Rollup merge of rust-lang#67323 - Centril:tes, r=davidtwco

make transparent enums more ordinary

By recognizing that structs & unions have one variant, we can make the treatment of transparent enums less ad-hoc.

cc rust-lang#60405

r? @davidtwco

src/librustc_typeck/check/mod.rs

@@ -2330,7 +2330,7 @@ fn bad_variant_count<'tcx>(tcx: TyCtxt<'tcx>, adt: &'tcx ty::AdtDef, sp: Span, d
     );
     let mut err = struct_span_err!(tcx.sess, sp, E0731, "transparent enum {}", msg);
     err.span_label(sp, &msg);
-    if let &[ref start @ .., ref end] = &variant_spans[..] {
+    if let [start @ .., end] = &*variant_spans {
         for variant_span in start {
             err.span_label(*variant_span, "");
         }
@@ -2372,23 +2372,14 @@ fn check_transparent(tcx: TyCtxt<'_>, sp: Span, def_id: DefId) {
     }
     let sp = tcx.sess.source_map().def_span(sp);
 
-    if adt.is_enum() {
-        if !tcx.features().transparent_enums {
-            feature_err(
-                &tcx.sess.parse_sess,
-                sym::transparent_enums,
-                sp,
-                "transparent enums are unstable",
-            )
-            .emit();
-        }
-        if adt.variants.len() != 1 {
-            bad_variant_count(tcx, adt, sp, def_id);
-            if adt.variants.is_empty() {
-                // Don't bother checking the fields. No variants (and thus no fields) exist.
-                return;
-            }
-        }
+    if adt.is_enum() && !tcx.features().transparent_enums {
+        feature_err(
+            &tcx.sess.parse_sess,
+            sym::transparent_enums,
+            sp,
+            "transparent enums are unstable",
+        )
+        .emit();
     }
 
     if adt.is_union() && !tcx.features().transparent_unions {
@@ -2401,6 +2392,14 @@ fn check_transparent(tcx: TyCtxt<'_>, sp: Span, def_id: DefId) {
         .emit();
     }
 
+    if adt.variants.len() != 1 {
+        bad_variant_count(tcx, adt, sp, def_id);
+        if adt.variants.is_empty() {
+            // Don't bother checking the fields. No variants (and thus no fields) exist.
+            return;
+        }
+    }
+
     // For each field, figure out if it's known to be a ZST and align(1)
     let field_infos = adt.all_fields().map(|field| {
         let ty = field.ty(tcx, InternalSubsts::identity_for_item(tcx, field.did));