minor: add a test for derive macros in core

Author: matklad

crates/hir_def/src/macro_expansion_tests/builtin_derive_macro.rs

@@ -20,6 +20,26 @@ impl < > core::marker::Copy for Foo< > {}"##]],
     );
 }
 
+#[test]
+fn test_copy_expand_in_core() {
+    cov_mark::check!(test_copy_expand_in_core);
+    check(
+        r#"
+#[rustc_builtin_macro]
+macro Copy {}
+#[derive(Copy)]
+struct Foo;
+"#,
+        expect![[r##"
+#[rustc_builtin_macro]
+macro Copy {}
+#[derive(Copy)]
+struct Foo;
+
+impl < > crate ::marker::Copy for Foo< > {}"##]],
+    );
+}
+
 #[test]
 fn test_copy_expand_with_type_params() {
     check(

crates/hir_expand/src/builtin_derive_macro.rs

@@ -176,6 +176,7 @@ fn find_builtin_crate(db: &dyn AstDatabase, id: MacroCallId) -> tt::TokenTree {
     let tt = if cg[krate].dependencies.iter().any(|dep| &*dep.name == "core") {
         quote! { core }
     } else {
+        cov_mark::hit!(test_copy_expand_in_core);
         quote! { crate }
     };