Auto merge of #52021 - nikomatsakis:nll-region-errors, r=estebank

refactor and cleanup region errors for NLL

This is a WIP commit. It simplifies some of the code from https://github.com/rust-lang/rust/pull/51536 and extends a few more steps towards the errors that @davidtwco and I were shooting for. These are intended as a replacement for the general "unable to infer lifetime" messages -- one that is actually actionable. We're certainly not there yet, but the overall shape hopefully gets a bit clearer.

I'm thinking about trying to open up an internals thread to sketch out the overall plan and perhaps discuss how to get the wording right, which special cases to handle, etc.

r? @estebank
cc @davidtwco

53 files changed, 694 insertions, 295 deletions

diff --git a/src/librustc/ich/impls_mir.rs b/src/librustc/ich/impls_mir.rs
index f43ac7ad0dd..6217f6c9fdd 100644
--- a/src/librustc/ich/impls_mir.rs
+++ b/src/librustc/ich/impls_mir.rs
@@ -30,7 +30,7 @@ impl_stable_hash_for!(struct mir::LocalDecl<'tcx> {
     internal,
     is_user_variable
 });
-impl_stable_hash_for!(struct mir::UpvarDecl { debug_name, by_ref, mutability });
+impl_stable_hash_for!(struct mir::UpvarDecl { debug_name, var_hir_id, by_ref, mutability });
 impl_stable_hash_for!(struct mir::BasicBlockData<'tcx> { statements, terminator, is_cleanup });
 impl_stable_hash_for!(struct mir::UnsafetyViolation { source_info, description, kind });
 impl_stable_hash_for!(struct mir::UnsafetyCheckResult { violations, unsafe_blocks });
diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index dca0d4f442a..cd4b32735e5 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -15,7 +15,7 @@
 use graphviz::IntoCow;
 use hir::def::CtorKind;
 use hir::def_id::DefId;
-use hir::{self, InlineAsm};
+use hir::{self, HirId, InlineAsm};
 use middle::region;
 use mir::interpret::{EvalErrorKind, Scalar, Value};
 use mir::visit::MirVisitable;
@@ -380,6 +380,15 @@ pub enum ClearCrossCrate<T> {
     Set(T),
 }
 
+impl<T> ClearCrossCrate<T> {
+    pub fn assert_crate_local(self) -> T {
+        match self {
+            ClearCrossCrate::Clear => bug!("unwrapping cross-crate data"),
+            ClearCrossCrate::Set(v) => v,
+        }
+    }
+}
+
 impl<T: serialize::Encodable> serialize::UseSpecializedEncodable for ClearCrossCrate<T> {}
 impl<T: serialize::Decodable> serialize::UseSpecializedDecodable for ClearCrossCrate<T> {}
 
@@ -785,6 +794,9 @@ impl<'tcx> LocalDecl<'tcx> {
 pub struct UpvarDecl {
     pub debug_name: Name,
 
+    /// `HirId` of the captured variable
+    pub var_hir_id: ClearCrossCrate<HirId>,
+
     /// If true, the capture is behind a reference.
     pub by_ref: bool,
 
diff --git a/src/librustc/ty/fold.rs b/src/librustc/ty/fold.rs
index f55a5129084..076a19fb4ed 100644
--- a/src/librustc/ty/fold.rs
+++ b/src/librustc/ty/fold.rs
@@ -253,13 +253,34 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
     }
 
-    pub fn for_each_free_region<T,F>(self,
-                                     value: &T,
-                                     callback: F)
-        where F: FnMut(ty::Region<'tcx>),
-              T: TypeFoldable<'tcx>,
-    {
-        value.visit_with(&mut RegionVisitor {
+    /// Invoke `callback` on every region appearing free in `value`.
+    pub fn for_each_free_region(
+        self,
+        value: &impl TypeFoldable<'tcx>,
+        mut callback: impl FnMut(ty::Region<'tcx>),
+    ) {
+        self.any_free_region_meets(value, |r| {
+            callback(r);
+            false
+        });
+    }
+
+    /// True if `callback` returns true for every region appearing free in `value`.
+    pub fn all_free_regions_meet(
+        self,
+        value: &impl TypeFoldable<'tcx>,
+        mut callback: impl FnMut(ty::Region<'tcx>) -> bool,
+    ) -> bool {
+        !self.any_free_region_meets(value, |r| !callback(r))
+    }
+
+    /// True if `callback` returns true for some region appearing free in `value`.
+    pub fn any_free_region_meets(
+        self,
+        value: &impl TypeFoldable<'tcx>,
+        callback: impl FnMut(ty::Region<'tcx>) -> bool,
+    ) -> bool {
+        return value.visit_with(&mut RegionVisitor {
             outer_index: ty::INNERMOST,
             callback
         });
@@ -287,25 +308,22 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
 
         impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<F>
-            where F : FnMut(ty::Region<'tcx>)
+            where F: FnMut(ty::Region<'tcx>) -> bool
         {
             fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
                 self.outer_index.shift_in(1);
-                t.skip_binder().visit_with(self);
+                let result = t.skip_binder().visit_with(self);
                 self.outer_index.shift_out(1);
-
-                false // keep visiting
+                result
             }
 
             fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
                 match *r {
                     ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
-                        /* ignore bound regions */
+                        false // ignore bound regions, keep visiting
                     }
                     _ => (self.callback)(r),
                 }
-
-                false // keep visiting
             }
         }
     }
diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index 83738d4ffd5..29f13d669fa 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -128,6 +128,8 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
     input_mir: &Mir<'gcx>,
     def_id: DefId,
 ) -> BorrowCheckResult<'gcx> {
+    debug!("do_mir_borrowck(def_id = {:?})", def_id);
+
     let tcx = infcx.tcx;
     let attributes = tcx.get_attrs(def_id);
     let param_env = tcx.param_env(def_id);
@@ -319,10 +321,14 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
         }
     }
 
-    BorrowCheckResult {
+    let result = BorrowCheckResult {
         closure_requirements: opt_closure_req,
         used_mut_upvars: mbcx.used_mut_upvars,
-    }
+    };
+
+    debug!("do_mir_borrowck: result = {:#?}", result);
+
+    result
 }
 
 #[allow(dead_code)]
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting.rs b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
index 6543516b9c2..786b6a77d2b 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
@@ -8,19 +8,21 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::fmt;
-use borrow_check::nll::region_infer::{ConstraintIndex, RegionInferenceContext};
 use borrow_check::nll::region_infer::values::ToElementIndex;
+use borrow_check::nll::region_infer::{ConstraintIndex, RegionInferenceContext};
 use borrow_check::nll::type_check::Locations;
 use rustc::hir::def_id::DefId;
-use rustc::infer::InferCtxt;
 use rustc::infer::error_reporting::nice_region_error::NiceRegionError;
-use rustc::mir::{self, Location, Mir, Place, StatementKind, TerminatorKind, Rvalue};
+use rustc::infer::InferCtxt;
+use rustc::mir::{self, Location, Mir, Place, Rvalue, StatementKind, TerminatorKind};
 use rustc::ty::RegionVid;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::indexed_vec::IndexVec;
+use std::fmt;
 use syntax_pos::Span;
 
+mod region_name;
+
 /// Constraints that are considered interesting can be categorized to
 /// determine why they are interesting. Order of variants indicates
 /// sort order of the category, thereby influencing diagnostic output.
@@ -47,115 +49,105 @@ impl fmt::Display for ConstraintCategory {
 }
 
 impl<'tcx> RegionInferenceContext<'tcx> {
-    /// When reporting an error, it is useful to be able to determine which constraints influenced
-    /// the region being reported as an error. This function finds all of the paths from the
+    /// Walks the graph of constraints (where `'a: 'b` is considered
+    /// an edge `'b -> 'a`) to find all paths from `from_region` to
+    /// `to_region`. The paths are accumulated into the vector
+    /// `results`. The paths are stored as a series of
+    /// `ConstraintIndex` values -- in other words, a list of *edges*.
+    ///
+    /// # Parameters
+    ///
+    /// - `from_region`
+    /// When reporting an error, it is useful to be able to determine
+    /// which constraints influenced the region being reported as an
+    /// error. This function finds all of the paths from the
     /// constraint.
-    fn find_constraint_paths_from_region(
+    fn find_constraint_paths_between_regions(
         &self,
-        r0: RegionVid
+        from_region: RegionVid,
+        target_test: impl Fn(RegionVid) -> bool,
     ) -> Vec<Vec<ConstraintIndex>> {
-        let constraints = self.constraints.clone();
-
-        // Mapping of regions to the previous region and constraint index that led to it.
-        let mut previous = FxHashMap();
-        // Regions yet to be visited.
-        let mut next = vec! [ r0 ];
-        // Regions that have been visited.
-        let mut visited = FxHashSet();
-        // Ends of paths.
-        let mut end_regions = FxHashSet();
-
-        // When we've still got points to visit...
-        while let Some(current) = next.pop() {
-            // ...take the next point...
-            debug!("find_constraint_paths_from_region: current={:?} visited={:?} next={:?}",
-                   current, visited, next);
-            // ...but make sure not to visit this point again...
-            visited.insert(current);
-
-            // ...find the edges containing it...
-            let mut upcoming = Vec::new();
-            for (index, constraint) in constraints.iter_enumerated() {
-                if constraint.sub == current {
-                    // ...add the regions that join us with to the path we've taken...
-                    debug!("find_constraint_paths_from_region: index={:?} constraint={:?}",
-                           index, constraint);
-                    let next_region = constraint.sup.clone();
-
-                    // ...unless we've visited it since this was added...
-                    if visited.contains(&next_region) {
-                        debug!("find_constraint_paths_from_region: skipping as visited");
-                        continue;
-                    }
-
-                    previous.insert(next_region, (index, Some(current)));
-                    upcoming.push(next_region);
-                }
-            }
+        let mut results = vec![];
+        self.find_constraint_paths_between_regions_helper(
+            from_region,
+            from_region,
+            &target_test,
+            &mut FxHashSet::default(),
+            &mut vec![],
+            &mut results,
+        );
+        results
+    }
 
-            if upcoming.is_empty() {
-                // If we didn't find any edges then this is the end of a path...
-                debug!("find_constraint_paths_from_region: new end region current={:?}", current);
-                end_regions.insert(current);
-            } else {
-                // ...but, if we did find edges, then add these to the regions yet to visit.
-                debug!("find_constraint_paths_from_region: extend next upcoming={:?}", upcoming);
-                next.extend(upcoming);
-            }
+    /// Helper for `find_constraint_paths_between_regions`.
+    fn find_constraint_paths_between_regions_helper(
+        &self,
+        from_region: RegionVid,
+        current_region: RegionVid,
+        target_test: &impl Fn(RegionVid) -> bool,
+        visited: &mut FxHashSet<RegionVid>,
+        stack: &mut Vec<ConstraintIndex>,
+        results: &mut Vec<Vec<ConstraintIndex>>,
+    ) {
+        let dependency_map = self.dependency_map.as_ref().unwrap();
 
+        // Check if we already visited this region.
+        if !visited.insert(current_region) {
+            return;
         }
 
-        // Now we've visited each point, compute the final paths.
-        let mut paths: Vec<Vec<ConstraintIndex>> = Vec::new();
-        debug!("find_constraint_paths_from_region: end_regions={:?}", end_regions);
-        for end_region in end_regions {
-            debug!("find_constraint_paths_from_region: end_region={:?}", end_region);
-
-            // Get the constraint and region that led to this end point.
-            // We can unwrap as we know if end_point was in the vector that it
-            // must also be in our previous map.
-            let (mut index, mut region) = previous.get(&end_region).unwrap();
-            debug!("find_constraint_paths_from_region: index={:?} region={:?}", index, region);
-
-            // Keep track of the indices.
-            let mut path: Vec<ConstraintIndex> = vec![index];
-
-            while region.is_some() && region != Some(r0) {
-                let p = previous.get(&region.unwrap()).unwrap();
-                index = p.0;
-                region = p.1;
-
-                debug!("find_constraint_paths_from_region: index={:?} region={:?}", index, region);
-                path.push(index);
+        // Check if we reached the region we were looking for.
+        if target_test(current_region) {
+            if !stack.is_empty() {
+                assert_eq!(self.constraints[stack[0]].sub, from_region);
+                results.push(stack.clone());
             }
-
-            // Add to our paths.
-            paths.push(path);
+            return;
         }
 
-        debug!("find_constraint_paths_from_region: paths={:?}", paths);
-        paths
+        self.constraints
+            .each_affected_by_dirty(dependency_map[current_region], |constraint| {
+                assert_eq!(self.constraints[constraint].sub, current_region);
+                stack.push(constraint);
+                self.find_constraint_paths_between_regions_helper(
+                    from_region,
+                    self.constraints[constraint].sup,
+                    target_test,
+                    visited,
+                    stack,
+                    results,
+                );
+                stack.pop();
+            });
     }
 
     /// This function will return true if a constraint is interesting and false if a constraint
     /// is not. It is useful in filtering constraint paths to only interesting points.
-    fn constraint_is_interesting(&self, index: &ConstraintIndex) -> bool {
-        self.constraints.get(*index).filter(|constraint| {
-            debug!("constraint_is_interesting: locations={:?} constraint={:?}",
-                   constraint.locations, constraint);
-            if let Locations::Interesting(_) = constraint.locations { true } else { false }
-        }).is_some()
+    fn constraint_is_interesting(&self, index: ConstraintIndex) -> bool {
+        let constraint = self.constraints[index];
+        debug!(
+            "constraint_is_interesting: locations={:?} constraint={:?}",
+            constraint.locations, constraint
+        );
+        if let Locations::Interesting(_) = constraint.locations {
+            true
+        } else {
+            false
+        }
     }
 
     /// This function classifies a constraint from a location.
-    fn classify_constraint(&self, index: &ConstraintIndex,
-                           mir: &Mir<'tcx>) -> Option<(ConstraintCategory, Span)> {
-        let constraint = self.constraints.get(*index)?;
+    fn classify_constraint(
+        &self,
+        index: ConstraintIndex,
+        mir: &Mir<'tcx>,
+    ) -> (ConstraintCategory, Span) {
+        let constraint = self.constraints[index];
         let span = constraint.locations.span(mir);
-        let location = constraint.locations.from_location()?;
+        let location = constraint.locations.from_location().unwrap_or(Location::START);
 
         if !self.constraint_is_interesting(index) {
-            return Some((ConstraintCategory::Boring, span));
+            return (ConstraintCategory::Boring, span);
         }
 
         let data = &mir[location.block];
@@ -182,12 +174,12 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                             _ => ConstraintCategory::Other,
                         }
                     }
-                },
+                }
                 _ => ConstraintCategory::Other,
             }
         };
 
-        Some((category, span))
+        (category, span)
     }
 
     /// Report an error because the universal region `fr` was required to outlive
@@ -207,12 +199,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         outlived_fr: RegionVid,
         blame_span: Span,
     ) {
-        // Obviously uncool error reporting.
-
-        let fr_name = self.to_error_region(fr);
-        let outlived_fr_name = self.to_error_region(outlived_fr);
+        debug!("report_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 
-        if let (Some(f), Some(o)) = (fr_name, outlived_fr_name) {
+        if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
             let tables = infcx.tcx.typeck_tables_of(mir_def_id);
             let nice = NiceRegionError::new_from_span(infcx.tcx, blame_span, o, f, Some(tables));
             if let Some(_error_reported) = nice.try_report() {
@@ -220,43 +209,52 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             }
         }
 
-        let fr_string = match fr_name {
-            Some(r) => format!("free region `{}`", r),
-            None => format!("free region `{:?}`", fr),
-        };
-
-        let outlived_fr_string = match outlived_fr_name {
-            Some(r) => format!("free region `{}`", r),
-            None => format!("free region `{:?}`", outlived_fr),
-        };
+        // Find all paths
+        let constraint_paths = self.find_constraint_paths_between_regions(outlived_fr, |r| r == fr);
+        debug!("report_error: constraint_paths={:#?}", constraint_paths);
 
-        let constraints = self.find_constraint_paths_from_region(fr.clone());
-        let path = constraints.iter().min_by_key(|p| p.len()).unwrap();
+        // Find the shortest such path.
+        let path = constraint_paths.iter().min_by_key(|p| p.len()).unwrap();
         debug!("report_error: shortest_path={:?}", path);
 
-        let mut categorized_path = path.iter().filter_map(|index| {
-            self.classify_constraint(index, mir)
-        }).collect::<Vec<(ConstraintCategory, Span)>>();
+        // Classify each of the constraints along the path.
+        let mut categorized_path: Vec<(ConstraintCategory, Span)> = path.iter()
+            .map(|&index| self.classify_constraint(index, mir))
+            .collect();
         debug!("report_error: categorized_path={:?}", categorized_path);
 
+        // Find what appears to be the most interesting path to report to the user.
         categorized_path.sort_by(|p0, p1| p0.0.cmp(&p1.0));
         debug!("report_error: sorted_path={:?}", categorized_path);
 
-        if let Some((category, span)) = &categorized_path.first() {
-            let mut diag = infcx.tcx.sess.struct_span_err(
-                *span, &format!("{} requires that data must outlive {}",
-                                category, outlived_fr_string),
-            );
-
-            diag.emit();
-        } else {
-            let mut diag = infcx.tcx.sess.struct_span_err(
-                blame_span,
-                &format!("{} does not outlive {}", fr_string, outlived_fr_string,),
-            );
-
-            diag.emit();
-        }
+        // Get a span
+        let (category, span) = categorized_path.first().unwrap();
+        let diag = &mut infcx.tcx.sess.struct_span_err(
+            *span,
+            &format!("unsatisfied lifetime constraints"), // FIXME
+        );
+
+        // Figure out how we can refer
+        let counter = &mut 1;
+        let fr_name = self.give_region_a_name(infcx.tcx, mir, mir_def_id, fr, counter, diag);
+        let outlived_fr_name = self.give_region_a_name(
+            infcx.tcx,
+            mir,
+            mir_def_id,
+            outlived_fr,
+            counter,
+            diag,
+        );
+
+        diag.span_label(
+            *span,
+            format!(
+                "{} requires that `{}` must outlive `{}`",
+                category, fr_name, outlived_fr_name,
+            ),
+        );
+
+        diag.emit();
     }
 
     // Find some constraint `X: Y` where:
@@ -269,8 +267,11 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
     /// Tries to finds a good span to blame for the fact that `fr1`
     /// contains `fr2`.
-    pub(super) fn blame_constraint(&self, fr1: RegionVid,
-                                   elem: impl ToElementIndex) -> ConstraintIndex {
+    pub(super) fn blame_constraint(
+        &self,
+        fr1: RegionVid,
+        elem: impl ToElementIndex,
+    ) -> ConstraintIndex {
         // Find everything that influenced final value of `fr`.
         let influenced_fr1 = self.dependencies(fr1);
 
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs
new file mode 100644
index 00000000000..bf8c9c8c3be
--- /dev/null
+++ b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs
@@ -0,0 +1,256 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use borrow_check::nll::region_infer::RegionInferenceContext;
+use borrow_check::nll::ToRegionVid;
+use rustc::hir::def_id::DefId;
+use rustc::mir::{Local, Mir};
+use rustc::ty::{self, RegionVid, TyCtxt};
+use rustc_data_structures::indexed_vec::Idx;
+use rustc_errors::DiagnosticBuilder;
+use syntax::ast::Name;
+use syntax::symbol::keywords;
+use syntax_pos::symbol::InternedString;
+
+impl<'tcx> RegionInferenceContext<'tcx> {
+    /// Maps from an internal MIR region vid to something that we can
+    /// report to the user. In some cases, the region vids will map
+    /// directly to lifetimes that the user has a name for (e.g.,
+    /// `'static`). But frequently they will not, in which case we
+    /// have to find some way to identify the lifetime to the user. To
+    /// that end, this function takes a "diagnostic" so that it can
+    /// create auxiliary notes as needed.
+    ///
+    /// Example (function arguments):
+    ///
+    /// Suppose we are trying to give a name to the lifetime of the
+    /// reference `x`:
+    ///
+    /// ```
+    /// fn foo(x: &u32) { .. }
+    /// ```
+    ///
+    /// This function would create a label like this:
+    ///
+    /// ```
+    ///  | fn foo(x: &u32) { .. }
+    ///           ------- fully elaborated type of `x` is `&'1 u32`
+    /// ```
+    ///
+    /// and then return the name `'1` for us to use.
+    crate fn give_region_a_name(
+        &self,
+        tcx: TyCtxt<'_, '_, 'tcx>,
+        mir: &Mir<'tcx>,
+        mir_def_id: DefId,
+        fr: RegionVid,
+        counter: &mut usize,
+        diag: &mut DiagnosticBuilder,
+    ) -> InternedString {
+        debug!("give_region_a_name(fr={:?}, counter={})", fr, counter);
+
+        assert!(self.universal_regions.is_universal_region(fr));
+
+        self.give_name_from_error_region(tcx, mir_def_id, fr, counter, diag)
+            .or_else(|| {
+                self.give_name_if_anonymous_region_appears_in_arguments(tcx, mir, fr, counter, diag)
+            })
+            .or_else(|| {
+                self.give_name_if_anonymous_region_appears_in_upvars(tcx, mir, fr, counter, diag)
+            })
+            .or_else(|| {
+                self.give_name_if_anonymous_region_appears_in_output(tcx, mir, fr, counter, diag)
+            })
+            .unwrap_or_else(|| span_bug!(mir.span, "can't make a name for free region {:?}", fr))
+    }
+
+    /// Check for the case where `fr` maps to something that the
+    /// *user* has a name for. In that case, we'll be able to map
+    /// `fr` to a `Region<'tcx>`, and that region will be one of
+    /// named variants.
+    fn give_name_from_error_region(
+        &self,
+        tcx: TyCtxt<'_, '_, 'tcx>,
+        mir_def_id: DefId,
+        fr: RegionVid,
+        counter: &mut usize,
+        diag: &mut DiagnosticBuilder<'_>,
+    ) -> Option<InternedString> {
+        let error_region = self.to_error_region(fr)?;
+        debug!("give_region_a_name: error_region = {:?}", error_region);
+        match error_region {
+            ty::ReEarlyBound(ebr) => Some(ebr.name),
+
+            ty::ReStatic => Some(keywords::StaticLifetime.name().as_interned_str()),
+
+            ty::ReFree(free_region) => match free_region.bound_region {
+                ty::BoundRegion::BrNamed(_, name) => Some(name),
+
+                ty::BoundRegion::BrEnv => {
+                    let closure_span = tcx.hir.span_if_local(mir_def_id).unwrap();
+                    let region_name = self.synthesize_region_name(counter);
+                    diag.span_label(
+                        closure_span,
+                        format!("lifetime `{}` represents the closure body", region_name),
+                    );
+                    Some(region_name)
+                }
+
+                ty::BoundRegion::BrAnon(_) | ty::BoundRegion::BrFresh(_) => None,
+            },
+
+            ty::ReLateBound(..)
+            | ty::ReScope(..)
+            | ty::ReVar(..)
+            | ty::ReSkolemized(..)
+            | ty::ReEmpty
+            | ty::ReErased
+            | ty::ReClosureBound(..)
+            | ty::ReCanonical(..) => None,
+        }
+    }
+
+    /// Find an argument that contains `fr` and label it with a fully
+    /// elaborated type, returning something like `'1`. Result looks
+    /// like:
+    ///
+    /// ```
+    ///  | fn foo(x: &u32) { .. }
+    ///           ------- fully elaborated type of `x` is `&'1 u32`
+    /// ```
+    fn give_name_if_anonymous_region_appears_in_arguments(
+        &self,
+        tcx: TyCtxt<'_, '_, 'tcx>,
+        mir: &Mir<'tcx>,
+        fr: RegionVid,
+        counter: &mut usize,
+        diag: &mut DiagnosticBuilder<'_>,
+    ) -> Option<InternedString> {
+        let implicit_inputs = self.universal_regions.defining_ty.implicit_inputs();
+        let argument_index = self.universal_regions
+            .unnormalized_input_tys
+            .iter()
+            .skip(implicit_inputs)
+            .position(|arg_ty| {
+                debug!("give_name_if_anonymous_region_appears_in_arguments: arg_ty = {:?}", arg_ty);
+                tcx.any_free_region_meets(arg_ty, |r| r.to_region_vid() == fr)
+            })?
+            + implicit_inputs;
+
+        debug!(
+            "give_name_if_anonymous_region_appears_in_arguments: \
+             found {:?} in argument {} which has type {:?}",
+            fr, argument_index, self.universal_regions.unnormalized_input_tys[argument_index],
+        );
+
+        let region_name = self.synthesize_region_name(counter);
+
+        let argument_local = Local::new(argument_index + 1);
+        let argument_span = mir.local_decls[argument_local].source_info.span;
+        diag.span_label(
+            argument_span,
+            format!("lifetime `{}` appears in this argument", region_name,),
+        );
+
+        Some(region_name)
+    }
+
+    /// Find a closure upvar that contains `fr` and label it with a
+    /// fully elaborated type, returning something like `'1`. Result
+    /// looks like:
+    ///
+    /// ```
+    ///  | let x = Some(&22);
+    ///        - fully elaborated type of `x` is `Option<&'1 u32>`
+    /// ```
+    fn give_name_if_anonymous_region_appears_in_upvars(
+        &self,
+        tcx: TyCtxt<'_, '_, 'tcx>,
+        mir: &Mir<'tcx>,
+        fr: RegionVid,
+        counter: &mut usize,
+        diag: &mut DiagnosticBuilder<'_>,
+    ) -> Option<InternedString> {
+        let upvar_index = self.universal_regions
+            .defining_ty
+            .upvar_tys(tcx)
+            .position(|upvar_ty| {
+                debug!(
+                    "give_name_if_anonymous_region_appears_in_upvars: upvar_ty = {:?}",
+                    upvar_ty,
+                );
+                tcx.any_free_region_meets(&upvar_ty, |r| r.to_region_vid() == fr)
+            })?;
+
+        debug!(
+            "give_name_if_anonymous_region_appears_in_upvars: \
+             found {:?} in upvar {} which has type {:?}",
+            fr,
+            upvar_index,
+            self.universal_regions
+                .defining_ty
+                .upvar_tys(tcx)
+                .nth(upvar_index),
+        );
+
+        let region_name = self.synthesize_region_name(counter);
+
+        let upvar_hir_id = mir.upvar_decls[upvar_index].var_hir_id.assert_crate_local();
+        let upvar_node_id = tcx.hir.hir_to_node_id(upvar_hir_id);
+        let upvar_span = tcx.hir.span(upvar_node_id);
+        let upvar_name = tcx.hir.name(upvar_node_id);
+        diag.span_label(
+            upvar_span,
+            format!(
+                "lifetime `{}` appears in the type of `{}`",
+                region_name, upvar_name,
+            ),
+        );
+
+        Some(region_name)
+    }
+
+    /// Check for arguments appearing in the (closure) return type. It
+    /// must be a closure since, in a free fn, such an argument would
+    /// have to either also appear in an argument (if using elision)
+    /// or be early bound (named, not in argument).
+    fn give_name_if_anonymous_region_appears_in_output(
+        &self,
+        tcx: TyCtxt<'_, '_, 'tcx>,
+        mir: &Mir<'tcx>,
+        fr: RegionVid,
+        counter: &mut usize,
+        diag: &mut DiagnosticBuilder<'_>,
+    ) -> Option<InternedString> {
+        let return_ty = self.universal_regions
+            .unnormalized_output_ty;
+        debug!("give_name_if_anonymous_region_appears_in_output: return_ty = {:?}", return_ty);
+        if !tcx.any_free_region_meets(&return_ty, |r| r.to_region_vid() == fr) {
+            return None;
+        }
+
+        let region_name = self.synthesize_region_name(counter);
+        diag.span_label(
+            mir.span,
+            format!("lifetime `{}` appears in return type", region_name),
+        );
+
+        Some(region_name)
+    }
+
+    /// Create a synthetic region named `'1`, incrementing the
+    /// counter.
+    fn synthesize_region_name(&self, counter: &mut usize) -> InternedString {
+        let c = *counter;
+        *counter += 1;
+
+        Name::intern(&format!("'{:?}", c)).as_interned_str()
+    }
+}
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/mod.rs b/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
index 13cc0c0419e..164941203e0 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
@@ -256,6 +256,11 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     fn init_universal_regions(&mut self) {
         // Update the names (if any)
         for (external_name, variable) in self.universal_regions.named_universal_regions() {
+            debug!(
+                "init_universal_regions: region {:?} has external name {:?}",
+                variable,
+                external_name
+            );
             self.definitions[variable].external_name = Some(external_name);
         }
 
diff --git a/src/librustc_mir/borrow_check/nll/universal_regions.rs b/src/librustc_mir/borrow_check/nll/universal_regions.rs
index ec8cd386679..8590e3d0765 100644
--- a/src/librustc_mir/borrow_check/nll/universal_regions.rs
+++ b/src/librustc_mir/borrow_check/nll/universal_regions.rs
@@ -22,15 +22,16 @@
 //! The code in this file doesn't *do anything* with those results; it
 //! just returns them for other code to use.
 
-use rustc::hir::{self, BodyOwnerKind, HirId};
+use either::Either;
 use rustc::hir::def_id::DefId;
-use rustc::infer::{InferCtxt, NLLRegionVariableOrigin};
-use rustc::infer::region_constraints::GenericKind;
+use rustc::hir::{self, BodyOwnerKind, HirId};
 use rustc::infer::outlives::bounds::{self, OutlivesBound};
 use rustc::infer::outlives::free_region_map::FreeRegionRelations;
-use rustc::ty::{self, RegionVid, Ty, TyCtxt, ClosureSubsts, GeneratorSubsts};
+use rustc::infer::region_constraints::GenericKind;
+use rustc::infer::{InferCtxt, NLLRegionVariableOrigin};
 use rustc::ty::fold::TypeFoldable;
 use rustc::ty::subst::Substs;
+use rustc::ty::{self, ClosureSubsts, GeneratorSubsts, RegionVid, Ty, TyCtxt};
 use rustc::util::nodemap::FxHashMap;
 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
 use rustc_data_structures::transitive_relation::TransitiveRelation;
@@ -128,6 +129,34 @@ pub enum DefiningTy<'tcx> {
     Const(DefId, &'tcx Substs<'tcx>),
 }
 
+impl<'tcx> DefiningTy<'tcx> {
+    /// Returns a list of all the upvar types for this MIR. If this is
+    /// not a closure or generator, there are no upvars, and hence it
+    /// will be an empty list. The order of types in this list will
+    /// match up with the `upvar_decls` field of `Mir`.
+    pub fn upvar_tys(self, tcx: TyCtxt<'_, '_, 'tcx>) -> impl Iterator<Item = Ty<'tcx>> + 'tcx {
+        match self {
+            DefiningTy::Closure(def_id, substs) => Either::Left(substs.upvar_tys(def_id, tcx)),
+            DefiningTy::Generator(def_id, substs, _) => {
+                Either::Right(Either::Left(substs.upvar_tys(def_id, tcx)))
+            }
+            DefiningTy::FnDef(..) | DefiningTy::Const(..) => {
+                Either::Right(Either::Right(iter::empty()))
+            }
+        }
+    }
+
+    /// Number of implicit inputs -- notably the "environment"
+    /// parameter for closures -- that appear in MIR but not in the
+    /// user's code.
+    pub fn implicit_inputs(self) -> usize {
+        match self {
+            DefiningTy::Closure(..) | DefiningTy::Generator(..) => 1,
+            DefiningTy::FnDef(..) | DefiningTy::Const(..) => 0,
+        }
+    }
+}
+
 #[derive(Debug)]
 struct UniversalRegionIndices<'tcx> {
     /// For those regions that may appear in the parameter environment
@@ -493,18 +522,15 @@ impl<'cx, 'gcx, 'tcx> UniversalRegionsBuilder<'cx, 'gcx, 'tcx> {
 
         debug!(
             "build: global regions = {}..{}",
-            FIRST_GLOBAL_INDEX,
-            first_extern_index
+            FIRST_GLOBAL_INDEX, first_extern_index
         );
         debug!(
             "build: extern regions = {}..{}",
-            first_extern_index,
-            first_local_index
+            first_extern_index, first_local_index
         );
         debug!(
             "build: local regions  = {}..{}",
-            first_local_index,
-            num_universals
+            first_local_index, num_universals
         );
 
         let yield_ty = match defining_ty {
@@ -545,10 +571,12 @@ impl<'cx, 'gcx, 'tcx> UniversalRegionsBuilder<'cx, 'gcx, 'tcx> {
                     tables.node_id_to_type(self.mir_hir_id)
                 };
 
+                debug!("defining_ty (pre-replacement): {:?}", defining_ty);
+
                 let defining_ty = self.infcx
                     .replace_free_regions_with_nll_infer_vars(FR, &defining_ty);
 
-                match defining_ty.sty  {
+                match defining_ty.sty {
                     ty::TyClosure(def_id, substs) => DefiningTy::Closure(def_id, substs),
                     ty::TyGenerator(def_id, substs, movability) => {
                         DefiningTy::Generator(def_id, substs, movability)
@@ -587,8 +615,8 @@ impl<'cx, 'gcx, 'tcx> UniversalRegionsBuilder<'cx, 'gcx, 'tcx> {
         let closure_base_def_id = tcx.closure_base_def_id(self.mir_def_id);
         let identity_substs = Substs::identity_for_item(gcx, closure_base_def_id);
         let fr_substs = match defining_ty {
-            DefiningTy::Closure(_, ClosureSubsts { ref substs }) |
-            DefiningTy::Generator(_, GeneratorSubsts { ref substs }, _) => {
+            DefiningTy::Closure(_, ClosureSubsts { ref substs })
+            | DefiningTy::Generator(_, GeneratorSubsts { ref substs }, _) => {
                 // In the case of closures, we rely on the fact that
                 // the first N elements in the ClosureSubsts are
                 // inherited from the `closure_base_def_id`.
@@ -726,8 +754,7 @@ impl UniversalRegionRelations {
     fn relate_universal_regions(&mut self, fr_a: RegionVid, fr_b: RegionVid) {
         debug!(
             "relate_universal_regions: fr_a={:?} outlives fr_b={:?}",
-            fr_a,
-            fr_b
+            fr_a, fr_b
         );
         self.outlives.add(fr_a, fr_b);
         self.inverse_outlives.add(fr_b, fr_a);
@@ -780,8 +807,7 @@ impl<'cx, 'gcx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'gcx, 'tcx> {
     {
         debug!(
             "replace_bound_regions_with_nll_infer_vars(value={:?}, all_outlive_scope={:?})",
-            value,
-            all_outlive_scope,
+            value, all_outlive_scope,
         );
         let (value, _map) = self.tcx.replace_late_bound_regions(value, |br| {
             let liberated_region = self.tcx.mk_region(ty::ReFree(ty::FreeRegion {
@@ -790,7 +816,10 @@ impl<'cx, 'gcx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'gcx, 'tcx> {
             }));
             let region_vid = self.next_nll_region_var(origin);
             indices.insert_late_bound_region(liberated_region, region_vid.to_region_vid());
-            debug!("liberated_region={:?} => {:?}", liberated_region, region_vid);
+            debug!(
+                "liberated_region={:?} => {:?}",
+                liberated_region, region_vid
+            );
             region_vid
         });
         value
@@ -803,9 +832,8 @@ impl<'tcx> UniversalRegionIndices<'tcx> {
     /// in later and instantiate the late-bound regions, and then we
     /// insert the `ReFree` version of those into the map as
     /// well. These are used for error reporting.
-    fn insert_late_bound_region(&mut self, r: ty::Region<'tcx>,
-                                vid: ty::RegionVid)
-    {
+    fn insert_late_bound_region(&mut self, r: ty::Region<'tcx>, vid: ty::RegionVid) {
+        debug!("insert_late_bound_region({:?}, {:?})", r, vid);
         self.indices.insert(r, vid);
     }
 
@@ -821,9 +849,9 @@ impl<'tcx> UniversalRegionIndices<'tcx> {
         if let ty::ReVar(..) = r {
             r.to_region_vid()
         } else {
-            *self.indices.get(&r).unwrap_or_else(|| {
-                bug!("cannot convert `{:?}` to a region vid", r)
-            })
+            *self.indices
+                .get(&r)
+                .unwrap_or_else(|| bug!("cannot convert `{:?}` to a region vid", r))
         }
     }
 
diff --git a/src/librustc_mir/build/mod.rs b/src/librustc_mir/build/mod.rs
index 8fb5b5c0fa4..a3f97376e90 100644
--- a/src/librustc_mir/build/mod.rs
+++ b/src/librustc_mir/build/mod.rs
@@ -534,6 +534,7 @@ fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
             };
             let mut decl = UpvarDecl {
                 debug_name: keywords::Invalid.name(),
+                var_hir_id: ClearCrossCrate::Set(var_hir_id),
                 by_ref,
                 mutability: Mutability::Not,
             };
diff --git a/src/test/compile-fail/borrowck/borrowck-describe-lvalue.rs b/src/test/compile-fail/borrowck/borrowck-describe-lvalue.rs
index 3a7e4a13740..b821c7cfa34 100644
--- a/src/test/compile-fail/borrowck/borrowck-describe-lvalue.rs
+++ b/src/test/compile-fail/borrowck/borrowck-describe-lvalue.rs
@@ -302,7 +302,7 @@ fn main() {
         // FIXME(#49824) -- the free region error below should probably not be there
         let mut x = 0;
            || {
-               || { //[mir]~ ERROR free region `` does not outlive
+               || { //[mir]~ ERROR unsatisfied lifetime constraints
                    let y = &mut x;
                    &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
                    //[mir]~^ ERROR cannot borrow `x` as mutable more than once at a time
diff --git a/src/test/compile-fail/mir_check_cast_closure.rs b/src/test/compile-fail/mir_check_cast_closure.rs
index d8f5956b585..2200c1569e5 100644
--- a/src/test/compile-fail/mir_check_cast_closure.rs
+++ b/src/test/compile-fail/mir_check_cast_closure.rs
@@ -14,7 +14,7 @@
 
 fn bar<'a, 'b>() -> fn(&'a u32, &'b u32) -> &'a u32 {
     let g: fn(_, _) -> _ = |_x, y| y;
-    //~^ ERROR free region `'b` does not outlive free region `'a`
+    //~^ ERROR unsatisfied lifetime constraints
     g
     //~^ WARNING not reporting region error due to nll
 }
diff --git a/src/test/compile-fail/mir_check_cast_reify.rs b/src/test/compile-fail/mir_check_cast_reify.rs
index 1f9174b3574..f85104dff86 100644
--- a/src/test/compile-fail/mir_check_cast_reify.rs
+++ b/src/test/compile-fail/mir_check_cast_reify.rs
@@ -45,7 +45,7 @@ fn bar<'a>(x: &'a u32) -> &'static u32 {
     // as part of checking the `ReifyFnPointer`.
     let f: fn(_) -> _ = foo;
     //~^ WARNING not reporting region error due to nll
-    //~| ERROR free region `'a` does not outlive free region `'static`
+    //~| ERROR unsatisfied lifetime constraints
     f(x)
 }
 
diff --git a/src/test/compile-fail/mir_check_cast_unsafe_fn.rs b/src/test/compile-fail/mir_check_cast_unsafe_fn.rs
index 27ca2728ddf..e90242f3f87 100644
--- a/src/test/compile-fail/mir_check_cast_unsafe_fn.rs
+++ b/src/test/compile-fail/mir_check_cast_unsafe_fn.rs
@@ -17,7 +17,7 @@ fn bar<'a>(input: &'a u32, f: fn(&'a u32) -> &'a u32) -> &'static u32 {
     // in `g`. These are related via the `UnsafeFnPointer` cast.
     let g: unsafe fn(_) -> _ = f;
     //~^ WARNING not reporting region error due to nll
-    //~| ERROR free region `'a` does not outlive free region `'static`
+    //~| ERROR unsatisfied lifetime constraints
     unsafe { g(input) }
 }
 
diff --git a/src/test/compile-fail/mir_check_cast_unsize.rs b/src/test/compile-fail/mir_check_cast_unsize.rs
index a2c840a7098..d242186a6f7 100644
--- a/src/test/compile-fail/mir_check_cast_unsize.rs
+++ b/src/test/compile-fail/mir_check_cast_unsize.rs
@@ -15,8 +15,7 @@
 use std::fmt::Debug;
 
 fn bar<'a>(x: &'a u32) -> &'static dyn Debug {
-    //~^ ERROR free region `'a` does not outlive free region `'static`
-    x
+    x //~ ERROR unsatisfied lifetime constraints
     //~^ WARNING not reporting region error due to nll
 }
 
diff --git a/src/test/compile-fail/regions-static-bound.rs b/src/test/compile-fail/regions-static-bound.rs
index a0097b9f6d7..0a37df4f0c7 100644
--- a/src/test/compile-fail/regions-static-bound.rs
+++ b/src/test/compile-fail/regions-static-bound.rs
@@ -18,7 +18,7 @@ fn static_id_indirect<'a,'b>(t: &'a ()) -> &'static ()
 fn static_id_wrong_way<'a>(t: &'a ()) -> &'static () where 'static: 'a {
     t //[ll]~ ERROR E0312
         //[nll]~^ WARNING not reporting region error due to nll
-        //[nll]~| ERROR free region `'a` does not outlive free region `'static`
+        //[nll]~| ERROR unsatisfied lifetime constraints
 }
 
 fn error(u: &(), v: &()) {
diff --git a/src/test/ui/borrowck/issue-45983.nll.stderr b/src/test/ui/borrowck/issue-45983.nll.stderr
index a008a408d97..68a039262c1 100644
--- a/src/test/ui/borrowck/issue-45983.nll.stderr
+++ b/src/test/ui/borrowck/issue-45983.nll.stderr
@@ -4,11 +4,15 @@ warning: not reporting region error due to nll
 LL |     give_any(|y| x = Some(y));
    |                           ^
 
-error: free region `` does not outlive free region `'_#2r`
-  --> $DIR/issue-45983.rs:17:27
+error: unsatisfied lifetime constraints
+  --> $DIR/issue-45983.rs:17:18
    |
+LL |     let x = None;
+   |         - lifetime `'2` appears in the type of `x`
 LL |     give_any(|y| x = Some(y));
-   |                           ^
+   |               -  ^^^^^^^^^^^ free region requires that `'1` must outlive `'2`
+   |               |
+   |               lifetime `'1` appears in this argument
 
 error[E0594]: cannot assign to immutable item `x`
   --> $DIR/issue-45983.rs:17:18
diff --git a/src/test/ui/borrowck/issue-7573.nll.stderr b/src/test/ui/borrowck/issue-7573.nll.stderr
index 84c6236eb0a..daa0a320b88 100644
--- a/src/test/ui/borrowck/issue-7573.nll.stderr
+++ b/src/test/ui/borrowck/issue-7573.nll.stderr
@@ -4,11 +4,17 @@ warning: not reporting region error due to nll
 LL |     let mut lines_to_use: Vec<&CrateId> = Vec::new();
    |                               ^
 
-error: free region `` does not outlive free region `'_#2r`
+error: unsatisfied lifetime constraints
   --> $DIR/issue-7573.rs:32:9
    |
+LL |     let mut lines_to_use: Vec<&CrateId> = Vec::new();
+   |         ---------------- lifetime `'2` appears in the type of `lines_to_use`
+LL |         //~^ NOTE cannot infer an appropriate lifetime
+LL |     let push_id = |installed_id: &CrateId| {
+   |                    ------------ lifetime `'1` appears in this argument
+...
 LL |         lines_to_use.push(installed_id);
-   |         ^^^^^^^^^^^^
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/borrowck/regions-escape-bound-fn-2.nll.stderr b/src/test/ui/borrowck/regions-escape-bound-fn-2.nll.stderr
index ee3970aa8fd..4f7843b7248 100644
--- a/src/test/ui/borrowck/regions-escape-bound-fn-2.nll.stderr
+++ b/src/test/ui/borrowck/regions-escape-bound-fn-2.nll.stderr
@@ -4,11 +4,15 @@ warning: not reporting region error due to nll
 LL |     with_int(|y| x = Some(y));
    |                           ^
 
-error: free region `` does not outlive free region `'_#2r`
-  --> $DIR/regions-escape-bound-fn-2.rs:18:27
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-escape-bound-fn-2.rs:18:18
    |
+LL |     let mut x = None;
+   |         ----- lifetime `'2` appears in the type of `x`
 LL |     with_int(|y| x = Some(y));
-   |                           ^
+   |               -  ^^^^^^^^^^^ free region requires that `'1` must outlive `'2`
+   |               |
+   |               lifetime `'1` appears in this argument
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/borrowck/regions-escape-bound-fn.nll.stderr b/src/test/ui/borrowck/regions-escape-bound-fn.nll.stderr
index 07a4ab1dbb1..9b107ae08b4 100644
--- a/src/test/ui/borrowck/regions-escape-bound-fn.nll.stderr
+++ b/src/test/ui/borrowck/regions-escape-bound-fn.nll.stderr
@@ -4,11 +4,15 @@ warning: not reporting region error due to nll
 LL |     with_int(|y| x = Some(y));
    |                      ^^^^^^^
 
-error: free region `` does not outlive free region `'_#2r`
-  --> $DIR/regions-escape-bound-fn.rs:18:27
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-escape-bound-fn.rs:18:18
    |
+LL |     let mut x: Option<&isize> = None;
+   |         ----- lifetime `'2` appears in the type of `x`
 LL |     with_int(|y| x = Some(y));
-   |                           ^
+   |               -  ^^^^^^^^^^^ free region requires that `'1` must outlive `'2`
+   |               |
+   |               lifetime `'1` appears in this argument
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/borrowck/regions-escape-unboxed-closure.nll.stderr b/src/test/ui/borrowck/regions-escape-unboxed-closure.nll.stderr
index 14c255ef527..8095330154d 100644
--- a/src/test/ui/borrowck/regions-escape-unboxed-closure.nll.stderr
+++ b/src/test/ui/borrowck/regions-escape-unboxed-closure.nll.stderr
@@ -4,11 +4,15 @@ warning: not reporting region error due to nll
 LL |     with_int(&mut |y| x = Some(y));
    |                           ^^^^^^^
 
-error: free region `` does not outlive free region `'_#2r`
-  --> $DIR/regions-escape-unboxed-closure.rs:16:32
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-escape-unboxed-closure.rs:16:23
    |
+LL |     let mut x: Option<&isize> = None;
+   |         ----- lifetime `'2` appears in the type of `x`
 LL |     with_int(&mut |y| x = Some(y));
-   |                                ^
+   |                    -  ^^^^^^^^^^^ free region requires that `'1` must outlive `'2`
+   |                    |
+   |                    lifetime `'1` appears in this argument
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/closure-expected-type/expect-region-supply-region.nll.stderr b/src/test/ui/closure-expected-type/expect-region-supply-region.nll.stderr
index bbae80e16ab..5487d34813b 100644
--- a/src/test/ui/closure-expected-type/expect-region-supply-region.nll.stderr
+++ b/src/test/ui/closure-expected-type/expect-region-supply-region.nll.stderr
@@ -22,23 +22,37 @@ warning: not reporting region error due to nll
 LL |         f = Some(x);
    |             ^^^^^^^
 
-error: free region `` does not outlive free region `'_#2r`
-  --> $DIR/expect-region-supply-region.rs:28:18
+error: unsatisfied lifetime constraints
+  --> $DIR/expect-region-supply-region.rs:28:9
    |
+LL |     let mut f: Option<&u32> = None;
+   |         ----- lifetime `'2` appears in the type of `f`
+LL |     closure_expecting_bound(|x| {
+   |                              - lifetime `'1` appears in this argument
 LL |         f = Some(x); //~ ERROR borrowed data cannot be stored outside of its closure
-   |                  ^
+   |         ^^^^^^^^^^^ free region requires that `'1` must outlive `'2`
 
-error: free region `` does not outlive free region `'_#2r`
-  --> $DIR/expect-region-supply-region.rs:38:18
+error: unsatisfied lifetime constraints
+  --> $DIR/expect-region-supply-region.rs:38:9
    |
+LL |     let mut f: Option<&u32> = None;
+   |         ----- lifetime `'2` appears in the type of `f`
+LL |     closure_expecting_bound(|x: &u32| {
+   |                              - lifetime `'1` appears in this argument
 LL |         f = Some(x); //~ ERROR borrowed data cannot be stored outside of its closure
-   |                  ^
+   |         ^^^^^^^^^^^ free region requires that `'1` must outlive `'2`
 
-error: free region `` does not outlive free region `'_#2r`
-  --> $DIR/expect-region-supply-region.rs:52:18
+error: unsatisfied lifetime constraints
+  --> $DIR/expect-region-supply-region.rs:52:9
    |
+LL |     let mut f: Option<&u32> = None;
+   |         ----- lifetime `'2` appears in the type of `f`
+...
+LL |     closure_expecting_bound(|x: &'x u32| {
+   |                              - lifetime `'1` appears in this argument
+...
 LL |         f = Some(x);
-   |                  ^
+   |         ^^^^^^^^^^^ free region requires that `'1` must outlive `'2`
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/error-codes/E0621-does-not-trigger-for-closures.nll.stderr b/src/test/ui/error-codes/E0621-does-not-trigger-for-closures.nll.stderr
index 5ae6afa7b17..9c7e3db67a6 100644
--- a/src/test/ui/error-codes/E0621-does-not-trigger-for-closures.nll.stderr
+++ b/src/test/ui/error-codes/E0621-does-not-trigger-for-closures.nll.stderr
@@ -4,11 +4,15 @@ warning: not reporting region error due to nll
 LL |     invoke(&x, |a, b| if a > b { a } else { b }); //~ ERROR E0495
    |     ^^^^^^
 
-error: free region `` does not outlive free region `'_#2r`
+error: unsatisfied lifetime constraints
   --> $DIR/E0621-does-not-trigger-for-closures.rs:25:26
    |
 LL |     invoke(&x, |a, b| if a > b { a } else { b }); //~ ERROR E0495
-   |                          ^^^^^
+   |                ----------^^^^^-----------------
+   |                |   |     |
+   |                |   |     free region requires that `'1` must outlive `'2`
+   |                |   lifetime `'1` appears in this argument
+   |                lifetime `'2` appears in return type
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/impl-trait/static-return-lifetime-infered.nll.stderr b/src/test/ui/impl-trait/static-return-lifetime-infered.nll.stderr
index 7099316d694..9aad7efdee5 100644
--- a/src/test/ui/impl-trait/static-return-lifetime-infered.nll.stderr
+++ b/src/test/ui/impl-trait/static-return-lifetime-infered.nll.stderr
@@ -10,17 +10,19 @@ warning: not reporting region error due to nll
 LL |         self.x.iter().map(|a| a.0)
    |                ^^^^
 
-error: free region `` does not outlive free region `'static`
+error: unsatisfied lifetime constraints
   --> $DIR/static-return-lifetime-infered.rs:17:9
    |
+LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
+   |                         ----- lifetime `'1` appears in this argument
 LL |         self.x.iter().map(|a| a.0)
-   |         ^^^^^^^^^^^^^
+   |         ^^^^^^ cast requires that `'1` must outlive `'static`
 
-error: free region `'a` does not outlive free region `'static`
+error: unsatisfied lifetime constraints
   --> $DIR/static-return-lifetime-infered.rs:21:9
    |
 LL |         self.x.iter().map(|a| a.0)
-   |         ^^^^^^^^^^^^^
+   |         ^^^^^^ cast requires that `'a` must outlive `'static`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/in-band-lifetimes/impl/dyn-trait.nll.stderr b/src/test/ui/in-band-lifetimes/impl/dyn-trait.nll.stderr
index 4cf7feddd46..19e10ba6da8 100644
--- a/src/test/ui/in-band-lifetimes/impl/dyn-trait.nll.stderr
+++ b/src/test/ui/in-band-lifetimes/impl/dyn-trait.nll.stderr
@@ -4,11 +4,11 @@ warning: not reporting region error due to nll
 LL |     static_val(x); //~ ERROR cannot infer
    |                ^
 
-error: free region `'a` does not outlive free region `'static`
+error: unsatisfied lifetime constraints
   --> $DIR/dyn-trait.rs:32:5
    |
 LL |     static_val(x); //~ ERROR cannot infer
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ argument requires that `'a` must outlive `'static`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/closure-requirements/escape-argument-callee.rs b/src/test/ui/nll/closure-requirements/escape-argument-callee.rs
index 1e168028c7c..78208d6d7db 100644
--- a/src/test/ui/nll/closure-requirements/escape-argument-callee.rs
+++ b/src/test/ui/nll/closure-requirements/escape-argument-callee.rs
@@ -34,7 +34,7 @@ fn test() {
     {
         let y = 22;
         let mut closure = expect_sig(|p, y| *p = y);
-        //~^ ERROR does not outlive free region
+        //~^ ERROR
         //~| WARNING not reporting region error due to nll
         closure(&mut p, &y);
     }
diff --git a/src/test/ui/nll/closure-requirements/escape-argument-callee.stderr b/src/test/ui/nll/closure-requirements/escape-argument-callee.stderr
index 067b5ebc6c4..d6f54218360 100644
--- a/src/test/ui/nll/closure-requirements/escape-argument-callee.stderr
+++ b/src/test/ui/nll/closure-requirements/escape-argument-callee.stderr
@@ -4,11 +4,14 @@ warning: not reporting region error due to nll
 LL |         let mut closure = expect_sig(|p, y| *p = y);
    |                                                  ^
 
-error: free region `ReFree(DefId(0/1:9 ~ escape_argument_callee[317d]::test[0]::{{closure}}[0]), BrAnon(3))` does not outlive free region `ReFree(DefId(0/1:9 ~ escape_argument_callee[317d]::test[0]::{{closure}}[0]), BrAnon(2))`
+error: unsatisfied lifetime constraints
   --> $DIR/escape-argument-callee.rs:36:45
    |
 LL |         let mut closure = expect_sig(|p, y| *p = y);
-   |                                             ^^^^^^
+   |                                       -  -  ^^^^^^ free region requires that `'1` must outlive `'2`
+   |                                       |  |
+   |                                       |  lifetime `'1` appears in this argument
+   |                                       lifetime `'2` appears in this argument
 
 note: No external requirements
   --> $DIR/escape-argument-callee.rs:36:38
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.rs b/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.rs
index e7ec0b9684d..b879f9a3398 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.rs
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.rs
@@ -54,8 +54,7 @@ fn supply<'a, 'b, 'c>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>, cell_c: Cell
             // Only works if 'x: 'y:
             let p = x.get();
             //~^ WARN not reporting region error due to nll
-            //~| ERROR does not outlive free region
-            demand_y(x, y, p)
+            demand_y(x, y, p) //~ ERROR
         },
     );
 }
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.stderr
index c88f0efba6f..a7a50a3a029 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.stderr
@@ -4,11 +4,16 @@ warning: not reporting region error due to nll
 LL |             let p = x.get();
    |                     ^^^^^^^
 
-error: free region `ReFree(DefId(0/1:20 ~ propagate_approximated_fail_no_postdom[317d]::supply[0]::{{closure}}[0]), BrAnon(1))` does not outlive free region `ReFree(DefId(0/1:20 ~ propagate_approximated_fail_no_postdom[317d]::supply[0]::{{closure}}[0]), BrAnon(2))`
-  --> $DIR/propagate-approximated-fail-no-postdom.rs:55:17
+error: unsatisfied lifetime constraints
+  --> $DIR/propagate-approximated-fail-no-postdom.rs:57:13
    |
-LL |             let p = x.get();
-   |                 ^
+LL |         |_outlives1, _outlives2, _outlives3, x, y| {
+   |          ----------              ---------- lifetime `'2` appears in this argument
+   |          |
+   |          lifetime `'1` appears in this argument
+...
+LL |             demand_y(x, y, p) //~ ERROR
+   |             ^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 note: No external requirements
   --> $DIR/propagate-approximated-fail-no-postdom.rs:53:9
@@ -17,8 +22,7 @@ LL | /         |_outlives1, _outlives2, _outlives3, x, y| {
 LL | |             // Only works if 'x: 'y:
 LL | |             let p = x.get();
 LL | |             //~^ WARN not reporting region error due to nll
-LL | |             //~| ERROR does not outlive free region
-LL | |             demand_y(x, y, p)
+LL | |             demand_y(x, y, p) //~ ERROR
 LL | |         },
    | |_________^
    |
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.rs b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.rs
index 5a71e75d4b2..a8ab41cebac 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.rs
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.rs
@@ -31,7 +31,7 @@ fn case1() {
     foo(cell, |cell_a, cell_x| {
         //~^ WARNING not reporting region error due to nll
         cell_a.set(cell_x.get()); // forces 'x: 'a, error in closure
-        //~^ ERROR argument requires that data must outlive free region
+        //~^ ERROR
     })
 }
 
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.stderr
index 656c1b46a3c..96f3d6a6a53 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.stderr
@@ -4,11 +4,16 @@ warning: not reporting region error due to nll
 LL |     foo(cell, |cell_a, cell_x| {
    |     ^^^
 
-error: argument requires that data must outlive free region `'_#1r`
-  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:33:20
+error: unsatisfied lifetime constraints
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:33:9
    |
+LL |     foo(cell, |cell_a, cell_x| {
+   |                ------  ------ lifetime `'1` appears in this argument
+   |                |
+   |                lifetime `'2` appears in this argument
+LL |         //~^ WARNING not reporting region error due to nll
 LL |         cell_a.set(cell_x.get()); // forces 'x: 'a, error in closure
-   |                    ^^^^^^^^^^^^
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 note: No external requirements
   --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:31:15
@@ -17,7 +22,7 @@ LL |       foo(cell, |cell_a, cell_x| {
    |  _______________^
 LL | |         //~^ WARNING not reporting region error due to nll
 LL | |         cell_a.set(cell_x.get()); // forces 'x: 'a, error in closure
-LL | |         //~^ ERROR argument requires that data must outlive free region
+LL | |         //~^ ERROR
 LL | |     })
    | |_____^
    |
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.rs b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.rs
index b25b0e25df2..26faccdde71 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.rs
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.rs
@@ -43,7 +43,7 @@ fn demand_y<'x, 'y>(_cell_x: &Cell<&'x u32>, _cell_y: &Cell<&'y u32>, _y: &'y u3
 #[rustc_regions]
 fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
-        //~^ ERROR argument requires that data must outlive free region
+        //~^ ERROR
 
         // Only works if 'x: 'y:
         demand_y(x, y, x.get()) //~ WARNING not reporting region error due to nll
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr
index 40f215619c6..f65e7161ca8 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr
@@ -9,7 +9,7 @@ note: External requirements
    |
 LL |       establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
    |  _______________________________________________^
-LL | |         //~^ ERROR argument requires that data must outlive free region
+LL | |         //~^ ERROR
 LL | |
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get()) //~ WARNING not reporting region error due to nll
@@ -23,23 +23,23 @@ LL | |     });
    = note: number of external vids: 2
    = note: where '_#1r: '_#0r
 
-error: argument requires that data must outlive free region `ReStatic`
+error: unsatisfied lifetime constraints
   --> $DIR/propagate-approximated-shorter-to-static-no-bound.rs:45:5
    |
 LL | /     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
-LL | |         //~^ ERROR argument requires that data must outlive free region
+LL | |         //~^ ERROR
 LL | |
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get()) //~ WARNING not reporting region error due to nll
 LL | |     });
-   | |______^
+   | |______^ argument requires that `'a` must outlive `'static`
 
 note: No external requirements
   --> $DIR/propagate-approximated-shorter-to-static-no-bound.rs:44:1
    |
 LL | / fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
 LL | |     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
-LL | |         //~^ ERROR argument requires that data must outlive free region
+LL | |         //~^ ERROR
 LL | |
 ...  |
 LL | |     });
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.rs b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.rs
index db9951bcc0f..703d60371cd 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.rs
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.rs
@@ -46,7 +46,7 @@ fn demand_y<'x, 'y>(_cell_x: &Cell<&'x u32>, _cell_y: &Cell<&'y u32>, _y: &'y u3
 #[rustc_regions]
 fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
-        //~^ ERROR argument requires that data must outlive free region
+        //~^ ERROR
         // Only works if 'x: 'y:
         demand_y(x, y, x.get())
         //~^ WARNING not reporting region error due to nll
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr
index d89ff028a50..f1b2c9f198d 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr
@@ -9,7 +9,7 @@ note: External requirements
    |
 LL |       establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
    |  _______________________________________________^
-LL | |         //~^ ERROR argument requires that data must outlive free region
+LL | |         //~^ ERROR
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
 LL | |         //~^ WARNING not reporting region error due to nll
@@ -23,23 +23,23 @@ LL | |     });
    = note: number of external vids: 3
    = note: where '_#1r: '_#0r
 
-error: argument requires that data must outlive free region `ReStatic`
+error: unsatisfied lifetime constraints
   --> $DIR/propagate-approximated-shorter-to-static-wrong-bound.rs:48:5
    |
 LL | /     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
-LL | |         //~^ ERROR argument requires that data must outlive free region
+LL | |         //~^ ERROR
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
 LL | |         //~^ WARNING not reporting region error due to nll
 LL | |     });
-   | |______^
+   | |______^ argument requires that `'a` must outlive `'static`
 
 note: No external requirements
   --> $DIR/propagate-approximated-shorter-to-static-wrong-bound.rs:47:1
    |
 LL | / fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
 LL | |     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
-LL | |         //~^ ERROR argument requires that data must outlive free region
+LL | |         //~^ ERROR
 LL | |         // Only works if 'x: 'y:
 ...  |
 LL | |     });
diff --git a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.rs b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.rs
index 316268e7e72..4b1f5231b3e 100644
--- a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.rs
+++ b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.rs
@@ -46,7 +46,7 @@ fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
         // Only works if 'x: 'y:
         demand_y(x, y, x.get())
         //~^ WARN not reporting region error due to nll
-        //~| ERROR argument requires that data must outlive free region
+        //~| ERROR
     });
 }
 
diff --git a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.stderr b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.stderr
index 74c0576e03b..fb98c506c7d 100644
--- a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.stderr
@@ -4,11 +4,16 @@ warning: not reporting region error due to nll
 LL |         demand_y(x, y, x.get())
    |         ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: argument requires that data must outlive free region `ReFree(DefId(0/1:18 ~ propagate_fail_to_approximate_longer_no_bounds[317d]::supply[0]::{{closure}}[0]), BrAnon(2))`
+error: unsatisfied lifetime constraints
   --> $DIR/propagate-fail-to-approximate-longer-no-bounds.rs:47:9
    |
+LL |     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
+   |                                                ---------  - lifetime `'1` appears in this argument
+   |                                                |
+   |                                                lifetime `'2` appears in this argument
+LL |         // Only works if 'x: 'y:
 LL |         demand_y(x, y, x.get())
-   |         ^^^^^^^^^^^^^^^^^^^^^^^
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 note: No external requirements
   --> $DIR/propagate-fail-to-approximate-longer-no-bounds.rs:45:47
@@ -18,7 +23,7 @@ LL |       establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
 LL | |         //~^ WARN not reporting region error due to nll
-LL | |         //~| ERROR argument requires that data must outlive free region
+LL | |         //~| ERROR
 LL | |     });
    | |_____^
    |
diff --git a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.rs b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.rs
index afb61b221be..62a20c1bfe7 100644
--- a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.rs
+++ b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.rs
@@ -50,7 +50,7 @@ fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
         // Only works if 'x: 'y:
         demand_y(x, y, x.get())
         //~^ WARN not reporting region error due to nll
-        //~| ERROR does not outlive free region
+        //~| ERROR
     });
 }
 
diff --git a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.stderr b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.stderr
index 2fd6ce50095..73d39a8502b 100644
--- a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.stderr
@@ -4,11 +4,16 @@ warning: not reporting region error due to nll
 LL |         demand_y(x, y, x.get())
    |         ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: free region `ReFree(DefId(0/1:18 ~ propagate_fail_to_approximate_longer_wrong_bounds[317d]::supply[0]::{{closure}}[0]), BrAnon(2))` does not outlive free region `ReFree(DefId(0/1:18 ~ propagate_fail_to_approximate_longer_wrong_bounds[317d]::supply[0]::{{closure}}[0]), BrAnon(4))`
-  --> $DIR/propagate-fail-to-approximate-longer-wrong-bounds.rs:51:18
+error: unsatisfied lifetime constraints
+  --> $DIR/propagate-fail-to-approximate-longer-wrong-bounds.rs:51:9
    |
+LL |     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
+   |                                                ----------  ---------- lifetime `'2` appears in this argument
+   |                                                |
+   |                                                lifetime `'1` appears in this argument
+LL |         // Only works if 'x: 'y:
 LL |         demand_y(x, y, x.get())
-   |                  ^
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 note: No external requirements
   --> $DIR/propagate-fail-to-approximate-longer-wrong-bounds.rs:49:47
@@ -18,7 +23,7 @@ LL |       establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x,
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
 LL | |         //~^ WARN not reporting region error due to nll
-LL | |         //~| ERROR does not outlive free region
+LL | |         //~| ERROR
 LL | |     });
    | |_____^
    |
diff --git a/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.rs b/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.rs
index dedbd8df41b..b5e8c95bc13 100644
--- a/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.rs
+++ b/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.rs
@@ -18,7 +18,7 @@
 fn foo<'a>(x: &'a u32) -> &'static u32 {
     &*x
         //~^ WARN not reporting region error due to nll
-        //~| ERROR does not outlive free region
+        //~| ERROR
 }
 
 fn main() { }
diff --git a/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.stderr b/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.stderr
index 9520b446303..d012dca2527 100644
--- a/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.stderr
+++ b/src/test/ui/nll/closure-requirements/region-lbr-named-does-not-outlive-static.stderr
@@ -4,11 +4,11 @@ warning: not reporting region error due to nll
 LL |     &*x
    |     ^^^
 
-error: free region `ReFree(DefId(0/0:3 ~ region_lbr_named_does_not_outlive_static[317d]::foo[0]), BrNamed(crate0:DefIndex(1:9), 'a))` does not outlive free region `ReStatic`
+error: unsatisfied lifetime constraints
   --> $DIR/region-lbr-named-does-not-outlive-static.rs:19:5
    |
 LL |     &*x
-   |     ^^^
+   |     ^^^ free region requires that `'a` must outlive `'static`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/closure-requirements/return-wrong-bound-region.rs b/src/test/ui/nll/closure-requirements/return-wrong-bound-region.rs
index 60f82ca0eef..d88729d04d9 100644
--- a/src/test/ui/nll/closure-requirements/return-wrong-bound-region.rs
+++ b/src/test/ui/nll/closure-requirements/return-wrong-bound-region.rs
@@ -20,7 +20,7 @@
 fn test() {
     expect_sig(|a, b| b); // ought to return `a`
     //~^ WARN not reporting region error due to nll
-    //~| ERROR does not outlive free region
+    //~| ERROR
 }
 
 fn expect_sig<F>(f: F) -> F
diff --git a/src/test/ui/nll/closure-requirements/return-wrong-bound-region.stderr b/src/test/ui/nll/closure-requirements/return-wrong-bound-region.stderr
index 04ff4aaadf8..5724cdbd8c9 100644
--- a/src/test/ui/nll/closure-requirements/return-wrong-bound-region.stderr
+++ b/src/test/ui/nll/closure-requirements/return-wrong-bound-region.stderr
@@ -4,11 +4,14 @@ warning: not reporting region error due to nll
 LL |     expect_sig(|a, b| b); // ought to return `a`
    |                       ^
 
-error: free region `ReFree(DefId(0/1:9 ~ return_wrong_bound_region[317d]::test[0]::{{closure}}[0]), BrAnon(2))` does not outlive free region `ReFree(DefId(0/1:9 ~ return_wrong_bound_region[317d]::test[0]::{{closure}}[0]), BrAnon(1))`
+error: unsatisfied lifetime constraints
   --> $DIR/return-wrong-bound-region.rs:21:23
    |
 LL |     expect_sig(|a, b| b); // ought to return `a`
-   |                       ^
+   |                 -  -  ^ free region requires that `'1` must outlive `'2`
+   |                 |  |
+   |                 |  lifetime `'1` appears in this argument
+   |                 lifetime `'2` appears in this argument
 
 note: No external requirements
   --> $DIR/return-wrong-bound-region.rs:21:16
@@ -27,7 +30,7 @@ note: No external requirements
 LL | / fn test() {
 LL | |     expect_sig(|a, b| b); // ought to return `a`
 LL | |     //~^ WARN not reporting region error due to nll
-LL | |     //~| ERROR does not outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
diff --git a/src/test/ui/nll/issue-48238.rs b/src/test/ui/nll/issue-48238.rs
index 6f7644da3d3..7c5527ae1e7 100644
--- a/src/test/ui/nll/issue-48238.rs
+++ b/src/test/ui/nll/issue-48238.rs
@@ -18,5 +18,5 @@ fn use_val<'a>(val: &'a u8) -> &'a u8 {
 
 fn main() {
     let orig: u8 = 5;
-    move || use_val(&orig); //~ ERROR free region `` does not outlive free region `'_#1r`
+    move || use_val(&orig); //~ ERROR
 }
diff --git a/src/test/ui/nll/issue-48238.stderr b/src/test/ui/nll/issue-48238.stderr
index 29385d9b243..7bdac345e90 100644
--- a/src/test/ui/nll/issue-48238.stderr
+++ b/src/test/ui/nll/issue-48238.stderr
@@ -1,8 +1,12 @@
-error: free region `` does not outlive free region `'_#1r`
-  --> $DIR/issue-48238.rs:21:21
+error: unsatisfied lifetime constraints
+  --> $DIR/issue-48238.rs:21:13
    |
-LL |     move || use_val(&orig); //~ ERROR free region `` does not outlive free region `'_#1r`
-   |                     ^^^^^
+LL |     move || use_val(&orig); //~ ERROR
+   |     --------^^^^^^^^^^^^^^
+   |     |       |
+   |     |       argument requires that `'1` must outlive `'2`
+   |     lifetime `'1` represents the closure body
+   |     lifetime `'2` appears in return type
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/issue-50716.rs b/src/test/ui/nll/issue-50716.rs
index a41af168c3e..beb2ff79e90 100644
--- a/src/test/ui/nll/issue-50716.rs
+++ b/src/test/ui/nll/issue-50716.rs
@@ -22,7 +22,7 @@ where
     for<'b> &'b T: A,
     <&'static T as A>::X: Sized
 {
-    let _x = *s; //~ ERROR assignment requires that data must outlive free region `'static`
+    let _x = *s; //~ ERROR
 }
 
 fn main() {}
diff --git a/src/test/ui/nll/issue-50716.stderr b/src/test/ui/nll/issue-50716.stderr
index de69f8cfbcb..48862166f89 100644
--- a/src/test/ui/nll/issue-50716.stderr
+++ b/src/test/ui/nll/issue-50716.stderr
@@ -1,8 +1,8 @@
-error: assignment requires that data must outlive free region `'static`
+error: unsatisfied lifetime constraints
   --> $DIR/issue-50716.rs:25:14
    |
-LL |     let _x = *s; //~ ERROR assignment requires that data must outlive free region `'static`
-   |              ^^
+LL |     let _x = *s; //~ ERROR
+   |              ^^ assignment requires that `'a` must outlive `'static`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/ty-outlives/projection-one-region-closure.rs b/src/test/ui/nll/ty-outlives/projection-one-region-closure.rs
index 3832be6288a..0a8801a3c4d 100644
--- a/src/test/ui/nll/ty-outlives/projection-one-region-closure.rs
+++ b/src/test/ui/nll/ty-outlives/projection-one-region-closure.rs
@@ -55,7 +55,7 @@ where
     with_signature(cell, t, |cell, t| require(cell, t));
     //~^ WARNING not reporting region error due to nll
     //~| ERROR the parameter type `T` may not live long enough
-    //~| ERROR argument requires that data must outlive free region
+    //~| ERROR
 }
 
 #[rustc_regions]
@@ -67,7 +67,7 @@ where
     with_signature(cell, t, |cell, t| require(cell, t));
     //~^ WARNING not reporting region error due to nll
     //~| ERROR the parameter type `T` may not live long enough
-    //~| ERROR argument requires that data must outlive free region
+    //~| ERROR
 }
 
 #[rustc_regions]
@@ -89,7 +89,7 @@ where
     with_signature(cell, t, |cell, t| require(cell, t));
     //~^ WARNING not reporting region error due to nll
     //~| ERROR the parameter type `T` may not live long enough
-    //~| ERROR argument requires that data must outlive free region
+    //~| ERROR
 }
 
 #[rustc_regions]
diff --git a/src/test/ui/nll/ty-outlives/projection-one-region-closure.stderr b/src/test/ui/nll/ty-outlives/projection-one-region-closure.stderr
index 898c995e09b..b84ee4de5a3 100644
--- a/src/test/ui/nll/ty-outlives/projection-one-region-closure.stderr
+++ b/src/test/ui/nll/ty-outlives/projection-one-region-closure.stderr
@@ -40,11 +40,11 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    |
    = help: consider adding an explicit lifetime bound `T: ReFree(DefId(0/0:8 ~ projection_one_region_closure[317d]::no_relationships_late[0]), BrNamed(crate0:DefIndex(1:16), 'a))`...
 
-error: argument requires that data must outlive free region `ReFree(DefId(0/0:8 ~ projection_one_region_closure[317d]::no_relationships_late[0]), BrNamed(crate0:DefIndex(1:16), 'a))`
+error: unsatisfied lifetime constraints
   --> $DIR/projection-one-region-closure.rs:55:5
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
 
 note: No external requirements
   --> $DIR/projection-one-region-closure.rs:51:1
@@ -54,7 +54,7 @@ LL | | where
 LL | |     T: Anything<'b>,
 LL | | {
 ...  |
-LL | |     //~| ERROR argument requires that data must outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
@@ -88,11 +88,11 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    |
    = help: consider adding an explicit lifetime bound `T: ReEarlyBound(0, 'a)`...
 
-error: argument requires that data must outlive free region `ReEarlyBound(0, 'a)`
+error: unsatisfied lifetime constraints
   --> $DIR/projection-one-region-closure.rs:67:5
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
 
 note: No external requirements
   --> $DIR/projection-one-region-closure.rs:62:1
@@ -102,7 +102,7 @@ LL | | where
 LL | |     T: Anything<'b>,
 LL | |     'a: 'a,
 ...  |
-LL | |     //~| ERROR argument requires that data must outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
@@ -137,11 +137,11 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    |
    = help: consider adding an explicit lifetime bound `T: ReEarlyBound(0, 'a)`...
 
-error: argument requires that data must outlive free region `ReEarlyBound(0, 'a)`
+error: unsatisfied lifetime constraints
   --> $DIR/projection-one-region-closure.rs:89:5
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
 
 note: No external requirements
   --> $DIR/projection-one-region-closure.rs:74:1
@@ -151,7 +151,7 @@ LL | | where
 LL | |     T: Anything<'b>,
 LL | |     T::AssocType: 'a,
 ...  |
-LL | |     //~| ERROR argument requires that data must outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
diff --git a/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.rs b/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.rs
index da76193459b..6317d6eca1f 100644
--- a/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.rs
+++ b/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.rs
@@ -46,7 +46,7 @@ where
 {
     with_signature(cell, t, |cell, t| require(cell, t));
     //~^ WARNING not reporting region error due to nll
-    //~| ERROR argument requires that data must outlive free region
+    //~| ERROR
 }
 
 #[rustc_regions]
@@ -57,7 +57,7 @@ where
 {
     with_signature(cell, t, |cell, t| require(cell, t));
     //~^ WARNING not reporting region error due to nll
-    //~| ERROR argument requires that data must outlive free region
+    //~| ERROR
 }
 
 #[rustc_regions]
@@ -78,7 +78,7 @@ where
 
     with_signature(cell, t, |cell, t| require(cell, t));
     //~^ WARNING not reporting region error due to nll
-    //~| ERROR argument requires that data must outlive free region
+    //~| ERROR
 }
 
 #[rustc_regions]
diff --git a/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.stderr b/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.stderr
index be11c6249f0..42ecd6f9860 100644
--- a/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.stderr
+++ b/src/test/ui/nll/ty-outlives/projection-one-region-trait-bound-closure.stderr
@@ -31,11 +31,11 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    = note: number of external vids: 3
    = note: where '_#1r: '_#2r
 
-error: argument requires that data must outlive free region `ReFree(DefId(0/0:8 ~ projection_one_region_trait_bound_closure[317d]::no_relationships_late[0]), BrNamed(crate0:DefIndex(1:16), 'a))`
+error: unsatisfied lifetime constraints
   --> $DIR/projection-one-region-trait-bound-closure.rs:47:5
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
 
 note: No external requirements
   --> $DIR/projection-one-region-trait-bound-closure.rs:43:1
@@ -45,7 +45,7 @@ LL | | where
 LL | |     T: Anything<'b>,
 LL | | {
 ...  |
-LL | |     //~| ERROR argument requires that data must outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
@@ -70,11 +70,11 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    = note: number of external vids: 4
    = note: where '_#2r: '_#3r
 
-error: argument requires that data must outlive free region `ReEarlyBound(0, 'a)`
+error: unsatisfied lifetime constraints
   --> $DIR/projection-one-region-trait-bound-closure.rs:58:5
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
 
 note: No external requirements
   --> $DIR/projection-one-region-trait-bound-closure.rs:53:1
@@ -84,7 +84,7 @@ LL | | where
 LL | |     T: Anything<'b>,
 LL | |     'a: 'a,
 ...  |
-LL | |     //~| ERROR argument requires that data must outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
@@ -110,11 +110,11 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    = note: number of external vids: 4
    = note: where '_#2r: '_#3r
 
-error: argument requires that data must outlive free region `ReEarlyBound(0, 'a)`
+error: unsatisfied lifetime constraints
   --> $DIR/projection-one-region-trait-bound-closure.rs:79:5
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
 
 note: No external requirements
   --> $DIR/projection-one-region-trait-bound-closure.rs:64:1
@@ -124,7 +124,7 @@ LL | | where
 LL | |     T: Anything<'b>,
 LL | |     T::AssocType: 'a,
 ...  |
-LL | |     //~| ERROR argument requires that data must outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
diff --git a/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.rs b/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.rs
index 5e481a9626f..2552054fd5f 100644
--- a/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.rs
+++ b/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.rs
@@ -107,7 +107,7 @@ where
 {
     with_signature(cell, t, |cell, t| require(cell, t));
     //~^ WARNING not reporting region error due to nll
-    //~| ERROR argument requires that data must outlive free region
+    //~| ERROR
 }
 
 #[rustc_regions]
diff --git a/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.stderr b/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.stderr
index 546384e8545..b0f823ad3d5 100644
--- a/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.stderr
+++ b/src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.stderr
@@ -239,11 +239,11 @@ LL |     with_signature(cell, t, |cell, t| require(cell, t));
    = note: number of external vids: 3
    = note: where <T as Anything<ReClosureBound('_#1r), ReClosureBound('_#1r)>>::AssocType: '_#2r
 
-error: argument requires that data must outlive free region `ReFree(DefId(0/0:13 ~ projection_two_region_trait_bound_closure[317d]::two_regions[0]), BrNamed(crate0:DefIndex(1:43), 'a))`
+error: unsatisfied lifetime constraints
   --> $DIR/projection-two-region-trait-bound-closure.rs:108:5
    |
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
 
 note: No external requirements
   --> $DIR/projection-two-region-trait-bound-closure.rs:104:1
@@ -253,7 +253,7 @@ LL | | where
 LL | |     T: Anything<'b, 'b>,
 LL | | {
 ...  |
-LL | |     //~| ERROR argument requires that data must outlive free region
+LL | |     //~| ERROR
 LL | | }
    | |_^
    |
diff --git a/src/test/ui/underscore-lifetime/dyn-trait-underscore.nll.stderr b/src/test/ui/underscore-lifetime/dyn-trait-underscore.nll.stderr
index 49f9044b19b..04c3ed2d6ee 100644
--- a/src/test/ui/underscore-lifetime/dyn-trait-underscore.nll.stderr
+++ b/src/test/ui/underscore-lifetime/dyn-trait-underscore.nll.stderr
@@ -22,11 +22,14 @@ warning: not reporting region error due to nll
 LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
-error: cast requires that data must outlive free region `'static`
+error: unsatisfied lifetime constraints
   --> $DIR/dyn-trait-underscore.rs:18:5
    |
+LL | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
+   |         ----- lifetime `'1` appears in this argument
+LL |     //                      ^^^^^^^^^^^^^^^^^^^^^ bound *here* defaults to `'static`
 LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
-   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^ cast requires that `'1` must outlive `'static`
 
 error: aborting due to previous error