Add regression test to ensure #[allow(unstable_name_collisions)] works

src/test/ui/inference/issue-81522.rs

@@ -0,0 +1,31 @@
+// Regression test for #81522.
+// Ensures that `#[allow(unstable_name_collisions)]` appended to things other than function
+// suppresses the corresponding diagnostics emitted from inside them.
+// But note that this attribute doesn't work for macro invocations if it is appended directly.
+
+// aux-build:inference_unstable_iterator.rs
+// aux-build:inference_unstable_itertools.rs
+// run-pass
+
+extern crate inference_unstable_iterator;
+extern crate inference_unstable_itertools;
+
+#[allow(unused_imports)]
+use inference_unstable_iterator::IpuIterator;
+use inference_unstable_itertools::IpuItertools;
+
+fn main() {
+    // expression statement
+    #[allow(unstable_name_collisions)]
+    'x'.ipu_flatten();
+
+    // let statement
+    #[allow(unstable_name_collisions)]
+    let _ = 'x'.ipu_flatten();
+
+    // block expression
+    #[allow(unstable_name_collisions)]
+    {
+        'x'.ipu_flatten();
+    }
+}