Rollup merge of #123797 - amandasystems:better-graphviz, r=oli-obk

Better graphviz output for SCCs and NLL constraints

This PR modifies the output for `-Z dump-mir-graphviz=yes`. Specifically, it changes the output of the files `.-------.nll.0.regioncx.all.dot` and `nll.0.regioncx.scc.dot` to be easier to read and contain some information that helped me during debugging. In particular:

- SCC indices are contracted to `SCC(n)` instead of `ConstraintSccIndex(n)` to compress the nodes
- SCC regions are in `{}` rather than `[]` (controversial since they are technically ordered by index, but I figured they're more sets than arrays conceptually since they're equivalence classes).
- For regions in other universes than the root, also show the region universe (as ?8/U1)
- For regions with external names, show the external name in parenthesis
- For the region graph where edges are locations, render the All variant of the enum without the file since it's extremely long and often destroys the rendering
- For region graph edge annotations for single locations, remove the wrapping around the Location variant and just add its contents since this can be unambiguously done

Example output (from the function `foo()` of `tests/ui/error-codes/E0582.rs`) for an SCC graph:
![a graph showing SCCs](https://github.com/rust-lang/rust/assets/102855/0b998338-0379-4829-b99e-d8105c094897)

...and for the constraints:
![a graph showing regions and their constraints](https://github.com/rust-lang/rust/assets/102855/e984c4ca-7aa2-4db2-9878-bf38fe8208d5)

This PR also gives `UniverseIndex`es the `is_root()` method since this is now an operation that happens three times in the borrowck crate.

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())
     }
 }
 

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

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)) => {

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 {