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 rustc::hir::def_id::DefId;
  14 use rustc::infer::error_reporting::nice_region_error::NiceRegionError;
  15 use rustc::infer::InferCtxt;
  16 use rustc::mir::{ConstraintCategory, Location, Mir};
  17 use rustc::ty::{self, RegionVid};
  18 use rustc_data_structures::indexed_vec::IndexVec;
  19 use rustc_errors::{Diagnostic, DiagnosticBuilder};
  20 use std::collections::VecDeque;
  21 use std::fmt;
  22 use syntax::symbol::keywords;
  23 use syntax_pos::Span;
  24 use syntax::errors::Applicability;
  25
  26 mod region_name;
  27 mod var_name;
  28
  29 use self::region_name::RegionName;
  30
  31 trait ConstraintDescription {
  32     fn description(&self) -> &'static str;
  33 }
  34
  35 impl ConstraintDescription for ConstraintCategory {
  36     fn description(&self) -> &'static str {
  37         // Must end with a space. Allows for empty names to be provided.
  38         match self {
  39             ConstraintCategory::Assignment => "assignment ",
  40             ConstraintCategory::Return => "returning this value ",
  41             ConstraintCategory::Cast => "cast ",
  42             ConstraintCategory::CallArgument => "argument ",
  43             ConstraintCategory::TypeAnnotation => "type annotation ",
  44             ConstraintCategory::ClosureBounds => "closure body ",
  45             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  46             ConstraintCategory::CopyBound => "copying this value ",
  47             ConstraintCategory::OpaqueType => "opaque type ",
  48             ConstraintCategory::Boring
  49             | ConstraintCategory::BoringNoLocation
  50             | ConstraintCategory::Internal => "",
  51         }
  52     }
  53 }
  54
  55 #[derive(Copy, Clone, PartialEq, Eq)]
  56 enum Trace {
  57     StartRegion,
  58     FromOutlivesConstraint(OutlivesConstraint),
  59     NotVisited,
  60 }
  61
  62 impl<'tcx> RegionInferenceContext<'tcx> {
  63     /// Tries to find the best constraint to blame for the fact that
  64     /// `R: from_region`, where `R` is some region that meets
  65     /// `target_test`. This works by following the constraint graph,
  66     /// creating a constraint path that forces `R` to outlive
  67     /// `from_region`, and then finding the best choices within that
  68     /// path to blame.
  69     fn best_blame_constraint(
  70         &self,
  71         mir: &Mir<'tcx>,
  72         from_region: RegionVid,
  73         target_test: impl Fn(RegionVid) -> bool,
  74     ) -> (ConstraintCategory, Span, RegionVid) {
  75         debug!("best_blame_constraint(from_region={:?})", from_region);
  76
  77         // Find all paths
  78         let (path, target_region) = self
  79             .find_constraint_paths_between_regions(from_region, target_test)
  80             .unwrap();
  81         debug!(
  82             "best_blame_constraint: path={:#?}",
  83             path.iter()
  84                 .map(|&c| format!(
  85                     "{:?} ({:?}: {:?})",
  86                     c,
  87                     self.constraint_sccs.scc(c.sup),
  88                     self.constraint_sccs.scc(c.sub),
  89                 ))
  90                 .collect::<Vec<_>>()
  91         );
  92
  93         // Classify each of the constraints along the path.
  94         let mut categorized_path: Vec<(ConstraintCategory, Span)> = path
  95             .iter()
  96             .map(|constraint| (constraint.category, constraint.locations.span(mir)))
  97             .collect();
  98         debug!(
  99             "best_blame_constraint: categorized_path={:#?}",
 100             categorized_path
 101         );
 102
 103         // To find the best span to cite, we first try to look for the
 104         // final constraint that is interesting and where the `sup` is
 105         // not unified with the ultimate target region. The reason
 106         // for this is that we have a chain of constraints that lead
 107         // from the source to the target region, something like:
 108         //
 109         //    '0: '1 ('0 is the source)
 110         //    '1: '2
 111         //    '2: '3
 112         //    '3: '4
 113         //    '4: '5
 114         //    '5: '6 ('6 is the target)
 115         //
 116         // Some of those regions are unified with `'6` (in the same
 117         // SCC).  We want to screen those out. After that point, the
 118         // "closest" constraint we have to the end is going to be the
 119         // most likely to be the point where the value escapes -- but
 120         // we still want to screen for an "interesting" point to
 121         // highlight (e.g., a call site or something).
 122         let target_scc = self.constraint_sccs.scc(target_region);
 123         let best_choice = (0..path.len()).rev().find(|&i| {
 124             let constraint = path[i];
 125
 126             let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
 127
 128             match categorized_path[i].0 {
 129                 ConstraintCategory::OpaqueType
 130                 | ConstraintCategory::Boring
 131                 | ConstraintCategory::BoringNoLocation
 132                 | ConstraintCategory::Internal => false,
 133                 _ => constraint_sup_scc != target_scc,
 134             }
 135         });
 136         if let Some(i) = best_choice {
 137             let (category, span) = categorized_path[i];
 138             return (category, span, target_region);
 139         }
 140
 141         // If that search fails, that is.. unusual. Maybe everything
 142         // is in the same SCC or something. In that case, find what
 143         // appears to be the most interesting point to report to the
 144         // user via an even more ad-hoc guess.
 145         categorized_path.sort_by(|p0, p1| p0.0.cmp(&p1.0));
 146         debug!("best_blame_constraint: sorted_path={:#?}", categorized_path);
 147
 148         let &(category, span) = categorized_path.first().unwrap();
 149
 150         (category, span, target_region)
 151     }
 152
 153     /// Walks the graph of constraints (where `'a: 'b` is considered
 154     /// an edge `'a -> 'b`) to find all paths from `from_region` to
 155     /// `to_region`. The paths are accumulated into the vector
 156     /// `results`. The paths are stored as a series of
 157     /// `ConstraintIndex` values -- in other words, a list of *edges*.
 158     ///
 159     /// Returns: a series of constraints as well as the region `R`
 160     /// that passed the target test.
 161     fn find_constraint_paths_between_regions(
 162         &self,
 163         from_region: RegionVid,
 164         target_test: impl Fn(RegionVid) -> bool,
 165     ) -> Option<(Vec<OutlivesConstraint>, RegionVid)> {
 166         let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
 167         context[from_region] = Trace::StartRegion;
 168
 169         // Use a deque so that we do a breadth-first search. We will
 170         // stop at the first match, which ought to be the shortest
 171         // path (fewest constraints).
 172         let mut deque = VecDeque::new();
 173         deque.push_back(from_region);
 174
 175         while let Some(r) = deque.pop_front() {
 176             // Check if we reached the region we were looking for. If so,
 177             // we can reconstruct the path that led to it and return it.
 178             if target_test(r) {
 179                 let mut result = vec![];
 180                 let mut p = r;
 181                 loop {
 182                     match context[p] {
 183                         Trace::NotVisited => {
 184                             bug!("found unvisited region {:?} on path to {:?}", p, r)
 185                         }
 186                         Trace::FromOutlivesConstraint(c) => {
 187                             result.push(c);
 188                             p = c.sup;
 189                         }
 190
 191                         Trace::StartRegion => {
 192                             result.reverse();
 193                             return Some((result, r));
 194                         }
 195                     }
 196                 }
 197             }
 198
 199             // Otherwise, walk over the outgoing constraints and
 200             // enqueue any regions we find, keeping track of how we
 201             // reached them.
 202             let fr_static = self.universal_regions.fr_static;
 203             for constraint in self.constraint_graph.outgoing_edges(r,
 204                                                                    &self.constraints,
 205                                                                    fr_static) {
 206                 assert_eq!(constraint.sup, r);
 207                 let sub_region = constraint.sub;
 208                 if let Trace::NotVisited = context[sub_region] {
 209                     context[sub_region] = Trace::FromOutlivesConstraint(constraint);
 210                     deque.push_back(sub_region);
 211                 }
 212             }
 213         }
 214
 215         None
 216     }
 217
 218     /// Report an error because the universal region `fr` was required to outlive
 219     /// `outlived_fr` but it is not known to do so. For example:
 220     ///
 221     /// ```
 222     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 223     /// ```
 224     ///
 225     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 226     pub(super) fn report_error(
 227         &self,
 228         mir: &Mir<'tcx>,
 229         infcx: &InferCtxt<'_, '_, 'tcx>,
 230         mir_def_id: DefId,
 231         fr: RegionVid,
 232         outlived_fr: RegionVid,
 233         errors_buffer: &mut Vec<Diagnostic>,
 234     ) {
 235         debug!("report_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 236
 237         let (category, span, _) = self.best_blame_constraint(
 238             mir,
 239             fr,
 240             |r| r == outlived_fr
 241         );
 242
 243         // Check if we can use one of the "nice region errors".
 244         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 245             let tables = infcx.tcx.typeck_tables_of(mir_def_id);
 246             let nice = NiceRegionError::new_from_span(infcx.tcx, span, o, f, Some(tables));
 247             if let Some(_error_reported) = nice.try_report_from_nll() {
 248                 return;
 249             }
 250         }
 251
 252         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 253             self.universal_regions.is_local_free_region(fr),
 254             self.universal_regions.is_local_free_region(outlived_fr),
 255         );
 256
 257         debug!("report_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 258                fr_is_local, outlived_fr_is_local, category);
 259         match (category, fr_is_local, outlived_fr_is_local) {
 260             (ConstraintCategory::Assignment, true, false) |
 261             (ConstraintCategory::CallArgument, true, false) =>
 262                 self.report_escaping_data_error(mir, infcx, mir_def_id, fr, outlived_fr,
 263                                                 category, span, errors_buffer),
 264             _ =>
 265                 self.report_general_error(mir, infcx, mir_def_id, fr, fr_is_local,
 266                                           outlived_fr, outlived_fr_is_local,
 267                                           category, span, errors_buffer),
 268         };
 269     }
 270
 271     fn report_escaping_data_error(
 272         &self,
 273         mir: &Mir<'tcx>,
 274         infcx: &InferCtxt<'_, '_, 'tcx>,
 275         mir_def_id: DefId,
 276         fr: RegionVid,
 277         outlived_fr: RegionVid,
 278         category: ConstraintCategory,
 279         span: Span,
 280         errors_buffer: &mut Vec<Diagnostic>,
 281     ) {
 282         let fr_name_and_span = self.get_var_name_and_span_for_region(infcx.tcx, mir, fr);
 283         let outlived_fr_name_and_span =
 284             self.get_var_name_and_span_for_region(infcx.tcx, mir, outlived_fr);
 285
 286         let escapes_from = if infcx.tcx.is_closure(mir_def_id) { "closure" } else { "function" };
 287
 288         // Revert to the normal error in these cases.
 289         // Assignments aren't "escapes" in function items.
 290         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 291             || (category == ConstraintCategory::Assignment && escapes_from == "function")
 292         {
 293             return self.report_general_error(mir, infcx, mir_def_id,
 294                                              fr, true, outlived_fr, false,
 295                                              category, span, errors_buffer);
 296         }
 297
 298         let mut diag = infcx.tcx.sess.struct_span_err(
 299             span, &format!("borrowed data escapes outside of {}", escapes_from),
 300         );
 301
 302         if let Some((outlived_fr_name, outlived_fr_span)) = outlived_fr_name_and_span {
 303             if let Some(name) = outlived_fr_name {
 304                 diag.span_label(
 305                     outlived_fr_span,
 306                     format!("`{}` is declared here, outside of the {} body", name, escapes_from),
 307                 );
 308             }
 309         }
 310
 311         if let Some((fr_name, fr_span)) = fr_name_and_span {
 312             if let Some(name) = fr_name {
 313                 diag.span_label(
 314                     fr_span,
 315                     format!("`{}` is a reference that is only valid in the {} body",
 316                             name, escapes_from),
 317                 );
 318
 319                 diag.span_label(span, format!("`{}` escapes the {} body here",
 320                                                name, escapes_from));
 321             }
 322         }
 323
 324         diag.buffer(errors_buffer);
 325     }
 326
 327     fn report_general_error(
 328         &self,
 329         mir: &Mir<'tcx>,
 330         infcx: &InferCtxt<'_, '_, 'tcx>,
 331         mir_def_id: DefId,
 332         fr: RegionVid,
 333         fr_is_local: bool,
 334         outlived_fr: RegionVid,
 335         outlived_fr_is_local: bool,
 336         category: ConstraintCategory,
 337         span: Span,
 338         errors_buffer: &mut Vec<Diagnostic>,
 339     ) {
 340         let mut diag = infcx.tcx.sess.struct_span_err(
 341             span,
 342             "unsatisfied lifetime constraints", // FIXME
 343         );
 344
 345         let counter = &mut 1;
 346         let fr_name = self.give_region_a_name(infcx, mir, mir_def_id, fr, counter);
 347         fr_name.highlight_region_name(&mut diag);
 348         let outlived_fr_name = self.give_region_a_name(
 349             infcx, mir, mir_def_id, outlived_fr, counter);
 350         outlived_fr_name.highlight_region_name(&mut diag);
 351
 352         let mir_def_name = if infcx.tcx.is_closure(mir_def_id) { "closure" } else { "function" };
 353
 354         match (category, outlived_fr_is_local, fr_is_local) {
 355             (ConstraintCategory::Return, true, _) => {
 356                 diag.span_label(span, format!(
 357                     "{} was supposed to return data with lifetime `{}` but it is returning \
 358                     data with lifetime `{}`",
 359                     mir_def_name, outlived_fr_name, fr_name
 360                 ));
 361             },
 362             _ => {
 363                 diag.span_label(span, format!(
 364                     "{}requires that `{}` must outlive `{}`",
 365                     category.description(), fr_name, outlived_fr_name,
 366                 ));
 367             },
 368         }
 369
 370         self.add_static_impl_trait_suggestion(
 371             infcx, &mut diag, fr, fr_name, outlived_fr,
 372         );
 373
 374         diag.buffer(errors_buffer);
 375     }
 376
 377     fn add_static_impl_trait_suggestion(
 378         &self,
 379         infcx: &InferCtxt<'_, '_, 'tcx>,
 380         diag: &mut DiagnosticBuilder<'_>,
 381         fr: RegionVid,
 382         // We need to pass `fr_name` - computing it again will label it twice.
 383         fr_name: RegionName,
 384         outlived_fr: RegionVid,
 385     ) {
 386         if let (
 387             Some(f),
 388             Some(ty::RegionKind::ReStatic)
 389         ) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 390             if let Some(ty::TyS {
 391                 sty: ty::TyKind::Opaque(did, substs),
 392                 ..
 393             }) = infcx.tcx.is_suitable_region(f)
 394                     .map(|r| r.def_id)
 395                     .map(|id| infcx.tcx.return_type_impl_trait(id))
 396                     .unwrap_or(None)
 397             {
 398                 // Check whether or not the impl trait return type is intended to capture
 399                 // data with the static lifetime.
 400                 //
 401                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 402                 let has_static_predicate = {
 403                     let predicates_of = infcx.tcx.predicates_of(*did);
 404                     let bounds = predicates_of.instantiate(infcx.tcx, substs);
 405
 406                     let mut found = false;
 407                     for predicate in bounds.predicates {
 408                         if let ty::Predicate::TypeOutlives(binder) = predicate {
 409                             if let ty::OutlivesPredicate(
 410                                 _,
 411                                 ty::RegionKind::ReStatic
 412                             ) = binder.skip_binder() {
 413                                 found = true;
 414                                 break;
 415                             }
 416                         }
 417                     }
 418
 419                     found
 420                 };
 421
 422                 debug!("add_static_impl_trait_suggestion: has_static_predicate={:?}",
 423                        has_static_predicate);
 424                 let static_str = keywords::StaticLifetime.name();
 425                 // If there is a static predicate, then the only sensible suggestion is to replace
 426                 // fr with `'static`.
 427                 if has_static_predicate {
 428                     diag.help(
 429                         &format!(
 430                             "consider replacing `{}` with `{}`",
 431                             fr_name, static_str,
 432                         ),
 433                     );
 434                 } else {
 435                     // Otherwise, we should suggest adding a constraint on the return type.
 436                     let span = infcx.tcx.def_span(*did);
 437                     if let Ok(snippet) = infcx.tcx.sess.source_map().span_to_snippet(span) {
 438                         let suggestable_fr_name = if fr_name.was_named() {
 439                             format!("{}", fr_name)
 440                         } else {
 441                             "'_".to_string()
 442                         };
 443
 444                         diag.span_suggestion_with_applicability(
 445                             span,
 446                             &format!(
 447                                 "to allow this impl Trait to capture borrowed data with lifetime \
 448                                  `{}`, add `{}` as a constraint",
 449                                 fr_name, suggestable_fr_name,
 450                             ),
 451                             format!("{} + {}", snippet, suggestable_fr_name),
 452                             Applicability::MachineApplicable,
 453                         );
 454                     }
 455                 }
 456             }
 457         }
 458     }
 459
 460     // Finds some region R such that `fr1: R` and `R` is live at
 461     // `elem`.
 462     crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
 463         // Find all paths
 464         let (_path, r) =
 465             self.find_constraint_paths_between_regions(fr1, |r| {
 466                 self.liveness_constraints.contains(r, elem)
 467             }).unwrap();
 468         r
 469     }
 470
 471     // Finds a good span to blame for the fact that `fr1` outlives `fr2`.
 472     crate fn find_outlives_blame_span(
 473         &self,
 474         mir: &Mir<'tcx>,
 475         fr1: RegionVid,
 476         fr2: RegionVid,
 477     ) -> Span {
 478         let (_, span, _) = self.best_blame_constraint(mir, fr1, |r| r == fr2);
 479         span
 480     }
 481 }