Add more ICEs due to malformed diagnostic::on_unimplemented

tests/ui/diagnostic_namespace/auxiliary/bad_on_unimplemented.rs

@@ -1,2 +1,26 @@
 #[diagnostic::on_unimplemented(aa = "broken")]
-pub trait Test {}
+pub trait MissingAttr {}
+
+#[diagnostic::on_unimplemented(label = "a", label = "b")]
+pub trait DuplicateAttr {}
+
+#[diagnostic::on_unimplemented = "broken"]
+pub trait NotMetaList {}
+
+#[diagnostic::on_unimplemented]
+pub trait Empty {}
+
+#[diagnostic::on_unimplemented {}]
+pub trait WrongDelim {}
+
+#[diagnostic::on_unimplemented(label = "{A:.3}")]
+pub trait BadFormatter<A> {}
+
+#[diagnostic::on_unimplemented(label = "test {}")]
+pub trait NoImplicitArgs {}
+
+#[diagnostic::on_unimplemented(label = "{missing}")]
+pub trait MissingArg {}
+
+#[diagnostic::on_unimplemented(label = "{_}")]
+pub trait BadArg {}

tests/ui/diagnostic_namespace/malformed_foreign_on_unimplemented.rs

@@ -0,0 +1,31 @@
+//@ edition:2021
+//@ 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::*;
+
+fn missing_attr<T: MissingAttr>(_: T) {}
+fn duplicate_attr<T: DuplicateAttr>(_: T) {}
+fn not_meta_list<T: NotMetaList>(_: T) {}
+fn empty<T: Empty>(_: T) {}
+fn wrong_delim<T: WrongDelim>(_: T) {}
+fn bad_formatter<T: BadFormatter<()>>(_: T) {}
+fn no_implicit_args<T: NoImplicitArgs>(_: T) {}
+fn missing_arg<T: MissingArg>(_: T) {}
+fn bad_arg<T: BadArg>(_: T) {}
+
+fn main() {
+    missing_attr(()); //~ ERROR E0277
+    duplicate_attr(()); //~ ERROR E0277
+    not_meta_list(()); //~ ERROR E0277
+    empty(()); //~ ERROR E0277
+    wrong_delim(()); //~ ERROR E0277
+    bad_formatter(()); //~ ERROR E0277
+    no_implicit_args(()); //~ ERROR E0277
+    missing_arg(()); //~ ERROR E0277
+    bad_arg(()); //~ ERROR E0277
+}