add on_unmatch_args

library/core/src/field.rs

@@ -45,6 +45,9 @@ fn clone(&self) -> Self {
 /// variant must also be specified. Only a single field is supported.
 #[unstable(feature = "field_projections", issue = "145383")]
 #[allow_internal_unstable(field_representing_type_raw, builtin_syntax)]
+#[diagnostic::on_unmatch_args(
+    note = "this macro expects a container type and a field path, like `field_of!(Type, field)` or `field_of!(Enum, Variant.field)`"
+)]
 // NOTE: when stabilizing this macro, we can never add new trait impls for `FieldRepresentingType`,
 // since it is `#[fundamental]` and thus could break users of this macro, since the compiler expands
 // it to `FieldRepresentingType<...>`. Thus stabilizing this requires careful thought about the

library/core/src/lib.rs

@@ -131,6 +131,7 @@
 #![feature(deprecated_suggestion)]
 #![feature(derive_const)]
 #![feature(diagnostic_on_const)]
+#![feature(diagnostic_on_unmatch_args)]
 #![feature(doc_cfg)]
 #![feature(doc_notable_trait)]
 #![feature(extern_types)]

library/core/src/mem/mod.rs

@@ -1567,6 +1567,9 @@ impl<T> SizedTypeProperties for T {}
 /// [`offset_of_enum`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-enum.html
 /// [`offset_of_slice`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-slice.html
 #[stable(feature = "offset_of", since = "1.77.0")]
+#[diagnostic::on_unmatch_args(
+    note = "this macro expects a container type and a (nested) field path, like `offset_of!(Type, field)`"
+)]
 #[allow_internal_unstable(builtin_syntax, core_intrinsics)]
 pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
     // The `{}` is for better error messages