[WIP] Expand free alias types during variance computation

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1533,8 +1533,7 @@ fn detect_discriminant_duplicate<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
 
 fn check_type_alias_type_params_are_used<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) {
     if tcx.type_alias_is_lazy(def_id) {
-        // Since we compute the variances for lazy type aliases and already reject bivariant
-        // parameters as unused, we can and should skip this check for lazy type aliases.
+        // FIXME(fmease): Explainer
         return;
     }
 

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -322,9 +322,7 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
         hir::ItemKind::TraitAlias(..) => check_trait(tcx, item),
         // `ForeignItem`s are handled separately.
         hir::ItemKind::ForeignMod { .. } => Ok(()),
-        hir::ItemKind::TyAlias(_, hir_ty, hir_generics)
-            if tcx.type_alias_is_lazy(item.owner_id) =>
-        {
+        hir::ItemKind::TyAlias(_, hir_ty, _) if tcx.type_alias_is_lazy(item.owner_id) => {
             let res = enter_wf_checking_ctxt(tcx, item.span, def_id, |wfcx| {
                 let ty = tcx.type_of(def_id).instantiate_identity();
                 let item_ty =
@@ -337,7 +335,6 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
                 check_where_clauses(wfcx, item.span, def_id);
                 Ok(())
             });
-            check_variances_for_type_defn(tcx, item, hir_generics);
             res
         }
         _ => Ok(()),
@@ -2044,15 +2041,7 @@ fn check_variances_for_type_defn<'tcx>(
     hir_generics: &hir::Generics<'tcx>,
 ) {
     match item.kind {
-        ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) => {
-            // Ok
-        }
-        ItemKind::TyAlias(..) => {
-            assert!(
-                tcx.type_alias_is_lazy(item.owner_id),
-                "should not be computing variance of non-free type alias"
-            );
-        }
+        ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) => {} // OK
         kind => span_bug!(item.span, "cannot compute the variances of {kind:?}"),
     }
 
@@ -2195,7 +2184,6 @@ fn report_bivariance<'tcx>(
                 errors::UnusedGenericParameterHelp::AdtNoPhantomData { param_name }
             }
         }
-        ItemKind::TyAlias(..) => errors::UnusedGenericParameterHelp::TyAlias { param_name },
         item_kind => bug!("report_bivariance: unexpected item kind: {item_kind:?}"),
     };
 
@@ -2268,9 +2256,6 @@ impl<'tcx> IsProbablyCyclical<'tcx> {
                     self.tcx.type_of(field.did).instantiate_identity().visit_with(self)
                 })
             }
-            DefKind::TyAlias if self.tcx.type_alias_is_lazy(def_id) => {
-                self.tcx.type_of(def_id).instantiate_identity().visit_with(self)
-            }
             _ => ControlFlow::Continue(()),
         }
     }
@@ -2280,17 +2265,12 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IsProbablyCyclical<'tcx> {
     type Result = ControlFlow<(), ()>;
 
     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<(), ()> {
-        let def_id = match ty.kind() {
-            ty::Adt(adt_def, _) => Some(adt_def.did()),
-            ty::Alias(ty::Free, alias_ty) => Some(alias_ty.def_id),
-            _ => None,
-        };
-        if let Some(def_id) = def_id {
-            if def_id == self.item_def_id {
+        if let Some(adt_def) = ty.ty_adt_def() {
+            if adt_def.did() == self.item_def_id {
                 return ControlFlow::Break(());
             }
-            if self.seen.insert(def_id) {
-                self.visit_def(def_id)?;
+            if self.seen.insert(adt_def.did()) {
+                self.visit_def(adt_def.did())?;
             }
         }
         ty.super_visit_with(self)

compiler/rustc_hir_analysis/src/variance/constraints.rs

@@ -79,9 +79,6 @@ pub(crate) fn add_constraints_from_crate<'a, 'tcx>(
                 }
             }
             DefKind::Fn | DefKind::AssocFn => constraint_cx.build_constraints_for_item(def_id),
-            DefKind::TyAlias if tcx.type_alias_is_lazy(def_id) => {
-                constraint_cx.build_constraints_for_item(def_id)
-            }
             _ => {}
         }
     }
@@ -107,15 +104,6 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
         let current_item = &CurrentItem { inferred_start };
         let ty = tcx.type_of(def_id).instantiate_identity();
 
-        // The type as returned by `type_of` is the underlying type and generally not a free alias.
-        // Therefore we need to check the `DefKind` first.
-        if let DefKind::TyAlias = tcx.def_kind(def_id)
-            && tcx.type_alias_is_lazy(def_id)
-        {
-            self.add_constraints_from_ty(current_item, ty, self.covariant);
-            return;
-        }
-
         match ty.kind() {
             ty::Adt(def, _) => {
                 // Not entirely obvious: constraints on structs/enums do not
@@ -223,6 +211,7 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
         variance: VarianceTermPtr<'a>,
     ) {
         debug!("add_constraints_from_ty(ty={:?}, variance={:?})", ty, variance);
+        let ty = self.tcx().expand_free_alias_tys(ty);
 
         match *ty.kind() {
             ty::Bool
@@ -273,12 +262,11 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
                 self.add_constraints_from_args(current, def.did(), args, variance);
             }
 
-            ty::Alias(ty::Projection | ty::Inherent | ty::Opaque, ref data) => {
-                self.add_constraints_from_invariant_args(current, data.args, variance);
-            }
+            // All free alias types should've been expanded beforehand.
+            ty::Alias(ty::Free, _) => panic!("unexpected free alias type"),
 
-            ty::Alias(ty::Free, ref data) => {
-                self.add_constraints_from_args(current, data.def_id, data.args, variance);
+            ty::Alias(_, ref alias_ty) => {
+                self.add_constraints_from_invariant_args(current, alias_ty.args, variance);
             }
 
             ty::Dynamic(data, r, _) => {

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -56,11 +56,6 @@ 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::TyAlias if tcx.type_alias_is_lazy(item_def_id) => {
-            // These are inferred.
-            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(

compiler/rustc_hir_analysis/src/variance/terms.rs

@@ -99,9 +99,6 @@ pub(crate) fn determine_parameters_to_be_inferred<'a, 'tcx>(
                 }
             }
             DefKind::Fn | DefKind::AssocFn => terms_cx.add_inferreds_for_item(def_id),
-            DefKind::TyAlias if tcx.type_alias_is_lazy(def_id) => {
-                terms_cx.add_inferreds_for_item(def_id)
-            }
             _ => {}
         }
     }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1123,6 +1123,7 @@ fn should_encode_variances<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, def_kind: Def
         | DefKind::Static { .. }
         | DefKind::Const
         | DefKind::ForeignMod
+        | DefKind::TyAlias
         | DefKind::Impl { .. }
         | DefKind::Trait
         | DefKind::TraitAlias
@@ -1136,7 +1137,6 @@ fn should_encode_variances<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, def_kind: Def
         | DefKind::Closure
         | DefKind::ExternCrate
         | DefKind::SyntheticCoroutineBody => false,
-        DefKind::TyAlias => tcx.type_alias_is_lazy(def_id),
     }
 }