rustdoc: Correctly classify enums/typedefs

Both of these items are surfaced as a DefTy, so some extra logic was needed in
the decoder module to figure out whether one is actually an enum or whether it's
a typedef.

Closes #14757

src/librustc/metadata/csearch.rs

@@ -321,3 +321,8 @@ pub fn get_reachable_extern_fns(cstore: &cstore::CStore, cnum: ast::CrateNum)
     let cdata = cstore.get_crate_data(cnum);
     decoder::get_reachable_extern_fns(&*cdata)
 }
+
+pub fn is_typedef(cstore: &cstore::CStore, did: ast::DefId) -> bool {
+    let cdata = cstore.get_crate_data(did.krate);
+    decoder::is_typedef(&*cdata, did.node)
+}

src/librustc/metadata/decoder.rs

@@ -1339,3 +1339,11 @@ pub fn get_reachable_extern_fns(cdata: Cmd) -> Vec<ast::DefId> {
     });
     return ret;
 }
+
+pub fn is_typedef(cdata: Cmd, id: ast::NodeId) -> bool {
+    let item_doc = lookup_item(id, cdata.data());
+    match item_family(item_doc) {
+        Type => true,
+        _ => false,
+    }
+}

src/librustdoc/clean/inline.rs

@@ -203,7 +203,7 @@ fn build_struct(tcx: &ty::ctxt, did: ast::DefId) -> clean::Struct {
 fn build_type(tcx: &ty::ctxt, did: ast::DefId) -> clean::ItemEnum {
     let t = ty::lookup_item_type(tcx, did);
     match ty::get(t.ty).sty {
-        ty::ty_enum(edid, _) => {
+        ty::ty_enum(edid, _) if !csearch::is_typedef(&tcx.sess.cstore, did) => {
             return clean::EnumItem(clean::Enum {
                 generics: t.generics.clean(),
                 variants_stripped: false,