Auto merge of #103171 - jackh726:gen-interior-hrtb-error, r=cjgillot

Better error for HRTB error from generator interior

cc #100013

This is just a first pass at an error. It could be better, and shouldn't really be emitted in the first place. But this is better than what was being emitted before.

23 files changed, 344 insertions, 63 deletions

diff --git a/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs
index dac6abe37f5..897a161f785 100644
--- a/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs
@@ -158,6 +158,7 @@ trait TypeOpInfo<'tcx> {
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>>;
 
+    #[instrument(level = "debug", skip(self, mbcx))]
     fn report_error(
         &self,
         mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
@@ -167,6 +168,7 @@ trait TypeOpInfo<'tcx> {
     ) {
         let tcx = mbcx.infcx.tcx;
         let base_universe = self.base_universe();
+        debug!(?base_universe);
 
         let Some(adjusted_universe) =
             placeholder.universe.as_u32().checked_sub(base_universe.as_u32())
@@ -389,6 +391,7 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
     )
 }
 
+#[instrument(level = "debug", skip(infcx, region_var_origin, universe_of_region))]
 fn try_extract_error_from_region_constraints<'tcx>(
     infcx: &InferCtxt<'tcx>,
     placeholder_region: ty::Region<'tcx>,
diff --git a/compiler/rustc_borrowck/src/diagnostics/region_name.rs b/compiler/rustc_borrowck/src/diagnostics/region_name.rs
index c044dbaba47..f9741bacd17 100644
--- a/compiler/rustc_borrowck/src/diagnostics/region_name.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/region_name.rs
@@ -355,7 +355,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                     })
                 }
 
-                ty::BoundRegionKind::BrAnon(_) => None,
+                ty::BoundRegionKind::BrAnon(..) => None,
             },
 
             ty::ReLateBound(..) | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReErased => None,
diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index 2665813478c..4518cf30acd 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -2980,7 +2980,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     ) {
         for br in referenced_regions.difference(&constrained_regions) {
             let br_name = match *br {
-                ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) | ty::BrEnv => {
+                ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(..) | ty::BrEnv => {
                     "an anonymous lifetime".to_string()
                 }
                 ty::BrNamed(_, name) => format!("lifetime `{}`", name),
@@ -2988,7 +2988,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
             let mut err = generate_err(&br_name);
 
-            if let ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) = *br {
+            if let ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(..) = *br {
                 // The only way for an anonymous lifetime to wind up
                 // in the return type but **also** be unconstrained is
                 // if it only appears in "associated types" in the
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index 609095c9cea..69e54b41d4c 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -134,15 +134,18 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
     let name_str = intrinsic_name.as_str();
 
     let bound_vars = tcx.mk_bound_variable_kinds(
-        [ty::BoundVariableKind::Region(ty::BrAnon(0)), ty::BoundVariableKind::Region(ty::BrEnv)]
-            .iter()
-            .copied(),
+        [
+            ty::BoundVariableKind::Region(ty::BrAnon(0, None)),
+            ty::BoundVariableKind::Region(ty::BrEnv),
+        ]
+        .iter()
+        .copied(),
     );
     let mk_va_list_ty = |mutbl| {
         tcx.lang_items().va_list().map(|did| {
             let region = tcx.mk_region(ty::ReLateBound(
                 ty::INNERMOST,
-                ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon(0) },
+                ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon(0, None) },
             ));
             let env_region = tcx.mk_region(ty::ReLateBound(
                 ty::INNERMOST,
@@ -364,7 +367,8 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
                 );
                 let discriminant_def_id = assoc_items[0];
 
-                let br = ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon(0) };
+                let br =
+                    ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon(0, None) };
                 (
                     1,
                     vec![
@@ -418,7 +422,8 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
             sym::nontemporal_store => (1, vec![tcx.mk_mut_ptr(param(0)), param(0)], tcx.mk_unit()),
 
             sym::raw_eq => {
-                let br = ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon(0) };
+                let br =
+                    ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon(0, None) };
                 let param_ty =
                     tcx.mk_imm_ref(tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br)), param(0));
                 (1, vec![param_ty; 2], tcx.types.bool)
diff --git a/compiler/rustc_hir_typeck/src/generator_interior/mod.rs b/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
index b7dd599cd43..7bbfb70f2c3 100644
--- a/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
+++ b/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
@@ -13,10 +13,13 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::hir_id::HirIdSet;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{Arm, Expr, ExprKind, Guard, HirId, Pat, PatKind};
+use rustc_infer::infer::RegionVariableOrigin;
 use rustc_middle::middle::region::{self, Scope, ScopeData, YieldData};
-use rustc_middle::ty::{self, RvalueScopes, Ty, TyCtxt, TypeVisitable};
+use rustc_middle::ty::fold::FnMutDelegate;
+use rustc_middle::ty::{self, BoundVariableKind, RvalueScopes, Ty, TyCtxt, TypeVisitable};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
+use smallvec::{smallvec, SmallVec};
 
 mod drop_ranges;
 
@@ -211,31 +214,57 @@ pub fn resolve_interior<'a, 'tcx>(
 
     debug!("types in generator {:?}, span = {:?}", types, body.value.span);
 
-    let mut counter = 0;
+    // We want to deduplicate if the lifetimes are the same modulo some non-informative counter.
+    // So, we need to actually do two passes: first by type to anonymize (preserving information
+    // required for diagnostics), then a second pass over all captured types to reassign disjoint
+    // region indices.
     let mut captured_tys = FxHashSet::default();
     let type_causes: Vec<_> = types
         .into_iter()
         .filter_map(|mut cause| {
-            // Erase regions and canonicalize late-bound regions to deduplicate as many types as we
-            // can.
+            // Replace all regions inside the generator interior with late bound regions.
+            // Note that each region slot in the types gets a new fresh late bound region,
+            // which means that none of the regions inside relate to any other, even if
+            // typeck had previously found constraints that would cause them to be related.
+
+            let mut counter = 0;
+            let mut mk_bound_region = |span| {
+                let kind = ty::BrAnon(counter, span);
+                let var = ty::BoundVar::from_u32(counter);
+                counter += 1;
+                ty::BoundRegion { var, kind }
+            };
             let ty = fcx.normalize_associated_types_in(cause.span, cause.ty);
-            let erased = fcx.tcx.erase_regions(ty);
-            if captured_tys.insert(erased) {
-                // Replace all regions inside the generator interior with late bound regions.
-                // Note that each region slot in the types gets a new fresh late bound region,
-                // which means that none of the regions inside relate to any other, even if
-                // typeck had previously found constraints that would cause them to be related.
-                let folded = fcx.tcx.fold_regions(erased, |_, current_depth| {
-                    let br = ty::BoundRegion {
-                        var: ty::BoundVar::from_u32(counter),
-                        kind: ty::BrAnon(counter),
-                    };
-                    let r = fcx.tcx.mk_region(ty::ReLateBound(current_depth, br));
-                    counter += 1;
-                    r
-                });
-
-                cause.ty = folded;
+            let ty = fcx.tcx.fold_regions(ty, |region, current_depth| {
+                let br = match region.kind() {
+                    ty::ReVar(vid) => {
+                        let origin = fcx.region_var_origin(vid);
+                        match origin {
+                            RegionVariableOrigin::EarlyBoundRegion(span, _) => {
+                                mk_bound_region(Some(span))
+                            }
+                            _ => mk_bound_region(None),
+                        }
+                    }
+                    // FIXME: these should use `BrNamed`
+                    ty::ReEarlyBound(region) => {
+                        mk_bound_region(Some(fcx.tcx.def_span(region.def_id)))
+                    }
+                    ty::ReLateBound(_, ty::BoundRegion { kind, .. })
+                    | ty::ReFree(ty::FreeRegion { bound_region: kind, .. }) => match kind {
+                        ty::BoundRegionKind::BrAnon(_, span) => mk_bound_region(span),
+                        ty::BoundRegionKind::BrNamed(def_id, _) => {
+                            mk_bound_region(Some(fcx.tcx.def_span(def_id)))
+                        }
+                        ty::BoundRegionKind::BrEnv => mk_bound_region(None),
+                    },
+                    _ => mk_bound_region(None),
+                };
+                let r = fcx.tcx.mk_region(ty::ReLateBound(current_depth, br));
+                r
+            });
+            if captured_tys.insert(ty) {
+                cause.ty = ty;
                 Some(cause)
             } else {
                 None
@@ -243,11 +272,38 @@ pub fn resolve_interior<'a, 'tcx>(
         })
         .collect();
 
+    let mut bound_vars: SmallVec<[BoundVariableKind; 4]> = smallvec![];
+    let mut counter = 0;
+    // Optimization: If there is only one captured type, then we don't actually
+    // need to fold and reindex (since the first type doesn't change).
+    let type_causes = if captured_tys.len() > 0 {
+        // Optimization: Use `replace_escaping_bound_vars_uncached` instead of
+        // `fold_regions`, since we only have late bound regions, and it skips
+        // types without bound regions.
+        fcx.tcx.replace_escaping_bound_vars_uncached(
+            type_causes,
+            FnMutDelegate {
+                regions: &mut |br| {
+                    let kind = match br.kind {
+                        ty::BrAnon(_, span) => ty::BrAnon(counter, span),
+                        _ => br.kind,
+                    };
+                    let var = ty::BoundVar::from_usize(bound_vars.len());
+                    bound_vars.push(ty::BoundVariableKind::Region(kind));
+                    counter += 1;
+                    fcx.tcx.mk_region(ty::ReLateBound(ty::INNERMOST, ty::BoundRegion { var, kind }))
+                },
+                types: &mut |b| bug!("unexpected bound ty in binder: {b:?}"),
+                consts: &mut |b, ty| bug!("unexpected bound ct in binder: {b:?} {ty}"),
+            },
+        )
+    } else {
+        type_causes
+    };
+
     // Extract type components to build the witness type.
     let type_list = fcx.tcx.mk_type_list(type_causes.iter().map(|cause| cause.ty));
-    let bound_vars = fcx.tcx.mk_bound_variable_kinds(
-        (0..counter).map(|i| ty::BoundVariableKind::Region(ty::BrAnon(i))),
-    );
+    let bound_vars = fcx.tcx.mk_bound_variable_kinds(bound_vars.iter());
     let witness =
         fcx.tcx.mk_generator_witness(ty::Binder::bind_with_vars(type_list, bound_vars.clone()));
 
diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index 052fdef2fc5..183e80f2e08 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/compiler/rustc_hir_typeck/src/lib.rs
@@ -209,6 +209,7 @@ fn diagnostic_only_typeck<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &ty::T
     typeck_with_fallback(tcx, def_id, fallback)
 }
 
+#[instrument(level = "debug", skip(tcx, fallback), ret)]
 fn typeck_with_fallback<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
diff --git a/compiler/rustc_infer/src/errors/note_and_explain.rs b/compiler/rustc_infer/src/errors/note_and_explain.rs
index 6a29d85627a..7aaa5ce2f42 100644
--- a/compiler/rustc_infer/src/errors/note_and_explain.rs
+++ b/compiler/rustc_infer/src/errors/note_and_explain.rs
@@ -89,10 +89,13 @@ impl<'a> DescriptionCtx<'a> {
                             };
                             me.span = Some(sp);
                         }
-                        ty::BrAnon(idx) => {
+                        ty::BrAnon(idx, span) => {
                             me.kind = "anon_num_here";
                             me.num_arg = idx+1;
-                            me.span = Some(tcx.def_span(scope));
+                            me.span = match span {
+                                Some(_) => span,
+                                None => Some(tcx.def_span(scope)),
+                            }
                         },
                         _ => {
                             me.kind = "defined_here_reg";
diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index aa44d582fd6..3dc0d60b1eb 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -738,7 +738,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         r: ty::Region<'tcx>,
     ) -> ty::Region<'tcx> {
         let var = self.canonical_var(info, r.into());
-        let br = ty::BoundRegion { var, kind: ty::BrAnon(var.as_u32()) };
+        let br = ty::BoundRegion { var, kind: ty::BrAnon(var.as_u32(), None) };
         let region = ty::ReLateBound(self.binder_index, br);
         self.tcx().mk_region(region)
     }
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 3dbc9b9f3f9..61519454a2c 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -207,9 +207,12 @@ fn msg_span_from_early_bound_and_free_regions<'tcx>(
                         };
                         (text, sp)
                     }
-                    ty::BrAnon(idx) => (
+                    ty::BrAnon(idx, span) => (
                         format!("the anonymous lifetime #{} defined here", idx + 1),
-                        tcx.def_span(scope)
+                        match span {
+                            Some(span) => span,
+                            None => tcx.def_span(scope)
+                        }
                     ),
                     _ => (
                         format!("the lifetime `{}` as defined here", region),
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs
index aaf5a7af00a..8a0e332f9c7 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs
@@ -10,6 +10,7 @@ pub mod find_anon_type;
 mod mismatched_static_lifetime;
 mod named_anon_conflict;
 mod placeholder_error;
+mod placeholder_relation;
 mod static_impl_trait;
 mod trait_impl_difference;
 mod util;
@@ -52,7 +53,9 @@ impl<'cx, 'tcx> NiceRegionError<'cx, 'tcx> {
     pub fn try_report_from_nll(&self) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
         // Due to the improved diagnostics returned by the MIR borrow checker, only a subset of
         // the nice region errors are required when running under the MIR borrow checker.
-        self.try_report_named_anon_conflict().or_else(|| self.try_report_placeholder_conflict())
+        self.try_report_named_anon_conflict()
+            .or_else(|| self.try_report_placeholder_conflict())
+            .or_else(|| self.try_report_placeholder_relation())
     }
 
     pub fn try_report(&self) -> Option<ErrorGuaranteed> {
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs
index 76cb76d9ff4..3fe7c1598fc 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs
@@ -68,7 +68,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         let is_impl_item = region_info.is_impl_item;
 
         match br {
-            ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) => {}
+            ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(..) => {}
             _ => {
                 /* not an anonymous region */
                 debug!("try_report_named_anon_conflict: not an anonymous region");
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_relation.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_relation.rs
new file mode 100644
index 00000000000..c42240f2172
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_relation.rs
@@ -0,0 +1,79 @@
+use crate::infer::{
+    error_reporting::nice_region_error::NiceRegionError, RegionResolutionError, SubregionOrigin,
+};
+use rustc_data_structures::intern::Interned;
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
+use rustc_middle::ty::{self, RePlaceholder, Region};
+
+impl<'tcx> NiceRegionError<'_, 'tcx> {
+    /// Emitted wwhen given a `ConcreteFailure` when relating two placeholders.
+    pub(super) fn try_report_placeholder_relation(
+        &self,
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
+        match &self.error {
+            Some(RegionResolutionError::ConcreteFailure(
+                SubregionOrigin::RelateRegionParamBound(span),
+                Region(Interned(RePlaceholder(ty::Placeholder { name: sub_name, .. }), _)),
+                Region(Interned(RePlaceholder(ty::Placeholder { name: sup_name, .. }), _)),
+            )) => {
+                let msg = "lifetime bound not satisfied";
+                let mut err = self.tcx().sess.struct_span_err(*span, msg);
+                let (sub_span, sub_symbol) = match sub_name {
+                    ty::BrNamed(def_id, symbol) => {
+                        (Some(self.tcx().def_span(def_id)), Some(symbol))
+                    }
+                    ty::BrAnon(_, span) => (*span, None),
+                    ty::BrEnv => (None, None),
+                };
+                let (sup_span, sup_symbol) = match sup_name {
+                    ty::BrNamed(def_id, symbol) => {
+                        (Some(self.tcx().def_span(def_id)), Some(symbol))
+                    }
+                    ty::BrAnon(_, span) => (*span, None),
+                    ty::BrEnv => (None, None),
+                };
+                match (sub_span, sup_span, sub_symbol, sup_symbol) {
+                    (Some(sub_span), Some(sup_span), Some(sub_symbol), Some(sup_symbol)) => {
+                        err.span_note(
+                            sub_span,
+                            format!("the lifetime `{sub_symbol}` defined here..."),
+                        );
+                        err.span_note(
+                            sup_span,
+                            format!("...must outlive the lifetime `{sup_symbol}` defined here"),
+                        );
+                    }
+                    (Some(sub_span), Some(sup_span), _, Some(sup_symbol)) => {
+                        err.span_note(sub_span, format!("the lifetime defined here..."));
+                        err.span_note(
+                            sup_span,
+                            format!("...must outlive the lifetime `{sup_symbol}` defined here"),
+                        );
+                    }
+                    (Some(sub_span), Some(sup_span), Some(sub_symbol), _) => {
+                        err.span_note(
+                            sub_span,
+                            format!("the lifetime `{sub_symbol}` defined here..."),
+                        );
+                        err.span_note(
+                            sup_span,
+                            format!("...must outlive the lifetime defined here"),
+                        );
+                    }
+                    (Some(sub_span), Some(sup_span), _, _) => {
+                        err.span_note(sub_span, format!("the lifetime defined here..."));
+                        err.span_note(
+                            sup_span,
+                            format!("...must outlive the lifetime defined here"),
+                        );
+                    }
+                    _ => {}
+                }
+                err.note("this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)");
+                Some(err)
+            }
+
+            _ => None,
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/infer/canonical.rs b/compiler/rustc_middle/src/infer/canonical.rs
index 8d1ed4b2a52..0331d764b38 100644
--- a/compiler/rustc_middle/src/infer/canonical.rs
+++ b/compiler/rustc_middle/src/infer/canonical.rs
@@ -336,8 +336,10 @@ impl<'tcx> CanonicalVarValues<'tcx> {
                         tcx.mk_ty(ty::Bound(ty::INNERMOST, ty::BoundVar::from_u32(i).into())).into()
                     }
                     GenericArgKind::Lifetime(..) => {
-                        let br =
-                            ty::BoundRegion { var: ty::BoundVar::from_u32(i), kind: ty::BrAnon(i) };
+                        let br = ty::BoundRegion {
+                            var: ty::BoundVar::from_u32(i),
+                            kind: ty::BrAnon(i, None),
+                        };
                         tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br)).into()
                     }
                     GenericArgKind::Const(ct) => tcx
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index a329753726e..2842b3c3102 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -598,7 +598,7 @@ impl<'tcx> TyCtxt<'tcx> {
             .replace_late_bound_regions(sig, |_| {
                 let br = ty::BoundRegion {
                     var: ty::BoundVar::from_u32(counter),
-                    kind: ty::BrAnon(counter),
+                    kind: ty::BrAnon(counter, None),
                 };
                 let r = self.mk_region(ty::ReLateBound(ty::INNERMOST, br));
                 counter += 1;
@@ -606,7 +606,7 @@ impl<'tcx> TyCtxt<'tcx> {
             })
             .0;
         let bound_vars = self.mk_bound_variable_kinds(
-            (0..counter).map(|i| ty::BoundVariableKind::Region(ty::BrAnon(i))),
+            (0..counter).map(|i| ty::BoundVariableKind::Region(ty::BrAnon(i, None))),
         );
         Binder::bind_with_vars(inner, bound_vars)
     }
@@ -626,7 +626,9 @@ impl<'tcx> TyCtxt<'tcx> {
                 let index = entry.index();
                 let var = ty::BoundVar::from_usize(index);
                 let kind = entry
-                    .or_insert_with(|| ty::BoundVariableKind::Region(ty::BrAnon(index as u32)))
+                    .or_insert_with(|| {
+                        ty::BoundVariableKind::Region(ty::BrAnon(index as u32, None))
+                    })
                     .expect_region();
                 let br = ty::BoundRegion { var, kind };
                 self.tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br))
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index fab85c39d25..c8815537835 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -2115,7 +2115,7 @@ impl<'a, 'tcx> ty::TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
                 // If this is an anonymous placeholder, don't rename. Otherwise, in some
                 // async fns, we get a `for<'r> Send` bound
                 match kind {
-                    ty::BrAnon(_) | ty::BrEnv => r,
+                    ty::BrAnon(..) | ty::BrEnv => r,
                     _ => {
                         // Index doesn't matter, since this is just for naming and these never get bound
                         let br = ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind };
@@ -2226,10 +2226,10 @@ impl<'tcx> FmtPrinter<'_, 'tcx> {
                     let ty::BoundVariableKind::Region(var) = var else {
                     // This doesn't really matter because it doesn't get used,
                     // it's just an empty value
-                    return ty::BrAnon(0);
+                    return ty::BrAnon(0, None);
                 };
                     match var {
-                        ty::BrAnon(_) | ty::BrEnv => {
+                        ty::BrAnon(..) | ty::BrEnv => {
                             start_or_continue(&mut self, "for<", ", ");
                             let name = next_name(&self);
                             debug!(?name);
@@ -2271,7 +2271,7 @@ impl<'tcx> FmtPrinter<'_, 'tcx> {
                             binder_level_idx: ty::DebruijnIndex,
                             br: ty::BoundRegion| {
                 let (name, kind) = match br.kind {
-                    ty::BrAnon(_) | ty::BrEnv => {
+                    ty::BrAnon(..) | ty::BrEnv => {
                         let name = next_name(&self);
 
                         if let Some(lt_idx) = lifetime_idx {
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index f2070869ce0..9f0598d0ba8 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -68,7 +68,7 @@ impl<'tcx> fmt::Debug for ty::adjustment::Adjustment<'tcx> {
 impl fmt::Debug for ty::BoundRegionKind {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
-            ty::BrAnon(n) => write!(f, "BrAnon({:?})", n),
+            ty::BrAnon(n, span) => write!(f, "BrAnon({n:?}, {span:?})"),
             ty::BrNamed(did, name) => {
                 if did.is_crate_root() {
                     write!(f, "BrNamed({})", name)
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 5f108bf0ef3..49d82b503a4 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -59,7 +59,7 @@ pub struct FreeRegion {
 #[derive(HashStable)]
 pub enum BoundRegionKind {
     /// An anonymous region parameter for a given fn (&T)
-    BrAnon(u32),
+    BrAnon(u32, Option<Span>),
 
     /// Named region parameters for functions (a in &'a T)
     ///
diff --git a/compiler/rustc_symbol_mangling/src/v0.rs b/compiler/rustc_symbol_mangling/src/v0.rs
index 2109b3c2496..e540e2f2a21 100644
--- a/compiler/rustc_symbol_mangling/src/v0.rs
+++ b/compiler/rustc_symbol_mangling/src/v0.rs
@@ -218,7 +218,7 @@ impl<'tcx> SymbolMangler<'tcx> {
         let lifetimes = regions
             .into_iter()
             .map(|br| match br {
-                ty::BrAnon(i) => i,
+                ty::BrAnon(i, _) => i,
                 _ => bug!("symbol_names: non-anonymized region `{:?}` in `{:?}`", br, value),
             })
             .max()
@@ -335,7 +335,7 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
 
             // Late-bound lifetimes use indices starting at 1,
             // see `BinderLevel` for more details.
-            ty::ReLateBound(debruijn, ty::BoundRegion { kind: ty::BrAnon(i), .. }) => {
+            ty::ReLateBound(debruijn, ty::BoundRegion { kind: ty::BrAnon(i, _), .. }) => {
                 let binder = &self.binders[self.binders.len() - 1 - debruijn.index()];
                 let depth = binder.lifetime_depths.start + i;
 
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 2954a2c163f..c6ff83120a7 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1971,6 +1971,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
     /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
     /// ```
+    #[instrument(level = "debug", skip(self), ret)]
     fn constituent_types_for_ty(
         &self,
         t: ty::Binder<'tcx, Ty<'tcx>>,
diff --git a/compiler/rustc_traits/src/chalk/db.rs b/compiler/rustc_traits/src/chalk/db.rs
index 2035252fe39..07f92299f72 100644
--- a/compiler/rustc_traits/src/chalk/db.rs
+++ b/compiler/rustc_traits/src/chalk/db.rs
@@ -728,7 +728,7 @@ fn bound_vars_for_item<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> SubstsRef<'tcx
         ty::GenericParamDefKind::Lifetime => {
             let br = ty::BoundRegion {
                 var: ty::BoundVar::from_usize(substs.len()),
-                kind: ty::BrAnon(substs.len() as u32),
+                kind: ty::BrAnon(substs.len() as u32, None),
             };
             tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br)).into()
         }
diff --git a/compiler/rustc_traits/src/chalk/lowering.rs b/compiler/rustc_traits/src/chalk/lowering.rs
index 0492e94b94e..b64d53e60de 100644
--- a/compiler/rustc_traits/src/chalk/lowering.rs
+++ b/compiler/rustc_traits/src/chalk/lowering.rs
@@ -498,13 +498,13 @@ impl<'tcx> LowerInto<'tcx, Region<'tcx>> for &chalk_ir::Lifetime<RustInterner<'t
                 ty::DebruijnIndex::from_u32(var.debruijn.depth()),
                 ty::BoundRegion {
                     var: ty::BoundVar::from_usize(var.index),
-                    kind: ty::BrAnon(var.index as u32),
+                    kind: ty::BrAnon(var.index as u32, None),
                 },
             ),
             chalk_ir::LifetimeData::InferenceVar(_var) => unimplemented!(),
             chalk_ir::LifetimeData::Placeholder(p) => ty::RePlaceholder(ty::Placeholder {
                 universe: ty::UniverseIndex::from_usize(p.ui.counter),
-                name: ty::BoundRegionKind::BrAnon(p.idx as u32),
+                name: ty::BoundRegionKind::BrAnon(p.idx as u32, None),
             }),
             chalk_ir::LifetimeData::Static => return interner.tcx.lifetimes.re_static,
             chalk_ir::LifetimeData::Empty(_) => {
@@ -933,7 +933,7 @@ impl<'tcx> TypeVisitor<'tcx> for BoundVarsCollector<'tcx> {
                     }
                 }
 
-                ty::BoundRegionKind::BrAnon(var) => match self.parameters.entry(var) {
+                ty::BoundRegionKind::BrAnon(var, _) => match self.parameters.entry(var) {
                     Entry::Vacant(entry) => {
                         entry.insert(chalk_ir::VariableKind::Lifetime);
                     }
@@ -991,13 +991,13 @@ impl<'a, 'tcx> TypeFolder<'tcx> for NamedBoundVarSubstitutor<'a, 'tcx> {
             ty::ReLateBound(index, br) if index == self.binder_index => match br.kind {
                 ty::BrNamed(def_id, _name) => match self.named_parameters.get(&def_id) {
                     Some(idx) => {
-                        let new_br = ty::BoundRegion { var: br.var, kind: ty::BrAnon(*idx) };
+                        let new_br = ty::BoundRegion { var: br.var, kind: ty::BrAnon(*idx, None) };
                         return self.tcx.mk_region(ty::ReLateBound(index, new_br));
                     }
                     None => panic!("Missing `BrNamed`."),
                 },
                 ty::BrEnv => unimplemented!(),
-                ty::BrAnon(_) => {}
+                ty::BrAnon(..) => {}
             },
             _ => (),
         };
@@ -1072,14 +1072,16 @@ impl<'tcx> TypeFolder<'tcx> for ParamsSubstitutor<'tcx> {
                 Some(idx) => {
                     let br = ty::BoundRegion {
                         var: ty::BoundVar::from_u32(*idx),
-                        kind: ty::BrAnon(*idx),
+                        kind: ty::BrAnon(*idx, None),
                     };
                     self.tcx.mk_region(ty::ReLateBound(self.binder_index, br))
                 }
                 None => {
                     let idx = self.named_regions.len() as u32;
-                    let br =
-                        ty::BoundRegion { var: ty::BoundVar::from_u32(idx), kind: ty::BrAnon(idx) };
+                    let br = ty::BoundRegion {
+                        var: ty::BoundVar::from_u32(idx),
+                        kind: ty::BrAnon(idx, None),
+                    };
                     self.named_regions.insert(_re.def_id, idx);
                     self.tcx.mk_region(ty::ReLateBound(self.binder_index, br))
                 }
@@ -1156,7 +1158,7 @@ impl<'tcx> TypeVisitor<'tcx> for PlaceholdersCollector {
     fn visit_region(&mut self, r: Region<'tcx>) -> ControlFlow<Self::BreakTy> {
         match *r {
             ty::RePlaceholder(p) if p.universe == self.universe_index => {
-                if let ty::BoundRegionKind::BrAnon(anon) = p.name {
+                if let ty::BoundRegionKind::BrAnon(anon, _) = p.name {
                     self.next_anon_region_placeholder = self.next_anon_region_placeholder.max(anon);
                 }
             }
diff --git a/src/test/ui/generic-associated-types/bugs/issue-100013.rs b/src/test/ui/generic-associated-types/bugs/issue-100013.rs
new file mode 100644
index 00000000000..fc4e47a3ba1
--- /dev/null
+++ b/src/test/ui/generic-associated-types/bugs/issue-100013.rs
@@ -0,0 +1,39 @@
+// check-fail
+// known-bug
+// edition: 2021
+
+// We really should accept this, but we need implied bounds between the regions
+// in a generator interior.
+
+pub trait FutureIterator {
+    type Future<'s, 'cx>: Send
+    where
+        's: 'cx;
+}
+
+fn call<I: FutureIterator>() -> impl Send {
+    async { // a generator checked for autotrait impl `Send`
+        //~^ lifetime bound not satisfied
+        let x = None::<I::Future<'_, '_>>; // a type referencing GAT
+        async {}.await; // a yield point
+    }
+}
+
+fn call2<'a, 'b, I: FutureIterator>() -> impl Send {
+    async { // a generator checked for autotrait impl `Send`
+        //~^ lifetime bound not satisfied
+        let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
+        //~^ lifetime may not live long enough
+        async {}.await; // a yield point
+    }
+}
+
+fn call3<'a: 'b, 'b, I: FutureIterator>() -> impl Send {
+    async { // a generator checked for autotrait impl `Send`
+        //~^ lifetime bound not satisfied
+        let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
+        async {}.await; // a yield point
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/generic-associated-types/bugs/issue-100013.stderr b/src/test/ui/generic-associated-types/bugs/issue-100013.stderr
new file mode 100644
index 00000000000..72ae288dcab
--- /dev/null
+++ b/src/test/ui/generic-associated-types/bugs/issue-100013.stderr
@@ -0,0 +1,82 @@
+error: lifetime bound not satisfied
+  --> $DIR/issue-100013.rs:15:5
+   |
+LL | /     async { // a generator checked for autotrait impl `Send`
+LL | |
+LL | |         let x = None::<I::Future<'_, '_>>; // a type referencing GAT
+LL | |         async {}.await; // a yield point
+LL | |     }
+   | |_____^
+   |
+note: the lifetime defined here...
+  --> $DIR/issue-100013.rs:17:38
+   |
+LL |         let x = None::<I::Future<'_, '_>>; // a type referencing GAT
+   |                                      ^^
+note: ...must outlive the lifetime defined here
+  --> $DIR/issue-100013.rs:17:34
+   |
+LL |         let x = None::<I::Future<'_, '_>>; // a type referencing GAT
+   |                                  ^^
+   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
+
+error: lifetime bound not satisfied
+  --> $DIR/issue-100013.rs:23:5
+   |
+LL | /     async { // a generator checked for autotrait impl `Send`
+LL | |
+LL | |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
+LL | |
+LL | |         async {}.await; // a yield point
+LL | |     }
+   | |_____^
+   |
+note: the lifetime defined here...
+  --> $DIR/issue-100013.rs:22:14
+   |
+LL | fn call2<'a, 'b, I: FutureIterator>() -> impl Send {
+   |              ^^
+note: ...must outlive the lifetime defined here
+  --> $DIR/issue-100013.rs:22:10
+   |
+LL | fn call2<'a, 'b, I: FutureIterator>() -> impl Send {
+   |          ^^
+   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
+
+error: lifetime may not live long enough
+  --> $DIR/issue-100013.rs:25:17
+   |
+LL | fn call2<'a, 'b, I: FutureIterator>() -> impl Send {
+   |          --  -- lifetime `'b` defined here
+   |          |
+   |          lifetime `'a` defined here
+...
+LL |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+error: lifetime bound not satisfied
+  --> $DIR/issue-100013.rs:32:5
+   |
+LL | /     async { // a generator checked for autotrait impl `Send`
+LL | |
+LL | |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
+LL | |         async {}.await; // a yield point
+LL | |     }
+   | |_____^
+   |
+note: the lifetime defined here...
+  --> $DIR/issue-100013.rs:31:18
+   |
+LL | fn call3<'a: 'b, 'b, I: FutureIterator>() -> impl Send {
+   |                  ^^
+note: ...must outlive the lifetime defined here
+  --> $DIR/issue-100013.rs:31:10
+   |
+LL | fn call3<'a: 'b, 'b, I: FutureIterator>() -> impl Send {
+   |          ^^
+   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
+
+error: aborting due to 4 previous errors
+