Don't allow transmuting ZSTs in dispatch_from_dyn impl

2 files changed, 50 insertions, 0 deletions

diff --git a/tests/ui/self/dispatch-from-dyn-zst-transmute.rs b/tests/ui/self/dispatch-from-dyn-zst-transmute.rs
new file mode 100644
index 00000000000..57c255b4d7b
--- /dev/null
+++ b/tests/ui/self/dispatch-from-dyn-zst-transmute.rs
@@ -0,0 +1,34 @@
+#![feature(arbitrary_self_types)]
+#![feature(unsize)]
+#![feature(dispatch_from_dyn)]
+
+use std::marker::PhantomData;
+use std::marker::Unsize;
+use std::ops::DispatchFromDyn;
+use std::ops::Deref;
+
+struct IsSendToken<T: ?Sized>(PhantomData<fn(T) -> T>);
+
+struct Foo<'a, U: ?Sized> {
+    token: IsSendToken<U>,
+    ptr: &'a U,
+}
+
+impl<'a, T, U> DispatchFromDyn<Foo<'a, U>> for Foo<'a, T>
+//~^ ERROR implementing the `DispatchFromDyn` trait requires multiple coercions
+where
+    T: Unsize<U> + ?Sized,
+    U: ?Sized {}
+
+trait Bar {
+    fn f(self: Foo<'_, Self>);
+}
+
+impl<U: ?Sized> Deref for Foo<'_, U> {
+    type Target = U;
+    fn deref(&self) -> &U {
+        self.ptr
+    }
+}
+
+fn main() {}
diff --git a/tests/ui/self/dispatch-from-dyn-zst-transmute.stderr b/tests/ui/self/dispatch-from-dyn-zst-transmute.stderr
new file mode 100644
index 00000000000..5a8ae88b5f1
--- /dev/null
+++ b/tests/ui/self/dispatch-from-dyn-zst-transmute.stderr
@@ -0,0 +1,16 @@
+error[E0378]: implementing the `DispatchFromDyn` trait requires multiple coercions
+  --> $DIR/dispatch-from-dyn-zst-transmute.rs:17:1
+   |
+LL | / impl<'a, T, U> DispatchFromDyn<Foo<'a, U>> for Foo<'a, T>
+LL | |
+LL | | where
+LL | |     T: Unsize<U> + ?Sized,
+LL | |     U: ?Sized {}
+   | |_____________^
+   |
+   = note: the trait `DispatchFromDyn` may only be implemented for a coercion between structures with a single field being coerced
+   = note: currently, 2 fields need coercions: `token` (`IsSendToken<T>` to `IsSendToken<U>`), `ptr` (`&'a T` to `&'a U`)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0378`.