Don't allow DispatchFromDyn impls that transmute ZST to non-ZST

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -267,6 +267,7 @@ fn visit_implementation_of_dispatch_from_dyn(checker: &Checker<'_>) -> Result<()
                     let ty_a = field.ty(tcx, args_a);
                     let ty_b = field.ty(tcx, args_b);
 
+                    if ty_a == ty_b {
                         // Allow 1-ZSTs that don't mention type params.
                         //
                         // Allowing type params here would allow us to possibly transmute
@@ -280,7 +281,6 @@ fn visit_implementation_of_dispatch_from_dyn(checker: &Checker<'_>) -> Result<()
                             return false;
                         }
 
-                    if ty_a == ty_b {
                         res = Err(tcx.dcx().emit_err(errors::DispatchFromDynZST {
                             span,
                             name: field.name,

tests/ui/self/dispatch-from-dyn-zst-transmute-zst-nonzst.rs

@@ -0,0 +1,25 @@
+// We used to allow erroneous `DispatchFromDyn` impls whose RHS type contained
+// fields that weren't ZSTs. I don't believe this was possible to abuse, but
+// it's at least nice to give users better errors.
+
+#![feature(arbitrary_self_types)]
+#![feature(unsize)]
+#![feature(dispatch_from_dyn)]
+
+use std::marker::Unsize;
+use std::ops::DispatchFromDyn;
+
+struct Dispatchable<T: ?Sized, Z> {
+    _ptr: Box<T>,
+    z: Z,
+}
+
+impl<T, U> DispatchFromDyn<Dispatchable<U, i32>> for Dispatchable<T, ()>
+//~^ ERROR implementing the `DispatchFromDyn` trait requires multiple coercions
+where
+    T: Unsize<U> + ?Sized,
+    U: ?Sized,
+{
+}
+
+fn main() {}

tests/ui/self/dispatch-from-dyn-zst-transmute-zst-nonzst.stderr

@@ -0,0 +1,16 @@
+error[E0378]: implementing the `DispatchFromDyn` trait requires multiple coercions
+  --> $DIR/dispatch-from-dyn-zst-transmute-zst-nonzst.rs:17:1
+   |
+LL | / impl<T, U> DispatchFromDyn<Dispatchable<U, i32>> for Dispatchable<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: `_ptr` (`Box<T>` to `Box<U>`), `z` (`()` to `i32`)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0378`.