Give RPITITs variance for precise capturing

Author: compiler-errors

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -61,6 +61,13 @@ fn variances_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Variance] {
             let crate_map = tcx.crate_variances(());
             return crate_map.variances.get(&item_def_id.to_def_id()).copied().unwrap_or(&[]);
         }
+        DefKind::AssocTy => match tcx.opt_rpitit_info(item_def_id.to_def_id()) {
+            Some(ty::ImplTraitInTraitData::Trait { opaque_def_id, .. }) => {
+                return variance_of_opaque(tcx, opaque_def_id.expect_local());
+            }
+            None => {}
+            Some(ty::ImplTraitInTraitData::Impl { .. }) => {}
+        },
         DefKind::OpaqueTy => {
             return variance_of_opaque(tcx, item_def_id);
         }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1086,9 +1086,12 @@ fn should_encode_variances<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, def_kind: Def
         | DefKind::Fn
         | DefKind::Ctor(..)
         | DefKind::AssocFn => true,
+        DefKind::AssocTy => {
+            // Only encode variances for RPITITs (for traits)
+            matches!(tcx.opt_rpitit_info(def_id), Some(ty::ImplTraitInTraitData::Trait { .. }))
+        }
         DefKind::Mod
         | DefKind::Field
-        | DefKind::AssocTy
         | DefKind::AssocConst
         | DefKind::TyParam
         | DefKind::ConstParam

compiler/rustc_middle/src/ty/relate.rs

@@ -233,6 +233,16 @@ impl<'tcx> Relate<'tcx> for ty::AliasTy<'tcx> {
                     b.args,
                     false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
                 )?,
+                DefKind::AssocTy if relation.tcx().is_impl_trait_in_trait(a.def_id) => {
+                    relate_args_with_variances(
+                        relation,
+                        a.def_id,
+                        relation.tcx().variances_of(a.def_id),
+                        a.args,
+                        b.args,
+                        false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
+                    )?
+                }
                 DefKind::AssocTy | DefKind::AssocConst | DefKind::TyAlias => {
                     relate_args_invariantly(relation, a.args, b.args)?
                 }