Improve opaque type higher-ranked region error message under NLL

compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs

@@ -2,9 +2,13 @@ use rustc_errors::DiagnosticBuilder;
 use rustc_infer::infer::canonical::Canonical;
 use rustc_infer::infer::error_reporting::nice_region_error::NiceRegionError;
 use rustc_infer::infer::region_constraints::Constraint;
+use rustc_infer::infer::region_constraints::RegionConstraintData;
+use rustc_infer::infer::RegionVariableOrigin;
 use rustc_infer::infer::{InferCtxt, RegionResolutionError, SubregionOrigin, TyCtxtInferExt as _};
 use rustc_infer::traits::{Normalized, ObligationCause, TraitEngine, TraitEngineExt};
 use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::RegionVid;
+use rustc_middle::ty::UniverseIndex;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc_span::Span;
 use rustc_trait_selection::traits::query::type_op;
@@ -78,6 +82,15 @@ crate trait ToUniverseInfo<'tcx> {
     fn to_universe_info(self, base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx>;
 }
 
+impl<'tcx> ToUniverseInfo<'tcx> for crate::type_check::InstantiateOpaqueType<'tcx> {
+    fn to_universe_info(self, base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
+        UniverseInfo(UniverseInfoInner::TypeOp(Rc::new(crate::type_check::InstantiateOpaqueType {
+            base_universe: Some(base_universe),
+            ..self
+        })))
+    }
+}
+
 impl<'tcx> ToUniverseInfo<'tcx>
     for Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::prove_predicate::ProvePredicate<'tcx>>>
 {
@@ -118,6 +131,12 @@ impl<'tcx, F, G> ToUniverseInfo<'tcx> for Canonical<'tcx, type_op::custom::Custo
     }
 }
 
+impl<'tcx> ToUniverseInfo<'tcx> for ! {
+    fn to_universe_info(self, _base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
+        self
+    }
+}
+
 #[allow(unused_lifetimes)]
 trait TypeOpInfo<'tcx> {
     /// Returns an error to be reported if rerunning the type op fails to
@@ -128,7 +147,7 @@ trait TypeOpInfo<'tcx> {
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
@@ -175,7 +194,7 @@ trait TypeOpInfo<'tcx> {
         debug!(?placeholder_region);
 
         let span = cause.span;
-        let nice_error = self.nice_error(tcx, cause, placeholder_region, error_region);
+        let nice_error = self.nice_error(mbcx, cause, placeholder_region, error_region);
 
         if let Some(nice_error) = nice_error {
             nice_error.buffer(&mut mbcx.errors_buffer);
@@ -204,16 +223,16 @@ impl<'tcx> TypeOpInfo<'tcx> for PredicateQuery<'tcx> {
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<DiagnosticBuilder<'tcx>> {
-        tcx.infer_ctxt().enter_with_canonical(
+        mbcx.infcx.tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
             |ref infcx, key, _| {
-                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(tcx);
+                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
                 type_op_prove_predicate_with_cause(infcx, &mut *fulfill_cx, key, cause);
                 try_extract_error_from_fulfill_cx(
                     fulfill_cx,
@@ -247,16 +266,16 @@ where
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<DiagnosticBuilder<'tcx>> {
-        tcx.infer_ctxt().enter_with_canonical(
+        mbcx.infcx.tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
             |ref infcx, key, _| {
-                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(tcx);
+                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 
                 let mut selcx = SelectionContext::new(infcx);
 
@@ -304,16 +323,16 @@ impl<'tcx> TypeOpInfo<'tcx> for AscribeUserTypeQuery<'tcx> {
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<DiagnosticBuilder<'tcx>> {
-        tcx.infer_ctxt().enter_with_canonical(
+        mbcx.infcx.tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
             |ref infcx, key, _| {
-                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(tcx);
+                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
                 type_op_ascribe_user_type_with_span(infcx, &mut *fulfill_cx, key, Some(cause.span))
                     .ok()?;
                 try_extract_error_from_fulfill_cx(
@@ -327,43 +346,90 @@ impl<'tcx> TypeOpInfo<'tcx> for AscribeUserTypeQuery<'tcx> {
     }
 }
 
+impl<'tcx> TypeOpInfo<'tcx> for crate::type_check::InstantiateOpaqueType<'tcx> {
+    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+        // FIXME: This error message isn't great, but it doesn't show up in the existing UI tests,
+        // and is only the fallback when the nice error fails. Consider improving this some more.
+        tcx.sess.struct_span_err(span, "higher-ranked lifetime error for opaque type!")
+    }
+
+    fn base_universe(&self) -> ty::UniverseIndex {
+        self.base_universe.unwrap()
+    }
+
+    fn nice_error(
+        &self,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
+        _cause: ObligationCause<'tcx>,
+        placeholder_region: ty::Region<'tcx>,
+        error_region: Option<ty::Region<'tcx>>,
+    ) -> Option<DiagnosticBuilder<'tcx>> {
+        try_extract_error_from_region_constraints(
+            mbcx.infcx,
+            placeholder_region,
+            error_region,
+            self.region_constraints.as_ref().unwrap(),
+            // We're using the original `InferCtxt` that we
+            // started MIR borrowchecking with, so the region
+            // constraints have already been taken. Use the data from
+            // our `mbcx` instead.
+            |vid| mbcx.regioncx.var_infos[vid].origin,
+            |vid| mbcx.regioncx.var_infos[vid].universe,
+        )
+    }
+}
+
 #[instrument(skip(fulfill_cx, infcx), level = "debug")]
 fn try_extract_error_from_fulfill_cx<'tcx>(
     mut fulfill_cx: Box<dyn TraitEngine<'tcx> + 'tcx>,
     infcx: &InferCtxt<'_, 'tcx>,
     placeholder_region: ty::Region<'tcx>,
     error_region: Option<ty::Region<'tcx>>,
 ) -> Option<DiagnosticBuilder<'tcx>> {
-    let tcx = infcx.tcx;
-
     // We generally shouldn't have errors here because the query was
     // already run, but there's no point using `delay_span_bug`
     // when we're going to emit an error here anyway.
     let _errors = fulfill_cx.select_all_or_error(infcx);
+    let region_constraints = infcx.with_region_constraints(|r| r.clone());
+    try_extract_error_from_region_constraints(
+        infcx,
+        placeholder_region,
+        error_region,
+        &region_constraints,
+        |vid| infcx.region_var_origin(vid),
+        |vid| infcx.universe_of_region(infcx.tcx.mk_region(ty::ReVar(vid))),
+    )
+}
 
-    let (sub_region, cause) = infcx.with_region_constraints(|region_constraints| {
-        debug!("{:#?}", region_constraints);
+fn try_extract_error_from_region_constraints<'tcx>(
+    infcx: &InferCtxt<'_, 'tcx>,
+    placeholder_region: ty::Region<'tcx>,
+    error_region: Option<ty::Region<'tcx>>,
+    region_constraints: &RegionConstraintData<'tcx>,
+    mut region_var_origin: impl FnMut(RegionVid) -> RegionVariableOrigin,
+    mut universe_of_region: impl FnMut(RegionVid) -> UniverseIndex,
+) -> Option<DiagnosticBuilder<'tcx>> {
+    let (sub_region, cause) =
         region_constraints.constraints.iter().find_map(|(constraint, cause)| {
             match *constraint {
                 Constraint::RegSubReg(sub, sup) if sup == placeholder_region && sup != sub => {
                     Some((sub, cause.clone()))
                 }
                 // FIXME: Should this check the universe of the var?
                 Constraint::VarSubReg(vid, sup) if sup == placeholder_region => {
-                    Some((tcx.mk_region(ty::ReVar(vid)), cause.clone()))
+                    Some((infcx.tcx.mk_region(ty::ReVar(vid)), cause.clone()))
                 }
                 _ => None,
             }
-        })
-    })?;
+        })?;
 
     debug!(?sub_region, "cause = {:#?}", cause);
     let nice_error = match (error_region, sub_region) {
         (Some(error_region), &ty::ReVar(vid)) => NiceRegionError::new(
             infcx,
             RegionResolutionError::SubSupConflict(
                 vid,
-                infcx.region_var_origin(vid),
+                region_var_origin(vid),
                 cause.clone(),
                 error_region,
                 cause.clone(),
@@ -380,8 +446,8 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
             infcx,
             RegionResolutionError::UpperBoundUniverseConflict(
                 vid,
-                infcx.region_var_origin(vid),
-                infcx.universe_of_region(sub_region),
+                region_var_origin(vid),
+                universe_of_region(vid),
                 cause.clone(),
                 placeholder_region,
             ),

compiler/rustc_borrowck/src/lib.rs

@@ -5,6 +5,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(let_else)]
 #![feature(min_specialization)]
+#![feature(never_type)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -44,6 +44,7 @@ mod reverse_sccs;
 pub mod values;
 
 pub struct RegionInferenceContext<'tcx> {
+    pub var_infos: VarInfos,
     /// Contains the definition for every region variable. Region
     /// variables are identified by their index (`RegionVid`). The
     /// definition contains information about where the region came
@@ -266,7 +267,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     ) -> Self {
         // Create a RegionDefinition for each inference variable.
         let definitions: IndexVec<_, _> = var_infos
-            .into_iter()
+            .iter()
             .map(|info| RegionDefinition::new(info.universe, info.origin))
             .collect();
 
@@ -291,6 +292,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
 
         let mut result = Self {
+            var_infos,
             definitions,
             liveness_constraints,
             constraints,

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -33,12 +33,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
     ) -> Fallible<R>
     where
         Op: type_op::TypeOp<'tcx, Output = R>,
-        Canonical<'tcx, Op>: ToUniverseInfo<'tcx>,
+        Op::ErrorInfo: ToUniverseInfo<'tcx>,
     {
         let old_universe = self.infcx.universe();
 
-        let TypeOpOutput { output, constraints, canonicalized_query } =
-            op.fully_perform(self.infcx)?;
+        let TypeOpOutput { output, constraints, error_info } = op.fully_perform(self.infcx)?;
 
         if let Some(data) = &constraints {
             self.push_region_constraints(locations, category, data);
@@ -47,8 +46,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         let universe = self.infcx.universe();
 
         if old_universe != universe {
-            let universe_info = match canonicalized_query {
-                Some(canonicalized_query) => canonicalized_query.to_universe_info(old_universe),
+            let universe_info = match error_info {
+                Some(error_info) => error_info.to_universe_info(old_universe),
                 None => UniverseInfo::other(),
             };
             for u in old_universe..universe {

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -268,7 +268,7 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
                         TypeOpOutput {
                             output: self.infcx.tcx.ty_error(),
                             constraints: None,
-                            canonicalized_query: None,
+                            error_info: None,
                         }
                     });
                 // Note: we need this in examples like

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -17,6 +17,7 @@ use rustc_hir::lang_items::LangItem;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_infer::infer::outlives::env::RegionBoundPairs;
+use rustc_infer::infer::region_constraints::RegionConstraintData;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{
     InferCtxt, InferOk, LateBoundRegionConversionTime, NllRegionVariableOrigin,
@@ -39,9 +40,11 @@ use rustc_target::abi::VariantIdx;
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::type_op;
+use rustc_trait_selection::traits::query::type_op::custom::scrape_region_constraints;
 use rustc_trait_selection::traits::query::type_op::custom::CustomTypeOp;
+use rustc_trait_selection::traits::query::type_op::{TypeOp, TypeOpOutput};
 use rustc_trait_selection::traits::query::Fallible;
-use rustc_trait_selection::traits::{self, ObligationCause};
+use rustc_trait_selection::traits::{self, ObligationCause, PredicateObligation};
 
 use rustc_const_eval::transform::{
     check_consts::ConstCx, promote_consts::is_const_fn_in_array_repeat_expression,
@@ -2637,3 +2640,32 @@ impl NormalizeLocation for Location {
         Locations::Single(self)
     }
 }
+
+/// Runs `infcx.instantiate_opaque_types`. Unlike other `TypeOp`s,
+/// this is not canonicalized - it directly affects the main `InferCtxt`
+/// that we use during MIR borrowchecking.
+#[derive(Debug)]
+pub(super) struct InstantiateOpaqueType<'tcx> {
+    pub base_universe: Option<ty::UniverseIndex>,
+    pub region_constraints: Option<RegionConstraintData<'tcx>>,
+    pub obligation: PredicateObligation<'tcx>,
+}
+
+impl<'tcx> TypeOp<'tcx> for InstantiateOpaqueType<'tcx> {
+    type Output = ();
+    /// We use this type itself to store the information used
+    /// when reporting errors. Since this is not a query, we don't
+    /// re-run anything during error reporting - we just use the information
+    /// we saved to help extract an error from the already-existing region
+    /// constraints in our `InferCtxt`
+    type ErrorInfo = InstantiateOpaqueType<'tcx>;
+
+    fn fully_perform(mut self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
+        let (mut output, region_constraints) = scrape_region_constraints(infcx, || {
+            Ok(InferOk { value: (), obligations: vec![self.obligation.clone()] })
+        })?;
+        self.region_constraints = Some(region_constraints);
+        output.error_info = Some(self);
+        Ok(output)
+    }
+}