[WIP] Move placeholder error handling to before region inference

compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs

@@ -24,7 +24,6 @@ use rustc_traits::{type_op_ascribe_user_type_with_span, type_op_prove_predicate_
 use tracing::{debug, instrument};
 
 use crate::MirBorrowckCtxt;
-use crate::region_infer::values::RegionElement;
 use crate::session_diagnostics::{
     HigherRankedErrorCause, HigherRankedLifetimeError, HigherRankedSubtypeError,
 };
@@ -49,11 +48,12 @@ impl<'tcx> UniverseInfo<'tcx> {
         UniverseInfo::RelateTys { expected, found }
     }
 
+    /// Report an error where an element erroneously made its way into `placeholder`.
     pub(crate) fn report_erroneous_element(
         &self,
         mbcx: &mut MirBorrowckCtxt<'_, '_, 'tcx>,
         placeholder: ty::PlaceholderRegion,
-        error_element: RegionElement,
+        error_element: Option<ty::PlaceholderRegion>,
         cause: ObligationCause<'tcx>,
     ) {
         match *self {
@@ -145,52 +145,54 @@ pub(crate) trait TypeOpInfo<'tcx> {
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<Diag<'infcx>>;
 
-    /// Constraints require that `error_element` appear in the
-    ///  values of `placeholder`, but this cannot be proven to
-    /// hold. Report an error.
+    /// Turn a placeholder region into a Region with its universe adjusted by
+    /// the base universe.
+    fn region_with_adjusted_universe(
+        &self,
+        placeholder: ty::PlaceholderRegion,
+        tcx: TyCtxt<'tcx>,
+    ) -> ty::Region<'tcx> {
+        let Some(adjusted_universe) =
+            placeholder.universe.as_u32().checked_sub(self.base_universe().as_u32())
+        else {
+            unreachable!(
+                "Could not adjust universe {:?} of {placeholder:?} by base universe {:?}",
+                placeholder.universe,
+                self.base_universe()
+            );
+        };
+        ty::Region::new_placeholder(
+            tcx,
+            ty::Placeholder { universe: adjusted_universe.into(), bound: placeholder.bound },
+        )
+    }
+
+    /// Report an error where an erroneous element reaches `placeholder`.
+    /// The erroneous element is either another placeholder that we provide,
+    /// or we reverse engineer what happened later anyway.
     #[instrument(level = "debug", skip(self, mbcx))]
     fn report_erroneous_element(
         &self,
         mbcx: &mut MirBorrowckCtxt<'_, '_, 'tcx>,
         placeholder: ty::PlaceholderRegion,
-        error_element: RegionElement,
+        error_element: Option<ty::PlaceholderRegion>,
         cause: ObligationCause<'tcx>,
     ) {
         let tcx = mbcx.infcx.tcx;
-        let base_universe = self.base_universe();
-        debug!(?base_universe);
-
-        let Some(adjusted_universe) =
-            placeholder.universe.as_u32().checked_sub(base_universe.as_u32())
-        else {
-            mbcx.buffer_error(self.fallback_error(tcx, cause.span));
-            return;
-        };
 
-        let placeholder_region = ty::Region::new_placeholder(
-            tcx,
-            ty::Placeholder { universe: adjusted_universe.into(), bound: placeholder.bound },
-        );
-
-        let error_region = if let RegionElement::PlaceholderRegion(error_placeholder) =
-            error_element
-        {
-            let adjusted_universe =
-                error_placeholder.universe.as_u32().checked_sub(base_universe.as_u32());
-            adjusted_universe.map(|adjusted| {
-                ty::Region::new_placeholder(
-                    tcx,
-                    ty::Placeholder { universe: adjusted.into(), bound: error_placeholder.bound },
-                )
-            })
-        } else {
-            None
-        };
+        // FIXME: these adjusted universes are not (always) the same ones as we compute
+        // earlier. They probably should be, but the logic downstream is complicated,
+        // and assumes they use whatever this is.
+        //
+        // In fact, this  function throws away a lot of interesting information that would
+        // probably allow bypassing lots of logic downstream for a much simpler flow.
+        let placeholder_region = self.region_with_adjusted_universe(placeholder, tcx);
+        let error_element = error_element.map(|e| self.region_with_adjusted_universe(e, tcx));
 
         debug!(?placeholder_region);
 
         let span = cause.span;
-        let nice_error = self.nice_error(mbcx, cause, placeholder_region, error_region);
+        let nice_error = self.nice_error(mbcx, cause, placeholder_region, error_element);
 
         debug!(?nice_error);
         mbcx.buffer_error(nice_error.unwrap_or_else(|| self.fallback_error(tcx, span)));

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -569,7 +569,7 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
         let (blame_constraint, path) = self.regioncx.best_blame_constraint(
             borrow_region,
             NllRegionVariableOrigin::FreeRegion,
-            |r| self.regioncx.provides_universal_region(r, borrow_region, outlived_region),
+            outlived_region,
         );
         let BlameConstraint { category, from_closure, cause, .. } = blame_constraint;
 

compiler/rustc_borrowck/src/diagnostics/opaque_suggestions.rs

@@ -174,10 +174,9 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for FindOpaqueRegion<'_, 'tcx> {
                 let opaque_region_vid = self.regioncx.to_region_vid(opaque_region);
 
                 // Find a path between the borrow region and our opaque capture.
-                if let Some((path, _)) =
-                    self.regioncx.find_constraint_paths_between_regions(self.borrow_region, |r| {
-                        r == opaque_region_vid
-                    })
+                if let Some(path) = self
+                    .regioncx
+                    .constraint_path_between_regions(self.borrow_region, opaque_region_vid)
                 {
                     for constraint in path {
                         // If we find a call in this path, then check if it defines the opaque.

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -62,7 +62,7 @@ impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
             | ConstraintCategory::Boring
             | ConstraintCategory::BoringNoLocation
             | ConstraintCategory::Internal
-            | ConstraintCategory::IllegalUniverse => "",
+            | ConstraintCategory::IllegalPlaceholder(..) => "",
         }
     }
 }
@@ -120,8 +120,31 @@ pub(crate) enum RegionErrorKind<'tcx> {
         member_region: ty::Region<'tcx>,
     },
 
-    /// Higher-ranked subtyping error.
-    BoundUniversalRegionError {
+    /// 'a outlives 'b, and both are placeholders.
+    PlaceholderOutlivesPlaceholder {
+        rvid_a: RegionVid,
+        rvid_b: RegionVid,
+        origin_a: ty::PlaceholderRegion,
+        origin_b: ty::PlaceholderRegion,
+    },
+
+    /// Indicates that a placeholder has a universe too large for one
+    /// of its member existentials, or, equivalently, that there is
+    /// a path through the outlives constraint graph from a placeholder
+    /// to an existential region that cannot name it.
+    PlaceholderOutlivesExistentialThatCannotNameIt {
+        /// the placeholder that transitively outlives an
+        /// existential that shouldn't leak into it
+        longer_fr: RegionVid,
+        /// The existential leaking into `longer_fr`.
+        existental_that_cannot_name_longer: RegionVid,
+        // `longer_fr`'s originating placeholder region.
+        placeholder: ty::PlaceholderRegion,
+    },
+
+    /// Higher-ranked subtyping error. A placeholder outlives
+    /// either a location or a universal region.
+    PlaceholderOutlivesLocationOrUniversal {
         /// The placeholder free region.
         longer_fr: RegionVid,
         /// The region element that erroneously must be outlived by `longer_fr`.
@@ -388,26 +411,56 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     }
                 }
 
-                RegionErrorKind::BoundUniversalRegionError {
+                RegionErrorKind::PlaceholderOutlivesLocationOrUniversal {
                     longer_fr,
                     placeholder,
                     error_element,
+                } => self.report_erroneous_rvid_reaches_placeholder(
+                    longer_fr,
+                    placeholder,
+                    self.regioncx.region_from_element(longer_fr, &error_element),
+                ),
+                RegionErrorKind::PlaceholderOutlivesPlaceholder {
+                    rvid_a,
+                    rvid_b,
+                    origin_a,
+                    origin_b,
                 } => {
-                    let error_vid = self.regioncx.region_from_element(longer_fr, &error_element);
-
-                    // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
-                    let (_, cause) = self.regioncx.find_outlives_blame_span(
-                        longer_fr,
-                        NllRegionVariableOrigin::Placeholder(placeholder),
-                        error_vid,
+                    debug!(
+                        "Placeholder mismatch: {rvid_a:?} ({origin_a:?}) reaches {rvid_b:?} ({origin_b:?})"
                     );
 
-                    let universe = placeholder.universe;
-                    let universe_info = self.regioncx.universe_info(universe);
-
-                    universe_info.report_erroneous_element(self, placeholder, error_element, cause);
+                    let cause = self
+                        .regioncx
+                        .best_blame_constraint(
+                            rvid_a,
+                            NllRegionVariableOrigin::Placeholder(origin_a),
+                            rvid_b,
+                        )
+                        .0
+                        .cause;
+
+                    // FIXME We may be able to shorten the code path here, and immediately
+                    // report a `RegionResolutionError::UpperBoundUniverseConflict`, but
+                    // that's left for a future refactoring.
+                    self.regioncx.universe_info(origin_a.universe).report_erroneous_element(
+                        self,
+                        origin_a,
+                        Some(origin_b),
+                        cause,
+                    );
                 }
 
+                RegionErrorKind::PlaceholderOutlivesExistentialThatCannotNameIt {
+                    longer_fr,
+                    existental_that_cannot_name_longer,
+                    placeholder,
+                } => self.report_erroneous_rvid_reaches_placeholder(
+                    longer_fr,
+                    placeholder,
+                    existental_that_cannot_name_longer,
+                ),
+
                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
                     if is_reported {
                         self.report_region_error(
@@ -456,9 +509,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
     ) {
         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 
-        let (blame_constraint, path) = self.regioncx.best_blame_constraint(fr, fr_origin, |r| {
-            self.regioncx.provides_universal_region(r, fr, outlived_fr)
-        });
+        let (blame_constraint, path) =
+            self.regioncx.best_blame_constraint(fr, fr_origin, outlived_fr);
         let BlameConstraint { category, cause, variance_info, .. } = blame_constraint;
 
         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);