Store Option<Region> as value for RegionVid

14 files changed, 170 insertions, 82 deletions

diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index c68705da413..27ac817463f 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -304,6 +304,15 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
     }
 
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        let tcx = self.tcx();
+        let r = self
+            .infcx
+            .unwrap()
+            .inner
+            .borrow_mut()
+            .unwrap_region_constraints()
+            .opportunistic_resolve_region(tcx, r);
+
         match *r {
             ty::ReLateBound(index, ..) => {
                 if index >= self.binder_index {
@@ -313,22 +322,7 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
                 }
             }
 
-            ty::ReVar(vid) => {
-                let resolved_vid = self
-                    .infcx
-                    .unwrap()
-                    .inner
-                    .borrow_mut()
-                    .unwrap_region_constraints()
-                    .opportunistic_resolve_var(vid);
-                debug!(
-                    "canonical: region var found with vid {:?}, \
-                     opportunistically resolved to {:?}",
-                    vid, r
-                );
-                let r = self.tcx.reuse_or_mk_region(r, ty::ReVar(resolved_vid));
-                self.canonicalize_region_mode.canonicalize_free_region(self, r)
-            }
+            ty::ReVar(_) => self.canonicalize_region_mode.canonicalize_free_region(self, r),
 
             ty::ReStatic
             | ty::ReEarlyBound(..)
diff --git a/compiler/rustc_infer/src/infer/region_constraints/mod.rs b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
index abbcff30cda..dfbb82bd657 100644
--- a/compiler/rustc_infer/src/infer/region_constraints/mod.rs
+++ b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
@@ -11,9 +11,9 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::undo_log::UndoLogs;
 use rustc_data_structures::unify as ut;
-use rustc_data_structures::unify::UnifyKey;
 use rustc_hir::def_id::DefId;
 use rustc_index::vec::IndexVec;
+use rustc_middle::infer::unify_key::{RegionVidKey, UnifiedRegion};
 use rustc_middle::ty::ReStatic;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{ReLateBound, ReVar};
@@ -47,13 +47,13 @@ pub struct RegionConstraintStorage<'tcx> {
 
     /// When we add a R1 == R2 constriant, we currently add (a) edges
     /// R1 <= R2 and R2 <= R1 and (b) we unify the two regions in this
-    /// table. You can then call `opportunistic_resolve_var` early
+    /// table. You can then call `opportunistic_resolve_region` early
     /// which will map R1 and R2 to some common region (i.e., either
     /// R1 or R2). This is important when fulfillment, dropck and other such
     /// code is iterating to a fixed point, because otherwise we sometimes
     /// would wind up with a fresh stream of region variables that have been
     /// equated but appear distinct.
-    pub(super) unification_table: ut::UnificationTableStorage<ty::RegionVid>,
+    pub(super) unification_table: ut::UnificationTableStorage<RegionVidKey<'tcx>>,
 
     /// a flag set to true when we perform any unifications; this is used
     /// to micro-optimize `take_and_reset_data`
@@ -406,8 +406,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         // `RegionConstraintData` contains the relationship here.
         if *any_unifications {
             *any_unifications = false;
-            self.unification_table()
-                .reset_unifications(|_| ());
+            self.unification_table().reset_unifications(|_| UnifiedRegion(None));
         }
 
         data
@@ -434,8 +433,8 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
     ) -> RegionVid {
         let vid = self.var_infos.push(RegionVariableInfo { origin, universe });
 
-        let u_vid = self.unification_table().new_key(());
-        assert_eq!(vid, u_vid);
+        let u_vid = self.unification_table().new_key(UnifiedRegion(None));
+        assert_eq!(vid, u_vid.vid);
         self.undo_log.push(AddVar(vid));
         debug!("created new region variable {:?} in {:?} with origin {:?}", vid, universe, origin);
         vid
@@ -497,10 +496,18 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
             self.make_subregion(origin.clone(), sub, sup);
             self.make_subregion(origin, sup, sub);
 
-            if let (ty::ReVar(sub), ty::ReVar(sup)) = (*sub, *sup) {
-                debug!("make_eqregion: uniying {:?} with {:?}", sub, sup);
-                self.unification_table().union(sub, sup);
-                self.any_unifications = true;
+            match (sub, sup) {
+                (&ty::ReVar(sub), &ty::ReVar(sup)) => {
+                    debug!("make_eqregion: unifying {:?} with {:?}", sub, sup);
+                    self.unification_table().union(sub, sup);
+                    self.any_unifications = true;
+                }
+                (&ty::ReVar(vid), value) | (value, &ty::ReVar(vid)) => {
+                    debug!("make_eqregion: unifying {:?} with {:?}", vid, value);
+                    self.unification_table().union_value(vid, UnifiedRegion(Some(value)));
+                    self.any_unifications = true;
+                }
+                (_, _) => {}
             }
         }
     }
@@ -616,8 +623,21 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         }
     }
 
-    pub fn opportunistic_resolve_var(&mut self, rid: RegionVid) -> ty::RegionVid {
-        self.unification_table().find(rid)
+    pub fn opportunistic_resolve_region(
+        &mut self,
+        tcx: TyCtxt<'tcx>,
+        region: ty::Region<'tcx>,
+    ) -> ty::Region<'tcx> {
+        match region {
+            ty::ReVar(rid) => {
+                let unified_region = self.unification_table().probe_value(*rid);
+                unified_region.0.unwrap_or_else(|| {
+                    let root = self.unification_table().find(*rid).vid;
+                    tcx.reuse_or_mk_region(region, ty::ReVar(root))
+                })
+            }
+            _ => region,
+        }
     }
 
     fn combine_map(&mut self, t: CombineMapType) -> &mut CombineMap<'tcx> {
@@ -672,8 +692,8 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         &self,
         value_count: usize,
     ) -> (Range<RegionVid>, Vec<RegionVariableOrigin>) {
-        let range = RegionVid::from_index(value_count as u32)
-            ..RegionVid::from_index(self.unification_table.len() as u32);
+        let range = RegionVid::from(value_count as u32)
+            ..RegionVid::from(self.unification_table.len() as u32);
         (
             range.clone(),
             (range.start.index()..range.end.index())
@@ -695,7 +715,7 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
     }
 
     #[inline]
-    fn unification_table(&mut self) -> super::UnificationTable<'_, 'tcx, ty::RegionVid> {
+    fn unification_table(&mut self) -> super::UnificationTable<'_, 'tcx, RegionVidKey<'tcx>> {
         ut::UnificationTable::with_log(&mut self.storage.unification_table, self.undo_log)
     }
 }
diff --git a/compiler/rustc_infer/src/infer/resolve.rs b/compiler/rustc_infer/src/infer/resolve.rs
index 48b8ee17594..4eec65b3d96 100644
--- a/compiler/rustc_infer/src/infer/resolve.rs
+++ b/compiler/rustc_infer/src/infer/resolve.rs
@@ -84,18 +84,12 @@ impl<'a, 'tcx> TypeFolder<'tcx> for OpportunisticRegionResolver<'a, 'tcx> {
     }
 
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        match *r {
-            ty::ReVar(rid) => {
-                let resolved = self
-                    .infcx
-                    .inner
-                    .borrow_mut()
-                    .unwrap_region_constraints()
-                    .opportunistic_resolve_var(rid);
-                self.tcx().reuse_or_mk_region(r, ty::ReVar(resolved))
-            }
-            _ => r,
-        }
+        let tcx = self.tcx();
+        self.infcx
+            .inner
+            .borrow_mut()
+            .unwrap_region_constraints()
+            .opportunistic_resolve_region(tcx, r)
     }
 
     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
diff --git a/compiler/rustc_infer/src/infer/undo_log.rs b/compiler/rustc_infer/src/infer/undo_log.rs
index f41e872e004..5ad2519a93c 100644
--- a/compiler/rustc_infer/src/infer/undo_log.rs
+++ b/compiler/rustc_infer/src/infer/undo_log.rs
@@ -3,6 +3,7 @@ use std::marker::PhantomData;
 use rustc_data_structures::snapshot_vec as sv;
 use rustc_data_structures::undo_log::{Rollback, UndoLogs};
 use rustc_data_structures::unify as ut;
+use rustc_middle::infer::unify_key::RegionVidKey;
 use rustc_middle::ty;
 
 use crate::{
@@ -22,7 +23,7 @@ pub(crate) enum UndoLog<'tcx> {
     IntUnificationTable(sv::UndoLog<ut::Delegate<ty::IntVid>>),
     FloatUnificationTable(sv::UndoLog<ut::Delegate<ty::FloatVid>>),
     RegionConstraintCollector(region_constraints::UndoLog<'tcx>),
-    RegionUnificationTable(sv::UndoLog<ut::Delegate<ty::RegionVid>>),
+    RegionUnificationTable(sv::UndoLog<ut::Delegate<RegionVidKey<'tcx>>>),
     ProjectionCache(traits::UndoLog<'tcx>),
     PushRegionObligation,
 }
@@ -55,7 +56,7 @@ impl_from! {
 
     ConstUnificationTable(sv::UndoLog<ut::Delegate<ty::ConstVid<'tcx>>>),
 
-    RegionUnificationTable(sv::UndoLog<ut::Delegate<ty::RegionVid>>),
+    RegionUnificationTable(sv::UndoLog<ut::Delegate<RegionVidKey<'tcx>>>),
     ProjectionCache(traits::UndoLog<'tcx>),
 }
 
diff --git a/compiler/rustc_middle/src/infer/unify_key.rs b/compiler/rustc_middle/src/infer/unify_key.rs
index 8209e3db739..2427b37f741 100644
--- a/compiler/rustc_middle/src/infer/unify_key.rs
+++ b/compiler/rustc_middle/src/infer/unify_key.rs
@@ -16,37 +16,45 @@ pub trait ToType {
 }
 
 #[derive(PartialEq, Copy, Clone, Debug)]
-pub struct RegionVidKey {
-    /// The minimum region vid in the unification set. This is needed
-    /// to have a canonical name for a type to prevent infinite
-    /// recursion.
-    pub min_vid: ty::RegionVid,
-}
-
-impl UnifyValue for RegionVidKey {
-    type Error = NoError;
+pub struct UnifiedRegion<'tcx>(pub Option<ty::Region<'tcx>>);
 
-    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
-        let min_vid = if value1.min_vid.index() < value2.min_vid.index() {
-            value1.min_vid
-        } else {
-            value2.min_vid
-        };
+#[derive(PartialEq, Copy, Clone, Debug)]
+pub struct RegionVidKey<'tcx> {
+    pub vid: ty::RegionVid,
+    pub phantom: PhantomData<UnifiedRegion<'tcx>>,
+}
 
-        Ok(RegionVidKey { min_vid })
+impl<'tcx> From<ty::RegionVid> for RegionVidKey<'tcx> {
+    fn from(vid: ty::RegionVid) -> Self {
+        RegionVidKey { vid, phantom: PhantomData }
     }
 }
 
-impl UnifyKey for ty::RegionVid {
-    type Value = ();
+impl<'tcx> UnifyKey for RegionVidKey<'tcx> {
+    type Value = UnifiedRegion<'tcx>;
     fn index(&self) -> u32 {
-        u32::from(*self)
+        self.vid.as_u32()
     }
-    fn from_index(i: u32) -> ty::RegionVid {
-        ty::RegionVid::from(i)
+    fn from_index(i: u32) -> Self {
+        RegionVidKey::from(ty::RegionVid::from_u32(i))
     }
     fn tag() -> &'static str {
-        "RegionVid"
+        "RegionVidKey"
+    }
+}
+
+impl<'tcx> UnifyValue for UnifiedRegion<'tcx> {
+    type Error = NoError;
+
+    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
+        Ok(match (value1.0, value2.0) {
+            (Some(_), Some(_)) => *value1,
+
+            (Some(_), _) => *value1,
+            (_, Some(_)) => *value2,
+
+            (None, None) => *value1,
+        })
     }
 }
 
diff --git a/src/test/ui/associated-types/associated-types-eq-hr.rs b/src/test/ui/associated-types/associated-types-eq-hr.rs
index fb391913c32..2cf868f5280 100644
--- a/src/test/ui/associated-types/associated-types-eq-hr.rs
+++ b/src/test/ui/associated-types/associated-types-eq-hr.rs
@@ -102,6 +102,8 @@ pub fn call_tuple_two() {
     tuple_two::<Tuple>();
     //~^ ERROR implementation of `TheTrait` is not general enough
     //~| ERROR implementation of `TheTrait` is not general enough
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
 }
 
 pub fn call_tuple_three() {
diff --git a/src/test/ui/associated-types/associated-types-eq-hr.stderr b/src/test/ui/associated-types/associated-types-eq-hr.stderr
index 6897b31fe46..281c3cf0fad 100644
--- a/src/test/ui/associated-types/associated-types-eq-hr.stderr
+++ b/src/test/ui/associated-types/associated-types-eq-hr.stderr
@@ -46,6 +46,34 @@ LL |     tuple_one::<Tuple>();
    = note: `Tuple` must implement `TheTrait<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
    = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
+error[E0308]: mismatched types
+  --> $DIR/associated-types-eq-hr.rs:102:5
+   |
+LL |     tuple_two::<Tuple>();
+   |     ^^^^^^^^^^^^^^^^^^ lifetime mismatch
+   |
+   = note: expected reference `&'x isize`
+              found reference `&'y isize`
+note: the lifetime requirement is introduced here
+  --> $DIR/associated-types-eq-hr.rs:66:53
+   |
+LL |     T: for<'x, 'y> TheTrait<(&'x isize, &'y isize), A = &'y isize>,
+   |                                                     ^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/associated-types-eq-hr.rs:102:5
+   |
+LL |     tuple_two::<Tuple>();
+   |     ^^^^^^^^^^^^^^^^^^ lifetime mismatch
+   |
+   = note: expected reference `&'x isize`
+              found reference `&'y isize`
+note: the lifetime requirement is introduced here
+  --> $DIR/associated-types-eq-hr.rs:66:53
+   |
+LL |     T: for<'x, 'y> TheTrait<(&'x isize, &'y isize), A = &'y isize>,
+   |                                                     ^^^^^^^^^^^^^
+
 error: implementation of `TheTrait` is not general enough
   --> $DIR/associated-types-eq-hr.rs:102:5
    |
@@ -65,7 +93,7 @@ LL |     tuple_two::<Tuple>();
    = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: implementation of `TheTrait` is not general enough
-  --> $DIR/associated-types-eq-hr.rs:112:5
+  --> $DIR/associated-types-eq-hr.rs:114:5
    |
 LL |     tuple_four::<Tuple>();
    |     ^^^^^^^^^^^^^^^^^^^ implementation of `TheTrait` is not general enough
@@ -73,6 +101,7 @@ LL |     tuple_four::<Tuple>();
    = note: `Tuple` must implement `TheTrait<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
    = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
-error: aborting due to 7 previous errors
+error: aborting due to 9 previous errors
 
-For more information about this error, try `rustc --explain E0271`.
+Some errors have detailed explanations: E0271, E0308.
+For more information about an error, try `rustc --explain E0271`.
diff --git a/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr b/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr
index 7b81beeed41..21feada38a8 100644
--- a/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr
+++ b/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr
@@ -9,7 +9,7 @@ LL |     let v = Unit2.m(
    = help: consider constraining the associated type `<_ as Ty<'_>>::V` to `Unit4`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
-error[E0271]: type mismatch resolving `<[closure@$DIR/issue-62203-hrtb-ice.rs:42:17: 42:39] as FnOnce<((&u8,),)>>::Output == Unit3`
+error[E0271]: type mismatch resolving `<[closure@$DIR/issue-62203-hrtb-ice.rs:42:17: 42:39] as FnOnce<((&'r u8,),)>>::Output == Unit3`
   --> $DIR/issue-62203-hrtb-ice.rs:38:19
    |
 LL |     let v = Unit2.m(
diff --git a/src/test/ui/rfc1623.rs b/src/test/ui/rfc1623.rs
index 9ff4813d112..b74a141bade 100644
--- a/src/test/ui/rfc1623.rs
+++ b/src/test/ui/rfc1623.rs
@@ -23,6 +23,10 @@ static SOME_STRUCT: &SomeStruct = &SomeStruct {
     bar: &Bar { bools: &[true, true] },
     f: &id,
     //~^ ERROR implementation of `FnOnce` is not general enough
+    //~^^ mismatched types
+    //~^^^ mismatched types
+    //~^^^^ mismatched types
+    //~^^^^^ mismatched types
 };
 
 // very simple test for a 'static static with default lifetime
diff --git a/src/test/ui/rfc1623.stderr b/src/test/ui/rfc1623.stderr
index e95e68c8e6d..67af4e045bb 100644
--- a/src/test/ui/rfc1623.stderr
+++ b/src/test/ui/rfc1623.stderr
@@ -1,3 +1,39 @@
+error[E0308]: mismatched types
+  --> $DIR/rfc1623.rs:24:8
+   |
+LL |     f: &id,
+   |        ^^^ one type is more general than the other
+   |
+   = note: expected reference `&'a Foo<'b>`
+              found reference `&'a Foo<'b>`
+
+error[E0308]: mismatched types
+  --> $DIR/rfc1623.rs:24:8
+   |
+LL |     f: &id,
+   |        ^^^ one type is more general than the other
+   |
+   = note: expected reference `&'a Foo<'b>`
+              found reference `&'a Foo<'b>`
+
+error[E0308]: mismatched types
+  --> $DIR/rfc1623.rs:24:8
+   |
+LL |     f: &id,
+   |        ^^^ one type is more general than the other
+   |
+   = note: expected reference `&'a Foo<'b>`
+              found reference `&'a Foo<'b>`
+
+error[E0308]: mismatched types
+  --> $DIR/rfc1623.rs:24:8
+   |
+LL |     f: &id,
+   |        ^^^ one type is more general than the other
+   |
+   = note: expected reference `&'a Foo<'b>`
+              found reference `&'a Foo<'b>`
+
 error: implementation of `FnOnce` is not general enough
   --> $DIR/rfc1623.rs:24:8
    |
@@ -7,5 +43,6 @@ LL |     f: &id,
    = note: `fn(&'2 Foo<'_>) -> &'2 Foo<'_> {id::<&'2 Foo<'_>>}` must implement `FnOnce<(&'1 Foo<'b>,)>`, for any lifetime `'1`...
    = note: ...but it actually implements `FnOnce<(&'2 Foo<'_>,)>`, for some specific lifetime `'2`
 
-error: aborting due to previous error
+error: aborting due to 5 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/traits/inductive-overflow/lifetime.rs b/src/test/ui/traits/inductive-overflow/lifetime.rs
index d66c30ed2dd..e23dfa57cd0 100644
--- a/src/test/ui/traits/inductive-overflow/lifetime.rs
+++ b/src/test/ui/traits/inductive-overflow/lifetime.rs
@@ -26,6 +26,4 @@ fn main() {
     // Should only be a few notes.
     is_send::<X<C<'static>>>();
     //~^ ERROR overflow evaluating
-    //~^^ 2 redundant
-    //~^^^ required because of
 }
diff --git a/src/test/ui/traits/inductive-overflow/lifetime.stderr b/src/test/ui/traits/inductive-overflow/lifetime.stderr
index cc913930395..c7397883525 100644
--- a/src/test/ui/traits/inductive-overflow/lifetime.stderr
+++ b/src/test/ui/traits/inductive-overflow/lifetime.stderr
@@ -1,4 +1,4 @@
-error[E0275]: overflow evaluating the requirement `Box<X<C<'_>>>: NotAuto`
+error[E0275]: overflow evaluating the requirement `Box<X<C<'static>>>: NotAuto`
   --> $DIR/lifetime.rs:27:5
    |
 LL | fn is_send<S: NotAuto>() {}
@@ -7,13 +7,11 @@ LL | fn is_send<S: NotAuto>() {}
 LL |     is_send::<X<C<'static>>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: required because of the requirements on the impl of `NotAuto` for `X<C<'_>>`
+note: required because of the requirements on the impl of `NotAuto` for `X<C<'static>>`
   --> $DIR/lifetime.rs:19:12
    |
 LL | impl<T: Y> NotAuto for X<T> where T::P: NotAuto {}
    |            ^^^^^^^     ^^^^
-   = note: 2 redundant requirements hidden
-   = note: required because of the requirements on the impl of `NotAuto` for `X<C<'static>>`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-60371.rs b/src/test/ui/type-alias-impl-trait/issue-60371.rs
index 4ac7f9423ff..959b637c850 100644
--- a/src/test/ui/type-alias-impl-trait/issue-60371.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-60371.rs
@@ -9,7 +9,7 @@ trait Bug {
 impl Bug for &() {
     type Item = impl Bug; //~ ERROR `impl Trait` in type aliases is unstable
     //~^ ERROR the trait bound `(): Bug` is not satisfied
-    //~^^ ERROR could not find defining uses
+    //~^^ ERROR the trait bound
 
     const FUN: fn() -> Self::Item = || ();
     //~^ ERROR type alias impl trait is not permitted here
diff --git a/src/test/ui/type-alias-impl-trait/issue-60371.stderr b/src/test/ui/type-alias-impl-trait/issue-60371.stderr
index 255d381bf06..6857d5264b6 100644
--- a/src/test/ui/type-alias-impl-trait/issue-60371.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-60371.stderr
@@ -25,11 +25,14 @@ LL |     type Item = impl Bug;
    = help: the following implementations were found:
              <&() as Bug>
 
-error: could not find defining uses
+error[E0277]: the trait bound `(): Bug` is not satisfied
   --> $DIR/issue-60371.rs:10:17
    |
 LL |     type Item = impl Bug;
-   |                 ^^^^^^^^
+   |                 ^^^^^^^^ the trait `Bug` is not implemented for `()`
+   |
+   = help: the following implementations were found:
+             <&() as Bug>
 
 error: aborting due to 4 previous errors