4 files changed, 43 insertions, 6 deletions
diff --git a/compiler/rustc_borrowck/src/region_infer/graphviz.rs b/compiler/rustc_borrowck/src/region_infer/graphviz.rs
index c103ba3c407..f145d30fe38 100644
--- a/compiler/rustc_borrowck/src/region_infer/graphviz.rs
+++ b/compiler/rustc_borrowck/src/region_infer/graphviz.rs
@@ -6,7 +6,38 @@ use std::borrow::Cow;
 use std::io::{self, Write};
 
 use super::*;
+use itertools::Itertools;
 use rustc_graphviz as dot;
+use rustc_middle::ty::UniverseIndex;
+
+fn render_outlives_constraint(constraint: &OutlivesConstraint<'_>) -> String {
+    match constraint.locations {
+        Locations::All(_) => "All(...)".to_string(),
+        Locations::Single(loc) => format!("{loc:?}"),
+    }
+}
+
+fn render_universe(u: UniverseIndex) -> String {
+    if u.is_root() {
+        return "".to_string();
+    }
+
+    format!("/{:?}", u)
+}
+
+fn render_region_vid(rvid: RegionVid, regioncx: &RegionInferenceContext<'_>) -> String {
+    let universe_str = render_universe(regioncx.region_definition(rvid).universe);
+
+    let external_name_str = if let Some(external_name) =
+        regioncx.region_definition(rvid).external_name.and_then(|e| e.get_name())
+    {
+        format!(" ({external_name})")
+    } else {
+        "".to_string()
+    };
+
+    format!("{:?}{universe_str}{external_name_str}", rvid)
+}
 
 impl<'tcx> RegionInferenceContext<'tcx> {
     /// Write out the region constraint graph.
@@ -46,10 +77,10 @@ impl<'a, 'this, 'tcx> dot::Labeller<'this> for RawConstraints<'a, 'tcx> {
         Some(dot::LabelText::LabelStr(Cow::Borrowed("box")))
     }
     fn node_label(&'this self, n: &RegionVid) -> dot::LabelText<'this> {
-        dot::LabelText::LabelStr(format!("{n:?}").into())
+        dot::LabelText::LabelStr(render_region_vid(*n, self.regioncx).into())
     }
     fn edge_label(&'this self, e: &OutlivesConstraint<'tcx>) -> dot::LabelText<'this> {
-        dot::LabelText::LabelStr(format!("{:?}", e.locations).into())
+        dot::LabelText::LabelStr(render_outlives_constraint(e).into())
     }
 }
 
@@ -96,8 +127,9 @@ impl<'a, 'this, 'tcx> dot::Labeller<'this> for SccConstraints<'a, 'tcx> {
         Some(dot::LabelText::LabelStr(Cow::Borrowed("box")))
     }
     fn node_label(&'this self, n: &ConstraintSccIndex) -> dot::LabelText<'this> {
-        let nodes = &self.nodes_per_scc[*n];
-        dot::LabelText::LabelStr(format!("{n:?} = {nodes:?}").into())
+        let nodes_str =
+            self.nodes_per_scc[*n].iter().map(|n| render_region_vid(*n, self.regioncx)).join(", ");
+        dot::LabelText::LabelStr(format!("SCC({n}) = {{{nodes_str}}}", n = n.as_usize()).into())
     }
 }
 
diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 599f7dd18c3..dd75548a15d 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -1562,7 +1562,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
         // Because this free region must be in the ROOT universe, we
         // know it cannot contain any bound universes.
-        assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
+        assert!(self.scc_universes[longer_fr_scc].is_root());
         debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
 
         // Only check all of the relations for the main representative of each
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index 9f0e54febe4..73ba5bee13b 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -213,7 +213,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 let scc = self.constraint_sccs.scc(vid);
 
                 // Special handling of higher-ranked regions.
-                if self.scc_universes[scc] != ty::UniverseIndex::ROOT {
+                if !self.scc_universes[scc].is_root() {
                     match self.scc_values.placeholders_contained_in(scc).enumerate().last() {
                         // If the region contains a single placeholder then they're equal.
                         Some((0, placeholder)) => {
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index 45e22b12a8b..a5b33a8125d 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -346,6 +346,11 @@ impl UniverseIndex {
     pub fn cannot_name(self, other: UniverseIndex) -> bool {
         self < other
     }
+
+    /// Returns `true` if `self` is the root universe, otherwise false.
+    pub fn is_root(self) -> bool {
+        self == Self::ROOT
+    }
 }
 
 impl Default for UniverseIndex {