Auto merge of rust-lang#6513 - nahuakang:fix/empty_enum_lint_never_type, r=flip1995

Fix: Empty enum never type suggested only if the feature is enabled

This PR addresses [Issue 6422](rust-lang/rust-clippy#6422). Instead of always recommending `never type` for empty enums, Clippy would only recommend [the lint](https://rust-lang.github.io/rust-clippy/master/index.html#empty_enum) if [LatePass.TyCtxt](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html) has `features().never_type` enabled.

- \[ ] Followed [lint naming conventions][lint_naming]
- \[x] Added passing UI tests (including committed `.stderr` file)
- \[x] `cargo test` passes locally
- \[x] Executed `cargo dev update_lints`
- \[x] Added lint documentation
- \[x] Run `cargo dev fmt`
---

*Please write a short comment explaining your change (or "none" for internal only changes)*
changelog: Only trigger [`empty_enum`] lint if `never_type` feature is enabled.

Author: bors

clippy_lints/src/empty_enum.rs

@@ -8,8 +8,12 @@ use rustc_session::{declare_lint_pass, declare_tool_lint};
 declare_clippy_lint! {
     /// **What it does:** Checks for `enum`s with no variants.
     ///
+    /// As of this writing, the `never_type` is still a
+    /// nightly-only experimental API. Therefore, this lint is only triggered
+    /// if the `never_type` is enabled.
+    ///
     /// **Why is this bad?** If you want to introduce a type which
-    /// can't be instantiated, you should use `!` (the never type),
+    /// can't be instantiated, you should use `!` (the primitive type "never"),
     /// or a wrapper around it, because `!` has more extensive
     /// compiler support (type inference, etc...) and wrappers
     /// around it are the conventional way to define an uninhabited type.
@@ -40,6 +44,11 @@ declare_lint_pass!(EmptyEnum => [EMPTY_ENUM]);
 
 impl<'tcx> LateLintPass<'tcx> for EmptyEnum {
     fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
+        // Only suggest the `never_type` if the feature is enabled
+        if !cx.tcx.features().never_type {
+            return;
+        }
+
         let did = cx.tcx.hir().local_def_id(item.hir_id);
         if let ItemKind::Enum(..) = item.kind {
             let ty = cx.tcx.type_of(did);

tests/ui/empty_enum.rs

@@ -1,6 +1,7 @@
 #![allow(dead_code)]
 #![warn(clippy::empty_enum)]
-
+// Enable never type to test empty enum lint
+#![feature(never_type)]
 enum Empty {}
 
 fn main() {}

tests/ui/empty_enum.stderr

@@ -1,5 +1,5 @@
 error: enum with no variants
-  --> $DIR/empty_enum.rs:4:1
+  --> $DIR/empty_enum.rs:5:1
    |
 LL | enum Empty {}
    | ^^^^^^^^^^^^^

tests/ui/empty_enum_without_never_type.rs

@@ -0,0 +1,7 @@
+#![allow(dead_code)]
+#![warn(clippy::empty_enum)]
+
+// `never_type` is not enabled; this test has no stderr file
+enum Empty {}
+
+fn main() {}