Add rustdoc JSON tests for #[doc(hidden)] handling of items.

tests/rustdoc-json/visibility/doc_hidden_default.rs

@@ -0,0 +1,19 @@
+#![no_std]
+
+// Without `--document-hidden-items`,
+// none of these items are present in rustdoc JSON.
+
+//@ !has "$.index[?(@.name=='func')]"
+#[doc(hidden)]
+pub fn func() {}
+
+//@ !has "$.index[?(@.name=='Unit')]"
+#[doc(hidden)]
+pub struct Unit;
+
+//@ !has "$.index[?(@.name=='hidden')]"
+#[doc(hidden)]
+pub mod hidden {
+    //@ !has "$.index[?(@.name=='Inner')]"
+    pub struct Inner;
+}

tests/rustdoc-json/visibility/doc_hidden_documented.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: --document-hidden-items
+#![no_std]
+
+//@ is "$.index[?(@.name=='func')].attrs" '["#[doc(hidden)]"]'
+#[doc(hidden)]
+pub fn func() {}
+
+//@ is "$.index[?(@.name=='Unit')].attrs" '["#[doc(hidden)]"]'
+#[doc(hidden)]
+pub struct Unit;
+
+//@ is "$.index[?(@.name=='hidden')].attrs" '["#[doc(hidden)]"]'
+#[doc(hidden)]
+pub mod hidden {
+    //@ is "$.index[?(@.name=='Inner')].attrs" '[]'
+    pub struct Inner;
+}