Rollup merge of #139669 - nnethercote:overhaul-AssocItem, r=oli-obk

Overhaul `AssocItem`

`AssocItem` has multiple fields that only make sense some of the time. E.g. the `name` can be empty if it's an RPITIT associated type. It's clearer and less error prone if these fields are moved to the relevant `kind` variants.

r? ``@fee1-dead``

2 files changed, 18 insertions, 17 deletions

diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 4cc12ca2e0b..3c2245347f9 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1332,29 +1332,30 @@ impl<'a> CrateMetadataRef<'a> {
     }
 
     fn get_associated_item(self, id: DefIndex, sess: &'a Session) -> ty::AssocItem {
-        let name = if self.root.tables.opt_rpitit_info.get(self, id).is_some() {
-            kw::Empty
-        } else {
-            self.item_name(id)
-        };
-        let (kind, has_self) = match self.def_kind(id) {
-            DefKind::AssocConst => (ty::AssocKind::Const, false),
-            DefKind::AssocFn => (ty::AssocKind::Fn, self.get_fn_has_self_parameter(id, sess)),
-            DefKind::AssocTy => (ty::AssocKind::Type, false),
+        let kind = match self.def_kind(id) {
+            DefKind::AssocConst => ty::AssocKind::Const { name: self.item_name(id) },
+            DefKind::AssocFn => ty::AssocKind::Fn {
+                name: self.item_name(id),
+                has_self: self.get_fn_has_self_parameter(id, sess),
+            },
+            DefKind::AssocTy => {
+                let data = if let Some(rpitit_info) = self.root.tables.opt_rpitit_info.get(self, id)
+                {
+                    ty::AssocTypeData::Rpitit(rpitit_info.decode(self))
+                } else {
+                    ty::AssocTypeData::Normal(self.item_name(id))
+                };
+                ty::AssocKind::Type { data }
+            }
             _ => bug!("cannot get associated-item of `{:?}`", self.def_key(id)),
         };
         let container = self.root.tables.assoc_container.get(self, id).unwrap();
-        let opt_rpitit_info =
-            self.root.tables.opt_rpitit_info.get(self, id).map(|d| d.decode(self));
 
         ty::AssocItem {
-            name,
             kind,
             def_id: self.local_def_id(id),
             trait_item_def_id: self.get_trait_item_def_id(id),
             container,
-            fn_has_self_parameter: has_self,
-            opt_rpitit_info,
         }
     }
 
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 5c8e2888ec9..177318bfe15 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1338,7 +1338,7 @@ fn should_encode_const(def_kind: DefKind) -> bool {
 fn should_encode_fn_impl_trait_in_trait<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
     if let Some(assoc_item) = tcx.opt_associated_item(def_id)
         && assoc_item.container == ty::AssocItemContainer::Trait
-        && assoc_item.kind == ty::AssocKind::Fn
+        && assoc_item.is_fn()
     {
         true
     } else {
@@ -1691,7 +1691,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
         match item.container {
             AssocItemContainer::Trait => {
-                if let ty::AssocKind::Type = item.kind {
+                if item.is_type() {
                     self.encode_explicit_item_bounds(def_id);
                     self.encode_explicit_item_self_bounds(def_id);
                     if tcx.is_conditionally_const(def_id) {
@@ -1706,7 +1706,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 }
             }
         }
-        if let Some(rpitit_info) = item.opt_rpitit_info {
+        if let ty::AssocKind::Type { data: ty::AssocTypeData::Rpitit(rpitit_info) } = item.kind {
             record!(self.tables.opt_rpitit_info[def_id] <- rpitit_info);
             if matches!(rpitit_info, ty::ImplTraitInTraitData::Trait { .. }) {
                 record_array!(