcx>) -> Option<Self> {
         match *ty.kind() {
             ty::Infer(ty::TyVar(v)) => Some(TyOrConstInferVar::Ty(v)),
             ty::Infer(ty::IntVar(v)) => Some(TyOrConstInferVar::TyInt(v)),
@@ -1825,7 +1856,7 @@ impl<'tcx> TyOrConstInferVar<'tcx> {
 
     /// Tries to extract an inference variable from a constant, returns `None`
     /// for constants other than `ty::ConstKind::Infer(_)` (or `InferConst::Fresh`).
-    pub fn maybe_from_const(ct: ty::Const<'tcx>) -> Option<Self> {
+    fn maybe_from_const(ct: ty::Const<'tcx>) -> Option<Self> {
         match ct.kind() {
             ty::ConstKind::Infer(InferConst::Var(v)) => Some(TyOrConstInferVar::Const(v)),
             _ => None,
@@ -1937,6 +1968,18 @@ impl<'tcx> TypeTrace<'tcx> {
         }
     }
 
+    pub fn poly_trait_refs(
+        cause: &ObligationCause<'tcx>,
+        a_is_expected: bool,
+        a: ty::PolyTraitRef<'tcx>,
+        b: ty::PolyTraitRef<'tcx>,
+    ) -> TypeTrace<'tcx> {
+        TypeTrace {
+            cause: cause.clone(),
+            values: PolyTraitRefs(ExpectedFound::new(a_is_expected, a.into(), b.into())),
+        }
+    }
+
     pub fn consts(
         cause: &ObligationCause<'tcx>,
         a_is_expected: bool,
@@ -1962,6 +2005,7 @@ impl<'tcx> SubregionOrigin<'tcx> {
             DataBorrowed(_, a) => a,
             ReferenceOutlivesReferent(_, a) => a,
             CompareImplItemObligation { span, .. } => span,
+            AscribeUserTypeProvePredicate(span) => span,
             CheckAssociatedTypeBounds { ref parent, .. } => parent.span(),
         }
     }
@@ -1994,6 +2038,10 @@ impl<'tcx> SubregionOrigin<'tcx> {
                 parent: Box::new(default()),
             },
 
+            traits::ObligationCauseCode::AscribeUserTypeProvePredicate(span) => {
+                SubregionOrigin::AscribeUserTypeProvePredicate(span)
+            }
+
             _ => default(),
         }
     }
@@ -2015,16 +2063,6 @@ impl RegionVariableOrigin {
     }
 }
 
-impl<'tcx> fmt::Debug for RegionObligation<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(
-            f,
-            "RegionObligation(sub_region={:?}, sup_type={:?})",
-            self.sub_region, self.sup_type
-        )
-    }
-}
-
 /// Replaces substs that reference param or infer variables with suitable
 /// placeholders. This function is meant to remove these param and infer
 /// substs when they're not actually needed to evaluate a constant.
diff --git a/compiler/rustc_infer/src/infer/nll_relate/mod.rs b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
index bab4f3e9e..00fc442d3 100644
--- a/compiler/rustc_infer/src/infer/nll_relate/mod.rs
+++ b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
@@ -396,6 +396,32 @@ where
 
         generalizer.relate(value, value)
     }
+
+    fn relate_opaques(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
+        let mut generalize = |ty, ty_is_expected| {
+            let var = self.infcx.next_ty_var_id_in_universe(
+                TypeVariableOrigin {
+                    kind: TypeVariableOriginKind::MiscVariable,
+                    span: self.delegate.span(),
+                },
+                ty::UniverseIndex::ROOT,
+            );
+            if ty_is_expected {
+                self.relate_ty_var((ty, var))
+            } else {
+                self.relate_ty_var((var, ty))
+            }
+        };
+        let (a, b) = match (a.kind(), b.kind()) {
+            (&ty::Opaque(..), _) => (a, generalize(b, false)?),
+            (_, &ty::Opaque(..)) => (generalize(a, true)?, b),
+            _ => unreachable!(),
+        };
+        self.delegate.register_opaque_type(a, b, true)?;
+        trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
+        Ok(a)
+    }
 }
 
 /// When we instantiate an inference variable with a value in
@@ -516,7 +542,7 @@ where
         true
     }
 
-    #[instrument(skip(self, info), level = "trace")]
+    #[instrument(skip(self, info), level = "trace", ret)]
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,
@@ -534,8 +560,6 @@ where
 
         self.ambient_variance = old_ambient_variance;
 
-        debug!(?r);
-
         Ok(r)
     }
 
@@ -572,32 +596,16 @@ where
             (&ty::Infer(ty::TyVar(vid)), _) => self.relate_ty_var((vid, b)),
 
             (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
-                self.infcx.super_combine_tys(self, a, b)
+                infcx.super_combine_tys(self, a, b).or_else(|err| {
+                    self.tcx().sess.delay_span_bug(
+                        self.delegate.span(),
+                        "failure to relate an opaque to itself should result in an error later on",
+                    );
+                    if a_def_id.is_local() { self.relate_opaques(a, b) } else { Err(err) }
+                })
             }
             (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..)) if did.is_local() => {
-                let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
-                let mut generalize = |ty, ty_is_expected| {
-                    let var = infcx.next_ty_var_id_in_universe(
-                        TypeVariableOrigin {
-                            kind: TypeVariableOriginKind::MiscVariable,
-                            span: self.delegate.span(),
-                        },
-                        ty::UniverseIndex::ROOT,
-                    );
-                    if ty_is_expected {
-                        self.relate_ty_var((ty, var))
-                    } else {
-                        self.relate_ty_var((var, ty))
-                    }
-                };
-                let (a, b) = match (a.kind(), b.kind()) {
-                    (&ty::Opaque(..), _) => (a, generalize(b, false)?),
-                    (_, &ty::Opaque(..)) => (generalize(a, true)?, b),
-                    _ => unreachable!(),
-                };
-                self.delegate.register_opaque_type(a, b, true)?;
-                trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
-                Ok(a)
+                self.relate_opaques(a, b)
             }
 
             (&ty::Projection(projection_ty), _)
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index e579afbf3..8c9ddf866 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -1,3 +1,4 @@
+use crate::errors::OpaqueHiddenTypeDiag;
 use crate::infer::{DefiningAnchor, InferCtxt, InferOk};
 use crate::traits;
 use hir::def_id::{DefId, LocalDefId};
@@ -72,7 +73,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     // for opaque types, and then use that kind to fix the spans for type errors
                     // that we see later on.
                     let ty_var = self.next_ty_var(TypeVariableOrigin {
-                        kind: TypeVariableOriginKind::TypeInference,
+                        kind: TypeVariableOriginKind::OpaqueTypeInference(def_id),
                         span,
                     });
                     obligations.extend(
@@ -153,22 +154,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     if let Some(OpaqueTyOrigin::TyAlias) =
                         did2.as_local().and_then(|did2| self.opaque_type_origin(did2, cause.span))
                     {
-                        self.tcx
-                                .sess
-                                .struct_span_err(
-                                    cause.span,
-                                    "opaque type's hidden type cannot be another opaque type from the same scope",
-                                )
-                                .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope")
-                                .span_note(
-                                    self.tcx.def_span(def_id),
-                                    "opaque type whose hidden type is being assigned",
-                                )
-                                .span_note(
-                                    self.tcx.def_span(did2),
-                                    "opaque type being used as hidden type",
-                                )
-                                .emit();
+                        self.tcx.sess.emit_err(OpaqueHiddenTypeDiag {
+                            span: cause.span,
+                            hidden_type: self.tcx.def_span(did2),
+                            opaque_type: self.tcx.def_span(def_id),
+                        });
                     }
                 }
                 Some(self.register_hidden_type(
@@ -400,7 +390,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         });
     }
 
-    #[instrument(skip(self), level = "trace")]
+    #[instrument(skip(self), level = "trace", ret)]
     pub fn opaque_type_origin(&self, def_id: LocalDefId, span: Span) -> Option<OpaqueTyOrigin> {
         let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
         let parent_def_id = match self.defining_use_anchor {
@@ -431,16 +421,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         in_definition_scope.then_some(*origin)
     }
 
-    #[instrument(skip(self), level = "trace")]
+    #[instrument(skip(self), level = "trace", ret)]
     fn opaque_ty_origin_unchecked(&self, def_id: LocalDefId, span: Span) -> OpaqueTyOrigin {
-        let origin = match self.tcx.hir().expect_item(def_id).kind {
+        match self.tcx.hir().expect_item(def_id).kind {
             hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
             ref itemkind => {
                 span_bug!(span, "weird opaque type: {:?}, {:#?}", def_id, itemkind)
             }
-        };
-        trace!(?origin);
-        origin
+        }
     }
 }
 
diff --git a/compiler/rustc_infer/src/infer/opaque_types/table.rs b/compiler/rustc_infer/src/infer/opaque_types/table.rs
index fb12da0cc..4d124554a 100644
--- a/compiler/rustc_infer/src/infer/opaque_types/table.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types/table.rs
@@ -29,7 +29,7 @@ impl<'tcx> OpaqueTypeStorage<'tcx> {
         }
     }
 
-    #[instrument(level = "debug")]
+    #[instrument(level = "debug", ret)]
     pub fn take_opaque_types(&mut self) -> OpaqueTypeMap<'tcx> {
         std::mem::take(&mut self.opaque_types)
     }
diff --git a/compiler/rustc_infer/src/infer/outlives/env.rs b/compiler/rustc_infer/src/infer/outlives/env.rs
index b2decd64f..9922b156e 100644
--- a/compiler/rustc_infer/src/infer/outlives/env.rs
+++ b/compiler/rustc_infer/src/infer/outlives/env.rs
@@ -2,6 +2,7 @@ use crate::infer::free_regions::FreeRegionMap;
 use crate::infer::{GenericKind, InferCtxt};
 use crate::traits::query::OutlivesBound;
 use rustc_data_structures::fx::FxIndexSet;
+use rustc_data_structures::transitive_relation::TransitiveRelationBuilder;
 use rustc_middle::ty::{self, ReEarlyBound, ReFree, ReVar, Region};
 
 use super::explicit_outlives_bounds;
@@ -51,23 +52,49 @@ pub struct OutlivesEnvironment<'tcx> {
     region_bound_pairs: RegionBoundPairs<'tcx>,
 }
 
+/// Builder of OutlivesEnvironment.
+#[derive(Debug)]
+struct OutlivesEnvironmentBuilder<'tcx> {
+    param_env: ty::ParamEnv<'tcx>,
+    region_relation: TransitiveRelationBuilder<Region<'tcx>>,
+    region_bound_pairs: RegionBoundPairs<'tcx>,
+}
+
 /// "Region-bound pairs" tracks outlives relations that are known to
 /// be true, either because of explicit where-clauses like `T: 'a` or
 /// because of implied bounds.
 pub type RegionBoundPairs<'tcx> =
     FxIndexSet<ty::OutlivesPredicate<GenericKind<'tcx>, Region<'tcx>>>;
 
-impl<'a, 'tcx> OutlivesEnvironment<'tcx> {
-    pub fn new(param_env: ty::ParamEnv<'tcx>) -> Self {
-        let mut env = OutlivesEnvironment {
+impl<'tcx> OutlivesEnvironment<'tcx> {
+    /// Create a builder using `ParamEnv` and add explicit outlives bounds into it.
+    fn builder(param_env: ty::ParamEnv<'tcx>) -> OutlivesEnvironmentBuilder<'tcx> {
+        let mut builder = OutlivesEnvironmentBuilder {
             param_env,
-            free_region_map: Default::default(),
+            region_relation: Default::default(),
             region_bound_pairs: Default::default(),
         };
 
-        env.add_outlives_bounds(None, explicit_outlives_bounds(param_env));
+        builder.add_outlives_bounds(None, explicit_outlives_bounds(param_env));
 
-        env
+        builder
+    }
+
+    #[inline]
+    /// Create a new `OutlivesEnvironment` without extra outlives bounds.
+    pub fn new(param_env: ty::ParamEnv<'tcx>) -> Self {
+        Self::builder(param_env).build()
+    }
+
+    /// Create a new `OutlivesEnvironment` with extra outlives bounds.
+    pub fn with_bounds<'a>(
+        param_env: ty::ParamEnv<'tcx>,
+        infcx: Option<&InferCtxt<'a, 'tcx>>,
+        extra_bounds: impl IntoIterator<Item = OutlivesBound<'tcx>>,
+    ) -> Self {
+        let mut builder = Self::builder(param_env);
+        builder.add_outlives_bounds(infcx, extra_bounds);
+        builder.build()
     }
 
     /// Borrows current value of the `free_region_map`.
@@ -79,6 +106,18 @@ impl<'a, 'tcx> OutlivesEnvironment<'tcx> {
     pub fn region_bound_pairs(&self) -> &RegionBoundPairs<'tcx> {
         &self.region_bound_pairs
     }
+}
+
+impl<'a, 'tcx> OutlivesEnvironmentBuilder<'tcx> {
+    #[inline]
+    #[instrument(level = "debug")]
+    fn build(self) -> OutlivesEnvironment<'tcx> {
+        OutlivesEnvironment {
+            param_env: self.param_env,
+            free_region_map: FreeRegionMap { relation: self.region_relation.freeze() },
+            region_bound_pairs: self.region_bound_pairs,
+        }
+    }
 
     /// Processes outlives bounds that are known to hold, whether from implied or other sources.
     ///
@@ -86,11 +125,8 @@ impl<'a, 'tcx> OutlivesEnvironment<'tcx> {
     /// contain inference variables, it must be supplied, in which
     /// case we will register "givens" on the inference context. (See
     /// `RegionConstraintData`.)
-    pub fn add_outlives_bounds<I>(
-        &mut self,
-        infcx: Option<&InferCtxt<'a, 'tcx>>,
-        outlives_bounds: I,
-    ) where
+    fn add_outlives_bounds<I>(&mut self, infcx: Option<&InferCtxt<'a, 'tcx>>, outlives_bounds: I)
+    where
         I: IntoIterator<Item = OutlivesBound<'tcx>>,
     {
         // Record relationships such as `T:'x` that don't go into the
@@ -122,7 +158,9 @@ impl<'a, 'tcx> OutlivesEnvironment<'tcx> {
                         // system to be more general and to make use
                         // of *every* relationship that arises here,
                         // but presently we do not.)
-                        self.free_region_map.relate_regions(r_a, r_b);
+                        if r_a.is_free_or_static() && r_b.is_free() {
+                            self.region_relation.add(r_a, r_b)
+                        }
                     }
                 }
             }
diff --git a/compiler/rustc_infer/src/infer/outlives/mod.rs b/compiler/rustc_infer/src/infer/outlives/mod.rs
index 2a085288f..2d19d1823 100644
--- a/compiler/rustc_infer/src/infer/outlives/mod.rs
+++ b/compiler/rustc_infer/src/infer/outlives/mod.rs
@@ -9,7 +9,7 @@ pub mod verify;
 use rustc_middle::traits::query::OutlivesBound;
 use rustc_middle::ty;
 
-#[instrument(level = "debug", skip(param_env))]
+#[instrument(level = "debug", skip(param_env), ret)]
 pub fn explicit_outlives_bounds<'tcx>(
     param_env: ty::ParamEnv<'tcx>,
 ) -> impl Iterator<Item = OutlivesBound<'tcx>> + 'tcx {
diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
index ad052f58c..5bd1774f6 100644
--- a/compiler/rustc_infer/src/infer/outlives/obligations.rs
+++ b/compiler/rustc_infer/src/infer/outlives/obligations.rs
@@ -69,6 +69,7 @@ use crate::infer::{
 use crate::traits::{ObligationCause, ObligationCauseCode};
 use rustc_data_structures::undo_log::UndoLogs;
 use rustc_hir::def_id::LocalDefId;
+use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{self, Region, Ty, TyCtxt, TypeVisitable};
 use smallvec::smallvec;
@@ -92,12 +93,14 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         sub_region: Region<'tcx>,
         cause: &ObligationCause<'tcx>,
     ) {
+        debug!(?sup_type, ?sub_region, ?cause);
         let origin = SubregionOrigin::from_obligation_cause(cause, || {
             infer::RelateParamBound(
                 cause.span,
                 sup_type,
                 match cause.code().peel_derives() {
-                    ObligationCauseCode::BindingObligation(_, span) => Some(*span),
+                    ObligationCauseCode::BindingObligation(_, span)
+                    | ObligationCauseCode::ExprBindingObligation(_, span, ..) => Some(*span),
                     _ => None,
                 },
             )
@@ -161,7 +164,8 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
 
             let outlives =
                 &mut TypeOutlives::new(self, self.tcx, &region_bound_pairs, None, param_env);
-            outlives.type_must_outlive(origin, sup_type, sub_region);
+            let category = origin.to_constraint_category();
+            outlives.type_must_outlive(origin, sup_type, sub_region, category);
         }
     }
 
@@ -205,6 +209,7 @@ pub trait TypeOutlivesDelegate<'tcx> {
         origin: SubregionOrigin<'tcx>,
         a: ty::Region<'tcx>,
         b: ty::Region<'tcx>,
+        constraint_category: ConstraintCategory<'tcx>,
     );
 
     fn push_verify(
@@ -247,19 +252,19 @@ where
     /// - `origin`, the reason we need this constraint
     /// - `ty`, the type `T`
     /// - `region`, the region `'a`
+    #[instrument(level = "debug", skip(self))]
     pub fn type_must_outlive(
         &mut self,
         origin: infer::SubregionOrigin<'tcx>,
         ty: Ty<'tcx>,
         region: ty::Region<'tcx>,
+        category: ConstraintCategory<'tcx>,
     ) {
-        debug!("type_must_outlive(ty={:?}, region={:?}, origin={:?})", ty, region, origin);
-
         assert!(!ty.has_escaping_bound_vars());
 
         let mut components = smallvec![];
         push_outlives_components(self.tcx, ty, &mut components);
-        self.components_must_outlive(origin, &components, region);
+        self.components_must_outlive(origin, &components, region, category);
     }
 
     fn components_must_outlive(
@@ -267,12 +272,13 @@ where
         origin: infer::SubregionOrigin<'tcx>,
         components: &[Component<'tcx>],
         region: ty::Region<'tcx>,
+        category: ConstraintCategory<'tcx>,
     ) {
         for component in components.iter() {
             let origin = origin.clone();
             match component {
                 Component::Region(region1) => {
-                    self.delegate.push_sub_region_constraint(origin, region, *region1);
+                    self.delegate.push_sub_region_constraint(origin, region, *region1, category);
                 }
                 Component::Param(param_ty) => {
                     self.param_ty_must_outlive(origin, region, *param_ty);
@@ -281,7 +287,7 @@ where
                     self.projection_must_outlive(origin, region, *projection_ty);
                 }
                 Component::EscapingProjection(subcomponents) => {
-                    self.components_must_outlive(origin, &subcomponents, region);
+                    self.components_must_outlive(origin, &subcomponents, region, category);
                 }
                 Component::UnresolvedInferenceVariable(v) => {
                     // ignore this, we presume it will yield an error
@@ -312,7 +318,7 @@ where
         self.delegate.push_verify(origin, generic, region, verify_bound);
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn projection_must_outlive(
         &mut self,
         origin: infer::SubregionOrigin<'tcx>,
@@ -388,13 +394,19 @@ where
         if approx_env_bounds.is_empty() && trait_bounds.is_empty() && needs_infer {
             debug!("projection_must_outlive: no declared bounds");
 
+            let constraint = origin.to_constraint_category();
             for k in projection_ty.substs {
                 match k.unpack() {
                     GenericArgKind::Lifetime(lt) => {
-                        self.delegate.push_sub_region_constraint(origin.clone(), region, lt);
+                        self.delegate.push_sub_region_constraint(
+                            origin.clone(),
+                            region,
+                            lt,
+                            constraint,
+                        );
                     }
                     GenericArgKind::Type(ty) => {
-                        self.type_must_outlive(origin.clone(), ty, region);
+                        self.type_must_outlive(origin.clone(), ty, region, constraint);
                     }
                     GenericArgKind::Const(_) => {
                         // Const parameters don't impose constraints.
@@ -432,7 +444,8 @@ where
             let unique_bound = trait_bounds[0];
             debug!("projection_must_outlive: unique trait bound = {:?}", unique_bound);
             debug!("projection_must_outlive: unique declared bound appears in trait ref");
-            self.delegate.push_sub_region_constraint(origin, region, unique_bound);
+            let category = origin.to_constraint_category();
+            self.delegate.push_sub_region_constraint(origin, region, unique_bound, category);
             return;
         }
 
@@ -454,6 +467,7 @@ impl<'cx, 'tcx> TypeOutlivesDelegate<'tcx> for &'cx InferCtxt<'cx, 'tcx> {
         origin: SubregionOrigin<'tcx>,
         a: ty::Region<'tcx>,
         b: ty::Region<'tcx>,
+        _constraint_category: ConstraintCategory<'tcx>,
     ) {
         self.sub_regions(origin, a, b)
     }
diff --git a/compiler/rustc_infer/src/infer/outlives/test_type_match.rs b/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
index 772e297b7..a5c21f0fb 100644
--- a/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
+++ b/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
@@ -34,7 +34,7 @@ use crate::infer::region_constraints::VerifyIfEq;
 /// like are used. This is a particular challenge since this function is invoked
 /// very late in inference and hence cannot make use of the normal inference
 /// machinery.
-#[tracing::instrument(level = "debug", skip(tcx, param_env))]
+#[instrument(level = "debug", skip(tcx, param_env))]
 pub fn extract_verify_if_eq<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
@@ -71,7 +71,7 @@ pub fn extract_verify_if_eq<'tcx>(
 }
 
 /// True if a (potentially higher-ranked) outlives
-#[tracing::instrument(level = "debug", skip(tcx, param_env))]
+#[instrument(level = "debug", skip(tcx, param_env))]
 pub(super) fn can_match_erased_ty<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
@@ -110,7 +110,7 @@ impl<'tcx> Match<'tcx> {
 
     /// Binds the pattern variable `br` to `value`; returns an `Err` if the pattern
     /// is already bound to a different value.
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn bind(
         &mut self,
         br: ty::BoundRegion,
@@ -174,7 +174,14 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     fn tys(&mut self, pattern: Ty<'tcx>, value: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        if pattern == value { Ok(pattern) } else { relate::super_relate_tys(self, pattern, value) }
+        if let ty::Error(_) = pattern.kind() {
+            // Unlike normal `TypeRelation` rules, `ty::Error` does not equal any type.
+            self.no_match()
+        } else if pattern == value {
+            Ok(pattern)
+        } else {
+            relate::super_relate_tys(self, pattern, value)
+        }
     }
 
     #[instrument(skip(self), level = "debug")]
diff --git a/compiler/rustc_infer/src/infer/outlives/verify.rs b/compiler/rustc_infer/src/infer/outlives/verify.rs
index c7d7ef40d..752334950 100644
--- a/compiler/rustc_infer/src/infer/outlives/verify.rs
+++ b/compiler/rustc_infer/src/infer/outlives/verify.rs
@@ -50,13 +50,13 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn param_bound(&self, param_ty: ty::ParamTy) -> VerifyBound<'tcx> {
-        debug!("param_bound(param_ty={:?})", param_ty);
-
         // Start with anything like `T: 'a` we can scrape from the
         // environment. If the environment contains something like
         // `for<'a> T: 'a`, then we know that `T` outlives everything.
         let declared_bounds_from_env = self.declared_generic_bounds_from_env(param_ty);
+        debug!(?declared_bounds_from_env);
         let mut param_bounds = vec![];
         for declared_bound in declared_bounds_from_env {
             let bound_region = declared_bound.map_bound(|outlives| outlives.1);
@@ -65,6 +65,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
                 param_bounds.push(VerifyBound::OutlivedBy(region));
             } else {
                 // This is `for<'a> T: 'a`. This means that `T` outlives everything! All done here.
+                debug!("found that {param_ty:?} outlives any lifetime, returning empty vector");
                 return VerifyBound::AllBounds(vec![]);
             }
         }
@@ -72,6 +73,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         // Add in the default bound of fn body that applies to all in
         // scope type parameters:
         if let Some(r) = self.implicit_region_bound {
+            debug!("adding implicit region bound of {r:?}");
             param_bounds.push(VerifyBound::OutlivedBy(r));
         }
 
diff --git a/compiler/rustc_infer/src/infer/region_constraints/mod.rs b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
index 0d4472a1c..e43d28ee5 100644
--- a/compiler/rustc_infer/src/infer/region_constraints/mod.rs
+++ b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
@@ -187,7 +187,7 @@ pub enum GenericKind<'tcx> {
 /// }
 /// ```
 /// This is described with an `AnyRegion('a, 'b)` node.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, TypeFoldable, TypeVisitable)]
 pub enum VerifyBound<'tcx> {
     /// See [`VerifyIfEq`] docs
     IfEq(ty::Binder<'tcx, VerifyIfEq<'tcx>>),
@@ -426,21 +426,21 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         data
     }
 
-    pub fn data(&self) -> &RegionConstraintData<'tcx> {
+    pub(super) fn data(&self) -> &RegionConstraintData<'tcx> {
         &self.data
     }
 
-    pub fn start_snapshot(&mut self) -> RegionSnapshot {
+    pub(super) fn start_snapshot(&mut self) -> RegionSnapshot {
         debug!("RegionConstraintCollector: start_snapshot");
         RegionSnapshot { any_unifications: self.any_unifications }
     }
 
-    pub fn rollback_to(&mut self, snapshot: RegionSnapshot) {
+    pub(super) fn rollback_to(&mut self, snapshot: RegionSnapshot) {
         debug!("RegionConstraintCollector: rollback_to({:?})", snapshot);
         self.any_unifications = snapshot.any_unifications;
     }
 
-    pub fn new_region_var(
+    pub(super) fn new_region_var(
         &mut self,
         universe: ty::UniverseIndex,
         origin: RegionVariableOrigin,
@@ -455,12 +455,12 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
     }
 
     /// Returns the universe for the given variable.
-    pub fn var_universe(&self, vid: RegionVid) -> ty::UniverseIndex {
+    pub(super) fn var_universe(&self, vid: RegionVid) -> ty::UniverseIndex {
         self.var_infos[vid].universe
     }
 
     /// Returns the origin for the given variable.
-    pub fn var_origin(&self, vid: RegionVid) -> RegionVariableOrigin {
+    pub(super) fn var_origin(&self, vid: RegionVid) -> RegionVariableOrigin {
         self.var_infos[vid].origin
     }
 
@@ -492,7 +492,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         self.undo_log.push(AddVerify(index));
     }
 
-    pub fn add_given(&mut self, sub: Region<'tcx>, sup: ty::RegionVid) {
+    pub(super) fn add_given(&mut self, sub: Region<'tcx>, sup: ty::RegionVid) {
         // cannot add givens once regions are resolved
         if self.data.givens.insert((sub, sup)) {
             debug!("add_given({:?} <= {:?})", sub, sup);
@@ -501,7 +501,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         }
     }
 
-    pub fn make_eqregion(
+    pub(super) fn make_eqregion(
         &mut self,
         origin: SubregionOrigin<'tcx>,
         sub: Region<'tcx>,
@@ -530,7 +530,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         }
     }
 
-    pub fn member_constraint(
+    pub(super) fn member_constraint(
         &mut self,
         key: ty::OpaqueTypeKey<'tcx>,
         definition_span: Span,
@@ -554,7 +554,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
     }
 
     #[instrument(skip(self, origin), level = "debug")]
-    pub fn make_subregion(
+    pub(super) fn make_subregion(
         &mut self,
         origin: SubregionOrigin<'tcx>,
         sub: Region<'tcx>,
@@ -585,7 +585,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         }
     }
 
-    pub fn verify_generic_bound(
+    pub(super) fn verify_generic_bound(
         &mut self,
         origin: SubregionOrigin<'tcx>,
         kind: GenericKind<'tcx>,
@@ -595,7 +595,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         self.add_verify(Verify { kind, origin, region: sub, bound });
     }
 
-    pub fn lub_regions(
+    pub(super) fn lub_regions(
         &mut self,
         tcx: TyCtxt<'tcx>,
         origin: SubregionOrigin<'tcx>,
@@ -613,7 +613,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         }
     }
 
-    pub fn glb_regions(
+    pub(super) fn glb_regions(
         &mut self,
         tcx: TyCtxt<'tcx>,
         origin: SubregionOrigin<'tcx>,
@@ -634,7 +634,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
     }
 
     /// Resolves the passed RegionVid to the root RegionVid in the unification table
-    pub fn opportunistic_resolve_var(&mut self, rid: ty::RegionVid) -> ty::RegionVid {
+    pub(super) fn opportunistic_resolve_var(&mut self, rid: ty::RegionVid) -> ty::RegionVid {
         self.unification_table().find(rid).vid
     }
 
@@ -699,7 +699,6 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
             ty::ReStatic | ty::ReErased | ty::ReFree(..) | ty::ReEarlyBound(..) => {
                 ty::UniverseIndex::ROOT
             }
-            ty::ReEmpty(ui) => ui,
             ty::RePlaceholder(placeholder) => placeholder.universe,
             ty::ReVar(vid) => self.var_universe(vid),
             ty::ReLateBound(..) => bug!("universe(): encountered bound region {:?}", region),
diff --git a/compiler/rustc_infer/src/infer/sub.rs b/compiler/rustc_infer/src/infer/sub.rs
index b27571275..b7eab5d43 100644
--- a/compiler/rustc_infer/src/infer/sub.rs
+++ b/compiler/rustc_infer/src/infer/sub.rs
@@ -4,6 +4,7 @@ use super::SubregionOrigin;
 use crate::infer::combine::ConstEquateRelation;
 use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::traits::Obligation;
+use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::visit::TypeVisitable;
 use rustc_middle::ty::TyVar;
@@ -141,17 +142,27 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
                     Ok(infcx.tcx.mk_ty_var(var))
                 };
                 let (a, b) = if self.a_is_expected { (a, b) } else { (b, a) };
-                let (a, b) = match (a.kind(), b.kind()) {
+                let (ga, gb) = match (a.kind(), b.kind()) {
                     (&ty::Opaque(..), _) => (a, generalize(b, true)?),
                     (_, &ty::Opaque(..)) => (generalize(a, false)?, b),
                     _ => unreachable!(),
                 };
                 self.fields.obligations.extend(
                     infcx
-                        .handle_opaque_type(a, b, true, &self.fields.trace.cause, self.param_env())?
+                        .handle_opaque_type(ga, gb, true, &self.fields.trace.cause, self.param_env())
+                        // Don't leak any generalized type variables out of this
+                        // subtyping relation in the case of a type error.
+                        .map_err(|err| {
+                            let (ga, gb) = self.fields.infcx.resolve_vars_if_possible((ga, gb));
+                            if let TypeError::Sorts(sorts) = err && sorts.expected == ga && sorts.found == gb {
+                                TypeError::Sorts(ExpectedFound { expected: a, found: b })
+                            } else {
+                                err
+                            }
+                        })?
                         .obligations,
                 );
-                Ok(a)
+                Ok(ga)
             }
 
             _ => {
diff --git a/compiler/rustc_infer/src/infer/type_variable.rs b/compiler/rustc_infer/src/infer/type_variable.rs
index a0e2965b6..7ff086452 100644
--- a/compiler/rustc_infer/src/infer/type_variable.rs
+++ b/compiler/rustc_infer/src/infer/type_variable.rs
@@ -122,6 +122,7 @@ pub enum TypeVariableOriginKind {
     MiscVariable,
     NormalizeProjectionType,
     TypeInference,
+    OpaqueTypeInference(DefId),
     TypeParameterDefinition(Symbol, Option<DefId>),
 
     /// One of the upvars or closure kind parameters in a `ClosureSubsts`
diff --git a/compiler/rustc_infer/src/infer/undo_log.rs b/compiler/rustc_infer/src/infer/undo_log.rs
index 74a26ebc3..611961ab1 100644
--- a/compiler/rustc_infer/src/infer/undo_log.rs
+++ b/compiler/rustc_infer/src/infer/undo_log.rs
@@ -100,7 +100,7 @@ impl Default for InferCtxtUndoLogs<'_> {
 }
 
 /// The UndoLogs trait defines how we undo a particular kind of action (of type T). We can undo any
-/// action that is convertable into an UndoLog (per the From impls above).
+/// action that is convertible into an UndoLog (per the From impls above).
 impl<'tcx, T> UndoLogs<T> for InferCtxtUndoLogs<'tcx>
 where
     UndoLog<'tcx>: From<T>,
diff --git a/compiler/rustc_infer/src/lib.rs b/compiler/rustc_infer/src/lib.rs
index 7769a68ba..ef60d2c91 100644
--- a/compiler/rustc_infer/src/lib.rs
+++ b/compiler/rustc_infer/src/lib.rs
@@ -17,9 +17,9 @@
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(extend_one)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(try_blocks)]
@@ -35,5 +35,6 @@ extern crate tracing;
 #[macro_use]
 extern crate rustc_middle;
 
+mod errors;
 pub mod infer;
 pub mod traits;