src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use borrow_check::nll::constraints::{OutlivesConstraint};
  12 use borrow_check::nll::region_infer::RegionInferenceContext;
  13 use borrow_check::nll::region_infer::error_reporting::region_name::RegionNameSource;
  14 use borrow_check::nll::type_check::Locations;
  15 use borrow_check::nll::universal_regions::DefiningTy;
  16 use rustc::hir::def_id::DefId;
  17 use rustc::infer::error_reporting::nice_region_error::NiceRegionError;
  18 use rustc::infer::InferCtxt;
  19 use rustc::mir::{ConstraintCategory, Location, Mir};
  20 use rustc::ty::{self, RegionVid};
  21 use rustc_data_structures::indexed_vec::IndexVec;
  22 use rustc_errors::{Diagnostic, DiagnosticBuilder};
  23 use std::collections::VecDeque;
  24 use syntax::symbol::keywords;
  25 use syntax_pos::Span;
  26 use syntax::errors::Applicability;
  27
  28 mod region_name;
  29 mod var_name;
  30
  31 use self::region_name::RegionName;
  32
  33 trait ConstraintDescription {
  34     fn description(&self) -> &'static str;
  35 }
  36
  37 impl ConstraintDescription for ConstraintCategory {
  38     fn description(&self) -> &'static str {
  39         // Must end with a space. Allows for empty names to be provided.
  40         match self {
  41             ConstraintCategory::Assignment => "assignment ",
  42             ConstraintCategory::Return => "returning this value ",
  43             ConstraintCategory::Cast => "cast ",
  44             ConstraintCategory::CallArgument => "argument ",
  45             ConstraintCategory::TypeAnnotation => "type annotation ",
  46             ConstraintCategory::ClosureBounds => "closure body ",
  47             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  48             ConstraintCategory::CopyBound => "copying this value ",
  49             ConstraintCategory::OpaqueType => "opaque type ",
  50             ConstraintCategory::Boring
  51             | ConstraintCategory::BoringNoLocation
  52             | ConstraintCategory::Internal => "",
  53         }
  54     }
  55 }
  56
  57 #[derive(Copy, Clone, PartialEq, Eq)]
  58 enum Trace {
  59     StartRegion,
  60     FromOutlivesConstraint(OutlivesConstraint),
  61     NotVisited,
  62 }
  63
  64 impl<'tcx> RegionInferenceContext<'tcx> {
  65     /// Tries to find the best constraint to blame for the fact that
  66     /// `R: from_region`, where `R` is some region that meets
  67     /// `target_test`. This works by following the constraint graph,
  68     /// creating a constraint path that forces `R` to outlive
  69     /// `from_region`, and then finding the best choices within that
  70     /// path to blame.
  71     fn best_blame_constraint(
  72         &self,
  73         mir: &Mir<'tcx>,
  74         from_region: RegionVid,
  75         target_test: impl Fn(RegionVid) -> bool,
  76     ) -> (ConstraintCategory, Span, RegionVid) {
  77         debug!("best_blame_constraint(from_region={:?})", from_region);
  78
  79         // Find all paths
  80         let (path, target_region) = self
  81             .find_constraint_paths_between_regions(from_region, target_test)
  82             .unwrap();
  83         debug!(
  84             "best_blame_constraint: path={:#?}",
  85             path.iter()
  86                 .map(|&c| format!(
  87                     "{:?} ({:?}: {:?})",
  88                     c,
  89                     self.constraint_sccs.scc(c.sup),
  90                     self.constraint_sccs.scc(c.sub),
  91                 ))
  92                 .collect::<Vec<_>>()
  93         );
  94
  95         // Classify each of the constraints along the path.
  96         let mut categorized_path: Vec<(ConstraintCategory, Span)> = path
  97             .iter()
  98             .map(|constraint| {
  99                 if constraint.category == ConstraintCategory::ClosureBounds {
 100                     self.retrieve_closure_constraint_info(mir, &constraint)
 101                 } else {
 102                     (constraint.category, constraint.locations.span(mir))
 103                 }
 104             })
 105             .collect();
 106         debug!(
 107             "best_blame_constraint: categorized_path={:#?}",
 108             categorized_path
 109         );
 110
 111         // To find the best span to cite, we first try to look for the
 112         // final constraint that is interesting and where the `sup` is
 113         // not unified with the ultimate target region. The reason
 114         // for this is that we have a chain of constraints that lead
 115         // from the source to the target region, something like:
 116         //
 117         //    '0: '1 ('0 is the source)
 118         //    '1: '2
 119         //    '2: '3
 120         //    '3: '4
 121         //    '4: '5
 122         //    '5: '6 ('6 is the target)
 123         //
 124         // Some of those regions are unified with `'6` (in the same
 125         // SCC).  We want to screen those out. After that point, the
 126         // "closest" constraint we have to the end is going to be the
 127         // most likely to be the point where the value escapes -- but
 128         // we still want to screen for an "interesting" point to
 129         // highlight (e.g., a call site or something).
 130         let target_scc = self.constraint_sccs.scc(target_region);
 131         let best_choice = (0..path.len()).rev().find(|&i| {
 132             let constraint = path[i];
 133
 134             let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
 135
 136             match categorized_path[i].0 {
 137                 ConstraintCategory::OpaqueType
 138                 | ConstraintCategory::Boring
 139                 | ConstraintCategory::BoringNoLocation
 140                 | ConstraintCategory::Internal => false,
 141                 _ => constraint_sup_scc != target_scc,
 142             }
 143         });
 144         if let Some(i) = best_choice {
 145             let (category, span) = categorized_path[i];
 146             return (category, span, target_region);
 147         }
 148
 149         // If that search fails, that is.. unusual. Maybe everything
 150         // is in the same SCC or something. In that case, find what
 151         // appears to be the most interesting point to report to the
 152         // user via an even more ad-hoc guess.
 153         categorized_path.sort_by(|p0, p1| p0.0.cmp(&p1.0));
 154         debug!("best_blame_constraint: sorted_path={:#?}", categorized_path);
 155
 156         let &(category, span) = categorized_path.first().unwrap();
 157
 158         (category, span, target_region)
 159     }
 160
 161     /// Walks the graph of constraints (where `'a: 'b` is considered
 162     /// an edge `'a -> 'b`) to find all paths from `from_region` to
 163     /// `to_region`. The paths are accumulated into the vector
 164     /// `results`. The paths are stored as a series of
 165     /// `ConstraintIndex` values -- in other words, a list of *edges*.
 166     ///
 167     /// Returns: a series of constraints as well as the region `R`
 168     /// that passed the target test.
 169     fn find_constraint_paths_between_regions(
 170         &self,
 171         from_region: RegionVid,
 172         target_test: impl Fn(RegionVid) -> bool,
 173     ) -> Option<(Vec<OutlivesConstraint>, RegionVid)> {
 174         let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
 175         context[from_region] = Trace::StartRegion;
 176
 177         // Use a deque so that we do a breadth-first search. We will
 178         // stop at the first match, which ought to be the shortest
 179         // path (fewest constraints).
 180         let mut deque = VecDeque::new();
 181         deque.push_back(from_region);
 182
 183         while let Some(r) = deque.pop_front() {
 184             // Check if we reached the region we were looking for. If so,
 185             // we can reconstruct the path that led to it and return it.
 186             if target_test(r) {
 187                 let mut result = vec![];
 188                 let mut p = r;
 189                 loop {
 190                     match context[p] {
 191                         Trace::NotVisited => {
 192                             bug!("found unvisited region {:?} on path to {:?}", p, r)
 193                         }
 194                         Trace::FromOutlivesConstraint(c) => {
 195                             result.push(c);
 196                             p = c.sup;
 197                         }
 198
 199                         Trace::StartRegion => {
 200                             result.reverse();
 201                             return Some((result, r));
 202                         }
 203                     }
 204                 }
 205             }
 206
 207             // Otherwise, walk over the outgoing constraints and
 208             // enqueue any regions we find, keeping track of how we
 209             // reached them.
 210             let fr_static = self.universal_regions.fr_static;
 211             for constraint in self.constraint_graph.outgoing_edges(r,
 212                                                                    &self.constraints,
 213                                                                    fr_static) {
 214                 assert_eq!(constraint.sup, r);
 215                 let sub_region = constraint.sub;
 216                 if let Trace::NotVisited = context[sub_region] {
 217                     context[sub_region] = Trace::FromOutlivesConstraint(constraint);
 218                     deque.push_back(sub_region);
 219                 }
 220             }
 221         }
 222
 223         None
 224     }
 225
 226     /// Report an error because the universal region `fr` was required to outlive
 227     /// `outlived_fr` but it is not known to do so. For example:
 228     ///
 229     /// ```
 230     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 231     /// ```
 232     ///
 233     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 234     pub(super) fn report_error(
 235         &self,
 236         mir: &Mir<'tcx>,
 237         infcx: &InferCtxt<'_, '_, 'tcx>,
 238         mir_def_id: DefId,
 239         fr: RegionVid,
 240         outlived_fr: RegionVid,
 241         errors_buffer: &mut Vec<Diagnostic>,
 242     ) {
 243         debug!("report_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 244
 245         let (category, span, _) = self.best_blame_constraint(
 246             mir,
 247             fr,
 248             |r| r == outlived_fr
 249         );
 250
 251         // Check if we can use one of the "nice region errors".
 252         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 253             let tables = infcx.tcx.typeck_tables_of(mir_def_id);
 254             let nice = NiceRegionError::new_from_span(infcx.tcx, span, o, f, Some(tables));
 255             if let Some(_error_reported) = nice.try_report_from_nll() {
 256                 return;
 257             }
 258         }
 259
 260         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 261             self.universal_regions.is_local_free_region(fr),
 262             self.universal_regions.is_local_free_region(outlived_fr),
 263         );
 264
 265         debug!("report_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 266                fr_is_local, outlived_fr_is_local, category);
 267         match (category, fr_is_local, outlived_fr_is_local) {
 268             (ConstraintCategory::Return, true, false) if self.is_closure_fn_mut(infcx, fr) =>
 269                 self.report_fnmut_error(mir, infcx, mir_def_id, fr, outlived_fr, span,
 270                                         errors_buffer),
 271             (ConstraintCategory::Assignment, true, false) |
 272             (ConstraintCategory::CallArgument, true, false) =>
 273                 self.report_escaping_data_error(mir, infcx, mir_def_id, fr, outlived_fr,
 274                                                 category, span, errors_buffer),
 275             _ =>
 276                 self.report_general_error(mir, infcx, mir_def_id, fr, fr_is_local,
 277                                           outlived_fr, outlived_fr_is_local,
 278                                           category, span, errors_buffer),
 279         };
 280     }
 281
 282     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 283     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 284     ///
 285     /// ```text
 286     /// error: captured variable cannot escape `FnMut` closure body
 287     ///   --> $DIR/issue-53040.rs:15:8
 288     ///    |
 289     /// LL |     || &mut v;
 290     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 291     ///    |     |
 292     ///    |     inferred to be a `FnMut` closure
 293     ///    |
 294     ///    = note: `FnMut` closures only have access to their captured variables while they are
 295     ///            executing...
 296     ///    = note: ...therefore, returned references to captured variables will escape the closure
 297     /// ```
 298     fn report_fnmut_error(
 299         &self,
 300         mir: &Mir<'tcx>,
 301         infcx: &InferCtxt<'_, '_, 'tcx>,
 302         mir_def_id: DefId,
 303         _fr: RegionVid,
 304         outlived_fr: RegionVid,
 305         span: Span,
 306         errors_buffer: &mut Vec<Diagnostic>,
 307     ) {
 308         let mut diag = infcx.tcx.sess.struct_span_err(
 309             span,
 310             "captured variable cannot escape `FnMut` closure body",
 311         );
 312
 313         // We should check if the return type of this closure is in fact a closure - in that
 314         // case, we can special case the error further.
 315         let return_type_is_closure = self.universal_regions.unnormalized_output_ty.is_closure();
 316         let message = if return_type_is_closure {
 317             "returns a closure that contains a reference to a captured variable, which then \
 318              escapes the closure body"
 319         } else {
 320             "returns a reference to a captured variable which escapes the closure body"
 321         };
 322
 323         diag.span_label(
 324             span,
 325             message,
 326         );
 327
 328         match self.give_region_a_name(infcx, mir, mir_def_id, outlived_fr, &mut 1).source {
 329             RegionNameSource::NamedEarlyBoundRegion(fr_span) |
 330             RegionNameSource::NamedFreeRegion(fr_span) |
 331             RegionNameSource::SynthesizedFreeEnvRegion(fr_span, _) |
 332             RegionNameSource::CannotMatchHirTy(fr_span, _) |
 333             RegionNameSource::MatchedHirTy(fr_span) |
 334             RegionNameSource::MatchedAdtAndSegment(fr_span) |
 335             RegionNameSource::AnonRegionFromUpvar(fr_span, _) |
 336             RegionNameSource::AnonRegionFromOutput(fr_span, _, _) => {
 337                 diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 338             },
 339             _ => {},
 340         }
 341
 342         diag.note("`FnMut` closures only have access to their captured variables while they are \
 343                    executing...");
 344         diag.note("...therefore, they cannot allow references to captured variables to escape");
 345
 346         diag.buffer(errors_buffer);
 347     }
 348
 349     /// Reports a error specifically for when data is escaping a closure.
 350     ///
 351     /// ```text
 352     /// error: borrowed data escapes outside of function
 353     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 354     ///    |
 355     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 356     ///    |              - `x` is a reference that is only valid in the function body
 357     /// LL |     // but ref_obj will not, so warn.
 358     /// LL |     ref_obj(x)
 359     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 360     /// ```
 361     fn report_escaping_data_error(
 362         &self,
 363         mir: &Mir<'tcx>,
 364         infcx: &InferCtxt<'_, '_, 'tcx>,
 365         mir_def_id: DefId,
 366         fr: RegionVid,
 367         outlived_fr: RegionVid,
 368         category: ConstraintCategory,
 369         span: Span,
 370         errors_buffer: &mut Vec<Diagnostic>,
 371     ) {
 372         let fr_name_and_span = self.get_var_name_and_span_for_region(infcx.tcx, mir, fr);
 373         let outlived_fr_name_and_span =
 374             self.get_var_name_and_span_for_region(infcx.tcx, mir, outlived_fr);
 375
 376         let escapes_from = if infcx.tcx.is_closure(mir_def_id) { "closure" } else { "function" };
 377
 378         // Revert to the normal error in these cases.
 379         // Assignments aren't "escapes" in function items.
 380         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 381             || (category == ConstraintCategory::Assignment && escapes_from == "function")
 382         {
 383             return self.report_general_error(mir, infcx, mir_def_id,
 384                                              fr, true, outlived_fr, false,
 385                                              category, span, errors_buffer);
 386         }
 387
 388         let mut diag = infcx.tcx.sess.struct_span_err(
 389             span, &format!("borrowed data escapes outside of {}", escapes_from),
 390         );
 391
 392         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 393             diag.span_label(
 394                 outlived_fr_span,
 395                 format!(
 396                     "`{}` is declared here, outside of the {} body",
 397                     outlived_fr_name, escapes_from
 398                 ),
 399             );
 400         }
 401
 402         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 403             diag.span_label(
 404                 fr_span,
 405                 format!(
 406                     "`{}` is a reference that is only valid in the {} body",
 407                     fr_name, escapes_from
 408                 ),
 409             );
 410
 411             diag.span_label(span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 412         }
 413
 414         diag.buffer(errors_buffer);
 415     }
 416
 417     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 418     /// explain what is happening.
 419     ///
 420     /// ```text
 421     /// error: unsatisfied lifetime constraints
 422     ///   --> $DIR/regions-creating-enums3.rs:17:5
 423     ///    |
 424     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 425     ///    |                -- -- lifetime `'b` defined here
 426     ///    |                |
 427     ///    |                lifetime `'a` defined here
 428     /// LL |     ast::add(x, y)
 429     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 430     ///    |                    is returning data with lifetime `'b`
 431     /// ```
 432     fn report_general_error(
 433         &self,
 434         mir: &Mir<'tcx>,
 435         infcx: &InferCtxt<'_, '_, 'tcx>,
 436         mir_def_id: DefId,
 437         fr: RegionVid,
 438         fr_is_local: bool,
 439         outlived_fr: RegionVid,
 440         outlived_fr_is_local: bool,
 441         category: ConstraintCategory,
 442         span: Span,
 443         errors_buffer: &mut Vec<Diagnostic>,
 444     ) {
 445         let mut diag = infcx.tcx.sess.struct_span_err(
 446             span,
 447             "unsatisfied lifetime constraints", // FIXME
 448         );
 449
 450         let counter = &mut 1;
 451         let fr_name = self.give_region_a_name(infcx, mir, mir_def_id, fr, counter);
 452         fr_name.highlight_region_name(&mut diag);
 453         let outlived_fr_name = self.give_region_a_name(
 454             infcx, mir, mir_def_id, outlived_fr, counter);
 455         outlived_fr_name.highlight_region_name(&mut diag);
 456
 457         let mir_def_name = if infcx.tcx.is_closure(mir_def_id) { "closure" } else { "function" };
 458
 459         match (category, outlived_fr_is_local, fr_is_local) {
 460             (ConstraintCategory::Return, true, _) => {
 461                 diag.span_label(span, format!(
 462                     "{} was supposed to return data with lifetime `{}` but it is returning \
 463                     data with lifetime `{}`",
 464                     mir_def_name, outlived_fr_name, fr_name
 465                 ));
 466             },
 467             _ => {
 468                 diag.span_label(span, format!(
 469                     "{}requires that `{}` must outlive `{}`",
 470                     category.description(), fr_name, outlived_fr_name,
 471                 ));
 472             },
 473         }
 474
 475         self.add_static_impl_trait_suggestion(
 476             infcx, &mut diag, fr, fr_name, outlived_fr,
 477         );
 478
 479         diag.buffer(errors_buffer);
 480     }
 481
 482     /// Adds a suggestion to errors where a `impl Trait` is returned.
 483     ///
 484     /// ```text
 485     /// help: to allow this impl Trait to capture borrowed data with lifetime `'1`, add `'_` as
 486     ///       a constraint
 487     ///    |
 488     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 489     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 490     /// ```
 491     fn add_static_impl_trait_suggestion(
 492         &self,
 493         infcx: &InferCtxt<'_, '_, 'tcx>,
 494         diag: &mut DiagnosticBuilder<'_>,
 495         fr: RegionVid,
 496         // We need to pass `fr_name` - computing it again will label it twice.
 497         fr_name: RegionName,
 498         outlived_fr: RegionVid,
 499     ) {
 500         if let (
 501             Some(f),
 502             Some(ty::RegionKind::ReStatic)
 503         ) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 504             if let Some(ty::TyS {
 505                 sty: ty::TyKind::Opaque(did, substs),
 506                 ..
 507             }) = infcx.tcx.is_suitable_region(f)
 508                     .map(|r| r.def_id)
 509                     .map(|id| infcx.tcx.return_type_impl_trait(id))
 510                     .unwrap_or(None)
 511             {
 512                 // Check whether or not the impl trait return type is intended to capture
 513                 // data with the static lifetime.
 514                 //
 515                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 516                 let has_static_predicate = {
 517                     let predicates_of = infcx.tcx.predicates_of(*did);
 518                     let bounds = predicates_of.instantiate(infcx.tcx, substs);
 519
 520                     let mut found = false;
 521                     for predicate in bounds.predicates {
 522                         if let ty::Predicate::TypeOutlives(binder) = predicate {
 523                             if let ty::OutlivesPredicate(
 524                                 _,
 525                                 ty::RegionKind::ReStatic
 526                             ) = binder.skip_binder() {
 527                                 found = true;
 528                                 break;
 529                             }
 530                         }
 531                     }
 532
 533                     found
 534                 };
 535
 536                 debug!("add_static_impl_trait_suggestion: has_static_predicate={:?}",
 537                        has_static_predicate);
 538                 let static_str = keywords::StaticLifetime.name();
 539                 // If there is a static predicate, then the only sensible suggestion is to replace
 540                 // fr with `'static`.
 541                 if has_static_predicate {
 542                     diag.help(
 543                         &format!(
 544                             "consider replacing `{}` with `{}`",
 545                             fr_name, static_str,
 546                         ),
 547                     );
 548                 } else {
 549                     // Otherwise, we should suggest adding a constraint on the return type.
 550                     let span = infcx.tcx.def_span(*did);
 551                     if let Ok(snippet) = infcx.tcx.sess.source_map().span_to_snippet(span) {
 552                         let suggestable_fr_name = if fr_name.was_named() {
 553                             fr_name.to_string()
 554                         } else {
 555                             "'_".to_string()
 556                         };
 557
 558                         diag.span_suggestion_with_applicability(
 559                             span,
 560                             &format!(
 561                                 "to allow this impl Trait to capture borrowed data with lifetime \
 562                                  `{}`, add `{}` as a constraint",
 563                                 fr_name, suggestable_fr_name,
 564                             ),
 565                             format!("{} + {}", snippet, suggestable_fr_name),
 566                             Applicability::MachineApplicable,
 567                         );
 568                     }
 569                 }
 570             }
 571         }
 572     }
 573
 574     // Finds some region R such that `fr1: R` and `R` is live at
 575     // `elem`.
 576     crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
 577         // Find all paths
 578         let (_path, r) =
 579             self.find_constraint_paths_between_regions(fr1, |r| {
 580                 self.liveness_constraints.contains(r, elem)
 581             }).unwrap();
 582         r
 583     }
 584
 585     // Finds a good span to blame for the fact that `fr1` outlives `fr2`.
 586     crate fn find_outlives_blame_span(
 587         &self,
 588         mir: &Mir<'tcx>,
 589         fr1: RegionVid,
 590         fr2: RegionVid,
 591     ) -> (ConstraintCategory, Span) {
 592         let (category, span, _) = self.best_blame_constraint(mir, fr1, |r| r == fr2);
 593         (category, span)
 594     }
 595
 596     fn retrieve_closure_constraint_info(
 597         &self,
 598         mir: &Mir<'tcx>,
 599         constraint: &OutlivesConstraint
 600     ) -> (ConstraintCategory, Span) {
 601         let loc = match constraint.locations {
 602             Locations::All(span) => return (constraint.category, span),
 603             Locations::Single(loc) => loc,
 604         };
 605
 606         let opt_span_category = self
 607             .closure_bounds_mapping[&loc]
 608             .get(&(constraint.sup, constraint.sub));
 609         *opt_span_category.unwrap_or(&(constraint.category, mir.source_info(loc).span))
 610     }
 611
 612     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 613     crate fn is_closure_fn_mut(
 614         &self,
 615         infcx: &InferCtxt<'_, '_, 'tcx>,
 616         fr: RegionVid,
 617     ) -> bool {
 618         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
 619             if let ty::BoundRegion::BrEnv = free_region.bound_region {
 620                 if let DefiningTy::Closure(def_id, substs) = self.universal_regions.defining_ty {
 621                     let closure_kind_ty = substs.closure_kind_ty(def_id, infcx.tcx);
 622                     return Some(ty::ClosureKind::FnMut) == closure_kind_ty.to_opt_closure_kind();
 623                 }
 624             }
 625         }
 626
 627         false
 628     }
 629 }