rustdoc: Allow linking from private items to private types

Fixes rust-lang#74134

After PR rust-lang#72771 this would trigger an intra_doc_link_resolution_failure warning
when rustdoc is invoked without --document-private-items. Links from private
items to private types are however never actually generated in that case and
thus shouldn't produce a warning. These links are in fact a very useful tool to
document crate internals.

Tests are added for all 4 combinations of public/private items and link
targets. Test 1 is the case mentioned above and fails without this commit. Tests
2 - 4 passed before already but are added nonetheless to prevent regressions.

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -796,6 +796,7 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
 
                     let hir_id = self.cx.tcx.hir().as_local_hir_id(local);
                     if !self.cx.tcx.privacy_access_levels(LOCAL_CRATE).is_exported(hir_id)
+                        && (item.visibility == Visibility::Public)
                         && !self.cx.render_options.document_private
                     {
                         let item_name = item.name.as_deref().unwrap_or("<unknown>");

src/test/rustdoc/issue-74134-1.rs

@@ -0,0 +1,10 @@
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a private item to a private type is fine without --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    private: Private,
+}

src/test/rustdoc/issue-74134-2.rs

@@ -0,0 +1,11 @@
+// compile-flags: --document-private-items
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a private item to a private type is fine with --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    private: Private,
+}

src/test/rustdoc/issue-74134-3.rs

@@ -0,0 +1,11 @@
+// should-fail
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a public item to a private type fails without --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    pub public: u32,
+}

src/test/rustdoc/issue-74134-4.rs

@@ -0,0 +1,11 @@
+// compile-flags: --document-private-items
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a public item to a private type is fine with --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    pub public: u32,
+}