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::ConstraintDescription;
  12 use borrow_check::nll::constraints::{OutlivesConstraint};
  13 use borrow_check::nll::region_infer::RegionInferenceContext;
  14 use borrow_check::nll::type_check::Locations;
  15 use borrow_check::nll::universal_regions::DefiningTy;
  16 use util::borrowck_errors::{BorrowckErrors, Origin};
  17 use rustc::hir::def_id::DefId;
  18 use rustc::infer::error_reporting::nice_region_error::NiceRegionError;
  19 use rustc::infer::InferCtxt;
  20 use rustc::mir::{ConstraintCategory, Location, Mir};
  21 use rustc::ty::{self, RegionVid};
  22 use rustc_data_structures::indexed_vec::IndexVec;
  23 use rustc_errors::{Diagnostic, DiagnosticBuilder};
  24 use std::collections::VecDeque;
  25 use syntax::symbol::keywords;
  26 use syntax_pos::Span;
  27 use syntax::errors::Applicability;
  28
  29 mod region_name;
  30 mod var_name;
  31
  32 crate use self::region_name::{RegionName, RegionNameSource};
  33
  34 impl ConstraintDescription for ConstraintCategory {
  35     fn description(&self) -> &'static str {
  36         // Must end with a space. Allows for empty names to be provided.
  37         match self {
  38             ConstraintCategory::Assignment => "assignment ",
  39             ConstraintCategory::Return => "returning this value ",
  40             ConstraintCategory::UseAsConst => "using this value as a constant ",
  41             ConstraintCategory::UseAsStatic => "using this value as a static ",
  42             ConstraintCategory::Cast => "cast ",
  43             ConstraintCategory::CallArgument => "argument ",
  44             ConstraintCategory::TypeAnnotation => "type annotation ",
  45             ConstraintCategory::ClosureBounds => "closure body ",
  46             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  47             ConstraintCategory::CopyBound => "copying this value ",
  48             ConstraintCategory::OpaqueType => "opaque type ",
  49             ConstraintCategory::Boring
  50             | ConstraintCategory::BoringNoLocation
  51             | ConstraintCategory::Internal => "",
  52         }
  53     }
  54 }
  55
  56 #[derive(Copy, Clone, PartialEq, Eq)]
  57 enum Trace {
  58     StartRegion,
  59     FromOutlivesConstraint(OutlivesConstraint),
  60     NotVisited,
  61 }
  62
  63 impl<'tcx> RegionInferenceContext<'tcx> {
  64     /// Tries to find the best constraint to blame for the fact that
  65     /// `R: from_region`, where `R` is some region that meets
  66     /// `target_test`. This works by following the constraint graph,
  67     /// creating a constraint path that forces `R` to outlive
  68     /// `from_region`, and then finding the best choices within that
  69     /// path to blame.
  70     fn best_blame_constraint(
  71         &self,
  72         mir: &Mir<'tcx>,
  73         from_region: RegionVid,
  74         target_test: impl Fn(RegionVid) -> bool,
  75     ) -> (ConstraintCategory, bool, Span) {
  76         debug!("best_blame_constraint(from_region={:?})", from_region);
  77
  78         // Find all paths
  79         let (path, target_region) = self
  80             .find_constraint_paths_between_regions(from_region, target_test)
  81             .unwrap();
  82         debug!(
  83             "best_blame_constraint: path={:#?}",
  84             path.iter()
  85                 .map(|&c| format!(
  86                     "{:?} ({:?}: {:?})",
  87                     c,
  88                     self.constraint_sccs.scc(c.sup),
  89                     self.constraint_sccs.scc(c.sub),
  90                 ))
  91                 .collect::<Vec<_>>()
  92         );
  93
  94         // Classify each of the constraints along the path.
  95         let mut categorized_path: Vec<(ConstraintCategory, bool, Span)> = path
  96             .iter()
  97             .map(|constraint| {
  98                 if constraint.category == ConstraintCategory::ClosureBounds {
  99                     self.retrieve_closure_constraint_info(mir, &constraint)
 100                 } else {
 101                     (constraint.category, false, constraint.locations.span(mir))
 102                 }
 103             })
 104             .collect();
 105         debug!(
 106             "best_blame_constraint: categorized_path={:#?}",
 107             categorized_path
 108         );
 109
 110         // To find the best span to cite, we first try to look for the
 111         // final constraint that is interesting and where the `sup` is
 112         // not unified with the ultimate target region. The reason
 113         // for this is that we have a chain of constraints that lead
 114         // from the source to the target region, something like:
 115         //
 116         //    '0: '1 ('0 is the source)
 117         //    '1: '2
 118         //    '2: '3
 119         //    '3: '4
 120         //    '4: '5
 121         //    '5: '6 ('6 is the target)
 122         //
 123         // Some of those regions are unified with `'6` (in the same
 124         // SCC).  We want to screen those out. After that point, the
 125         // "closest" constraint we have to the end is going to be the
 126         // most likely to be the point where the value escapes -- but
 127         // we still want to screen for an "interesting" point to
 128         // highlight (e.g., a call site or something).
 129         let target_scc = self.constraint_sccs.scc(target_region);
 130         let best_choice = (0..path.len()).rev().find(|&i| {
 131             let constraint = path[i];
 132
 133             let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
 134
 135             match categorized_path[i].0 {
 136                 ConstraintCategory::OpaqueType
 137                 | ConstraintCategory::Boring
 138                 | ConstraintCategory::BoringNoLocation
 139                 | ConstraintCategory::Internal => false,
 140                 ConstraintCategory::TypeAnnotation
 141                 | ConstraintCategory::Return => true,
 142                 _ => constraint_sup_scc != target_scc,
 143             }
 144         });
 145         if let Some(i) = best_choice {
 146             return categorized_path[i]
 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!("`: sorted_path={:#?}", categorized_path);
 155
 156         *categorized_path.first().unwrap()
 157     }
 158
 159     /// Walks the graph of constraints (where `'a: 'b` is considered
 160     /// an edge `'a -> 'b`) to find all paths from `from_region` to
 161     /// `to_region`. The paths are accumulated into the vector
 162     /// `results`. The paths are stored as a series of
 163     /// `ConstraintIndex` values -- in other words, a list of *edges*.
 164     ///
 165     /// Returns: a series of constraints as well as the region `R`
 166     /// that passed the target test.
 167     fn find_constraint_paths_between_regions(
 168         &self,
 169         from_region: RegionVid,
 170         target_test: impl Fn(RegionVid) -> bool,
 171     ) -> Option<(Vec<OutlivesConstraint>, RegionVid)> {
 172         let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
 173         context[from_region] = Trace::StartRegion;
 174
 175         // Use a deque so that we do a breadth-first search. We will
 176         // stop at the first match, which ought to be the shortest
 177         // path (fewest constraints).
 178         let mut deque = VecDeque::new();
 179         deque.push_back(from_region);
 180
 181         while let Some(r) = deque.pop_front() {
 182             // Check if we reached the region we were looking for. If so,
 183             // we can reconstruct the path that led to it and return it.
 184             if target_test(r) {
 185                 let mut result = vec![];
 186                 let mut p = r;
 187                 loop {
 188                     match context[p] {
 189                         Trace::NotVisited => {
 190                             bug!("found unvisited region {:?} on path to {:?}", p, r)
 191                         }
 192                         Trace::FromOutlivesConstraint(c) => {
 193                             result.push(c);
 194                             p = c.sup;
 195                         }
 196
 197                         Trace::StartRegion => {
 198                             result.reverse();
 199                             return Some((result, r));
 200                         }
 201                     }
 202                 }
 203             }
 204
 205             // Otherwise, walk over the outgoing constraints and
 206             // enqueue any regions we find, keeping track of how we
 207             // reached them.
 208             let fr_static = self.universal_regions.fr_static;
 209             for constraint in self.constraint_graph.outgoing_edges(r,
 210                                                                    &self.constraints,
 211                                                                    fr_static) {
 212                 assert_eq!(constraint.sup, r);
 213                 let sub_region = constraint.sub;
 214                 if let Trace::NotVisited = context[sub_region] {
 215                     context[sub_region] = Trace::FromOutlivesConstraint(constraint);
 216                     deque.push_back(sub_region);
 217                 }
 218             }
 219         }
 220
 221         None
 222     }
 223
 224     /// Report an error because the universal region `fr` was required to outlive
 225     /// `outlived_fr` but it is not known to do so. For example:
 226     ///
 227     /// ```
 228     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 229     /// ```
 230     ///
 231     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 232     pub(super) fn report_error(
 233         &self,
 234         mir: &Mir<'tcx>,
 235         infcx: &InferCtxt<'_, '_, 'tcx>,
 236         mir_def_id: DefId,
 237         fr: RegionVid,
 238         outlived_fr: RegionVid,
 239         errors_buffer: &mut Vec<Diagnostic>,
 240     ) {
 241         debug!("report_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 242
 243         let (category, _, span) = self.best_blame_constraint(
 244             mir,
 245             fr,
 246             |r| r == outlived_fr
 247         );
 248
 249         // Check if we can use one of the "nice region errors".
 250         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 251             let tables = infcx.tcx.typeck_tables_of(mir_def_id);
 252             let nice = NiceRegionError::new_from_span(infcx.tcx, span, o, f, Some(tables));
 253             if let Some(_error_reported) = nice.try_report_from_nll() {
 254                 return;
 255             }
 256         }
 257
 258         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 259             self.universal_regions.is_local_free_region(fr),
 260             self.universal_regions.is_local_free_region(outlived_fr),
 261         );
 262
 263         debug!("report_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 264                fr_is_local, outlived_fr_is_local, category);
 265         match (category, fr_is_local, outlived_fr_is_local) {
 266             (ConstraintCategory::Return, true, false) if self.is_closure_fn_mut(infcx, fr) =>
 267                 self.report_fnmut_error(mir, infcx, mir_def_id, fr, outlived_fr, span,
 268                                         errors_buffer),
 269             (ConstraintCategory::Assignment, true, false) |
 270             (ConstraintCategory::CallArgument, true, false) =>
 271                 self.report_escaping_data_error(mir, infcx, mir_def_id, fr, outlived_fr,
 272                                                 category, span, errors_buffer),
 273             _ =>
 274                 self.report_general_error(mir, infcx, mir_def_id, fr, fr_is_local,
 275                                           outlived_fr, outlived_fr_is_local,
 276                                           category, span, errors_buffer),
 277         };
 278     }
 279
 280     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 281     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 282     ///
 283     /// ```text
 284     /// error: captured variable cannot escape `FnMut` closure body
 285     ///   --> $DIR/issue-53040.rs:15:8
 286     ///    |
 287     /// LL |     || &mut v;
 288     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 289     ///    |     |
 290     ///    |     inferred to be a `FnMut` closure
 291     ///    |
 292     ///    = note: `FnMut` closures only have access to their captured variables while they are
 293     ///            executing...
 294     ///    = note: ...therefore, returned references to captured variables will escape the closure
 295     /// ```
 296     fn report_fnmut_error(
 297         &self,
 298         mir: &Mir<'tcx>,
 299         infcx: &InferCtxt<'_, '_, 'tcx>,
 300         mir_def_id: DefId,
 301         _fr: RegionVid,
 302         outlived_fr: RegionVid,
 303         span: Span,
 304         errors_buffer: &mut Vec<Diagnostic>,
 305     ) {
 306         let mut diag = infcx.tcx.sess.struct_span_err(
 307             span,
 308             "captured variable cannot escape `FnMut` closure body",
 309         );
 310
 311         // We should check if the return type of this closure is in fact a closure - in that
 312         // case, we can special case the error further.
 313         let return_type_is_closure = self.universal_regions.unnormalized_output_ty.is_closure();
 314         let message = if return_type_is_closure {
 315             "returns a closure that contains a reference to a captured variable, which then \
 316              escapes the closure body"
 317         } else {
 318             "returns a reference to a captured variable which escapes the closure body"
 319         };
 320
 321         diag.span_label(
 322             span,
 323             message,
 324         );
 325
 326         match self.give_region_a_name(infcx, mir, mir_def_id, outlived_fr, &mut 1).source {
 327             RegionNameSource::NamedEarlyBoundRegion(fr_span) |
 328             RegionNameSource::NamedFreeRegion(fr_span) |
 329             RegionNameSource::SynthesizedFreeEnvRegion(fr_span, _) |
 330             RegionNameSource::CannotMatchHirTy(fr_span, _) |
 331             RegionNameSource::MatchedHirTy(fr_span) |
 332             RegionNameSource::MatchedAdtAndSegment(fr_span) |
 333             RegionNameSource::AnonRegionFromUpvar(fr_span, _) |
 334             RegionNameSource::AnonRegionFromOutput(fr_span, _, _) => {
 335                 diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 336             },
 337             _ => {},
 338         }
 339
 340         diag.note("`FnMut` closures only have access to their captured variables while they are \
 341                    executing...");
 342         diag.note("...therefore, they cannot allow references to captured variables to escape");
 343
 344         diag.buffer(errors_buffer);
 345     }
 346
 347     /// Reports a error specifically for when data is escaping a closure.
 348     ///
 349     /// ```text
 350     /// error: borrowed data escapes outside of function
 351     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 352     ///    |
 353     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 354     ///    |              - `x` is a reference that is only valid in the function body
 355     /// LL |     // but ref_obj will not, so warn.
 356     /// LL |     ref_obj(x)
 357     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 358     /// ```
 359     fn report_escaping_data_error(
 360         &self,
 361         mir: &Mir<'tcx>,
 362         infcx: &InferCtxt<'_, '_, 'tcx>,
 363         mir_def_id: DefId,
 364         fr: RegionVid,
 365         outlived_fr: RegionVid,
 366         category: ConstraintCategory,
 367         span: Span,
 368         errors_buffer: &mut Vec<Diagnostic>,
 369     ) {
 370         let fr_name_and_span = self.get_var_name_and_span_for_region(infcx.tcx, mir, fr);
 371         let outlived_fr_name_and_span =
 372             self.get_var_name_and_span_for_region(infcx.tcx, mir, outlived_fr);
 373
 374         let escapes_from = match self.universal_regions.defining_ty {
 375             DefiningTy::Closure(..) => "closure",
 376             DefiningTy::Generator(..) => "generator",
 377             DefiningTy::FnDef(..) => "function",
 378             DefiningTy::Const(..) => "const"
 379         };
 380
 381         // Revert to the normal error in these cases.
 382         // Assignments aren't "escapes" in function items.
 383         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 384             || (category == ConstraintCategory::Assignment && escapes_from == "function")
 385             || escapes_from == "const"
 386         {
 387             return self.report_general_error(mir, infcx, mir_def_id,
 388                                              fr, true, outlived_fr, false,
 389                                              category, span, errors_buffer);
 390         }
 391
 392         let mut diag = infcx.tcx.borrowed_data_escapes_closure(span, escapes_from, Origin::Mir);
 393
 394         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 395             diag.span_label(
 396                 outlived_fr_span,
 397                 format!(
 398                     "`{}` is declared here, outside of the {} body",
 399                     outlived_fr_name, escapes_from
 400                 ),
 401             );
 402         }
 403
 404         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 405             diag.span_label(
 406                 fr_span,
 407                 format!(
 408                     "`{}` is a reference that is only valid in the {} body",
 409                     fr_name, escapes_from
 410                 ),
 411             );
 412
 413             diag.span_label(span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 414         }
 415
 416         diag.buffer(errors_buffer);
 417     }
 418
 419     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 420     /// explain what is happening.
 421     ///
 422     /// ```text
 423     /// error: unsatisfied lifetime constraints
 424     ///   --> $DIR/regions-creating-enums3.rs:17:5
 425     ///    |
 426     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 427     ///    |                -- -- lifetime `'b` defined here
 428     ///    |                |
 429     ///    |                lifetime `'a` defined here
 430     /// LL |     ast::add(x, y)
 431     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 432     ///    |                    is returning data with lifetime `'b`
 433     /// ```
 434     fn report_general_error(
 435         &self,
 436         mir: &Mir<'tcx>,
 437         infcx: &InferCtxt<'_, '_, 'tcx>,
 438         mir_def_id: DefId,
 439         fr: RegionVid,
 440         fr_is_local: bool,
 441         outlived_fr: RegionVid,
 442         outlived_fr_is_local: bool,
 443         category: ConstraintCategory,
 444         span: Span,
 445         errors_buffer: &mut Vec<Diagnostic>,
 446     ) {
 447         let mut diag = infcx.tcx.sess.struct_span_err(
 448             span,
 449             "unsatisfied lifetime constraints", // FIXME
 450         );
 451
 452         let counter = &mut 1;
 453         let fr_name = self.give_region_a_name(infcx, mir, mir_def_id, fr, counter);
 454         fr_name.highlight_region_name(&mut diag);
 455         let outlived_fr_name = self.give_region_a_name(
 456             infcx, mir, mir_def_id, outlived_fr, counter);
 457         outlived_fr_name.highlight_region_name(&mut diag);
 458
 459         let mir_def_name = if infcx.tcx.is_closure(mir_def_id) { "closure" } else { "function" };
 460
 461         match (category, outlived_fr_is_local, fr_is_local) {
 462             (ConstraintCategory::Return, true, _) => {
 463                 diag.span_label(span, format!(
 464                     "{} was supposed to return data with lifetime `{}` but it is returning \
 465                     data with lifetime `{}`",
 466                     mir_def_name, outlived_fr_name, fr_name
 467                 ));
 468             },
 469             _ => {
 470                 diag.span_label(span, format!(
 471                     "{}requires that `{}` must outlive `{}`",
 472                     category.description(), fr_name, outlived_fr_name,
 473                 ));
 474             },
 475         }
 476
 477         self.add_static_impl_trait_suggestion(
 478             infcx, &mut diag, fr, fr_name, outlived_fr,
 479         );
 480
 481         diag.buffer(errors_buffer);
 482     }
 483
 484     /// Adds a suggestion to errors where a `impl Trait` is returned.
 485     ///
 486     /// ```text
 487     /// help: to allow this impl Trait to capture borrowed data with lifetime `'1`, add `'_` as
 488     ///       a constraint
 489     ///    |
 490     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 491     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 492     /// ```
 493     fn add_static_impl_trait_suggestion(
 494         &self,
 495         infcx: &InferCtxt<'_, '_, 'tcx>,
 496         diag: &mut DiagnosticBuilder<'_>,
 497         fr: RegionVid,
 498         // We need to pass `fr_name` - computing it again will label it twice.
 499         fr_name: RegionName,
 500         outlived_fr: RegionVid,
 501     ) {
 502         if let (
 503             Some(f),
 504             Some(ty::RegionKind::ReStatic)
 505         ) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 506             if let Some(ty::TyS {
 507                 sty: ty::TyKind::Opaque(did, substs),
 508                 ..
 509             }) = infcx.tcx.is_suitable_region(f)
 510                     .map(|r| r.def_id)
 511                     .map(|id| infcx.tcx.return_type_impl_trait(id))
 512                     .unwrap_or(None)
 513             {
 514                 // Check whether or not the impl trait return type is intended to capture
 515                 // data with the static lifetime.
 516                 //
 517                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 518                 let has_static_predicate = {
 519                     let predicates_of = infcx.tcx.predicates_of(*did);
 520                     let bounds = predicates_of.instantiate(infcx.tcx, substs);
 521
 522                     let mut found = false;
 523                     for predicate in bounds.predicates {
 524                         if let ty::Predicate::TypeOutlives(binder) = predicate {
 525                             if let ty::OutlivesPredicate(
 526                                 _,
 527                                 ty::RegionKind::ReStatic
 528                             ) = binder.skip_binder() {
 529                                 found = true;
 530                                 break;
 531                             }
 532                         }
 533                     }
 534
 535                     found
 536                 };
 537
 538                 debug!("add_static_impl_trait_suggestion: has_static_predicate={:?}",
 539                        has_static_predicate);
 540                 let static_str = keywords::StaticLifetime.name();
 541                 // If there is a static predicate, then the only sensible suggestion is to replace
 542                 // fr with `'static`.
 543                 if has_static_predicate {
 544                     diag.help(
 545                         &format!(
 546                             "consider replacing `{}` with `{}`",
 547                             fr_name, static_str,
 548                         ),
 549                     );
 550                 } else {
 551                     // Otherwise, we should suggest adding a constraint on the return type.
 552                     let span = infcx.tcx.def_span(*did);
 553                     if let Ok(snippet) = infcx.tcx.sess.source_map().span_to_snippet(span) {
 554                         let suggestable_fr_name = if fr_name.was_named() {
 555                             fr_name.to_string()
 556                         } else {
 557                             "'_".to_string()
 558                         };
 559
 560                         diag.span_suggestion_with_applicability(
 561                             span,
 562                             &format!(
 563                                 "to allow this impl Trait to capture borrowed data with lifetime \
 564                                  `{}`, add `{}` as a constraint",
 565                                 fr_name, suggestable_fr_name,
 566                             ),
 567                             format!("{} + {}", snippet, suggestable_fr_name),
 568                             Applicability::MachineApplicable,
 569                         );
 570                     }
 571                 }
 572             }
 573         }
 574     }
 575
 576     crate fn free_region_constraint_info(
 577         &self,
 578         mir: &Mir<'tcx>,
 579         mir_def_id: DefId,
 580         infcx: &InferCtxt<'_, '_, 'tcx>,
 581         borrow_region: RegionVid,
 582         outlived_region: RegionVid,
 583     ) -> (ConstraintCategory, bool, Span, RegionName) {
 584         let (category, from_closure, span) = self.best_blame_constraint(
 585             mir,
 586             borrow_region,
 587             |r| r == outlived_region
 588         );
 589         let outlived_fr_name = self.give_region_a_name(
 590             infcx, mir, mir_def_id, outlived_region, &mut 1);
 591         (category, from_closure, span, outlived_fr_name)
 592     }
 593
 594     // Finds some region R such that `fr1: R` and `R` is live at
 595     // `elem`.
 596     crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
 597         // Find all paths
 598         let (_path, r) =
 599             self.find_constraint_paths_between_regions(fr1, |r| {
 600                 self.liveness_constraints.contains(r, elem)
 601             }).unwrap();
 602         r
 603     }
 604
 605     // Finds a good span to blame for the fact that `fr1` outlives `fr2`.
 606     crate fn find_outlives_blame_span(
 607         &self,
 608         mir: &Mir<'tcx>,
 609         fr1: RegionVid,
 610         fr2: RegionVid,
 611     ) -> (ConstraintCategory, Span) {
 612         let (category, _, span) = self.best_blame_constraint(mir, fr1, |r| r == fr2);
 613         (category, span)
 614     }
 615
 616     fn retrieve_closure_constraint_info(
 617         &self,
 618         mir: &Mir<'tcx>,
 619         constraint: &OutlivesConstraint
 620     ) -> (ConstraintCategory, bool, Span) {
 621         let loc = match constraint.locations {
 622             Locations::All(span) => return (constraint.category, false, span),
 623             Locations::Single(loc) => loc,
 624         };
 625
 626         let opt_span_category = self
 627             .closure_bounds_mapping[&loc]
 628             .get(&(constraint.sup, constraint.sub));
 629         opt_span_category
 630             .map(|&(category, span)| (category, true, span))
 631             .unwrap_or((constraint.category, false, mir.source_info(loc).span))
 632     }
 633
 634     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 635     crate fn is_closure_fn_mut(
 636         &self,
 637         infcx: &InferCtxt<'_, '_, 'tcx>,
 638         fr: RegionVid,
 639     ) -> bool {
 640         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
 641             if let ty::BoundRegion::BrEnv = free_region.bound_region {
 642                 if let DefiningTy::Closure(def_id, substs) = self.universal_regions.defining_ty {
 643                     let closure_kind_ty = substs.closure_kind_ty(def_id, infcx.tcx);
 644                     return Some(ty::ClosureKind::FnMut) == closure_kind_ty.to_opt_closure_kind();
 645                 }
 646             }
 647         }
 648
 649         false
 650     }
 651 }