restore the old behavior for derive(Default) on fieldless types

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -69,7 +69,15 @@ fn default_struct_substructure(
     variant_data: &VariantData,
     type_ident: Ident,
 ) -> BlockOrExpr {
+    // FIXME: This is currently needed to avoid a regression with attribute macros after
+    // derives that incorrectly rely on the generated code using `Unit {}` instead of `Unit`.
+    // See https://github.com/rust-lang/rust/issues/164078.
+    if variant_data.fields().is_empty() {
+        return BlockOrExpr::new_expr(cx.expr_struct_ident(trait_span, type_ident, ThinVec::new()));
+    }
+
     let expr = match variant_data {
+        // This is currently unreachable due to the above hack.
         VariantData::Unit(_) => cx.expr_ident(trait_span, type_ident),
         VariantData::Tuple(fields, _) => {
             let exprs = fields

tests/ui/derives/attr-macro-changes-type-kind.rs

@@ -8,7 +8,7 @@
 extern crate struct_to_enum;
 use struct_to_enum::*;
 
-#[derive(Clone, Copy, Default, Hash, PartialEq, Eq, PartialOrd, Ord, Debug)]
+#[derive(Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord, Debug)]
 #[struct_to_enum::phantom]
 struct Spooky<T>;
 

tests/ui/derives/default/attr-macros-fieldless-types.rs

@@ -0,0 +1,47 @@
+//! Regression test for https://github.com/rust-lang/rust/issues/164078,
+//! where some attribute macros relied on the generated code always being
+//! `Unit {}` and not `Unit()` or `Unit`.
+
+//@ check-pass
+//@ proc-macro: rewrite-fieldless-type-kind.rs
+
+extern crate rewrite_fieldless_type_kind;
+use rewrite_fieldless_type_kind::*;
+
+#[derive(Default)]
+#[to_unit]
+struct UnitToUnit;
+
+#[derive(Default)]
+#[to_tuple]
+struct UnitToTuple;
+
+#[derive(Default)]
+#[to_braced]
+struct UnitToBraced;
+
+#[derive(Default)]
+#[to_unit]
+struct TupleToUnit();
+
+#[derive(Default)]
+#[to_tuple]
+struct TupleToTuple();
+
+#[derive(Default)]
+#[to_braced]
+struct TupleToBraced();
+
+#[derive(Default)]
+#[to_unit]
+struct BracedToUnit {}
+
+#[derive(Default)]
+#[to_tuple]
+struct BracedToTuple {}
+
+#[derive(Default)]
+#[to_braced]
+struct BracedToBraced {}
+
+fn main() {}

tests/ui/derives/default/auxiliary/rewrite-fieldless-type-kind.rs

@@ -0,0 +1,20 @@
+extern crate proc_macro;
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn to_unit(_: TokenStream, struct_item: TokenStream) -> TokenStream {
+    rewrite(struct_item, ";")
+}
+#[proc_macro_attribute]
+pub fn to_tuple(_: TokenStream, struct_item: TokenStream) -> TokenStream {
+    rewrite(struct_item, "();")
+}
+#[proc_macro_attribute]
+pub fn to_braced(_: TokenStream, struct_item: TokenStream) -> TokenStream {
+    rewrite(struct_item, "{}")
+}
+
+fn rewrite(struct_item: TokenStream, suffix: &str) -> TokenStream {
+    let ident = struct_item.into_iter().nth(1).unwrap();
+    format!("struct {ident}{suffix}").parse().unwrap()
+}

tests/ui/derives/deriving-all-codegen.stdout

@@ -46,7 +46,7 @@ impl ::core::fmt::Debug for Empty {
 #[automatically_derived]
 impl ::core::default::Default for Empty {
     #[inline]
-    fn default() -> Self { Empty }
+    fn default() -> Self { Empty {} }
 }
 #[automatically_derived]
 impl ::core::hash::Hash for Empty {