Regression tests for issue 81211.

src/test/ui/derives/derive-Debug-use-ufcs-struct.rs

@@ -0,0 +1,40 @@
+// run-pass
+#![allow(warnings)]
+
+#[derive(Debug)]
+pub struct Bar { pub t: () }
+
+impl<T> Access for T {}
+pub trait Access {
+    fn field(&self, _: impl Sized, _: impl Sized) {
+        panic!("got into Access::field");
+    }
+
+    fn finish(&self) -> Result<(), std::fmt::Error> {
+        panic!("got into Access::finish");
+    }
+
+    fn debug_struct(&self, _: impl Sized, _: impl Sized) {
+        panic!("got into Access::debug_struct");
+    }
+}
+
+impl<T> MutAccess for T {}
+pub trait MutAccess {
+    fn field(&mut self, _: impl Sized, _: impl Sized) {
+        panic!("got into MutAccess::field");
+    }
+
+    fn finish(&mut self) -> Result<(), std::fmt::Error> {
+        panic!("got into MutAccess::finish");
+    }
+
+    fn debug_struct(&mut self, _: impl Sized, _: impl Sized) {
+        panic!("got into MutAccess::debug_struct");
+    }
+}
+
+fn main() {
+    let bar = Bar { t: () };
+    assert_eq!("Bar { t: () }", format!("{:?}", bar));
+}

src/test/ui/derives/derive-Debug-use-ufcs-tuple.rs

@@ -0,0 +1,32 @@
+// run-pass
+#![allow(warnings)]
+
+#[derive(Debug)]
+pub struct Foo<T>(pub T);
+
+use std::fmt;
+
+impl<T> Field for T {}
+impl<T> Finish for T {}
+impl Dt for &mut fmt::Formatter<'_> {}
+
+pub trait Field {
+    fn field(&self, _: impl Sized) {
+        panic!("got into field");
+    }
+}
+pub trait Finish {
+    fn finish(&self) -> Result<(), std::fmt::Error> {
+        panic!("got into finish");
+    }
+}
+pub trait Dt {
+    fn debug_tuple(&self, _: &str) {
+        panic!("got into debug_tuple");
+    }
+}
+
+fn main() {
+    let foo = Foo(());
+    assert_eq!("Foo(())", format!("{:?}", foo));
+}