compiler/rustc_mir/src/borrow_check/diagnostics/region_errors.rs - third_party/rust - Git at Google

 //! Error reporting machinery for lifetime errors.

 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_infer::infer::{
     error_reporting::nice_region_error::NiceRegionError,
     error_reporting::unexpected_hidden_region_diagnostic, NLLRegionVariableOrigin,
 };
 use rustc_middle::mir::{ConstraintCategory, ReturnConstraint};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, RegionVid, Ty};
 use rustc_span::symbol::{kw, sym};
 use rustc_span::Span;

 use crate::util::borrowck_errors;

 use crate::borrow_check::{
     nll::ConstraintDescription,
     region_infer::{values::RegionElement, TypeTest},
     universal_regions::DefiningTy,
     MirBorrowckCtxt,
 };

 use super::{OutlivesSuggestionBuilder, RegionName};

 impl ConstraintDescription for ConstraintCategory {
     fn description(&self) -> &'static str {
         // Must end with a space. Allows for empty names to be provided.
         match self {
             ConstraintCategory::Assignment => "assignment ",
             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 ",
             ConstraintCategory::Cast => "cast ",
             ConstraintCategory::CallArgument => "argument ",
             ConstraintCategory::TypeAnnotation => "type annotation ",
             ConstraintCategory::ClosureBounds => "closure body ",
             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 => "",
         }
     }
 }

 /// A collection of errors encountered during region inference. This is needed to efficiently
 /// report errors after borrow checking.
 ///
 /// Usually we expect this to either be empty or contain a small number of items, so we can avoid
 /// allocation most of the time.
 crate type RegionErrors<'tcx> = Vec<RegionErrorKind<'tcx>>;

 #[derive(Clone, Debug)]
 crate enum RegionErrorKind<'tcx> {
     /// A generic bound failure for a type test (`T: 'a`).
     TypeTestError { type_test: TypeTest<'tcx> },

     /// An unexpected hidden region for an opaque type.
     UnexpectedHiddenRegion {
         /// The span for the member constraint.
         span: Span,
         /// The hidden type.
         hidden_ty: Ty<'tcx>,
         /// The unexpected region.
         member_region: ty::Region<'tcx>,
     },

     /// Higher-ranked subtyping error.
     BoundUniversalRegionError {
         /// The placeholder free region.
         longer_fr: RegionVid,
         /// The region element that erroneously must be outlived by `longer_fr`.
         error_element: RegionElement,
         /// The origin of the placeholder region.
         fr_origin: NLLRegionVariableOrigin,
     },

     /// Any other lifetime error.
     RegionError {
         /// The origin of the region.
         fr_origin: NLLRegionVariableOrigin,
         /// The region that should outlive `shorter_fr`.
         longer_fr: RegionVid,
         /// The region that should be shorter, but we can't prove it.
         shorter_fr: RegionVid,
         /// Indicates whether this is a reported error. We currently only report the first error
         /// encountered and leave the rest unreported so as not to overwhelm the user.
         is_reported: bool,
     },
 }

 /// Information about the various region constraints involved in a borrow checker error.
 #[derive(Clone, Debug)]
 pub struct ErrorConstraintInfo {
     // fr: outlived_fr
     pub(super) fr: RegionVid,
     pub(super) fr_is_local: bool,
     pub(super) outlived_fr: RegionVid,
     pub(super) outlived_fr_is_local: bool,

     // Category and span for best blame constraint
     pub(super) category: ConstraintCategory,
     pub(super) span: Span,
 }

 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// Converts a region inference variable into a `ty::Region` that
     /// we can use for error reporting. If `r` is universally bound,
     /// then we use the name that we have on record for it. If `r` is
     /// existentially bound, then we check its inferred value and try
     /// to find a good name from that. Returns `None` if we can't find
     /// one (e.g., this is just some random part of the CFG).
     pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
         self.to_error_region_vid(r).and_then(|r| self.regioncx.region_definition(r).external_name)
     }

     /// Returns the `RegionVid` corresponding to the region returned by
     /// `to_error_region`.
     pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
         if self.regioncx.universal_regions().is_universal_region(r) {
             Some(r)
         } else {
             // We just want something nameable, even if it's not
             // actually an upper bound.
             let upper_bound = self.regioncx.approx_universal_upper_bound(r);

             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
                 self.to_error_region_vid(upper_bound)
             } else {
                 None
             }
         }
     }

     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
             if let ty::BoundRegion::BrEnv = free_region.bound_region {
                 if let DefiningTy::Closure(_, substs) =
                     self.regioncx.universal_regions().defining_ty
                 {
                     return substs.as_closure().kind() == ty::ClosureKind::FnMut;
                 }
             }
         }

         false
     }

     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
     pub(in crate::borrow_check) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
         // buffered in the `MirBorrowckCtxt`.

         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();

         for nll_error in nll_errors.into_iter() {
             match nll_error {
                 RegionErrorKind::TypeTestError { type_test } => {
                     // Try to convert the lower-bound region into something named we can print for the user.
                     let lower_bound_region = self.to_error_region(type_test.lower_bound);

                     let type_test_span = type_test.locations.span(&self.body);

                     if let Some(lower_bound_region) = lower_bound_region {
                         self.infcx
                             .construct_generic_bound_failure(
                                 type_test_span,
                                 None,
                                 type_test.generic_kind,
                                 lower_bound_region,
                             )
                             .buffer(&mut self.errors_buffer);
                     } else {
                         // FIXME. We should handle this case better. It
                         // indicates that we have e.g., some region variable
                         // whose value is like `'a+'b` where `'a` and `'b` are
                         // distinct unrelated univesal regions that are not
                         // known to outlive one another. It'd be nice to have
                         // some examples where this arises to decide how best
                         // to report it; we could probably handle it by
                         // iterating over the universal regions and reporting
                         // an error that multiple bounds are required.
                         self.infcx
                             .tcx
                             .sess
                             .struct_span_err(
                                 type_test_span,
                                 &format!("`{}` does not live long enough", type_test.generic_kind),
                             )
                             .buffer(&mut self.errors_buffer);
                     }
                 }

                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
                     unexpected_hidden_region_diagnostic(
                         self.infcx.tcx,
                         span,
                         named_ty,
                         named_region,
                     )
                     .buffer(&mut self.errors_buffer);
                 }

                 RegionErrorKind::BoundUniversalRegionError {
                     longer_fr,
                     fr_origin,
                     error_element,
                 } => {
                     let error_region = self.regioncx.region_from_element(longer_fr, error_element);

                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
                     let (_, span) = self.regioncx.find_outlives_blame_span(
                         &self.body,
                         longer_fr,
                         fr_origin,
                         error_region,
                     );

                     // FIXME: improve this error message
                     self.infcx
                         .tcx
                         .sess
                         .struct_span_err(span, "higher-ranked subtype error")
                         .buffer(&mut self.errors_buffer);
                 }

                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
                     if is_reported {
                         self.report_region_error(
                             longer_fr,
                             fr_origin,
                             shorter_fr,
                             &mut outlives_suggestion,
                         );
                     } else {
                         // We only report the first error, so as not to overwhelm the user. See
                         // `RegRegionErrorKind` docs.
                         //
                         // FIXME: currently we do nothing with these, but perhaps we can do better?
                         // FIXME: try collecting these constraints on the outlives suggestion
                         // builder. Does it make the suggestions any better?
                         debug!(
                             "Unreported region error: can't prove that {:?}: {:?}",
                             longer_fr, shorter_fr
                         );
                     }
                 }
             }
         }

         // Emit one outlives suggestions for each MIR def we borrowck
         outlives_suggestion.add_suggestion(self);
     }

     /// Report an error because the universal region `fr` was required to outlive
     /// `outlived_fr` but it is not known to do so. For example:
     ///
     /// ```
     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
     /// ```
     ///
     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
     pub(in crate::borrow_check) fn report_region_error(
         &mut self,
         fr: RegionVid,
         fr_origin: NLLRegionVariableOrigin,
         outlived_fr: RegionVid,
         outlives_suggestion: &mut OutlivesSuggestionBuilder,
     ) {
         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);

         let (category, _, span) =
             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
             });

         debug!("report_region_error: category={:?} {:?}", category, span);
         // Check if we can use one of the "nice region errors".
         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
             let nice = NiceRegionError::new_from_span(self.infcx, span, o, f);
             if let Some(diag) = nice.try_report_from_nll() {
                 diag.buffer(&mut self.errors_buffer);
                 return;
             }
         }

         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
             self.regioncx.universal_regions().is_local_free_region(fr),
             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
         );

         debug!(
             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
             fr_is_local, outlived_fr_is_local, category
         );

         let errci = ErrorConstraintInfo {
             fr,
             outlived_fr,
             fr_is_local,
             outlived_fr_is_local,
             category,
             span,
         };

         let diag = match (category, fr_is_local, outlived_fr_is_local) {
             (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) => {
                 let mut db = self.report_escaping_data_error(&errci);

                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
                 outlives_suggestion.collect_constraint(fr, outlived_fr);

                 db
             }
             _ => {
                 let mut db = self.report_general_error(&errci);

                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
                 outlives_suggestion.collect_constraint(fr, outlived_fr);

                 db
             }
         };

         diag.buffer(&mut self.errors_buffer);
     }

     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
     /// This function expects `fr` to be local and `outlived_fr` to not be local.
     ///
     /// ```text
     /// error: captured variable cannot escape `FnMut` closure body
     ///   --> $DIR/issue-53040.rs:15:8
     ///    |
     /// LL |     || &mut v;
     ///    |     -- ^^^^^^ creates 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...
     ///    = note: ...therefore, returned references to captured variables will escape the closure
     /// ```
     fn report_fnmut_error(
         &self,
         errci: &ErrorConstraintInfo,
         kind: ReturnConstraint,
     ) -> DiagnosticBuilder<'tcx> {
         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;

         let mut diag = self
             .infcx
             .tcx
             .sess
             .struct_span_err(*span, "captured variable cannot escape `FnMut` 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");
         }

         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
             diag.span_label(fr_span, "inferred to be a `FnMut` closure");
         }

         diag.note(
             "`FnMut` closures only have access to their captured variables while they are \
              executing...",
         );
         diag.note("...therefore, they cannot allow references to captured variables to escape");

         diag
     }

     /// Reports a error specifically for when data is escaping a closure.
     ///
     /// ```text
     /// error: borrowed data escapes outside of function
     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
     ///    |
     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
     ///    |              - `x` is a reference that is only valid in the function body
     /// LL |     // but ref_obj will not, so warn.
     /// LL |     ref_obj(x)
     ///    |     ^^^^^^^^^^ `x` escapes the function body here
     /// ```
     fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
         let ErrorConstraintInfo { span, category, .. } = errci;

         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
             self.infcx.tcx,
             &self.body,
             &self.local_names,
             &self.upvars,
             errci.fr,
         );
         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
             self.infcx.tcx,
             &self.body,
             &self.local_names,
             &self.upvars,
             errci.outlived_fr,
         );

         let (_, escapes_from) = self
             .infcx
             .tcx
             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());

         // Revert to the normal error in these cases.
         // Assignments aren't "escapes" in function items.
         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
             || (*category == ConstraintCategory::Assignment
                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
             || self.regioncx.universal_regions().defining_ty.is_const()
         {
             return self.report_general_error(&ErrorConstraintInfo {
                 fr_is_local: true,
                 outlived_fr_is_local: false,
                 ..*errci
             });
         }

         let mut diag =
             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);

         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
             diag.span_label(
                 outlived_fr_span,
                 format!(
                     "`{}` declared here, outside of the {} body",
                     outlived_fr_name, escapes_from
                 ),
             );
         }

         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
             diag.span_label(
                 fr_span,
                 format!(
                     "`{}` is a reference that is only valid in the {} body",
                     fr_name, escapes_from
                 ),
             );

             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
         }

         diag
     }

     /// Reports a region inference error for the general case with named/synthesized lifetimes to
     /// explain what is happening.
     ///
     /// ```text
     /// error: unsatisfied lifetime constraints
     ///   --> $DIR/regions-creating-enums3.rs:17:5
     ///    |
     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
     ///    |                -- -- lifetime `'b` defined here
     ///    |                |
     ///    |                lifetime `'a` defined here
     /// LL |     ast::add(x, y)
     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
     ///    |                    is returning data with lifetime `'b`
     /// ```
     fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
         let ErrorConstraintInfo {
             fr,
             fr_is_local,
             outlived_fr,
             outlived_fr_is_local,
             span,
             category,
             ..
         } = errci;

         let mut diag =
             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");

         let (_, mir_def_name) =
             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());

         let fr_name = self.give_region_a_name(*fr).unwrap();
         fr_name.highlight_region_name(&mut diag);
         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
         outlived_fr_name.highlight_region_name(&mut diag);

         match (category, outlived_fr_is_local, fr_is_local) {
             (ConstraintCategory::Return(_), true, _) => {
                 diag.span_label(
                     *span,
                     format!(
                         "{} was supposed to return data with lifetime `{}` but it is returning \
                          data with lifetime `{}`",
                         mir_def_name, outlived_fr_name, fr_name
                     ),
                 );
             }
             _ => {
                 diag.span_label(
                     *span,
                     format!(
                         "{}requires that `{}` must outlive `{}`",
                         category.description(),
                         fr_name,
                         outlived_fr_name,
                     ),
                 );
             }
         }

         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);

         diag
     }

     /// Adds a suggestion to errors where a `impl Trait` is returned.
     ///
     /// ```text
     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
     ///       a constraint
     ///    |
     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     /// ```
     fn add_static_impl_trait_suggestion(
         &self,
         diag: &mut DiagnosticBuilder<'tcx>,
         fr: RegionVid,
         // We need to pass `fr_name` - computing it again will label it twice.
         fr_name: RegionName,
         outlived_fr: RegionVid,
     ) {
         if let (Some(f), Some(ty::RegionKind::ReStatic)) =
             (self.to_error_region(fr), self.to_error_region(outlived_fr))
         {
             if let Some(&ty::Opaque(did, substs)) = self
                 .infcx
                 .tcx
                 .is_suitable_region(f)
                 .map(|r| r.def_id)
                 .and_then(|id| self.infcx.tcx.return_type_impl_trait(id))
                 .map(|(ty, _)| ty.kind())
             {
                 // Check whether or not the impl trait return type is intended to capture
                 // data with the static lifetime.
                 //
                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
                 let has_static_predicate = {
                     let bounds = self.infcx.tcx.explicit_item_bounds(did);

                     let mut found = false;
                     for (bound, _) in bounds {
                         if let ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(_, r)) =
                             bound.skip_binders()
                         {
                             let r = r.subst(self.infcx.tcx, substs);
                             if let ty::RegionKind::ReStatic = r {
                                 found = true;
                                 break;
                             } else {
                                 // If there's already a lifetime bound, don't
                                 // suggest anything.
                                 return;
                             }
                         }
                     }

                     found
                 };

                 debug!(
                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
                     has_static_predicate
                 );
                 let static_str = kw::StaticLifetime;
                 // If there is a static predicate, then the only sensible suggestion is to replace
                 // fr with `'static`.
                 if has_static_predicate {
                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
                 } else {
                     // Otherwise, we should suggest adding a constraint on the return type.
                     let span = self.infcx.tcx.def_span(did);
                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
                         let suggestable_fr_name = if fr_name.was_named() {
                             fr_name.to_string()
                         } else {
                             "'_".to_string()
                         };
                         let suggestion = if snippet.ends_with(';') {
                             // `type X = impl Trait;`
                             format!("{} + {};", &snippet[..snippet.len() - 1], suggestable_fr_name)
                         } else {
                             format!("{} + {}", snippet, suggestable_fr_name)
                         };
                         diag.span_suggestion(
                             span,
                             &format!(
                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
                                  `{}`, add `{}` as a bound",
                                 fr_name, suggestable_fr_name,
                             ),
                             suggestion,
                             Applicability::MachineApplicable,
                         );
                     }
                 }
             }
         }
     }
 }
	//! Error reporting machinery for lifetime errors.

	use rustc_errors::{Applicability, DiagnosticBuilder};
	use rustc_infer::infer::{
	error_reporting::nice_region_error::NiceRegionError,
	error_reporting::unexpected_hidden_region_diagnostic, NLLRegionVariableOrigin,
	};
	use rustc_middle::mir::{ConstraintCategory, ReturnConstraint};
	use rustc_middle::ty::subst::Subst;
	use rustc_middle::ty::{self, RegionVid, Ty};
	use rustc_span::symbol::{kw, sym};
	use rustc_span::Span;

	use crate::util::borrowck_errors;

	use crate::borrow_check::{
	nll::ConstraintDescription,
	region_infer::{values::RegionElement, TypeTest},
	universal_regions::DefiningTy,
	MirBorrowckCtxt,
	};

	use super::{OutlivesSuggestionBuilder, RegionName};

	impl ConstraintDescription for ConstraintCategory {
	fn description(&self) -> &'static str {
	// Must end with a space. Allows for empty names to be provided.
	match self {
	ConstraintCategory::Assignment => "assignment ",
	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 ",
	ConstraintCategory::Cast => "cast ",
	ConstraintCategory::CallArgument => "argument ",
	ConstraintCategory::TypeAnnotation => "type annotation ",
	ConstraintCategory::ClosureBounds => "closure body ",
	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 => "",
	}
	}
	}

	/// A collection of errors encountered during region inference. This is needed to efficiently
	/// report errors after borrow checking.
	///
	/// Usually we expect this to either be empty or contain a small number of items, so we can avoid
	/// allocation most of the time.
	crate type RegionErrors<'tcx> = Vec<RegionErrorKind<'tcx>>;

	#[derive(Clone, Debug)]
	crate enum RegionErrorKind<'tcx> {
	/// A generic bound failure for a type test (`T: 'a`).
	TypeTestError { type_test: TypeTest<'tcx> },

	/// An unexpected hidden region for an opaque type.
	UnexpectedHiddenRegion {
	/// The span for the member constraint.
	span: Span,
	/// The hidden type.
	hidden_ty: Ty<'tcx>,
	/// The unexpected region.
	member_region: ty::Region<'tcx>,
	},

	/// Higher-ranked subtyping error.
	BoundUniversalRegionError {
	/// The placeholder free region.
	longer_fr: RegionVid,
	/// The region element that erroneously must be outlived by `longer_fr`.
	error_element: RegionElement,
	/// The origin of the placeholder region.
	fr_origin: NLLRegionVariableOrigin,
	},

	/// Any other lifetime error.
	RegionError {
	/// The origin of the region.
	fr_origin: NLLRegionVariableOrigin,
	/// The region that should outlive `shorter_fr`.
	longer_fr: RegionVid,
	/// The region that should be shorter, but we can't prove it.
	shorter_fr: RegionVid,
	/// Indicates whether this is a reported error. We currently only report the first error
	/// encountered and leave the rest unreported so as not to overwhelm the user.
	is_reported: bool,
	},
	}

	/// Information about the various region constraints involved in a borrow checker error.
	#[derive(Clone, Debug)]
	pub struct ErrorConstraintInfo {
	// fr: outlived_fr
	pub(super) fr: RegionVid,
	pub(super) fr_is_local: bool,
	pub(super) outlived_fr: RegionVid,
	pub(super) outlived_fr_is_local: bool,

	// Category and span for best blame constraint
	pub(super) category: ConstraintCategory,
	pub(super) span: Span,
	}

	impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
	/// Converts a region inference variable into a `ty::Region` that
	/// we can use for error reporting. If `r` is universally bound,
	/// then we use the name that we have on record for it. If `r` is
	/// existentially bound, then we check its inferred value and try
	/// to find a good name from that. Returns `None` if we can't find
	/// one (e.g., this is just some random part of the CFG).
	pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
	self.to_error_region_vid(r).and_then(\|r\| self.regioncx.region_definition(r).external_name)
	}

	/// Returns the `RegionVid` corresponding to the region returned by
	/// `to_error_region`.
	pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
	if self.regioncx.universal_regions().is_universal_region(r) {
	Some(r)
	} else {
	// We just want something nameable, even if it's not
	// actually an upper bound.
	let upper_bound = self.regioncx.approx_universal_upper_bound(r);

	if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
	self.to_error_region_vid(upper_bound)
	} else {
	None
	}
	}
	}

	/// Returns `true` if a closure is inferred to be an `FnMut` closure.
	fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
	if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
	if let ty::BoundRegion::BrEnv = free_region.bound_region {
	if let DefiningTy::Closure(_, substs) =
	self.regioncx.universal_regions().defining_ty
	{
	return substs.as_closure().kind() == ty::ClosureKind::FnMut;
	}
	}
	}

	false
	}

	/// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
	pub(in crate::borrow_check) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
	// Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
	// buffered in the `MirBorrowckCtxt`.

	let mut outlives_suggestion = OutlivesSuggestionBuilder::default();

	for nll_error in nll_errors.into_iter() {
	match nll_error {
	RegionErrorKind::TypeTestError { type_test } => {
	// Try to convert the lower-bound region into something named we can print for the user.
	let lower_bound_region = self.to_error_region(type_test.lower_bound);

	let type_test_span = type_test.locations.span(&self.body);

	if let Some(lower_bound_region) = lower_bound_region {
	self.infcx
	.construct_generic_bound_failure(
	type_test_span,
	None,
	type_test.generic_kind,
	lower_bound_region,
	)
	.buffer(&mut self.errors_buffer);
	} else {
	// FIXME. We should handle this case better. It
	// indicates that we have e.g., some region variable
	// whose value is like `'a+'b` where `'a` and `'b` are
	// distinct unrelated univesal regions that are not
	// known to outlive one another. It'd be nice to have
	// some examples where this arises to decide how best
	// to report it; we could probably handle it by
	// iterating over the universal regions and reporting
	// an error that multiple bounds are required.
	self.infcx
	.tcx
	.sess
	.struct_span_err(
	type_test_span,
	&format!("`{}` does not live long enough", type_test.generic_kind),
	)
	.buffer(&mut self.errors_buffer);
	}
	}

	RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
	let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
	let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
	unexpected_hidden_region_diagnostic(
	self.infcx.tcx,
	span,
	named_ty,
	named_region,
	)
	.buffer(&mut self.errors_buffer);
	}

	RegionErrorKind::BoundUniversalRegionError {
	longer_fr,
	fr_origin,
	error_element,
	} => {
	let error_region = self.regioncx.region_from_element(longer_fr, error_element);

	// Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
	let (_, span) = self.regioncx.find_outlives_blame_span(
	&self.body,
	longer_fr,
	fr_origin,
	error_region,
	);

	// FIXME: improve this error message
	self.infcx
	.tcx
	.sess
	.struct_span_err(span, "higher-ranked subtype error")
	.buffer(&mut self.errors_buffer);
	}

	RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
	if is_reported {
	self.report_region_error(
	longer_fr,
	fr_origin,
	shorter_fr,
	&mut outlives_suggestion,
	);
	} else {
	// We only report the first error, so as not to overwhelm the user. See
	// `RegRegionErrorKind` docs.
	//
	// FIXME: currently we do nothing with these, but perhaps we can do better?
	// FIXME: try collecting these constraints on the outlives suggestion
	// builder. Does it make the suggestions any better?
	debug!(
	"Unreported region error: can't prove that {:?}: {:?}",
	longer_fr, shorter_fr
	);
	}
	}
	}
	}

	// Emit one outlives suggestions for each MIR def we borrowck
	outlives_suggestion.add_suggestion(self);
	}

	/// Report an error because the universal region `fr` was required to outlive
	/// `outlived_fr` but it is not known to do so. For example:
	///
	/// ```
	/// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
	/// ```
	///
	/// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
	pub(in crate::borrow_check) fn report_region_error(
	&mut self,
	fr: RegionVid,
	fr_origin: NLLRegionVariableOrigin,
	outlived_fr: RegionVid,
	outlives_suggestion: &mut OutlivesSuggestionBuilder,
	) {
	debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);

	let (category, _, span) =
	self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, \|r\| {
	self.regioncx.provides_universal_region(r, fr, outlived_fr)
	});

	debug!("report_region_error: category={:?} {:?}", category, span);
	// Check if we can use one of the "nice region errors".
	if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
	let nice = NiceRegionError::new_from_span(self.infcx, span, o, f);
	if let Some(diag) = nice.try_report_from_nll() {
	diag.buffer(&mut self.errors_buffer);
	return;
	}
	}

	let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
	self.regioncx.universal_regions().is_local_free_region(fr),
	self.regioncx.universal_regions().is_local_free_region(outlived_fr),
	);

	debug!(
	"report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
	fr_is_local, outlived_fr_is_local, category
	);

	let errci = ErrorConstraintInfo {
	fr,
	outlived_fr,
	fr_is_local,
	outlived_fr_is_local,
	category,
	span,
	};

	let diag = match (category, fr_is_local, outlived_fr_is_local) {
	(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) => {
	let mut db = self.report_escaping_data_error(&errci);

	outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
	outlives_suggestion.collect_constraint(fr, outlived_fr);

	db
	}
	_ => {
	let mut db = self.report_general_error(&errci);

	outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
	outlives_suggestion.collect_constraint(fr, outlived_fr);

	db
	}
	};

	diag.buffer(&mut self.errors_buffer);
	}

	/// Report a specialized error when `FnMut` closures return a reference to a captured variable.
	/// This function expects `fr` to be local and `outlived_fr` to not be local.
	///
	/// ```text
	/// error: captured variable cannot escape `FnMut` closure body
	/// --> $DIR/issue-53040.rs:15:8
	/// \|
	/// LL \| \|\| &mut v;
	/// \| -- ^^^^^^ creates 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...
	/// = note: ...therefore, returned references to captured variables will escape the closure
	/// ```
	fn report_fnmut_error(
	&self,
	errci: &ErrorConstraintInfo,
	kind: ReturnConstraint,
	) -> DiagnosticBuilder<'tcx> {
	let ErrorConstraintInfo { outlived_fr, span, .. } = errci;

	let mut diag = self
	.infcx
	.tcx
	.sess
	.struct_span_err(*span, "captured variable cannot escape `FnMut` 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");
	}

	if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
	diag.span_label(fr_span, "inferred to be a `FnMut` closure");
	}

	diag.note(
	"`FnMut` closures only have access to their captured variables while they are \
	executing...",
	);
	diag.note("...therefore, they cannot allow references to captured variables to escape");

	diag
	}

	/// Reports a error specifically for when data is escaping a closure.
	///
	/// ```text
	/// error: borrowed data escapes outside of function
	/// --> $DIR/lifetime-bound-will-change-warning.rs:44:5
	/// \|
	/// LL \| fn test2<'a>(x: &'a Box<Fn()+'a>) {
	/// \| - `x` is a reference that is only valid in the function body
	/// LL \| // but ref_obj will not, so warn.
	/// LL \| ref_obj(x)
	/// \| ^^^^^^^^^^ `x` escapes the function body here
	/// ```
	fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
	let ErrorConstraintInfo { span, category, .. } = errci;

	let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
	self.infcx.tcx,
	&self.body,
	&self.local_names,
	&self.upvars,
	errci.fr,
	);
	let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
	self.infcx.tcx,
	&self.body,
	&self.local_names,
	&self.upvars,
	errci.outlived_fr,
	);

	let (_, escapes_from) = self
	.infcx
	.tcx
	.article_and_description(self.regioncx.universal_regions().defining_ty.def_id());

	// Revert to the normal error in these cases.
	// Assignments aren't "escapes" in function items.
	if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
	\|\| (*category == ConstraintCategory::Assignment
	&& self.regioncx.universal_regions().defining_ty.is_fn_def())
	\|\| self.regioncx.universal_regions().defining_ty.is_const()
	{
	return self.report_general_error(&ErrorConstraintInfo {
	fr_is_local: true,
	outlived_fr_is_local: false,
	..*errci
	});
	}

	let mut diag =
	borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);

	if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
	diag.span_label(
	outlived_fr_span,
	format!(
	"`{}` declared here, outside of the {} body",
	outlived_fr_name, escapes_from
	),
	);
	}

	if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
	diag.span_label(
	fr_span,
	format!(
	"`{}` is a reference that is only valid in the {} body",
	fr_name, escapes_from
	),
	);

	diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
	}

	diag
	}

	/// Reports a region inference error for the general case with named/synthesized lifetimes to
	/// explain what is happening.
	///
	/// ```text
	/// error: unsatisfied lifetime constraints
	/// --> $DIR/regions-creating-enums3.rs:17:5
	/// \|
	/// LL \| fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
	/// \| -- -- lifetime `'b` defined here
	/// \| \|
	/// \| lifetime `'a` defined here
	/// LL \| ast::add(x, y)
	/// \| ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
	/// \| is returning data with lifetime `'b`
	/// ```
	fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
	let ErrorConstraintInfo {
	fr,
	fr_is_local,
	outlived_fr,
	outlived_fr_is_local,
	span,
	category,
	..
	} = errci;

	let mut diag =
	self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");

	let (_, mir_def_name) =
	self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());

	let fr_name = self.give_region_a_name(*fr).unwrap();
	fr_name.highlight_region_name(&mut diag);
	let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
	outlived_fr_name.highlight_region_name(&mut diag);

	match (category, outlived_fr_is_local, fr_is_local) {
	(ConstraintCategory::Return(_), true, _) => {
	diag.span_label(
	*span,
	format!(
	"{} was supposed to return data with lifetime `{}` but it is returning \
	data with lifetime `{}`",
	mir_def_name, outlived_fr_name, fr_name
	),
	);
	}
	_ => {
	diag.span_label(
	*span,
	format!(
	"{}requires that `{}` must outlive `{}`",
	category.description(),
	fr_name,
	outlived_fr_name,
	),
	);
	}
	}

	self.add_static_impl_trait_suggestion(&mut diag, fr, fr_name, outlived_fr);

	diag
	}

	/// Adds a suggestion to errors where a `impl Trait` is returned.
	///
	/// ```text
	/// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
	/// a constraint
	/// \|
	/// LL \| fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
	/// \| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	/// ```
	fn add_static_impl_trait_suggestion(
	&self,
	diag: &mut DiagnosticBuilder<'tcx>,
	fr: RegionVid,
	// We need to pass `fr_name` - computing it again will label it twice.
	fr_name: RegionName,
	outlived_fr: RegionVid,
	) {
	if let (Some(f), Some(ty::RegionKind::ReStatic)) =
	(self.to_error_region(fr), self.to_error_region(outlived_fr))
	{
	if let Some(&ty::Opaque(did, substs)) = self
	.infcx
	.tcx
	.is_suitable_region(f)
	.map(\|r\| r.def_id)
	.and_then(\|id\| self.infcx.tcx.return_type_impl_trait(id))
	.map(\|(ty, _)\| ty.kind())
	{
	// Check whether or not the impl trait return type is intended to capture
	// data with the static lifetime.
	//
	// eg. check for `impl Trait + 'static` instead of `impl Trait`.
	let has_static_predicate = {
	let bounds = self.infcx.tcx.explicit_item_bounds(did);

	let mut found = false;
	for (bound, _) in bounds {
	if let ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(_, r)) =
	bound.skip_binders()
	{
	let r = r.subst(self.infcx.tcx, substs);
	if let ty::RegionKind::ReStatic = r {
	found = true;
	break;
	} else {
	// If there's already a lifetime bound, don't
	// suggest anything.
	return;
	}
	}
	}

	found
	};

	debug!(
	"add_static_impl_trait_suggestion: has_static_predicate={:?}",
	has_static_predicate
	);
	let static_str = kw::StaticLifetime;
	// If there is a static predicate, then the only sensible suggestion is to replace
	// fr with `'static`.
	if has_static_predicate {
	diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
	} else {
	// Otherwise, we should suggest adding a constraint on the return type.
	let span = self.infcx.tcx.def_span(did);
	if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
	let suggestable_fr_name = if fr_name.was_named() {
	fr_name.to_string()
	} else {
	"'_".to_string()
	};
	let suggestion = if snippet.ends_with(';') {
	// `type X = impl Trait;`
	format!("{} + {};", &snippet[..snippet.len() - 1], suggestable_fr_name)
	} else {
	format!("{} + {}", snippet, suggestable_fr_name)
	};
	diag.span_suggestion(
	span,
	&format!(
	"to allow this `impl Trait` to capture borrowed data with lifetime \
	`{}`, add `{}` as a bound",
	fr_name, suggestable_fr_name,
	),
	suggestion,
	Applicability::MachineApplicable,
	);
	}
	}
	}
	}
	}
	}