Borrowck: simplify diagnostics for placeholders.

This essentially folds the call to `region_from_element` into `RegionInferenceContext`,
and simplifies the error variant for this case. It also clarifies the type
information on the methods called to emphasise the fact that they only ever use
placeholder regions in the diagnostics, and completely ignore any other element.

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>,