Final bits

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -272,9 +272,12 @@ impl<'tcx> BorrowExplanation<'tcx> {
 
                 for extra in extra_info {
                     match extra {
-                        _ => {}
+                        ExtraConstraintInfo::PlaceholderFromPredicate(span) => {
+                            err.span_note(*span, format!("due to current limitations in the borrow checker, this implies a `'static` lifetime"));
+                        }
                     }
                 }
+
                 self.add_lifetime_bound_suggestion_to_diagnostic(err, &category, span, region_name);
             }
             _ => {}

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -246,7 +246,9 @@ enum Trace<'tcx> {
 }
 
 #[derive(Clone, PartialEq, Eq, Debug)]
-pub enum ExtraConstraintInfo {}
+pub enum ExtraConstraintInfo {
+    PlaceholderFromPredicate(Span),
+}
 
 impl<'tcx> RegionInferenceContext<'tcx> {
     /// Creates a new region inference context with a total of
@@ -2028,7 +2030,17 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 .collect::<Vec<_>>()
         );
 
-        let extra_info = vec![];
+        let mut extra_info = vec![];
+        for constraint in path.iter() {
+            let outlived = constraint.sub;
+            let Some(origin) = self.var_infos.get(outlived) else { continue; };
+            let RegionVariableOrigin::Nll(NllRegionVariableOrigin::Placeholder(p)) = origin.origin else { continue; };
+            debug!(?constraint, ?p);
+            let ConstraintCategory::Predicate(span) = constraint.category else { continue; };
+            extra_info.push(ExtraConstraintInfo::PlaceholderFromPredicate(span));
+            // We only want to point to one
+            break;
+        }
 
         // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
         // Instead, we use it to produce an improved `ObligationCauseCode`.

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -25,7 +25,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
     /// constraints should occur within this method so that those
     /// constraints can be properly localized!**
     #[instrument(skip(self, op), level = "trace")]
-    pub(super) fn fully_perform_op<R, Op>(
+    pub(super) fn fully_perform_op<R: fmt::Debug, Op>(
         &mut self,
         locations: Locations,
         category: ConstraintCategory<'tcx>,
@@ -39,6 +39,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         let TypeOpOutput { output, constraints, error_info } = op.fully_perform(self.infcx)?;
 
+        debug!(?output, ?constraints);
+
         if let Some(data) = constraints {
             self.push_region_constraints(locations, category, data);
         }

compiler/rustc_borrowck/src/type_check/constraint_conversion.rs

@@ -86,7 +86,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
         }
     }
 
-    pub(super) fn convert(&mut self, query_constraint: &QueryOutlivesConstraint<'tcx>) {
+    fn convert(&mut self, query_constraint: &QueryOutlivesConstraint<'tcx>) {
         debug!("generate: constraints at: {:#?}", self.locations);
 
         // Extract out various useful fields we'll need below.