fix(empty_enum): don't lint if all variants happen to be cfg-d out (rust-lang#15911)

Fixes rust-lang/rust-clippy#15910

changelog: [`empty_enum`]: don't lint if all variants happen to be
`cfg`-d out

Author: samueltardieu

clippy_lints/src/empty_enum.rs

@@ -1,4 +1,5 @@
 use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::span_contains_cfg;
 use rustc_hir::{Item, ItemKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
@@ -25,10 +26,6 @@ declare_clippy_lint! {
     ///   matched to mark code unreachable. If the field is not visible, then the struct
     ///   acts like any other struct with private fields.
     ///
-    /// * If the enum has no variants only because all variants happen to be
-    ///   [disabled by conditional compilation][cfg], then it would be appropriate
-    ///   to allow the lint, with `#[allow(empty_enum)]`.
-    ///
     /// For further information, visit
     /// [the never type’s documentation][`!`].
     ///
@@ -62,11 +59,11 @@ declare_lint_pass!(EmptyEnum => [EMPTY_ENUM]);
 
 impl LateLintPass<'_> for EmptyEnum {
     fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
-        if let ItemKind::Enum(..) = item.kind
+        if let ItemKind::Enum(.., def) = item.kind
+            && def.variants.is_empty()
             // Only suggest the `never_type` if the feature is enabled
             && cx.tcx.features().never_type()
-            && let Some(adt) = cx.tcx.type_of(item.owner_id).instantiate_identity().ty_adt_def()
-            && adt.variants().is_empty()
+            && !span_contains_cfg(cx, item.span)
         {
             span_lint_and_help(
                 cx,

tests/ui/empty_enum.rs

@@ -1,8 +1,25 @@
-#![allow(dead_code)]
 #![warn(clippy::empty_enum)]
 // Enable never type to test empty enum lint
 #![feature(never_type)]
+
 enum Empty {}
 //~^ empty_enum
 
+mod issue15910 {
+    enum NotReallyEmpty {
+        #[cfg(false)]
+        Hidden,
+    }
+
+    enum OneVisibleVariant {
+        #[cfg(false)]
+        Hidden,
+        Visible,
+    }
+
+    enum CfgInsideVariant {
+        Variant(#[cfg(false)] String),
+    }
+}
+
 fn main() {}

tests/ui/empty_enum_without_never_type.rs

@@ -1,6 +1,5 @@
 //@ check-pass
 
-#![allow(dead_code)]
 #![warn(clippy::empty_enum)]
 
 // `never_type` is not enabled; this test has no stderr file