Report nicer errors for HRTB NLL errors from queries

compiler/rustc_infer/src/infer/mod.rs

@@ -1111,7 +1111,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// etc) this is the root universe U0. For inference variables or
     /// placeholders, however, it will return the universe which which
     /// they are associated.
-    fn universe_of_region(&self, r: ty::Region<'tcx>) -> ty::UniverseIndex {
+    pub fn universe_of_region(&self, r: ty::Region<'tcx>) -> ty::UniverseIndex {
         self.inner.borrow_mut().unwrap_region_constraints().universe(r)
     }
 
@@ -1289,6 +1289,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         op(inner.unwrap_region_constraints().data())
     }
 
+    pub fn region_var_origin(&self, vid: ty::RegionVid) -> RegionVariableOrigin {
+        let mut inner = self.inner.borrow_mut();
+        let inner = &mut *inner;
+        inner
+            .region_constraint_storage
+            .as_mut()
+            .expect("regions already resolved")
+            .with_log(&mut inner.undo_log)
+            .var_origin(vid)
+    }
+
     /// Takes ownership of the list of variable regions. This implies
     /// that all the region constraints have already been taken, and
     /// hence that `resolve_regions_and_report_errors` can never be

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -445,6 +445,11 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         self.var_infos[vid].universe
     }
 
+    /// Returns the origin for the given variable.
+    pub fn var_origin(&self, vid: RegionVid) -> RegionVariableOrigin {
+        self.var_infos[vid].origin
+    }
+
     fn add_constraint(&mut self, constraint: Constraint<'tcx>, origin: SubregionOrigin<'tcx>) {
         // cannot add constraints once regions are resolved
         debug!("RegionConstraintCollector: add_constraint({:?})", constraint);

compiler/rustc_mir/src/borrow_check/diagnostics/bound_region_errors.rs

@@ -1,9 +1,14 @@
+use rustc_errors::DiagnosticBuilder;
 use rustc_infer::infer::canonical::Canonical;
-use rustc_infer::traits::ObligationCause;
+use rustc_infer::infer::error_reporting::nice_region_error::NiceRegionError;
+use rustc_infer::infer::region_constraints::Constraint;
+use rustc_infer::infer::{InferCtxt, RegionResolutionError, SubregionOrigin, TyCtxtInferExt as _};
+use rustc_infer::traits::{Normalized, Obligation, ObligationCause, TraitEngine, TraitEngineExt};
 use rustc_middle::ty::error::TypeError;
-use rustc_middle::ty::{self, Ty, TypeFoldable};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc_span::Span;
 use rustc_trait_selection::traits::query::type_op;
+use rustc_trait_selection::traits::{SelectionContext, TraitEngineExt as _};
 
 use std::fmt;
 use std::rc::Rc;
@@ -37,11 +42,10 @@ impl UniverseInfo<'tcx> {
     crate fn report_error(
         &self,
         mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
-        _placeholder: ty::PlaceholderRegion,
-        _error_element: RegionElement,
+        placeholder: ty::PlaceholderRegion,
+        error_element: RegionElement,
         span: Span,
     ) {
-        // FIXME: improve this error message
         match self.0 {
             UniverseInfoInner::RelateTys { expected, found } => {
                 let body_id = mbcx.infcx.tcx.hir().local_def_id_to_hir_id(mbcx.mir_def_id());
@@ -53,7 +57,13 @@ impl UniverseInfo<'tcx> {
                 );
                 err.buffer(&mut mbcx.errors_buffer);
             }
-            UniverseInfoInner::TypeOp(_) | UniverseInfoInner::Other => {
+            UniverseInfoInner::TypeOp(ref type_op_info) => {
+                type_op_info.report_error(mbcx, placeholder, error_element, span);
+            }
+            UniverseInfoInner::Other => {
+                // FIXME: This error message isn't great, but it doesn't show
+                // up in the existing UI tests. Consider investigating this
+                // some more.
                 mbcx.infcx
                     .tcx
                     .sess
@@ -73,8 +83,8 @@ impl<'tcx> ToUniverseInfo<'tcx>
 {
     fn to_universe_info(self, base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
         UniverseInfo(UniverseInfoInner::TypeOp(Rc::new(PredicateQuery {
-            _canonical_query: self,
-            _base_universe: base_universe,
+            canonical_query: self,
+            base_universe,
         })))
     }
 }
@@ -84,8 +94,8 @@ impl<'tcx, T: Copy + fmt::Display + TypeFoldable<'tcx> + 'tcx> ToUniverseInfo<'t
 {
     fn to_universe_info(self, base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
         UniverseInfo(UniverseInfoInner::TypeOp(Rc::new(NormalizeQuery {
-            _canonical_query: self,
-            _base_universe: base_universe,
+            canonical_query: self,
+            base_universe,
         })))
     }
 }
@@ -109,23 +119,229 @@ impl<'tcx, F, G> ToUniverseInfo<'tcx> for Canonical<'tcx, type_op::custom::Custo
 
 #[allow(unused_lifetimes)]
 trait TypeOpInfo<'tcx> {
-    // TODO: Methods for rerunning type op and reporting an error
+    /// Returns an rrror to be reported if rerunning the type op fails to
+    /// recover the error's cause.
+    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx>;
+
+    fn base_universe(&self) -> ty::UniverseIndex;
+
+    fn nice_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+        placeholder_region: ty::Region<'tcx>,
+        error_region: Option<ty::Region<'tcx>>,
+    ) -> Option<DiagnosticBuilder<'tcx>>;
+
+    fn report_error(
+        &self,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
+        placeholder: ty::PlaceholderRegion,
+        error_element: RegionElement,
+        span: Span,
+    ) {
+        let tcx = mbcx.infcx.tcx;
+        let base_universe = self.base_universe();
+
+        let adjusted_universe = if let Some(adjusted) =
+            placeholder.universe.as_u32().checked_sub(base_universe.as_u32())
+        {
+            adjusted
+        } else {
+            self.fallback_error(tcx, span).buffer(&mut mbcx.errors_buffer);
+            return;
+        };
+
+        let placeholder_region = tcx.mk_region(ty::RePlaceholder(ty::Placeholder {
+            name: placeholder.name,
+            universe: adjusted_universe.into(),
+        }));
+
+        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| {
+                    tcx.mk_region(ty::RePlaceholder(ty::Placeholder {
+                        name: error_placeholder.name,
+                        universe: adjusted.into(),
+                    }))
+                })
+            } else {
+                None
+            };
+
+        debug!(?placeholder_region);
+
+        let nice_error = self.nice_error(tcx, span, placeholder_region, error_region);
+
+        if let Some(nice_error) = nice_error {
+            nice_error.buffer(&mut mbcx.errors_buffer);
+        } else {
+            self.fallback_error(tcx, span).buffer(&mut mbcx.errors_buffer);
+        }
+    }
 }
 
 struct PredicateQuery<'tcx> {
-    _canonical_query:
+    canonical_query:
         Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::prove_predicate::ProvePredicate<'tcx>>>,
-    _base_universe: ty::UniverseIndex,
+    base_universe: ty::UniverseIndex,
 }
 
-impl TypeOpInfo<'tcx> for PredicateQuery<'tcx> {}
+impl TypeOpInfo<'tcx> for PredicateQuery<'tcx> {
+    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+        let mut err = tcx.sess.struct_span_err(span, "higher-ranked lifetime error");
+        err.note(&format!("could not prove {}", self.canonical_query.value.value.predicate));
+        err
+    }
+
+    fn base_universe(&self) -> ty::UniverseIndex {
+        self.base_universe
+    }
+
+    fn nice_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+        placeholder_region: ty::Region<'tcx>,
+        error_region: Option<ty::Region<'tcx>>,
+    ) -> Option<DiagnosticBuilder<'tcx>> {
+        tcx.infer_ctxt().enter_with_canonical(span, &self.canonical_query, |ref infcx, key, _| {
+            let mut fulfill_cx = TraitEngine::new(tcx);
+
+            let (param_env, prove_predicate) = key.into_parts();
+            fulfill_cx.register_predicate_obligation(
+                infcx,
+                Obligation::new(
+                    ObligationCause::dummy_with_span(span),
+                    param_env,
+                    prove_predicate.predicate,
+                ),
+            );
+
+            try_extract_error_from_fulfill_cx(fulfill_cx, infcx, placeholder_region, error_region)
+        })
+    }
+}
 
 struct NormalizeQuery<'tcx, T> {
-    _canonical_query: Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::Normalize<T>>>,
-    _base_universe: ty::UniverseIndex,
+    canonical_query: Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::Normalize<T>>>,
+    base_universe: ty::UniverseIndex,
 }
 
-impl<T> TypeOpInfo<'tcx> for NormalizeQuery<'tcx, T> where
-    T: Copy + fmt::Display + TypeFoldable<'tcx> + 'tcx
+impl<T> TypeOpInfo<'tcx> for NormalizeQuery<'tcx, T>
+where
+    T: Copy + fmt::Display + TypeFoldable<'tcx> + 'tcx,
 {
+    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+        let mut err = tcx.sess.struct_span_err(span, "higher-ranked lifetime error");
+        err.note(&format!("could not normalize `{}`", self.canonical_query.value.value.value));
+        err
+    }
+
+    fn base_universe(&self) -> ty::UniverseIndex {
+        self.base_universe
+    }
+
+    fn nice_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+        placeholder_region: ty::Region<'tcx>,
+        error_region: Option<ty::Region<'tcx>>,
+    ) -> Option<DiagnosticBuilder<'tcx>> {
+        tcx.infer_ctxt().enter_with_canonical(span, &self.canonical_query, |ref infcx, key, _| {
+            let mut fulfill_cx = TraitEngine::new(tcx);
+
+            let mut selcx = SelectionContext::new(infcx);
+            let (param_env, value) = key.into_parts();
+
+            let Normalized { value: _, obligations } = rustc_trait_selection::traits::normalize(
+                &mut selcx,
+                param_env,
+                ObligationCause::dummy_with_span(span),
+                value.value,
+            );
+            fulfill_cx.register_predicate_obligations(infcx, obligations);
+
+            try_extract_error_from_fulfill_cx(fulfill_cx, infcx, placeholder_region, error_region)
+        })
+    }
+}
+
+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 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).err().unwrap_or_else(Vec::new);
+
+    let region_obligations = infcx.take_registered_region_obligations();
+    debug!(?region_obligations);
+
+    let (sub_region, cause) = infcx.with_region_constraints(|region_constraints| {
+        debug!(?region_constraints);
+        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()))
+                }
+                _ => None,
+            }
+        })
+    })?;
+
+    debug!(?sub_region, ?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),
+                cause.clone(),
+                error_region,
+                cause.clone(),
+                placeholder_region,
+            ),
+        ),
+        (Some(error_region), _) => NiceRegionError::new(
+            infcx,
+            RegionResolutionError::ConcreteFailure(cause.clone(), error_region, placeholder_region),
+        ),
+        // Note universe here is wrong...
+        (None, &ty::ReVar(vid)) => NiceRegionError::new(
+            infcx,
+            RegionResolutionError::UpperBoundUniverseConflict(
+                vid,
+                infcx.region_var_origin(vid),
+                infcx.universe_of_region(sub_region),
+                cause.clone(),
+                placeholder_region,
+            ),
+        ),
+        (None, _) => NiceRegionError::new(
+            infcx,
+            RegionResolutionError::ConcreteFailure(cause.clone(), sub_region, placeholder_region),
+        ),
+    };
+    nice_error.try_report_from_nll().or_else(|| {
+        if let SubregionOrigin::Subtype(trace) = cause {
+            Some(
+                infcx.report_and_explain_type_error(*trace, &TypeError::RegionsPlaceholderMismatch),
+            )
+        } else {
+            None
+        }
+    })
 }

src/test/ui/associated-types/higher-ranked-projection.bad.nll.stderr

@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/higher-ranked-projection.rs:25:5
    |
 LL |     foo(());
-   |     ^^^^^^^
+   |     ^^^^^^^ one type is more general than the other
+   |
+   = note:   expected type `&'a ()`
+           found reference `&()`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/generator/auto-trait-regions.nll.stderr

@@ -24,17 +24,23 @@ LL |         assert_foo(a);
    |
    = note: consider using a `let` binding to create a longer lived value
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/auto-trait-regions.rs:31:5
    |
 LL |     assert_foo(gen);
-   |     ^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `&'0 OnlyFooIfStaticRef` must implement `Foo`, for any lifetime `'0`...
+   = note: ...but `Foo` is actually implemented for the type `&'static OnlyFooIfStaticRef`
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/auto-trait-regions.rs:50:5
    |
 LL |     assert_foo(gen);
-   |     ^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo` would have to be implemented for the type `A<'0, '1>`, for any two lifetimes `'0` and `'1`...
+   = note: ...but `Foo` is actually implemented for the type `A<'_, '2>`, for some specific lifetime `'2`
 
 error: aborting due to 4 previous errors