Add regression test for #137857 to ensure that we generate intra doc links for extern crate items.

tests/rustdoc/intra-doc/deps.rs

@@ -0,0 +1,23 @@
+// Checks that links to crates are correctly generated and only existing crates
+// have a link generated.
+// Regression test for <https://github.com/rust-lang/rust/issues/137857>.
+
+//@ compile-flags: --document-private-items -Z unstable-options
+//@ compile-flags: --extern-html-root-url=empty=https://empty.example/
+// This one is to ensure that we don't link to any item we see which has
+// an external html root URL unless it actually exists.
+//@ compile-flags: --extern-html-root-url=non_existant=https://non-existant.example/
+//@ aux-build: empty.rs
+
+#![crate_name = "foo"]
+#![expect(rustdoc::broken_intra_doc_links)]
+
+//@ has 'foo/index.html'
+//@ has - '//a[@href="https://empty.example/empty/index.html"]' 'empty'
+// There should only be one intra doc links, we should not link `non_existant`.
+//@ count - '//*[@class="docblock"]//a' 1
+//! [`empty`]
+//!
+//! [`non_existant`]
+
+extern crate empty;