Auto merge of #110399 - cjgillot:infer-variance, r=aliemjay

Account for opaque variance in outlives

Fixes https://github.com/rust-lang/rust/issues/108591
Fixes https://github.com/rust-lang/rust/issues/108592
cc `@aliemjay`

compiler/rustc_infer/src/infer/outlives/components.rs

@@ -143,7 +143,7 @@ fn compute_components<'tcx>(
                     // through and constrain Pi.
                     let mut subcomponents = smallvec![];
                     let mut subvisited = SsoHashSet::new();
-                    compute_components_recursive(tcx, ty.into(), &mut subcomponents, &mut subvisited);
+                    compute_alias_components_recursive(tcx, ty, &mut subcomponents, &mut subvisited);
                     out.push(Component::EscapingAlias(subcomponents.into_iter().collect()));
                 }
             }
@@ -193,7 +193,43 @@ fn compute_components<'tcx>(
 ///
 /// This should not be used to get the components of `parent` itself.
 /// Use [push_outlives_components] instead.
-pub(super) fn compute_components_recursive<'tcx>(
+pub(super) fn compute_alias_components_recursive<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    alias_ty: Ty<'tcx>,
+    out: &mut SmallVec<[Component<'tcx>; 4]>,
+    visited: &mut SsoHashSet<GenericArg<'tcx>>,
+) {
+    let ty::Alias(kind, alias_ty) = alias_ty.kind() else { bug!() };
+    let opt_variances = if *kind == ty::Opaque { tcx.variances_of(alias_ty.def_id) } else { &[] };
+    for (index, child) in alias_ty.substs.iter().enumerate() {
+        if opt_variances.get(index) == Some(&ty::Bivariant) {
+            continue;
+        }
+        if !visited.insert(child) {
+            continue;
+        }
+        match child.unpack() {
+            GenericArgKind::Type(ty) => {
+                compute_components(tcx, ty, out, visited);
+            }
+            GenericArgKind::Lifetime(lt) => {
+                // Ignore late-bound regions.
+                if !lt.is_late_bound() {
+                    out.push(Component::Region(lt));
+                }
+            }
+            GenericArgKind::Const(_) => {
+                compute_components_recursive(tcx, child, out, visited);
+            }
+        }
+    }
+}
+
+/// Collect [Component]s for *all* the substs of `parent`.
+///
+/// This should not be used to get the components of `parent` itself.
+/// Use [push_outlives_components] instead.
+fn compute_components_recursive<'tcx>(
     tcx: TyCtxt<'tcx>,
     parent: GenericArg<'tcx>,
     out: &mut SmallVec<[Component<'tcx>; 4]>,

compiler/rustc_infer/src/infer/outlives/obligations.rs

@@ -344,12 +344,14 @@ where
         // the problem is to add `T: 'r`, which isn't true. So, if there are no
         // inference variables, we use a verify constraint instead of adding
         // edges, which winds up enforcing the same condition.
+        let is_opaque = alias_ty.kind(self.tcx) == ty::Opaque;
         if approx_env_bounds.is_empty()
             && trait_bounds.is_empty()
-            && (alias_ty.needs_infer() || alias_ty.kind(self.tcx) == ty::Opaque)
+            && (alias_ty.needs_infer() || is_opaque)
         {
             debug!("no declared bounds");
-            self.substs_must_outlive(alias_ty.substs, origin, region);
+            let opt_variances = is_opaque.then(|| self.tcx.variances_of(alias_ty.def_id));
+            self.substs_must_outlive(alias_ty.substs, origin, region, opt_variances);
             return;
         }
 
@@ -395,22 +397,31 @@ where
         self.delegate.push_verify(origin, GenericKind::Alias(alias_ty), region, verify_bound);
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn substs_must_outlive(
         &mut self,
         substs: SubstsRef<'tcx>,
         origin: infer::SubregionOrigin<'tcx>,
         region: ty::Region<'tcx>,
+        opt_variances: Option<&[ty::Variance]>,
     ) {
         let constraint = origin.to_constraint_category();
-        for k in substs {
+        for (index, k) in substs.iter().enumerate() {
             match k.unpack() {
                 GenericArgKind::Lifetime(lt) => {
-                    self.delegate.push_sub_region_constraint(
-                        origin.clone(),
-                        region,
-                        lt,
-                        constraint,
-                    );
+                    let variance = if let Some(variances) = opt_variances {
+                        variances[index]
+                    } else {
+                        ty::Invariant
+                    };
+                    if variance == ty::Invariant {
+                        self.delegate.push_sub_region_constraint(
+                            origin.clone(),
+                            region,
+                            lt,
+                            constraint,
+                        );
+                    }
                 }
                 GenericArgKind::Type(ty) => {
                     self.type_must_outlive(origin.clone(), ty, region, constraint);

compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -1,4 +1,4 @@
-use crate::infer::outlives::components::{compute_components_recursive, Component};
+use crate::infer::outlives::components::{compute_alias_components_recursive, Component};
 use crate::infer::outlives::env::RegionBoundPairs;
 use crate::infer::region_constraints::VerifyIfEq;
 use crate::infer::VerifyBound;
@@ -130,7 +130,12 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         // see the extensive comment in projection_must_outlive
         let recursive_bound = {
             let mut components = smallvec![];
-            compute_components_recursive(self.tcx, alias_ty_as_ty.into(), &mut components, visited);
+            compute_alias_components_recursive(
+                self.tcx,
+                alias_ty_as_ty.into(),
+                &mut components,
+                visited,
+            );
             self.bound_from_components(&components, visited)
         };