Regression test for incremental treatment of rustc_on_unimplemented.

Author: pnkfelix

src/test/incremental/issue-59523-on-implemented-is-not-unused.rs

@@ -0,0 +1,27 @@
+// We should not see the unused_attributes lint fire for
+// rustc_on_unimplemented, but with this bug we are seeing it fire (on
+// subsequent runs) if incremental compilation is enabled.
+
+// revisions: rpass1 rpass2
+// compile-pass
+
+#![feature(on_unimplemented)]
+#![deny(unused_attributes)]
+
+#[rustc_on_unimplemented = "invalid"]
+trait Index<Idx: ?Sized> {
+    type Output: ?Sized;
+    fn index(&self, index: Idx) -> &Self::Output;
+}
+
+#[rustc_on_unimplemented = "a usize is required to index into a slice"]
+impl Index<usize> for [i32] {
+    type Output = i32;
+    fn index(&self, index: usize) -> &i32 {
+        &self[index]
+    }
+}
+
+fn main() {
+    Index::<usize>::index(&[1, 2, 3] as &[i32], 2);
+}