add tests for broken links in unused doc strings

tests/rustdoc-ui/intra-doc/broken-link-in-unused-doc-string.rs

@@ -0,0 +1,16 @@
+// Test that we don't ICE with broken links that don't show up in the docs.
+
+// check-pass
+// edition: 2021
+
+/// [1]
+//~^ WARN unresolved link to `1`
+//~| WARN unresolved link to `1`
+pub use {std, core};
+
+/// [2]
+pub use {};
+
+/// [3]
+//~^ WARN unresolved link to `3`
+pub extern crate alloc;

tests/rustdoc-ui/intra-doc/broken-link-in-unused-doc-string.stderr

@@ -0,0 +1,27 @@
+warning: unresolved link to `3`
+  --> $DIR/broken-link-in-unused-doc-string.rs:14:6
+   |
+LL | /// [3]
+   |      ^ no item named `3` in scope
+   |
+   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
+   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
+
+warning: unresolved link to `1`
+  --> $DIR/broken-link-in-unused-doc-string.rs:6:6
+   |
+LL | /// [1]
+   |      ^ no item named `1` in scope
+   |
+   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
+
+warning: unresolved link to `1`
+  --> $DIR/broken-link-in-unused-doc-string.rs:6:6
+   |
+LL | /// [1]
+   |      ^ no item named `1` in scope
+   |
+   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
+
+warning: 3 warnings emitted
+