name regions in expected/found borrowck errors

Author: aliemjay

compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs

@@ -56,6 +56,8 @@ impl<'tcx> UniverseInfo<'tcx> {
     ) {
         match self.0 {
             UniverseInfoInner::RelateTys { expected, found } => {
+                let (expected, found) =
+                    mbcx.regioncx.name_regions(mbcx.infcx.tcx, (expected, found));
                 let err = mbcx.infcx.report_mismatched_types(
                     &cause,
                     expected,

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -208,9 +208,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 }
 
                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, key, member_region } => {
-                    let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
-                    let named_key = self.regioncx.name_regions(self.infcx.tcx, key);
-                    let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
+                    let named_ty = self.regioncx.name_regions_opaque(self.infcx.tcx, hidden_ty);
+                    let named_key = self.regioncx.name_regions_opaque(self.infcx.tcx, key);
+                    let named_region =
+                        self.regioncx.name_regions_opaque(self.infcx.tcx, member_region);
                     self.buffer_error(unexpected_hidden_region_diagnostic(
                         self.infcx.tcx,
                         span,

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -2271,6 +2271,24 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         categorized_path.remove(0)
     }
 
+    /// Replaces region variables with equal universal regions where possible.
+    pub(crate) fn name_regions<T>(&self, tcx: TyCtxt<'tcx>, ty: T) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        tcx.fold_regions(ty, |region, _| match *region {
+            ty::ReVar(vid) => {
+                let scc = self.constraint_sccs.scc(vid);
+                let normalized = self.scc_representatives[scc];
+                match &self.definitions[normalized].external_name {
+                    Some(universal_r) => *universal_r,
+                    None => region,
+                }
+            }
+            _ => region,
+        })
+    }
+
     pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
         self.universe_causes[&universe].clone()
     }

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -153,7 +153,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// that the regions produced are in fact equal to the named region they are
     /// replaced with. This is fine because this function is only to improve the
     /// region names in error messages.
-    pub(crate) fn name_regions<T>(&self, tcx: TyCtxt<'tcx>, ty: T) -> T
+    pub(crate) fn name_regions_opaque<T>(&self, tcx: TyCtxt<'tcx>, ty: T) -> T
     where
         T: TypeFoldable<'tcx>,
     {

src/test/ui/closure-expected-type/expect-fn-supply-fn.stderr

@@ -34,7 +34,7 @@ error[E0308]: mismatched types
 LL |     with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
    |                                                  ^ one type is more general than the other
    |
-   = note: expected fn pointer `fn(&u32)`
+   = note: expected fn pointer `fn(&'x u32)`
               found fn pointer `for<'r> fn(&'r u32)`
 
 error[E0308]: mismatched types

src/test/ui/const-generics/invariant.stderr

@@ -18,8 +18,8 @@ error[E0308]: mismatched types
 LL |     v
    |     ^ one type is more general than the other
    |
-   = note: expected reference `&Foo<fn(&())>`
-              found reference `&Foo<for<'a> fn(&'a ())>`
+   = note: expected reference `&'static Foo<fn(&'static ())>`
+              found reference `&'static Foo<for<'a> fn(&'a ())>`
 
 error: aborting due to previous error; 1 warning emitted
 

src/test/ui/nll/relate_tys/fn-subtype.stderr

@@ -5,7 +5,7 @@ LL |     let y: for<'a> fn(&'a ()) = x;
    |                                 ^ one type is more general than the other
    |
    = note: expected fn pointer `for<'a> fn(&'a ())`
-              found fn pointer `fn(&())`
+              found fn pointer `fn(&'static ())`
 
 error: aborting due to previous error
 

src/test/ui/nll/relate_tys/universe-violation.stderr

@@ -5,7 +5,7 @@ LL |     let b: fn(&u32) -> &u32 = a;
    |                               ^ one type is more general than the other
    |
    = note: expected fn pointer `for<'r> fn(&'r u32) -> &'r u32`
-              found fn pointer `fn(&u32) -> &u32`
+              found fn pointer `fn(&'static u32) -> &'static u32`
 
 error: aborting due to previous error