Only allow ZSTs with 1 byte alignment

Author: TimDiekmann

src/librustc_typeck/coherence/builtin.rs

@@ -227,8 +227,8 @@ fn visit_implementation_of_dispatch_from_dyn<'a, 'tcx>(
                         let ty_b = field.ty(tcx, substs_b);
 
                         if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
-                            if layout.is_zst() {
-                                // ignore ZST fields
+                            if layout.is_zst() && layout.details.align.abi.bytes() == 1 {
+                                // ignore ZST fields with alignment of 1 byte
                                 return None;
                             }
                         }
@@ -238,7 +238,7 @@ fn visit_implementation_of_dispatch_from_dyn<'a, 'tcx>(
                                 create_err(
                                     "the trait `DispatchFromDyn` may only be implemented \
                                      for structs containing the field being coerced, \
-                                     ZST fields, and nothing else"
+                                     ZST fields with 1 byte alignment, and nothing else"
                                 ).note(
                                     &format!(
                                         "extra field `{}` of type `{}` is not allowed",

src/test/ui/invalid_dispatch_from_dyn_impls.rs

@@ -39,4 +39,13 @@ where
     T: Unsize<U>,
 {} //~^^^ ERROR [E0378]
 
+#[repr(align(64))]
+struct OverAlignedZst;
+struct OverAligned<T: ?Sized>(Box<T>, OverAlignedZst);
+
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<OverAligned<U>> for OverAligned<T>
+    where
+        T: Unsize<U>,
+{} //~^^^ ERROR [E0378]
+
 fn main() {}

src/test/ui/invalid_dispatch_from_dyn_impls.stderr

@@ -1,4 +1,4 @@
-error[E0378]: the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields, and nothing else
+error[E0378]: the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields with 1 byte alignment, and nothing else
   --> $DIR/invalid_dispatch_from_dyn_impls.rs:10:1
    |
 LL | / impl<T, U> DispatchFromDyn<WrapperWithExtraField<U>> for WrapperWithExtraField<T>
@@ -36,6 +36,17 @@ LL | |     T: Unsize<U>,
 LL | | {}
    | |__^
 
-error: aborting due to 4 previous errors
+error[E0378]: the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields with 1 byte alignment, and nothing else
+  --> $DIR/invalid_dispatch_from_dyn_impls.rs:46:1
+   |
+LL | / impl<T: ?Sized, U: ?Sized> DispatchFromDyn<OverAligned<U>> for OverAligned<T>
+LL | |     where
+LL | |         T: Unsize<U>,
+LL | | {}
+   | |__^
+   |
+   = note: extra field `1` of type `OverAlignedZst` is not allowed
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0378`.