Borrowck: simplify diagnostics for placeholders

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<'tcx>,
-        error_element: RegionElement<'tcx>,
+        error_element: Option<ty::PlaceholderRegion<'tcx>>,
         cause: ObligationCause<'tcx>,
     ) {
         match *self {
@@ -146,14 +146,14 @@ pub(crate) trait TypeOpInfo<'tcx> {
     ) -> Option<Diag<'infcx>>;
 
     /// Constraints require that `error_element` appear in the
-    ///  values of `placeholder`, but this cannot be proven to
+    /// values of `placeholder`, but this cannot be proven to
     /// hold. Report an error.
     #[instrument(level = "debug", skip(self, mbcx))]
     fn report_erroneous_element(
         &self,
         mbcx: &mut MirBorrowckCtxt<'_, '_, 'tcx>,
         placeholder: ty::PlaceholderRegion<'tcx>,
-        error_element: RegionElement<'tcx>,
+        error_element: Option<ty::PlaceholderRegion<'tcx>>,
         cause: ObligationCause<'tcx>,
     ) {
         let tcx = mbcx.infcx.tcx;
@@ -172,19 +172,17 @@ pub(crate) trait TypeOpInfo<'tcx> {
             ty::PlaceholderRegion::new(adjusted_universe.into(), 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::PlaceholderRegion::new(adjusted.into(), error_placeholder.bound),
-                    )
-                })
-            } else {
-                None
-            };
+        // FIXME: one day this should just be error_element,
+        // and this method shouldn't do anything.
+        let error_region = error_element.and_then(|e| {
+            let adjusted_universe = e.universe.as_u32().checked_sub(base_universe.as_u32());
+            adjusted_universe.map(|adjusted| {
+                ty::Region::new_placeholder(
+                    tcx,
+                    ty::PlaceholderRegion::new(adjusted.into(), e.bound),
+                )
+            })
+        });
 
         debug!(?placeholder_region);
 

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -29,7 +29,6 @@ use tracing::{debug, instrument, trace};
 
 use super::{OutlivesSuggestionBuilder, RegionName, RegionNameSource};
 use crate::nll::ConstraintDescription;
-use crate::region_infer::values::RegionElement;
 use crate::region_infer::{BlameConstraint, TypeTest};
 use crate::session_diagnostics::{
     FnMutError, FnMutReturnTypeErr, GenericDoesNotLiveLongEnough, LifetimeOutliveErr,
@@ -104,15 +103,9 @@ pub(crate) enum RegionErrorKind<'tcx> {
     /// A generic bound failure for a type test (`T: 'a`).
     TypeTestError { type_test: TypeTest<'tcx> },
 
-    /// Higher-ranked subtyping error.
-    BoundUniversalRegionError {
-        /// The placeholder free region.
-        longer_fr: RegionVid,
-        /// The region element that erroneously must be outlived by `longer_fr`.
-        error_element: RegionElement<'tcx>,
-        /// The placeholder region.
-        placeholder: ty::PlaceholderRegion<'tcx>,
-    },
+    /// 'p outlives 'r, which does not hold. 'p is always a placeholder
+    /// and 'r is some other region.
+    PlaceholderOutlivesIllegalRegion { longer_fr: RegionVid, illegally_outlived_r: RegionVid },
 
     /// Any other lifetime error.
     RegionError {
@@ -360,28 +353,11 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     }
                 }
 
-                RegionErrorKind::BoundUniversalRegionError {
+                RegionErrorKind::PlaceholderOutlivesIllegalRegion {
                     longer_fr,
-                    placeholder,
-                    error_element,
+                    illegally_outlived_r,
                 } => {
-                    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
-                        .best_blame_constraint(
-                            longer_fr,
-                            NllRegionVariableOrigin::Placeholder(placeholder),
-                            error_vid,
-                        )
-                        .0
-                        .cause;
-
-                    let universe = placeholder.universe;
-                    let universe_info = self.regioncx.universe_info(universe);
-
-                    universe_info.report_erroneous_element(self, placeholder, error_element, cause);
+                    self.report_erroneous_rvid_reaches_placeholder(longer_fr, illegally_outlived_r)
                 }
 
                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
@@ -412,6 +388,43 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         outlives_suggestion.add_suggestion(self);
     }
 
+    /// Report that `longer_fr: error_vid`, which doesn't hold,
+    /// where `longer_fr` is a placeholder.
+    fn report_erroneous_rvid_reaches_placeholder(
+        &mut self,
+        longer_fr: RegionVid,
+        error_vid: RegionVid,
+    ) {
+        use NllRegionVariableOrigin::*;
+
+        let origin_longer = self.regioncx.definitions[longer_fr].origin;
+
+        let Placeholder(placeholder) = origin_longer else {
+            bug!("Expected {longer_fr:?} to come from placeholder!");
+        };
+
+        // FIXME: Is throwing away the existential region really the best here?
+        let error_region = match self.regioncx.definitions[error_vid].origin {
+            FreeRegion | Existential { .. } => None,
+            Placeholder(other_placeholder) => Some(other_placeholder),
+        };
+
+        // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
+        let cause =
+            self.regioncx.best_blame_constraint(longer_fr, origin_longer, error_vid).0.cause;
+
+        // FIXME these methods should have better names, and also probably not be this generic.
+        // FIXME note that we *throw away* the error element here! We probably want to
+        // thread it through the computation further down and use it, but there currently isn't
+        // anything there to receive it.
+        self.regioncx.universe_info(placeholder.universe).report_erroneous_element(
+            self,
+            placeholder,
+            error_region,
+            cause,
+        );
+    }
+
     /// Report an error because the universal region `fr` was required to outlive
     /// `outlived_fr` but it is not known to do so. For example:
     ///

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -1379,11 +1379,11 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             .elements_contained_in(longer_fr_scc)
             .find(|e| *e != RegionElement::PlaceholderRegion(placeholder))
         {
+            let illegally_outlived_r = self.region_from_element(longer_fr, &error_element);
             // Stop after the first error, it gets too noisy otherwise, and does not provide more information.
-            errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
+            errors_buffer.push(RegionErrorKind::PlaceholderOutlivesIllegalRegion {
                 longer_fr,
-                error_element,
-                placeholder,
+                illegally_outlived_r,
             });
         } else {
             debug!("check_bound_universal_region: all bounds satisfied");
@@ -1572,7 +1572,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     }
 
     /// Get the region outlived by `longer_fr` and live at `element`.
-    pub(crate) fn region_from_element(
+    fn region_from_element(
         &self,
         longer_fr: RegionVid,
         element: &RegionElement<'tcx>,