Add test case for doc_cfg_hide feature gate

Author: jyn514

compiler/rustc_feature/src/active.rs

@@ -370,9 +370,6 @@ declare_features! (
     /// Allows `#[doc(cfg(...))]`.
     (active, doc_cfg, "1.21.0", Some(43781), None),
 
-    /// Allows `#[doc(cfg_hide(...))]`.
-    (active, doc_cfg_hide, "1.49.0", Some(43781), None),
-
     /// Allows `#[doc(masked)]`.
     (active, doc_masked, "1.21.0", Some(44027), None),
 
@@ -658,6 +655,9 @@ declare_features! (
     /// Allows unsizing coercions in `const fn`.
     (active, const_fn_unsize, "1.53.0", Some(64992), None),
 
+    /// Allows `#[doc(cfg_hide(...))]`.
+    (active, doc_cfg_hide, "1.53.0", Some(43781), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/test/rustdoc-ui/feature-gate-doc_cfg_hide.rs

@@ -0,0 +1,7 @@
+#![doc(cfg_hide(test))]
+//~^ ERROR `#[doc(cfg_hide)]` is experimental
+
+#[cfg(not(test))]
+pub fn public_fn() {}
+#[cfg(test)]
+pub fn internal_use_only() {}

src/test/rustdoc-ui/feature-gate-doc_cfg_hide.stderr

@@ -0,0 +1,14 @@
+error[E0658]: `#[doc(cfg_hide)]` is experimental
+  --> $DIR/feature-gate-doc_cfg_hide.rs:1:1
+   |
+LL | #![doc(cfg_hide(test))]
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #43781 <https://github.com/rust-lang/rust/issues/43781> for more information
+   = help: add `#![feature(doc_cfg_hide)]` to the crate attributes to enable
+
+error: Compilation failed, aborting rustdoc
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.