promote subject even if it has unnamed regions

Don't require a region to have an `external_name` in order to be
promoted.

Author: aliemjay

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -12,8 +12,9 @@ use rustc_infer::infer::outlives::test_type_match;
 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq};
 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
 use rustc_middle::mir::{
-    Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
-    ConstraintCategory, Local, Location, ReturnConstraint, TerminatorKind,
+    Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureOutlivesSubjectTy,
+    ClosureRegionRequirements, ConstraintCategory, Local, Location, ReturnConstraint,
+    TerminatorKind,
 };
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::traits::ObligationCauseCode;
@@ -1084,18 +1085,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         true
     }
 
-    /// When we promote a type test `T: 'r`, we have to convert the
-    /// type `T` into something we can store in a query result (so
-    /// something allocated for `'tcx`). This is problematic if `ty`
-    /// contains regions. During the course of NLL region checking, we
-    /// will have replaced all of those regions with fresh inference
-    /// variables. To create a test subject, we want to replace those
-    /// inference variables with some region from the closure
-    /// signature -- this is not always possible, so this is a
-    /// fallible process. Presuming we do find a suitable region, we
-    /// will use it's *external name*, which will be a `RegionKind`
-    /// variant that can be used in query responses such as
-    /// `ReEarlyBound`.
+    /// When we promote a type test `T: 'r`, we have to replace all region
+    /// variables in the type `T` with an equal universal region from the
+    /// closure signature.
+    /// This is not always possible, so this is a fallible process.
     #[instrument(level = "debug", skip(self, infcx))]
     fn try_promote_type_test_subject(
         &self,
@@ -1144,22 +1137,22 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             // find an equivalent.
             let upper_bound = self.non_local_universal_upper_bound(region_vid);
             if self.region_contains(region_vid, upper_bound) {
-                self.definitions[upper_bound].external_name.unwrap_or(r)
+                tcx.mk_re_var(upper_bound)
             } else {
-                // In the case of a failure, use a `ReVar` result. This will
-                // cause the `needs_infer` later on to return `None`.
-                r
+                // In the case of a failure, use `ReErased`. We will eventually
+                // return `None` in this case.
+                tcx.lifetimes.re_erased
             }
         });
 
         debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
 
-        // `needs_infer` will only be true if we failed to promote some region.
-        if ty.needs_infer() {
+        // This will be true if we failed to promote some region.
+        if ty.has_erased_regions() {
             return None;
         }
 
-        Some(ClosureOutlivesSubject::Ty(ty))
+        Some(ClosureOutlivesSubject::Ty(ClosureOutlivesSubjectTy::new(tcx, ty)))
     }
 
     /// Given some universal or existential region `r`, finds a

compiler/rustc_borrowck/src/type_check/constraint_conversion.rs

@@ -116,7 +116,9 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
             let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
             let subject = match outlives_requirement.subject {
                 ClosureOutlivesSubject::Region(re) => closure_mapping[re].into(),
-                ClosureOutlivesSubject::Ty(ty) => ty.into(),
+                ClosureOutlivesSubject::Ty(subject_ty) => {
+                    subject_ty.instantiate(self.tcx, |vid| closure_mapping[vid]).into()
+                }
             };
 
             self.category = outlives_requirement.category;

compiler/rustc_middle/src/mir/query.rs

@@ -8,7 +8,7 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_index::bit_set::BitMatrix;
-use rustc_index::vec::IndexVec;
+use rustc_index::vec::{Idx, IndexVec};
 use rustc_span::Span;
 use rustc_target::abi::VariantIdx;
 use smallvec::SmallVec;
@@ -289,13 +289,6 @@ pub struct ConstQualifs {
 /// instance of the closure is created, the corresponding free regions
 /// can be extracted from its type and constrained to have the given
 /// outlives relationship.
-///
-/// In some cases, we have to record outlives requirements between types and
-/// regions as well. In that case, if those types include any regions, those
-/// regions are recorded using their external names (`ReStatic`,
-/// `ReEarlyBound`, `ReFree`). We use these because in a query response we
-/// cannot use `ReVar` (which is what we use internally within the rest of the
-/// NLL code).
 #[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
 pub struct ClosureRegionRequirements<'tcx> {
     /// The number of external regions defined on the closure. In our
@@ -392,16 +385,56 @@ pub enum ClosureOutlivesSubject<'tcx> {
     /// Subject is a type, typically a type parameter, but could also
     /// be a projection. Indicates a requirement like `T: 'a` being
     /// passed to the caller, where the type here is `T`.
-    ///
-    /// The type here is guaranteed not to contain any free regions at
-    /// present.
-    Ty(Ty<'tcx>),
+    Ty(ClosureOutlivesSubjectTy<'tcx>),
 
     /// Subject is a free region from the closure. Indicates a requirement
     /// like `'a: 'b` being passed to the caller; the region here is `'a`.
     Region(ty::RegionVid),
 }
 
+/// Represents a `ty::Ty` for use in [`ClosureOutlivesSubject`].
+///
+/// This indirection is necessary because the type may include `ReVar` regions,
+/// which is what we use internally within NLL code,
+/// and we can't use `ReVar`s in a query response.
+#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct ClosureOutlivesSubjectTy<'tcx> {
+    inner: Ty<'tcx>,
+}
+
+impl<'tcx> ClosureOutlivesSubjectTy<'tcx> {
+    // All regions of `ty` must be of kind `ReVar`
+    // and must point to an early-bound region in the closure's signature.
+    pub fn new(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Self {
+        let inner = tcx.fold_regions(ty, |r, depth| match r.kind() {
+            ty::ReVar(vid) => {
+                let br = ty::BoundRegion {
+                    var: ty::BoundVar::new(vid.index()),
+                    kind: ty::BrAnon(0u32, None),
+                };
+                tcx.mk_re_late_bound(depth, br)
+            }
+            _ => bug!("unexpected region in ClosureOutlivesSubjectTy: {r:?}"),
+        });
+
+        Self { inner }
+    }
+
+    pub fn instantiate(
+        self,
+        tcx: TyCtxt<'tcx>,
+        mut map: impl FnMut(ty::RegionVid) -> ty::Region<'tcx>,
+    ) -> Ty<'tcx> {
+        tcx.fold_regions(self.inner, |r, depth| match r.kind() {
+            ty::ReLateBound(debruijn, br) => {
+                debug_assert_eq!(debruijn, depth);
+                map(ty::RegionVid::new(br.var.index()))
+            }
+            _ => bug!("unexpected region {r:?}"),
+        })
+    }
+}
+
 /// The constituent parts of a mir constant of kind ADT or array.
 #[derive(Copy, Clone, Debug, HashStable)]
 pub struct DestructuredConstant<'tcx> {

tests/ui/nll/closure-requirements/type-test-subject-opaque-1.rs

@@ -1,5 +1,5 @@
-// check-fail
-// known-bug: #107426
+// Regression test for #107426.
+// check-pass
 
 use std::marker::PhantomData;
 #[derive(Clone, Copy)]

tests/ui/nll/closure-requirements/type-test-subject-opaque-1.stderr

@@ -1,5 +1,5 @@
-// check-fail
-// known-bug: #108635
+// See #108635 for description.
+// check-pass
 
 trait Trait {
     type Item<'a>: 'a;