Add a new constraint category for our outlives static constraints

This makes the origin of these synthetic constraints clearer and allows us to make sure that they are literally never blamed during error reporting, massively reducing the number of failing UI tests.

Author: amandasystems

compiler/rustc_borrowck/src/constraints/mod.rs

@@ -82,7 +82,7 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
         let outlives_static = |rvid: RegionVid| OutlivesConstraint {
             sup: rvid,
             sub: universal_regions.fr_static, // All the following values are made up ex nihil
-            category: ConstraintCategory::BoringNoLocation,
+            category: ConstraintCategory::IllegalUniverse,
             locations: Locations::All(rustc_span::DUMMY_SP),
             span: rustc_span::DUMMY_SP,
             variance_info: VarianceDiagInfo::None,

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -63,7 +63,8 @@ impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
             ConstraintCategory::Predicate(_)
             | ConstraintCategory::Boring
             | ConstraintCategory::BoringNoLocation
-            | ConstraintCategory::Internal => "",
+            | ConstraintCategory::Internal
+            | ConstraintCategory::IllegalUniverse => "",
         }
     }
 }

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -1919,6 +1919,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
             // This loop can be hot.
             for constraint in outgoing_edges_from_graph {
+                if matches!(constraint.category, ConstraintCategory::IllegalUniverse) {
+                    debug!("Ignoring illegal universe constraint: {constraint:?}");
+                    continue;
+                }
                 handle_constraint(constraint);
             }
 

compiler/rustc_middle/src/mir/query.rs

@@ -273,6 +273,8 @@ pub enum ConstraintCategory<'tcx> {
 
     /// A constraint that doesn't correspond to anything the user sees.
     Internal,
+    /// An internal constraint derived from an illegal universe relation.
+    IllegalUniverse,
 }
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]