Display information about captured variable in `FnMut` error

Fixes #69446

When we encounter a region error involving an `FnMut` closure, we
display a specialized error message. However, we currently do not
tell the user which upvar was captured. This makes it difficult to
determine the cause of the error, especially when the closure is large.

This commit records marks constraints involving closure upvars
with `ConstraintCategory::ClosureUpvar`. When we decide to 'blame'
a `ConstraintCategory::Return`, we additionall store
the captured upvar if we found a `ConstraintCategory::ClosureUpvar` in
the path.

When generating an error message, we point to relevant spans if we have
closure upvar information available. We further customize the message if
an `async` closure is being returned, to make it clear that the captured
variable is being returned indirectly.

17 files changed, 203 insertions, 58 deletions

diff --git a/src/libcore/future/mod.rs b/src/libcore/future/mod.rs
index 6f6009b47e6..9dbc23f5c04 100644
--- a/src/libcore/future/mod.rs
+++ b/src/libcore/future/mod.rs
@@ -56,6 +56,7 @@ pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
 where
     T: Generator<ResumeTy, Yield = ()>,
 {
+    #[rustc_diagnostic_item = "gen_future"]
     struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);
 
     // We rely on the fact that async/await futures are immovable in order to create
diff --git a/src/librustc_middle/mir/query.rs b/src/librustc_middle/mir/query.rs
index 63b8d8c8da7..aed93e86863 100644
--- a/src/librustc_middle/mir/query.rs
+++ b/src/librustc_middle/mir/query.rs
@@ -171,7 +171,7 @@ pub struct ClosureOutlivesRequirement<'tcx> {
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]
 #[derive(RustcEncodable, RustcDecodable, HashStable)]
 pub enum ConstraintCategory {
-    Return,
+    Return(ReturnConstraint),
     Yield,
     UseAsConst,
     UseAsStatic,
@@ -187,6 +187,7 @@ pub enum ConstraintCategory {
     SizedBound,
     Assignment,
     OpaqueType,
+    ClosureUpvar(hir::HirId),
 
     /// A "boring" constraint (caused by the given location) is one that
     /// the user probably doesn't want to see described in diagnostics,
@@ -204,6 +205,13 @@ pub enum ConstraintCategory {
     Internal,
 }
 
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]
+#[derive(RustcEncodable, RustcDecodable, HashStable)]
+pub enum ReturnConstraint {
+    Normal,
+    ClosureUpvar(hir::HirId),
+}
+
 /// The subject of a `ClosureOutlivesRequirement` -- that is, the thing
 /// that must outlive some region.
 #[derive(Copy, Clone, Debug, RustcEncodable, RustcDecodable, HashStable)]
diff --git a/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs b/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs
index 5f1c0911da2..b0b5a819983 100644
--- a/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs
+++ b/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs
@@ -766,7 +766,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     category:
                         category
                         @
-                        (ConstraintCategory::Return
+                        (ConstraintCategory::Return(_)
                         | ConstraintCategory::CallArgument
                         | ConstraintCategory::OpaqueType),
                     from_closure: false,
@@ -1096,7 +1096,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         opt_place_desc: Option<&String>,
     ) -> Option<DiagnosticBuilder<'cx>> {
         let return_kind = match category {
-            ConstraintCategory::Return => "return",
+            ConstraintCategory::Return(_) => "return",
             ConstraintCategory::Yield => "yield",
             _ => return None,
         };
@@ -1210,7 +1210,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         );
 
         let msg = match category {
-            ConstraintCategory::Return | ConstraintCategory::OpaqueType => {
+            ConstraintCategory::Return(_) | ConstraintCategory::OpaqueType => {
                 format!("{} is returned here", kind)
             }
             ConstraintCategory::CallArgument => {
diff --git a/src/librustc_mir/borrow_check/diagnostics/region_errors.rs b/src/librustc_mir/borrow_check/diagnostics/region_errors.rs
index e19fab89eab..681f999e048 100644
--- a/src/librustc_mir/borrow_check/diagnostics/region_errors.rs
+++ b/src/librustc_mir/borrow_check/diagnostics/region_errors.rs
@@ -5,9 +5,9 @@ use rustc_infer::infer::{
     error_reporting::nice_region_error::NiceRegionError,
     error_reporting::unexpected_hidden_region_diagnostic, NLLRegionVariableOrigin,
 };
-use rustc_middle::mir::ConstraintCategory;
+use rustc_middle::mir::{ConstraintCategory, ReturnConstraint};
 use rustc_middle::ty::{self, RegionVid, Ty};
-use rustc_span::symbol::kw;
+use rustc_span::symbol::{kw, sym};
 use rustc_span::Span;
 
 use crate::util::borrowck_errors;
@@ -26,7 +26,7 @@ impl ConstraintDescription for ConstraintCategory {
         // Must end with a space. Allows for empty names to be provided.
         match self {
             ConstraintCategory::Assignment => "assignment ",
-            ConstraintCategory::Return => "returning this value ",
+            ConstraintCategory::Return(_) => "returning this value ",
             ConstraintCategory::Yield => "yielding this value ",
             ConstraintCategory::UseAsConst => "using this value as a constant ",
             ConstraintCategory::UseAsStatic => "using this value as a static ",
@@ -37,6 +37,7 @@ impl ConstraintDescription for ConstraintCategory {
             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
             ConstraintCategory::CopyBound => "copying this value ",
             ConstraintCategory::OpaqueType => "opaque type ",
+            ConstraintCategory::ClosureUpvar(_) => "closure capture ",
             ConstraintCategory::Boring
             | ConstraintCategory::BoringNoLocation
             | ConstraintCategory::Internal => "",
@@ -306,8 +307,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         };
 
         let diag = match (category, fr_is_local, outlived_fr_is_local) {
-            (ConstraintCategory::Return, true, false) if self.is_closure_fn_mut(fr) => {
-                self.report_fnmut_error(&errci)
+            (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
+                self.report_fnmut_error(&errci, kind)
             }
             (ConstraintCategory::Assignment, true, false)
             | (ConstraintCategory::CallArgument, true, false) => {
@@ -347,7 +348,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     ///            executing...
     ///    = note: ...therefore, returned references to captured variables will escape the closure
     /// ```
-    fn report_fnmut_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
+    fn report_fnmut_error(
+        &self,
+        errci: &ErrorConstraintInfo,
+        kind: ReturnConstraint,
+    ) -> DiagnosticBuilder<'tcx> {
         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 
         let mut diag = self
@@ -356,19 +361,39 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             .sess
             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 
-        // We should check if the return type of this closure is in fact a closure - in that
-        // case, we can special case the error further.
-        let return_type_is_closure =
-            self.regioncx.universal_regions().unnormalized_output_ty.is_closure();
-        let message = if return_type_is_closure {
-            "returns a closure that contains a reference to a captured variable, which then \
-             escapes the closure body"
-        } else {
-            "returns a reference to a captured variable which escapes the closure body"
+        let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
+        if let ty::Opaque(def_id, _) = output_ty.kind {
+            output_ty = self.infcx.tcx.type_of(def_id)
+        };
+
+        debug!("report_fnmut_error: output_ty={:?}", output_ty);
+
+        let message = match output_ty.kind {
+            ty::Closure(_, _) => {
+                "returns a closure that contains a reference to a captured variable, which then \
+                 escapes the closure body"
+            }
+            ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did) => {
+                "returns an `async` block that contains a reference to a captured variable, which then \
+                 escapes the closure body"
+            }
+            _ => "returns a reference to a captured variable which escapes the closure body",
         };
 
         diag.span_label(*span, message);
 
+        if let ReturnConstraint::ClosureUpvar(upvar) = kind {
+            let def_id = match self.regioncx.universal_regions().defining_ty {
+                DefiningTy::Closure(def_id, _) => def_id,
+                ty @ _ => bug!("unexpected DefiningTy {:?}", ty),
+            };
+
+            let upvar_def_span = self.infcx.tcx.hir().span(upvar);
+            let upvar_span = self.infcx.tcx.upvars_mentioned(def_id).unwrap()[&upvar].span;
+            diag.span_label(upvar_def_span, "variable defined here");
+            diag.span_label(upvar_span, "variable captured here");
+        }
+
         match self.give_region_a_name(*outlived_fr).unwrap().source {
             RegionNameSource::NamedEarlyBoundRegion(fr_span)
             | RegionNameSource::NamedFreeRegion(fr_span)
@@ -506,7 +531,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         outlived_fr_name.highlight_region_name(&mut diag);
 
         match (category, outlived_fr_is_local, fr_is_local) {
-            (ConstraintCategory::Return, true, _) => {
+            (ConstraintCategory::Return(_), true, _) => {
                 diag.span_label(
                     *span,
                     format!(
diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index a0c1d96bb47..78d88ac81aa 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -218,6 +218,7 @@ fn do_mir_borrowck<'a, 'tcx>(
         &mut flow_inits,
         &mdpe.move_data,
         &borrow_set,
+        &upvars,
     );
 
     // Dump MIR results into a file, if that is enabled. This let us
@@ -2301,30 +2302,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     /// be `self` in the current MIR, because that is the only time we directly access the fields
     /// of a closure type.
     pub fn is_upvar_field_projection(&self, place_ref: PlaceRef<'tcx>) -> Option<Field> {
-        let mut place_projection = place_ref.projection;
-        let mut by_ref = false;
-
-        if let [proj_base @ .., ProjectionElem::Deref] = place_projection {
-            place_projection = proj_base;
-            by_ref = true;
-        }
-
-        match place_projection {
-            [base @ .., ProjectionElem::Field(field, _ty)] => {
-                let tcx = self.infcx.tcx;
-                let base_ty = Place::ty_from(place_ref.local, base, self.body(), tcx).ty;
-
-                if (base_ty.is_closure() || base_ty.is_generator())
-                    && (!by_ref || self.upvars[field.index()].by_ref)
-                {
-                    Some(*field)
-                } else {
-                    None
-                }
-            }
-
-            _ => None,
-        }
+        path_utils::is_upvar_field_projection(self.infcx.tcx, &self.upvars, place_ref, self.body())
     }
 }
 
diff --git a/src/librustc_mir/borrow_check/nll.rs b/src/librustc_mir/borrow_check/nll.rs
index b820b79c47f..5bae31cafb9 100644
--- a/src/librustc_mir/borrow_check/nll.rs
+++ b/src/librustc_mir/borrow_check/nll.rs
@@ -39,6 +39,7 @@ use crate::borrow_check::{
     renumber,
     type_check::{self, MirTypeckRegionConstraints, MirTypeckResults},
     universal_regions::UniversalRegions,
+    Upvar,
 };
 
 crate type PoloniusOutput = Output<RustcFacts>;
@@ -166,6 +167,7 @@ pub(in crate::borrow_check) fn compute_regions<'cx, 'tcx>(
     flow_inits: &mut ResultsCursor<'cx, 'tcx, MaybeInitializedPlaces<'cx, 'tcx>>,
     move_data: &MoveData<'tcx>,
     borrow_set: &BorrowSet<'tcx>,
+    upvars: &[Upvar],
 ) -> NllOutput<'tcx> {
     let mut all_facts = AllFacts::enabled(infcx.tcx).then_some(AllFacts::default());
 
@@ -188,6 +190,7 @@ pub(in crate::borrow_check) fn compute_regions<'cx, 'tcx>(
             flow_inits,
             move_data,
             elements,
+            upvars,
         );
 
     if let Some(all_facts) = &mut all_facts {
diff --git a/src/librustc_mir/borrow_check/path_utils.rs b/src/librustc_mir/borrow_check/path_utils.rs
index f5238e7b7be..934729553a7 100644
--- a/src/librustc_mir/borrow_check/path_utils.rs
+++ b/src/librustc_mir/borrow_check/path_utils.rs
@@ -1,10 +1,11 @@
 use crate::borrow_check::borrow_set::{BorrowData, BorrowSet, TwoPhaseActivation};
 use crate::borrow_check::places_conflict;
 use crate::borrow_check::AccessDepth;
+use crate::borrow_check::Upvar;
 use crate::dataflow::indexes::BorrowIndex;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_middle::mir::BorrowKind;
-use rustc_middle::mir::{BasicBlock, Body, Location, Place};
+use rustc_middle::mir::{BasicBlock, Body, Field, Location, Place, PlaceRef, ProjectionElem};
 use rustc_middle::ty::TyCtxt;
 
 /// Returns `true` if the borrow represented by `kind` is
@@ -135,3 +136,38 @@ pub(super) fn borrow_of_local_data(place: Place<'_>) -> bool {
     // Any errors will be caught on the initial borrow
     !place.is_indirect()
 }
+
+/// If `place` is a field projection, and the field is being projected from a closure type,
+/// then returns the index of the field being projected. Note that this closure will always
+/// be `self` in the current MIR, because that is the only time we directly access the fields
+/// of a closure type.
+pub(crate) fn is_upvar_field_projection(
+    tcx: TyCtxt<'tcx>,
+    upvars: &[Upvar],
+    place_ref: PlaceRef<'tcx>,
+    body: &Body<'tcx>,
+) -> Option<Field> {
+    let mut place_projection = place_ref.projection;
+    let mut by_ref = false;
+
+    if let [proj_base @ .., ProjectionElem::Deref] = place_projection {
+        place_projection = proj_base;
+        by_ref = true;
+    }
+
+    match place_projection {
+        [base @ .., ProjectionElem::Field(field, _ty)] => {
+            let base_ty = Place::ty_from(place_ref.local, base, body, tcx).ty;
+
+            if (base_ty.is_closure() || base_ty.is_generator())
+                && (!by_ref || upvars[field.index()].by_ref)
+            {
+                Some(*field)
+            } else {
+                None
+            }
+        }
+
+        _ => None,
+    }
+}
diff --git a/src/librustc_mir/borrow_check/region_infer/mod.rs b/src/librustc_mir/borrow_check/region_infer/mod.rs
index fe113843800..3e459bd52f7 100644
--- a/src/librustc_mir/borrow_check/region_infer/mod.rs
+++ b/src/librustc_mir/borrow_check/region_infer/mod.rs
@@ -12,7 +12,7 @@ use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound}
 use rustc_infer::infer::{InferCtxt, NLLRegionVariableOrigin, RegionVariableOrigin};
 use rustc_middle::mir::{
     Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
-    ConstraintCategory, Local, Location,
+    ConstraintCategory, Local, Location, ReturnConstraint,
 };
 use rustc_middle::ty::{self, subst::SubstsRef, RegionVid, Ty, TyCtxt, TypeFoldable};
 use rustc_span::Span;
@@ -2017,7 +2017,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                     | ConstraintCategory::BoringNoLocation
                     | ConstraintCategory::Internal => false,
                     ConstraintCategory::TypeAnnotation
-                    | ConstraintCategory::Return
+                    | ConstraintCategory::Return(_)
                     | ConstraintCategory::Yield => true,
                     _ => constraint_sup_scc != target_scc,
                 }
@@ -2042,7 +2042,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
         if let Some(i) = best_choice {
             if let Some(next) = categorized_path.get(i + 1) {
-                if categorized_path[i].0 == ConstraintCategory::Return
+                if matches!(categorized_path[i].0, ConstraintCategory::Return(_))
                     && next.0 == ConstraintCategory::OpaqueType
                 {
                     // The return expression is being influenced by the return type being
@@ -2050,6 +2050,18 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                     return *next;
                 }
             }
+
+            if categorized_path[i].0 == ConstraintCategory::Return(ReturnConstraint::Normal) {
+                let field = categorized_path.iter().find_map(|p| {
+                    if let ConstraintCategory::ClosureUpvar(f) = p.0 { Some(f) } else { None }
+                });
+
+                if let Some(field) = field {
+                    categorized_path[i].0 =
+                        ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
+                }
+            }
+
             return categorized_path[i];
         }
 
diff --git a/src/librustc_mir/borrow_check/type_check/mod.rs b/src/librustc_mir/borrow_check/type_check/mod.rs
index bdbce1de745..d6b06bc1dfa 100644
--- a/src/librustc_mir/borrow_check/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/type_check/mod.rs
@@ -55,6 +55,7 @@ use crate::borrow_check::{
     location::LocationTable,
     member_constraints::MemberConstraintSet,
     nll::ToRegionVid,
+    path_utils,
     region_infer::values::{
         LivenessValues, PlaceholderIndex, PlaceholderIndices, RegionValueElements,
     },
@@ -62,6 +63,7 @@ use crate::borrow_check::{
     renumber,
     type_check::free_region_relations::{CreateResult, UniversalRegionRelations},
     universal_regions::{DefiningTy, UniversalRegions},
+    Upvar,
 };
 
 macro_rules! span_mirbug {
@@ -132,6 +134,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
     flow_inits: &mut ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
     move_data: &MoveData<'tcx>,
     elements: &Rc<RegionValueElements>,
+    upvars: &[Upvar],
 ) -> MirTypeckResults<'tcx> {
     let implicit_region_bound = infcx.tcx.mk_region(ty::ReVar(universal_regions.fr_fn_body));
     let mut constraints = MirTypeckRegionConstraints {
@@ -162,6 +165,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
         borrow_set,
         all_facts,
         constraints: &mut constraints,
+        upvars,
     };
 
     let opaque_type_values = type_check_internal(
@@ -577,7 +581,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
         for constraint in constraints.outlives().iter() {
             let mut constraint = *constraint;
             constraint.locations = locations;
-            if let ConstraintCategory::Return
+            if let ConstraintCategory::Return(_)
             | ConstraintCategory::UseAsConst
             | ConstraintCategory::UseAsStatic = constraint.category
             {
@@ -827,6 +831,7 @@ struct BorrowCheckContext<'a, 'tcx> {
     all_facts: &'a mut Option<AllFacts>,
     borrow_set: &'a BorrowSet<'tcx>,
     constraints: &'a mut MirTypeckRegionConstraints<'tcx>,
+    upvars: &'a [Upvar],
 }
 
 crate struct MirTypeckResults<'tcx> {
@@ -1420,7 +1425,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                 ConstraintCategory::UseAsConst
                             }
                         } else {
-                            ConstraintCategory::Return
+                            ConstraintCategory::Return(ReturnConstraint::Normal)
                         }
                     }
                     Some(l) if !body.local_decls[l].is_user_variable() => {
@@ -1703,7 +1708,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                 ConstraintCategory::UseAsConst
                             }
                         } else {
-                            ConstraintCategory::Return
+                            ConstraintCategory::Return(ReturnConstraint::Normal)
                         }
                     }
                     Some(l) if !body.local_decls[l].is_user_variable() => {
@@ -2487,6 +2492,19 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         );
 
         let mut cursor = borrowed_place.projection.as_ref();
+        let tcx = self.infcx.tcx;
+        let field = path_utils::is_upvar_field_projection(
+            tcx,
+            &self.borrowck_context.upvars,
+            borrowed_place.as_ref(),
+            body,
+        );
+        let category = if let Some(field) = field {
+            ConstraintCategory::ClosureUpvar(self.borrowck_context.upvars[field.index()].var_hir_id)
+        } else {
+            ConstraintCategory::Boring
+        };
+
         while let [proj_base @ .., elem] = cursor {
             cursor = proj_base;
 
@@ -2494,7 +2512,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
             match elem {
                 ProjectionElem::Deref => {
-                    let tcx = self.infcx.tcx;
                     let base_ty = Place::ty_from(borrowed_place.local, proj_base, body, tcx).ty;
 
                     debug!("add_reborrow_constraint - base_ty = {:?}", base_ty);
@@ -2504,7 +2521,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                 sup: ref_region.to_region_vid(),
                                 sub: borrow_region.to_region_vid(),
                                 locations: location.to_locations(),
-                                category: ConstraintCategory::Boring,
+                                category,
                             });
 
                             match mutbl {
diff --git a/src/test/ui/async-await/issue-69446-fnmut-capture.rs b/src/test/ui/async-await/issue-69446-fnmut-capture.rs
new file mode 100644
index 00000000000..842115538c9
--- /dev/null
+++ b/src/test/ui/async-await/issue-69446-fnmut-capture.rs
@@ -0,0 +1,22 @@
+// Regression test for issue #69446 - we should display
+// which variable is captured
+// edition:2018
+
+use core::future::Future;
+
+struct Foo;
+impl Foo {
+    fn foo(&mut self) {}
+}
+
+async fn bar<T>(_: impl FnMut() -> T)
+where
+    T: Future<Output = ()>,
+{}
+
+fn main() {
+    let mut x = Foo;
+    bar(move || async { //~ ERROR captured
+        x.foo();
+    });
+}
diff --git a/src/test/ui/async-await/issue-69446-fnmut-capture.stderr b/src/test/ui/async-await/issue-69446-fnmut-capture.stderr
new file mode 100644
index 00000000000..3d2b0402bc5
--- /dev/null
+++ b/src/test/ui/async-await/issue-69446-fnmut-capture.stderr
@@ -0,0 +1,19 @@
+error: captured variable cannot escape `FnMut` closure body
+  --> $DIR/issue-69446-fnmut-capture.rs:19:17
+   |
+LL |       let mut x = Foo;
+   |           ----- variable defined here
+LL |       bar(move || async {
+   |  _______________-_^
+   | |               |
+   | |               inferred to be a `FnMut` closure
+LL | |         x.foo();
+   | |         - variable captured here
+LL | |     });
+   | |_____^ returns an `async` block that contains a reference to a captured variable, which then escapes the closure body
+   |
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/borrowck/borrowck-describe-lvalue.stderr b/src/test/ui/borrowck/borrowck-describe-lvalue.stderr
index 075e0e2e451..4144d70cc16 100644
--- a/src/test/ui/borrowck/borrowck-describe-lvalue.stderr
+++ b/src/test/ui/borrowck/borrowck-describe-lvalue.stderr
@@ -21,10 +21,13 @@ LL |                    *y = 1;
 error: captured variable cannot escape `FnMut` closure body
   --> $DIR/borrowck-describe-lvalue.rs:264:16
    |
+LL |           let mut x = 0;
+   |               ----- variable defined here
 LL |              || {
    |               - inferred to be a `FnMut` closure
 LL | /                || {
 LL | |                    let y = &mut x;
+   | |                                 - variable captured here
 LL | |                    &mut x;
 LL | |                    *y = 1;
 LL | |                    drop(y);
diff --git a/src/test/ui/issues/issue-40510-1.stderr b/src/test/ui/issues/issue-40510-1.stderr
index f4fda0abc20..54df40b6e3d 100644
--- a/src/test/ui/issues/issue-40510-1.stderr
+++ b/src/test/ui/issues/issue-40510-1.stderr
@@ -1,10 +1,16 @@
 error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-40510-1.rs:7:9
    |
+LL |     let mut x: Box<()> = Box::new(());
+   |         ----- variable defined here
+LL | 
 LL |     || {
    |      - inferred to be a `FnMut` closure
 LL |         &mut x
-   |         ^^^^^^ returns a reference to a captured variable which escapes the closure body
+   |         ^^^^^-
+   |         |    |
+   |         |    variable captured here
+   |         returns a reference to a captured variable which escapes the closure body
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
    = note: ...therefore, they cannot allow references to captured variables to escape
diff --git a/src/test/ui/issues/issue-40510-3.stderr b/src/test/ui/issues/issue-40510-3.stderr
index 4bc7d0f5dea..cb885ec7d95 100644
--- a/src/test/ui/issues/issue-40510-3.stderr
+++ b/src/test/ui/issues/issue-40510-3.stderr
@@ -1,10 +1,14 @@
 error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-40510-3.rs:7:9
    |
+LL |       let mut x: Vec<()> = Vec::new();
+   |           ----- variable defined here
+LL | 
 LL |       || {
    |        - inferred to be a `FnMut` closure
 LL | /         || {
 LL | |             x.push(())
+   | |             - variable captured here
 LL | |         }
    | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
diff --git a/src/test/ui/issues/issue-49824.stderr b/src/test/ui/issues/issue-49824.stderr
index 6b486aafcdf..2fec482543d 100644
--- a/src/test/ui/issues/issue-49824.stderr
+++ b/src/test/ui/issues/issue-49824.stderr
@@ -1,11 +1,14 @@
 error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-49824.rs:4:9
    |
+LL |       let mut x = 0;
+   |           ----- variable defined here
 LL |       || {
    |        - inferred to be a `FnMut` closure
 LL | /         || {
 LL | |
 LL | |             let _y = &mut x;
+   | |                           - variable captured here
 LL | |         }
    | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
diff --git a/src/test/ui/nll/issue-53040.stderr b/src/test/ui/nll/issue-53040.stderr
index 7cba32c6743..87ffe9b1abf 100644
--- a/src/test/ui/nll/issue-53040.stderr
+++ b/src/test/ui/nll/issue-53040.stderr
@@ -1,9 +1,13 @@
 error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-53040.rs:3:8
    |
+LL |     let mut v: Vec<()> = Vec::new();
+   |         ----- variable defined here
 LL |     || &mut v;
-   |      - ^^^^^^ returns a reference to a captured variable which escapes the closure body
-   |      |
+   |      - ^^^^^-
+   |      | |    |
+   |      | |    variable captured here
+   |      | returns a reference to a captured variable which escapes the closure body
    |      inferred to be a `FnMut` closure
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
diff --git a/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.stderr b/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.stderr
index 4c275b19492..b087e03b464 100644
--- a/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.stderr
+++ b/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.stderr
@@ -1,9 +1,13 @@
 error: captured variable cannot escape `FnMut` closure body
   --> $DIR/regions-return-ref-to-upvar-issue-17403.rs:7:24
    |
+LL |         let mut x = 0;
+   |             ----- variable defined here
 LL |         let mut f = || &mut x;
-   |                      - ^^^^^^ returns a reference to a captured variable which escapes the closure body
-   |                      |
+   |                      - ^^^^^-
+   |                      | |    |
+   |                      | |    variable captured here
+   |                      | returns a reference to a captured variable which escapes the closure body
    |                      inferred to be a `FnMut` closure
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...