Rollup merge of #60369 - TimDiekmann:dispatch-zst, r=eddyb

Support ZSTs in DispatchFromDyn

Allows to use ZSTs with 1 byte alignment in `DispatchFromDyn` implementation. This is required for `Box<T, A: Alloc>`

cc #58457

src/librustc_typeck/coherence/builtin.rs

@@ -223,19 +223,22 @@ fn visit_implementation_of_dispatch_from_dyn<'a, 'tcx>(
                     let fields = &def_a.non_enum_variant().fields;
 
                     let coerced_fields = fields.iter().filter_map(|field| {
-                        if tcx.type_of(field.did).is_phantom_data() {
-                            // ignore PhantomData fields
-                            return None
-                        }
-
                         let ty_a = field.ty(tcx, substs_a);
                         let ty_b = field.ty(tcx, substs_b);
+
+                        if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
+                            if layout.is_zst() && layout.details.align.abi.bytes() == 1 {
+                                // ignore ZST fields with alignment of 1 byte
+                                return None;
+                            }
+                        }
+
                         if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
                             if ok.obligations.is_empty() {
                                 create_err(
                                     "the trait `DispatchFromDyn` may only be implemented \
                                      for structs containing the field being coerced, \
-                                     `PhantomData` fields, and nothing else"
+                                     ZST fields with 1 byte alignment, and nothing else"
                                 ).note(
                                     &format!(
                                         "extra field `{}` of type `{}` is not allowed",

src/test/run-pass/dispatch_from_dyn_zst.rs

@@ -0,0 +1,49 @@
+#![feature(unsize, dispatch_from_dyn, never_type)]
+
+#![allow(dead_code)]
+
+use std::{
+    ops::DispatchFromDyn,
+    marker::{Unsize, PhantomData},
+};
+
+struct Zst;
+struct NestedZst(PhantomData<()>, Zst);
+
+
+struct WithUnit<T: ?Sized>(Box<T>, ());
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<WithUnit<U>> for WithUnit<T>
+    where T: Unsize<U> {}
+
+struct WithPhantom<T: ?Sized>(Box<T>, PhantomData<()>);
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<WithPhantom<U>> for WithPhantom<T>
+    where T: Unsize<U> {}
+
+struct WithNever<T: ?Sized>(Box<T>, !);
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<WithNever<U>> for WithNever<T>
+    where T: Unsize<U> {}
+
+struct WithZst<T: ?Sized>(Box<T>, Zst);
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<WithZst<U>> for WithZst<T>
+    where T: Unsize<U> {}
+
+struct WithNestedZst<T: ?Sized>(Box<T>, NestedZst);
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<WithNestedZst<U>> for WithNestedZst<T>
+    where T: Unsize<U> {}
+
+
+struct Generic<T: ?Sized, A>(Box<T>, A);
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Generic<U, ()>> for Generic<T, ()>
+    where T: Unsize<U> {}
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Generic<U, PhantomData<()>>>
+    for Generic<T, PhantomData<()>>
+    where T: Unsize<U> {}
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Generic<U, !>> for Generic<T, !>
+    where T: Unsize<U> {}
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Generic<U, Zst>> for Generic<T, Zst>
+    where T: Unsize<U> {}
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Generic<U, NestedZst>> for Generic<T, NestedZst>
+    where T: Unsize<U> {}
+
+
+fn main() {}

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, `PhantomData` 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`.