Auto merge of #116891 - aliemjay:opaque-region-infer-rework-2, r=<try>

rework opaque type region inference

fixes #113971 Pass -> Error

fixes #111906 ICE -> Pass
fixes #110623 ==
fixes #109059 ==

fixes #112841 Pass -> Error

fixes #110726 ICE->Error

r? `@ghost`

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -97,9 +97,11 @@ pub struct RegionInferenceContext<'tcx> {
     /// visible from this index.
     scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
 
-    /// Contains a "representative" from each SCC. This will be the
-    /// minimal RegionVid belonging to that universe. It is used as a
-    /// kind of hacky way to manage checking outlives relationships,
+    /// Contains the "representative" region of each SCC.
+    /// It is defined as the one with the minimal RegionVid, favoring
+    /// free regions, then placeholders, then existential regions.
+    ///
+    /// It is a hacky way to manage checking regions for equality,
     /// since we can 'canonicalize' each region to the representative
     /// of its SCC and be sure that -- if they have the same repr --
     /// they *must* be equal (though not having the same repr does not
@@ -487,22 +489,27 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         scc_universes
     }
 
-    /// For each SCC, we compute a unique `RegionVid` (in fact, the
-    /// minimal one that belongs to the SCC). See
+    /// For each SCC, we compute a unique `RegionVid`. See
     /// `scc_representatives` field of `RegionInferenceContext` for
     /// more details.
     fn compute_scc_representatives(
         constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
         definitions: &IndexSlice<RegionVid, RegionDefinition<'tcx>>,
     ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
         let num_sccs = constraints_scc.num_sccs();
-        let next_region_vid = definitions.next_index();
-        let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
-
-        for region_vid in definitions.indices() {
-            let scc = constraints_scc.scc(region_vid);
-            let prev_min = scc_representatives[scc];
-            scc_representatives[scc] = region_vid.min(prev_min);
+        let mut scc_representatives = IndexVec::from_elem_n(ty::RegionVid::MAX, num_sccs);
+
+        for (vid, def) in definitions.iter_enumerated() {
+            use NllRegionVariableOrigin as VarOrigin;
+            let scc = constraints_scc.scc(vid);
+            let repr = &mut scc_representatives[scc];
+            if *repr == ty::RegionVid::MAX {
+                *repr = vid;
+            } else if matches!(def.origin, VarOrigin::Placeholder(_))
+                && matches!(definitions[*repr].origin, VarOrigin::Existential { .. })
+            {
+                *repr = vid;
+            }
         }
 
         scc_representatives

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -1,10 +1,10 @@
-use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::OpaqueTyOrigin;
-use rustc_infer::infer::InferCtxt;
 use rustc_infer::infer::TyCtxtInferExt as _;
+use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin};
 use rustc_infer::traits::{Obligation, ObligationCause};
 use rustc_middle::traits::DefiningAnchor;
 use rustc_middle::ty::visit::TypeVisitableExt;
@@ -66,85 +66,60 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     ) -> FxIndexMap<LocalDefId, OpaqueHiddenType<'tcx>> {
         let mut result: FxIndexMap<LocalDefId, OpaqueHiddenType<'tcx>> = FxIndexMap::default();
 
-        let member_constraints: FxIndexMap<_, _> = self
-            .member_constraints
-            .all_indices()
-            .map(|ci| (self.member_constraints[ci].key, ci))
-            .collect();
-        debug!(?member_constraints);
-
         for (opaque_type_key, concrete_type) in opaque_ty_decls {
-            let args = opaque_type_key.args;
-            debug!(?concrete_type, ?args);
+            debug!(?opaque_type_key, ?concrete_type);
 
-            let mut subst_regions = vec![self.universal_regions.fr_static];
+            let mut arg_regions: Vec<(ty::RegionVid, ty::Region<'_>)> =
+                vec![(self.universal_regions.fr_static, infcx.tcx.lifetimes.re_static)];
 
-            let to_universal_region = |vid, subst_regions: &mut Vec<_>| {
-                trace!(?vid);
-                let scc = self.constraint_sccs.scc(vid);
-                trace!(?scc);
-                match self.scc_values.universal_regions_outlived_by(scc).find_map(|lb| {
-                    self.eval_equal(vid, lb).then_some(self.definitions[lb].external_name?)
-                }) {
-                    Some(region) => {
-                        let vid = self.universal_regions.to_region_vid(region);
-                        subst_regions.push(vid);
-                        region
+            let opaque_type_key =
+                opaque_type_key.fold_captured_lifetime_args(infcx.tcx, |region| {
+                    let scc = self.constraint_sccs.scc(self.to_region_vid(region));
+                    let vid = self.scc_representatives[scc];
+                    let named = match self.definitions[vid].origin {
+                        NllRegionVariableOrigin::FreeRegion => self
+                            .universal_regions
+                            .universal_regions()
+                            .filter(|&ur| {
+                                use crate::universal_regions::RegionClassification as Class;
+                                matches!(
+                                    self.universal_regions.region_classification(ur),
+                                    Some(Class::Global | Class::Local)
+                                )
+                            })
+                            .filter(|&ur| ur != self.universal_regions.fr_fn_body)
+                            .find(|&ur| self.universal_region_relations.equal(vid, ur))
+                            .map(|ur| self.definitions[ur].external_name.unwrap()),
+                        NllRegionVariableOrigin::Placeholder(placeholder) => {
+                            Some(ty::Region::new_placeholder(infcx.tcx, placeholder))
+                        }
+                        NllRegionVariableOrigin::Existential { .. } => None,
                     }
-                    None => {
-                        subst_regions.push(vid);
+                    .unwrap_or_else(|| {
                         ty::Region::new_error_with_message(
                             infcx.tcx,
                             concrete_type.span,
                             "opaque type with non-universal region args",
                         )
-                    }
-                }
-            };
+                    });
 
-            // Start by inserting universal regions from the member_constraint choice regions.
-            // This will ensure they get precedence when folding the regions in the concrete type.
-            if let Some(&ci) = member_constraints.get(&opaque_type_key) {
-                for &vid in self.member_constraints.choice_regions(ci) {
-                    to_universal_region(vid, &mut subst_regions);
-                }
-            }
-            debug!(?subst_regions);
-
-            // Next, insert universal regions from args, so we can translate regions that appear
-            // in them but are not subject to member constraints, for instance closure args.
-            let universal_args = infcx.tcx.fold_regions(args, |region, _| {
-                if let ty::RePlaceholder(..) = region.kind() {
-                    // Higher kinded regions don't need remapping, they don't refer to anything outside of this the args.
-                    return region;
-                }
+                    arg_regions.push((vid, named));
+                    named
+                });
+            debug!(?opaque_type_key, ?arg_regions);
+
+            let concrete_type = infcx.tcx.fold_regions(concrete_type, |region, _| {
                 let vid = self.to_region_vid(region);
-                to_universal_region(vid, &mut subst_regions)
+                arg_regions
+                    .iter()
+                    .find(|&&(ur_vid, _)| self.eval_equal(vid, ur_vid))
+                    .map(|&(_, ur_name)| ur_name)
+                    .unwrap_or(infcx.tcx.lifetimes.re_erased)
             });
-            debug!(?universal_args);
-            debug!(?subst_regions);
-
-            // Deduplicate the set of regions while keeping the chosen order.
-            let subst_regions = subst_regions.into_iter().collect::<FxIndexSet<_>>();
-            debug!(?subst_regions);
-
-            let universal_concrete_type =
-                infcx.tcx.fold_regions(concrete_type, |region, _| match *region {
-                    ty::ReVar(vid) => subst_regions
-                        .iter()
-                        .find(|ur_vid| self.eval_equal(vid, **ur_vid))
-                        .and_then(|ur_vid| self.definitions[*ur_vid].external_name)
-                        .unwrap_or(infcx.tcx.lifetimes.re_erased),
-                    _ => region,
-                });
-            debug!(?universal_concrete_type);
+            debug!(?concrete_type);
 
-            let opaque_type_key =
-                OpaqueTypeKey { def_id: opaque_type_key.def_id, args: universal_args };
-            let ty = infcx.infer_opaque_definition_from_instantiation(
-                opaque_type_key,
-                universal_concrete_type,
-            );
+            let ty =
+                infcx.infer_opaque_definition_from_instantiation(opaque_type_key, concrete_type);
             // Sometimes two opaque types are the same only after we remap the generic parameters
             // back to the opaque type definition. E.g. we may have `OpaqueType<X, Y>` mapped to `(X, Y)`
             // and `OpaqueType<Y, X>` mapped to `(Y, X)`, and those are the same, but we only know that
@@ -365,9 +340,9 @@ fn check_opaque_type_well_formed<'tcx>(
     }
 }
 
-fn check_opaque_type_parameter_valid(
-    tcx: TyCtxt<'_>,
-    opaque_type_key: OpaqueTypeKey<'_>,
+fn check_opaque_type_parameter_valid<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    opaque_type_key: OpaqueTypeKey<'tcx>,
     span: Span,
 ) -> Result<(), ErrorGuaranteed> {
     let opaque_ty_hir = tcx.hir().expect_item(opaque_type_key.def_id);
@@ -378,25 +353,28 @@ fn check_opaque_type_parameter_valid(
 
     let opaque_generics = tcx.generics_of(opaque_type_key.def_id);
     let mut seen_params: FxIndexMap<_, Vec<_>> = FxIndexMap::default();
-    for (i, arg) in opaque_type_key.args.iter().enumerate() {
+    for (i, arg) in opaque_type_key.iter_captured_args(tcx) {
         if let Err(guar) = arg.error_reported() {
             return Err(guar);
         }
 
         let arg_is_param = match arg.unpack() {
             GenericArgKind::Type(ty) => matches!(ty.kind(), ty::Param(_)),
-            GenericArgKind::Lifetime(lt) if is_ty_alias => {
-                matches!(*lt, ty::ReEarlyBound(_) | ty::ReFree(_))
-            }
-            // FIXME(#113916): we can't currently check for unique lifetime params,
-            // see that issue for more. We will also have to ignore unused lifetime
-            // params for RPIT, but that's comparatively trivial ✨
-            GenericArgKind::Lifetime(_) => continue,
+            GenericArgKind::Lifetime(lt) => match is_ty_alias {
+                true => matches!(*lt, ty::ReEarlyBound(_) | ty::ReFree(_)),
+                // FIXME(#111935, #113916): For RPIT, we currently accept ReStatic as well.
+                // This is a back-compat hack, see the issue for more.
+                false => matches!(*lt, ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic),
+            },
             GenericArgKind::Const(ct) => matches!(ct.kind(), ty::ConstKind::Param(_)),
         };
 
         if arg_is_param {
-            seen_params.entry(arg).or_default().push(i);
+            // FIXME(#113916): For RPIT, we can't currently check for unique lifetime
+            // params, see that issue for more. We limit this to TAIT for now.
+            if is_ty_alias {
+                seen_params.entry(arg).or_default().push(i);
+            }
         } else {
             // Prevent `fn foo() -> Foo<u32>` from being defining.
             let opaque_param = opaque_generics.param_at(i, tcx);

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -159,6 +159,13 @@ impl UniversalRegionRelations<'_> {
         self.outlives.contains(fr1, fr2)
     }
 
+    /// Returns `true` if fr1 is known to equal fr2.
+    ///
+    /// This will only ever be true for universally quantified regions.
+    pub(crate) fn equal(&self, fr1: RegionVid, fr2: RegionVid) -> bool {
+        self.outlives.contains(fr1, fr2) && self.outlives.contains(fr2, fr1)
+    }
+
     /// Returns a vector of free regions `x` such that `fr1: x` is
     /// known to hold.
     pub(crate) fn regions_outlived_by(&self, fr1: RegionVid) -> Vec<RegionVid> {

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -243,6 +243,22 @@ pub(crate) fn type_check<'mir, 'tcx>(
                 hidden_type.ty = Ty::new_error(infcx.tcx, reported);
             }
 
+            // Convert all regions to nll vars.
+            let (opaque_type_key, hidden_type) =
+                infcx.tcx.fold_regions((opaque_type_key, hidden_type), |region, _| {
+                    match region.kind() {
+                        ty::ReVar(_) => region,
+                        ty::RePlaceholder(placeholder) => checker
+                            .borrowck_context
+                            .constraints
+                            .placeholder_region(infcx, placeholder),
+                        _ => ty::Region::new_var(
+                            infcx.tcx,
+                            checker.borrowck_context.universal_regions.to_region_vid(region),
+                        ),
+                    }
+                });
+
             (opaque_type_key, hidden_type)
         })
         .collect();

compiler/rustc_middle/src/ty/mod.rs

@@ -1504,6 +1504,38 @@ pub struct OpaqueTypeKey<'tcx> {
     pub args: GenericArgsRef<'tcx>,
 }
 
+impl<'tcx> OpaqueTypeKey<'tcx> {
+    pub fn fold_captured_lifetime_args(
+        self,
+        tcx: TyCtxt<'tcx>,
+        mut f: impl FnMut(Region<'tcx>) -> Region<'tcx>,
+    ) -> Self {
+        let Self { def_id, args } = self;
+        let args = std::iter::zip(args, tcx.variances_of(def_id)).map(|(arg, v)| {
+            match (arg.unpack(), v) {
+                (ty::GenericArgKind::Lifetime(_), ty::Bivariant) => arg,
+                (ty::GenericArgKind::Lifetime(lt), _) => f(lt).into(),
+                _ => arg,
+            }
+        });
+        let args = tcx.mk_args_from_iter(args);
+        Self { def_id, args }
+    }
+
+    pub fn iter_captured_args(
+        self,
+        tcx: TyCtxt<'tcx>,
+    ) -> impl Iterator<Item = (usize, GenericArg<'tcx>)> {
+        std::iter::zip(self.args, tcx.variances_of(self.def_id)).enumerate().filter_map(
+            |(i, (arg, v))| match (arg.unpack(), v) {
+                (_, ty::Invariant) => Some((i, arg)),
+                (ty::GenericArgKind::Lifetime(_), ty::Bivariant) => None,
+                _ => bug!("unexpected opaque type arg variance"),
+            },
+        )
+    }
+}
+
 #[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable, HashStable, TyEncodable, TyDecodable)]
 pub struct OpaqueHiddenType<'tcx> {
     /// The span of this particular definition of the opaque type. So

tests/ui/impl-trait/defining-use-captured-non-universal-region.infer.stderr

@@ -0,0 +1,12 @@
+error[E0792]: expected generic lifetime parameter, found `'_`
+  --> $DIR/defining-use-captured-non-universal-region.rs:15:18
+   |
+LL | fn foo<'a>() -> impl Sized + 'a {
+   |                              -- this generic parameter must be used with a generic lifetime parameter
+...
+LL |     let i: i32 = foo::<'_>();
+   |                  ^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0792`.

tests/ui/impl-trait/defining-use-captured-non-universal-region.rs

@@ -0,0 +1,24 @@
+// This was an ICE. See #110726.
+//
+// FIXME(#111935) revision `statik` should not pass.
+
+// revisions: statik infer fixed
+//[fixed] check-pass
+//[statik] check-pass
+#![allow(unconditional_recursion)]
+
+fn foo<'a>() -> impl Sized + 'a {
+    #[cfg(statik)]
+    let i: i32 = foo::<'static>();
+
+    #[cfg(infer)]
+    let i: i32 = foo::<'_>();
+    //[infer]~^ ERROR expected generic lifetime parameter, found `'_`
+
+    #[cfg(fixed)]
+    let i: i32 = foo::<'a>();
+
+    i
+}
+
+fn main() {}