Add test for #124651

tests/ui/diagnostic_namespace/auxiliary/bad_on_unimplemented.rs

@@ -0,0 +1,2 @@
+#[diagnostic::on_unimplemented(aa = "broken")]
+pub trait Test {}

tests/ui/diagnostic_namespace/on_unimplemented_ice.rs

@@ -0,0 +1,17 @@
+//@ edition:2021
+//@ compile-flags:--test
+//@ aux-build:bad_on_unimplemented.rs
+
+// Do not ICE when encountering a malformed `#[diagnostic::on_unimplemented]` annotation in a
+// dependency when incorrectly used (#124651).
+
+extern crate bad_on_unimplemented;
+
+use bad_on_unimplemented::Test;
+
+fn breakage<T: Test>(_: T) {}
+
+#[test]
+fn test() {
+    breakage(1); //~ ERROR E0277
+}

tests/ui/diagnostic_namespace/on_unimplemented_ice.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `{integer}: Test` is not satisfied
+  --> $DIR/on_unimplemented_ice.rs:16:14
+   |
+LL |     breakage(1);
+   |     -------- ^ the trait `Test` is not implemented for `{integer}`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `breakage`
+  --> $DIR/on_unimplemented_ice.rs:12:16
+   |
+LL | fn breakage<T: Test>(_: T) {}
+   |                ^^^^ required by this bound in `breakage`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.