Rollup merge of #66679 - mark-i-m:fix-anon-lifetime-errors, r=matthewjasper

Improve lifetime errors with implicit trait object lifetimes

r? @matthewjasper

cc @estebank

I still think the ideal solution would be to construct a `BrAnon`, but that seems like a more invasive change, and can be done later. This at least gets rid of the hack in `OutliveSuggestion` and is slightly more principled.

10 files changed, 120 insertions, 165 deletions

diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index aa5b1c7315a..6cb0d1e9946 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -69,7 +69,7 @@ pub enum BoundRegion {
 impl BoundRegion {
     pub fn is_named(&self) -> bool {
         match *self {
-            BoundRegion::BrNamed(..) => true,
+            BoundRegion::BrNamed(_, name) => name != kw::UnderscoreLifetime,
             _ => false,
         }
     }
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/outlives_suggestion.rs b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/outlives_suggestion.rs
index c0cf4eb5285..938059c2a92 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/outlives_suggestion.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/outlives_suggestion.rs
@@ -78,17 +78,7 @@ impl OutlivesSuggestionBuilder<'a> {
         match name.source {
             RegionNameSource::NamedEarlyBoundRegion(..)
             | RegionNameSource::NamedFreeRegion(..)
-            | RegionNameSource::Static => {
-                // FIXME: This is a bit hacky. We should ideally have a semantic way for checking
-                // if the name is `'_`...
-                if name.name().with(|name| name != "'_") {
-                    debug!("Region {:?} is suggestable", name);
-                    true
-                } else {
-                    debug!("Region {:?} is NOT suggestable", name);
-                    false
-                }
-            }
+            | RegionNameSource::Static => true,
 
             // Don't give suggestions for upvars, closure return types, or other unnamable
             // regions.
@@ -98,7 +88,8 @@ impl OutlivesSuggestionBuilder<'a> {
             | RegionNameSource::MatchedAdtAndSegment(..)
             | RegionNameSource::AnonRegionFromUpvar(..)
             | RegionNameSource::AnonRegionFromOutput(..)
-            | RegionNameSource::AnonRegionFromYieldTy(..) => {
+            | RegionNameSource::AnonRegionFromYieldTy(..)
+            | RegionNameSource::AnonRegionFromAsyncFn(..) => {
                 debug!("Region {:?} is NOT suggestable", name);
                 false
             }
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
index e59928987a0..0f5d1c5edc4 100644
--- 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
@@ -13,13 +13,13 @@ use rustc::hir::def_id::DefId;
 use rustc::infer::InferCtxt;
 use rustc::mir::{Local, Body};
 use rustc::ty::subst::{SubstsRef, GenericArgKind};
-use rustc::ty::{self, RegionKind, RegionVid, Ty, TyCtxt};
+use rustc::ty::{self, RegionVid, Ty, TyCtxt};
 use rustc::ty::print::RegionHighlightMode;
 use rustc_index::vec::IndexVec;
 use rustc_errors::DiagnosticBuilder;
 use syntax::symbol::kw;
 use rustc_data_structures::fx::FxHashMap;
-use syntax_pos::{Span, symbol::Symbol};
+use syntax_pos::{Span, symbol::Symbol, DUMMY_SP};
 
 /// A name for a particular region used in emitting diagnostics. This name could be a generated
 /// name like `'1`, a name used by the user like `'a`, or a name like `'static`.
@@ -55,7 +55,10 @@ crate enum RegionNameSource {
     AnonRegionFromUpvar(Span, String),
     /// The region corresponding to the return type of a closure.
     AnonRegionFromOutput(Span, String, String),
+    /// The region from a type yielded by a generator.
     AnonRegionFromYieldTy(Span, String),
+    /// An anonymous region from an async fn.
+    AnonRegionFromAsyncFn(Span),
 }
 
 /// Records region names that have been assigned before so that we can use the same ones in later
@@ -113,14 +116,11 @@ impl RegionName {
             RegionNameSource::MatchedAdtAndSegment(..) |
             RegionNameSource::AnonRegionFromUpvar(..) |
             RegionNameSource::AnonRegionFromOutput(..) |
-            RegionNameSource::AnonRegionFromYieldTy(..) => false,
+            RegionNameSource::AnonRegionFromYieldTy(..) |
+            RegionNameSource::AnonRegionFromAsyncFn(..) => false,
         }
     }
 
-    crate fn name(&self) -> Symbol {
-        self.name
-    }
-
     crate fn highlight_region_name(&self, diag: &mut DiagnosticBuilder<'_>) {
         match &self.source {
             RegionNameSource::NamedFreeRegion(span)
@@ -137,7 +137,8 @@ impl RegionName {
             RegionNameSource::CannotMatchHirTy(span, type_name) => {
                 diag.span_label(*span, format!("has type `{}`", type_name));
             }
-            RegionNameSource::MatchedHirTy(span) => {
+            RegionNameSource::MatchedHirTy(span) |
+            RegionNameSource::AnonRegionFromAsyncFn(span) => {
                 diag.span_label(
                     *span,
                     format!("let's call the lifetime of this reference `{}`", self),
@@ -270,7 +271,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         match error_region {
             ty::ReEarlyBound(ebr) => {
                 if ebr.has_name() {
-                    let span = self.get_named_span(tcx, error_region, ebr.name);
+                    let span = tcx.hir().span_if_local(ebr.def_id).unwrap_or(DUMMY_SP);
                     Some(RegionName {
                         name: ebr.name,
                         source: RegionNameSource::NamedEarlyBoundRegion(span),
@@ -286,12 +287,30 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             }),
 
             ty::ReFree(free_region) => match free_region.bound_region {
-                ty::BoundRegion::BrNamed(_, name) => {
-                    let span = self.get_named_span(tcx, error_region, name);
-                    Some(RegionName {
-                        name,
-                        source: RegionNameSource::NamedFreeRegion(span),
-                    })
+                ty::BoundRegion::BrNamed(region_def_id, name) => {
+                    // Get the span to point to, even if we don't use the name.
+                    let span = tcx.hir().span_if_local(region_def_id).unwrap_or(DUMMY_SP);
+                    debug!("bound region named: {:?}, is_named: {:?}",
+                        name, free_region.bound_region.is_named());
+
+                    if free_region.bound_region.is_named() {
+                        // A named region that is actually named.
+                        Some(RegionName {
+                            name,
+                            source: RegionNameSource::NamedFreeRegion(span),
+                        })
+                    } else {
+                        // If we spuriously thought that the region is named, we should let the
+                        // system generate a true name for error messages. Currently this can
+                        // happen if we have an elided name in an async fn for example: the
+                        // compiler will generate a region named `'_`, but reporting such a name is
+                        // not actually useful, so we synthesize a name for it instead.
+                        let name = renctx.synthesize_region_name();
+                        Some(RegionName {
+                            name,
+                            source: RegionNameSource::AnonRegionFromAsyncFn(span),
+                        })
+                    }
                 }
 
                 ty::BoundRegion::BrEnv => {
@@ -350,40 +369,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         }
     }
 
-    /// Gets a span of a named region to provide context for error messages that
-    /// mention that span, for example:
-    ///
-    /// ```
-    ///  |
-    ///  | fn two_regions<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
-    ///  |                --  -- lifetime `'b` defined here
-    ///  |                |
-    ///  |                lifetime `'a` defined here
-    ///  |
-    ///  |     with_signature(cell, t, |cell, t| require(cell, t));
-    ///  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must
-    ///  |                                                         outlive `'a`
-    /// ```
-    fn get_named_span(
-        &self,
-        tcx: TyCtxt<'tcx>,
-        error_region: &RegionKind,
-        name: Symbol,
-    ) -> Span {
-        let scope = error_region.free_region_binding_scope(tcx);
-        let node = tcx.hir().as_local_hir_id(scope).unwrap_or(hir::DUMMY_HIR_ID);
-
-        let span = tcx.sess.source_map().def_span(tcx.hir().span(node));
-        if let Some(param) = tcx.hir()
-            .get_generics(scope)
-            .and_then(|generics| generics.get_named(name))
-        {
-            param.span
-        } else {
-            span
-        }
-    }
-
     /// Finds an argument that contains `fr` and label it with a fully
     /// elaborated type, returning something like `'1`. Result looks
     /// like:
diff --git a/src/test/ui/async-await/issues/issue-63388-1.nll.stderr b/src/test/ui/async-await/issues/issue-63388-1.nll.stderr
index 22610fe54a4..4ae3971e90e 100644
--- a/src/test/ui/async-await/issues/issue-63388-1.nll.stderr
+++ b/src/test/ui/async-await/issues/issue-63388-1.nll.stderr
@@ -12,12 +12,12 @@ error: lifetime may not live long enough
 LL |       async fn do_sth<'a>(
    |                       -- lifetime `'a` defined here
 LL |           &'a self, foo: &dyn Foo
-   |                          - lifetime `'_` defined here
+   |                          - let's call the lifetime of this reference `'1`
 LL |       ) -> &dyn Foo
 LL | /     {
 LL | |         foo
 LL | |     }
-   | |_____^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'_`
+   | |_____^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'1`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/self/arbitrary_self_types_pin_lifetime_impl_trait-async.nll.stderr b/src/test/ui/self/arbitrary_self_types_pin_lifetime_impl_trait-async.nll.stderr
index 713d4b75909..5a63553adc5 100644
--- a/src/test/ui/self/arbitrary_self_types_pin_lifetime_impl_trait-async.nll.stderr
+++ b/src/test/ui/self/arbitrary_self_types_pin_lifetime_impl_trait-async.nll.stderr
@@ -2,11 +2,11 @@ error: lifetime may not live long enough
   --> $DIR/arbitrary_self_types_pin_lifetime_impl_trait-async.rs:8:48
    |
 LL |     async fn f(self: Pin<&Self>) -> impl Clone { self }
-   |                          -                     ^^^^^^^^ returning this value requires that `'_` must outlive `'static`
+   |                          -                     ^^^^^^^^ returning this value requires that `'1` must outlive `'static`
    |                          |
-   |                          lifetime `'_` defined here
+   |                          let's call the lifetime of this reference `'1`
    |
-help: to allow this `impl Trait` to capture borrowed data with lifetime `'_`, add `'_` as a constraint
+help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as a constraint
    |
 LL |     async fn f(self: Pin<&Self>) -> impl Clone + '_ { self }
    |                                     ^^^^^^^^^^^^^^^
diff --git a/src/test/ui/self/arbitrary_self_types_pin_lifetime_mismatch-async.nll.stderr b/src/test/ui/self/arbitrary_self_types_pin_lifetime_mismatch-async.nll.stderr
index 7eec31d36e3..b05940fd273 100644
--- a/src/test/ui/self/arbitrary_self_types_pin_lifetime_mismatch-async.nll.stderr
+++ b/src/test/ui/self/arbitrary_self_types_pin_lifetime_mismatch-async.nll.stderr
@@ -10,19 +10,19 @@ error: lifetime may not live long enough
   --> $DIR/arbitrary_self_types_pin_lifetime_mismatch-async.rs:8:52
    |
 LL |     async fn a(self: Pin<&Foo>, f: &Foo) -> &Foo { f }
-   |                          -                         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
-   |                          |
-   |                          lifetime `'_` defined here
-   |                          lifetime `'_` defined here
+   |                          -         -               ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
+   |                          |         |
+   |                          |         let's call the lifetime of this reference `'1`
+   |                          let's call the lifetime of this reference `'2`
 
 error: lifetime may not live long enough
   --> $DIR/arbitrary_self_types_pin_lifetime_mismatch-async.rs:11:75
    |
 LL |     async fn c(self: Pin<&Self>, f: &Foo, g: &Foo) -> (Pin<&Foo>, &Foo) { (self, f) }
-   |                          -                                                ^^^^^^^^^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
-   |                          |
-   |                          lifetime `'_` defined here
-   |                          lifetime `'_` defined here
+   |                          -          -                                     ^^^^^^^^^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
+   |                          |          |
+   |                          |          let's call the lifetime of this reference `'1`
+   |                          let's call the lifetime of this reference `'2`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/arbitrary_self_types_pin_lifetime_mismatch-async.rs:17:58
@@ -36,8 +36,9 @@ error: lifetime may not live long enough
   --> $DIR/arbitrary_self_types_pin_lifetime_mismatch-async.rs:17:64
    |
 LL |     async fn bar<'a>(self: Alias<&Self>, arg: &'a ()) -> &() { arg }
-   |                  --              - lifetime `'_` defined here  ^^^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'a`
-   |                  |
+   |                  --              -                             ^^^ function was supposed to return data with lifetime `'1` but it is returning data with lifetime `'a`
+   |                  |               |
+   |                  |               let's call the lifetime of this reference `'1`
    |                  lifetime `'a` defined here
 
 error: aborting due to 5 previous errors
diff --git a/src/test/ui/self/elision/lt-ref-self-async.nll.stderr b/src/test/ui/self/elision/lt-ref-self-async.nll.stderr
index 1288759703f..8dd823a2204 100644
--- a/src/test/ui/self/elision/lt-ref-self-async.nll.stderr
+++ b/src/test/ui/self/elision/lt-ref-self-async.nll.stderr
@@ -10,12 +10,11 @@ error: lifetime may not live long enough
   --> $DIR/lt-ref-self-async.rs:13:9
    |
 LL |     async fn ref_self(&self, f: &u32) -> &u32 {
-   |                       -
+   |                       -         - let's call the lifetime of this reference `'1`
    |                       |
-   |                       lifetime `'_` defined here
-   |                       lifetime `'_` defined here
+   |                       let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/lt-ref-self-async.rs:18:48
@@ -29,12 +28,11 @@ error: lifetime may not live long enough
   --> $DIR/lt-ref-self-async.rs:19:9
    |
 LL |     async fn ref_Self(self: &Self, f: &u32) -> &u32 {
-   |                             -
+   |                             -         - let's call the lifetime of this reference `'1`
    |                             |
-   |                             lifetime `'_` defined here
-   |                             lifetime `'_` defined here
+   |                             let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/lt-ref-self-async.rs:22:57
@@ -48,12 +46,11 @@ error: lifetime may not live long enough
   --> $DIR/lt-ref-self-async.rs:23:9
    |
 LL |     async fn box_ref_Self(self: Box<&Self>, f: &u32) -> &u32 {
-   |                                     -
+   |                                     -          - let's call the lifetime of this reference `'1`
    |                                     |
-   |                                     lifetime `'_` defined here
-   |                                     lifetime `'_` defined here
+   |                                     let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/lt-ref-self-async.rs:26:57
@@ -67,12 +64,11 @@ error: lifetime may not live long enough
   --> $DIR/lt-ref-self-async.rs:27:9
    |
 LL |     async fn pin_ref_Self(self: Pin<&Self>, f: &u32) -> &u32 {
-   |                                     -
+   |                                     -          - let's call the lifetime of this reference `'1`
    |                                     |
-   |                                     lifetime `'_` defined here
-   |                                     lifetime `'_` defined here
+   |                                     let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/lt-ref-self-async.rs:30:66
@@ -86,12 +82,11 @@ error: lifetime may not live long enough
   --> $DIR/lt-ref-self-async.rs:31:9
    |
 LL |     async fn box_box_ref_Self(self: Box<Box<&Self>>, f: &u32) -> &u32 {
-   |                                             -
+   |                                             -           - let's call the lifetime of this reference `'1`
    |                                             |
-   |                                             lifetime `'_` defined here
-   |                                             lifetime `'_` defined here
+   |                                             let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/lt-ref-self-async.rs:34:62
@@ -105,12 +100,11 @@ error: lifetime may not live long enough
   --> $DIR/lt-ref-self-async.rs:35:9
    |
 LL |     async fn box_pin_Self(self: Box<Pin<&Self>>, f: &u32) -> &u32 {
-   |                                         -
+   |                                         -           - let's call the lifetime of this reference `'1`
    |                                         |
-   |                                         lifetime `'_` defined here
-   |                                         lifetime `'_` defined here
+   |                                         let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error: aborting due to 12 previous errors
 
diff --git a/src/test/ui/self/elision/ref-mut-self-async.nll.stderr b/src/test/ui/self/elision/ref-mut-self-async.nll.stderr
index 24e3f7a098f..768f532c183 100644
--- a/src/test/ui/self/elision/ref-mut-self-async.nll.stderr
+++ b/src/test/ui/self/elision/ref-mut-self-async.nll.stderr
@@ -10,12 +10,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-self-async.rs:13:9
    |
 LL |     async fn ref_self(&mut self, f: &u32) -> &u32 {
-   |                       -
+   |                       -             - let's call the lifetime of this reference `'1`
    |                       |
-   |                       lifetime `'_` defined here
-   |                       lifetime `'_` defined here
+   |                       let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-self-async.rs:18:52
@@ -29,12 +28,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-self-async.rs:19:9
    |
 LL |     async fn ref_Self(self: &mut Self, f: &u32) -> &u32 {
-   |                             -
+   |                             -             - let's call the lifetime of this reference `'1`
    |                             |
-   |                             lifetime `'_` defined here
-   |                             lifetime `'_` defined here
+   |                             let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-self-async.rs:22:61
@@ -48,12 +46,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-self-async.rs:23:9
    |
 LL |     async fn box_ref_Self(self: Box<&mut Self>, f: &u32) -> &u32 {
-   |                                     -
+   |                                     -              - let's call the lifetime of this reference `'1`
    |                                     |
-   |                                     lifetime `'_` defined here
-   |                                     lifetime `'_` defined here
+   |                                     let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-self-async.rs:26:61
@@ -67,12 +64,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-self-async.rs:27:9
    |
 LL |     async fn pin_ref_Self(self: Pin<&mut Self>, f: &u32) -> &u32 {
-   |                                     -
+   |                                     -              - let's call the lifetime of this reference `'1`
    |                                     |
-   |                                     lifetime `'_` defined here
-   |                                     lifetime `'_` defined here
+   |                                     let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-self-async.rs:30:70
@@ -86,12 +82,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-self-async.rs:31:9
    |
 LL |     async fn box_box_ref_Self(self: Box<Box<&mut Self>>, f: &u32) -> &u32 {
-   |                                             -
+   |                                             -               - let's call the lifetime of this reference `'1`
    |                                             |
-   |                                             lifetime `'_` defined here
-   |                                             lifetime `'_` defined here
+   |                                             let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-self-async.rs:34:70
@@ -105,12 +100,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-self-async.rs:35:9
    |
 LL |     async fn box_pin_ref_Self(self: Box<Pin<&mut Self>>, f: &u32) -> &u32 {
-   |                                             -
+   |                                             -               - let's call the lifetime of this reference `'1`
    |                                             |
-   |                                             lifetime `'_` defined here
-   |                                             lifetime `'_` defined here
+   |                                             let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error: aborting due to 12 previous errors
 
diff --git a/src/test/ui/self/elision/ref-mut-struct-async.nll.stderr b/src/test/ui/self/elision/ref-mut-struct-async.nll.stderr
index c0423d1d3e6..9e26e411d30 100644
--- a/src/test/ui/self/elision/ref-mut-struct-async.nll.stderr
+++ b/src/test/ui/self/elision/ref-mut-struct-async.nll.stderr
@@ -10,12 +10,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-struct-async.rs:13:9
    |
 LL |     async fn ref_Struct(self: &mut Struct, f: &u32) -> &u32 {
-   |                               -
+   |                               -               - let's call the lifetime of this reference `'1`
    |                               |
-   |                               lifetime `'_` defined here
-   |                               lifetime `'_` defined here
+   |                               let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-struct-async.rs:16:65
@@ -29,12 +28,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-struct-async.rs:17:9
    |
 LL |     async fn box_ref_Struct(self: Box<&mut Struct>, f: &u32) -> &u32 {
-   |                                       -
+   |                                       -                - let's call the lifetime of this reference `'1`
    |                                       |
-   |                                       lifetime `'_` defined here
-   |                                       lifetime `'_` defined here
+   |                                       let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-struct-async.rs:20:65
@@ -48,12 +46,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-struct-async.rs:21:9
    |
 LL |     async fn pin_ref_Struct(self: Pin<&mut Struct>, f: &u32) -> &u32 {
-   |                                       -
+   |                                       -                - let's call the lifetime of this reference `'1`
    |                                       |
-   |                                       lifetime `'_` defined here
-   |                                       lifetime `'_` defined here
+   |                                       let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-struct-async.rs:24:74
@@ -67,12 +64,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-struct-async.rs:25:9
    |
 LL |     async fn box_box_ref_Struct(self: Box<Box<&mut Struct>>, f: &u32) -> &u32 {
-   |                                               -
+   |                                               -                 - let's call the lifetime of this reference `'1`
    |                                               |
-   |                                               lifetime `'_` defined here
-   |                                               lifetime `'_` defined here
+   |                                               let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-mut-struct-async.rs:28:74
@@ -86,12 +82,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-mut-struct-async.rs:29:9
    |
 LL |     async fn box_pin_ref_Struct(self: Box<Pin<&mut Struct>>, f: &u32) -> &u32 {
-   |                                               -
+   |                                               -                 - let's call the lifetime of this reference `'1`
    |                                               |
-   |                                               lifetime `'_` defined here
-   |                                               lifetime `'_` defined here
+   |                                               let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error: aborting due to 10 previous errors
 
diff --git a/src/test/ui/self/elision/ref-struct-async.nll.stderr b/src/test/ui/self/elision/ref-struct-async.nll.stderr
index 6f413a7f49f..cbf051205ed 100644
--- a/src/test/ui/self/elision/ref-struct-async.nll.stderr
+++ b/src/test/ui/self/elision/ref-struct-async.nll.stderr
@@ -10,12 +10,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-struct-async.rs:13:9
    |
 LL |     async fn ref_Struct(self: &Struct, f: &u32) -> &u32 {
-   |                               -
+   |                               -           - let's call the lifetime of this reference `'1`
    |                               |
-   |                               lifetime `'_` defined here
-   |                               lifetime `'_` defined here
+   |                               let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-struct-async.rs:16:61
@@ -29,12 +28,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-struct-async.rs:17:9
    |
 LL |     async fn box_ref_Struct(self: Box<&Struct>, f: &u32) -> &u32 {
-   |                                       -
+   |                                       -            - let's call the lifetime of this reference `'1`
    |                                       |
-   |                                       lifetime `'_` defined here
-   |                                       lifetime `'_` defined here
+   |                                       let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-struct-async.rs:20:61
@@ -48,12 +46,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-struct-async.rs:21:9
    |
 LL |     async fn pin_ref_Struct(self: Pin<&Struct>, f: &u32) -> &u32 {
-   |                                       -
+   |                                       -            - let's call the lifetime of this reference `'1`
    |                                       |
-   |                                       lifetime `'_` defined here
-   |                                       lifetime `'_` defined here
+   |                                       let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-struct-async.rs:24:70
@@ -67,12 +64,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-struct-async.rs:25:9
    |
 LL |     async fn box_box_ref_Struct(self: Box<Box<&Struct>>, f: &u32) -> &u32 {
-   |                                               -
+   |                                               -             - let's call the lifetime of this reference `'1`
    |                                               |
-   |                                               lifetime `'_` defined here
-   |                                               lifetime `'_` defined here
+   |                                               let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
   --> $DIR/ref-struct-async.rs:28:66
@@ -86,12 +82,11 @@ error: lifetime may not live long enough
   --> $DIR/ref-struct-async.rs:29:9
    |
 LL |     async fn box_pin_Struct(self: Box<Pin<&Struct>>, f: &u32) -> &u32 {
-   |                                           -
+   |                                           -             - let's call the lifetime of this reference `'1`
    |                                           |
-   |                                           lifetime `'_` defined here
-   |                                           lifetime `'_` defined here
+   |                                           let's call the lifetime of this reference `'2`
 LL |         f
-   |         ^ function was supposed to return data with lifetime `'_` but it is returning data with lifetime `'_`
+   |         ^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
 
 error: aborting due to 10 previous errors