Fix ICE when negative impl is collected during eager mono

compiler/rustc_monomorphize/src/collector.rs

@@ -1336,6 +1336,10 @@ fn create_mono_items_for_default_impls<'tcx>(
 ) {
     match item.kind {
         hir::ItemKind::Impl(ref impl_) => {
+            if matches!(impl_.polarity, hir::ImplPolarity::Negative(_)) {
+                return;
+            }
+
             for param in impl_.generics.params {
                 match param.kind {
                     hir::GenericParamKind::Lifetime { .. } => {}

src/test/ui/traits/negative-impls/eager-mono.rs

@@ -0,0 +1,12 @@
+// build-pass
+// compile-flags:-C link-dead-code=y
+
+#![feature(negative_impls)]
+
+trait Foo {
+    fn foo() {}
+}
+
+impl !Foo for () {}
+
+fn main() {}