Skip PhantomData in Unsize checks

This patch proposes relaxation on the type unsizing
condition.
`PhantomData` has been in the current type system
exempted from being treated as carrying any data,
including dropck, virtual call dispatch by
`DispatchFromDyn` and unsizing container by
`CoerceUnsize`.
`PhantomData` is a special 1-ZST that really
carries no "data" of the types it captures.

I propose that we should also extend this
interpretation to `Unsize`.

Signed-off-by: Xiangfei Ding <dingxiangfei2009@protonmail.ch>

Author: dingxiangfei2009

compiler/rustc_ty_utils/src/ty.rs

@@ -332,7 +332,11 @@ fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> DenseBitSe
     // Ensure none of the other fields mention the parameters used
     // in unsizing.
     for field in prefix_fields {
-        for arg in tcx.type_of(field.did).instantiate_identity().walk() {
+        let field_ty = tcx.type_of(field.did).instantiate_identity();
+        if field_ty.is_phantom_data() {
+            continue;
+        }
+        for arg in field_ty.walk() {
             if let Some(i) = maybe_unsizing_param_idx(arg) {
                 unsizing_params.remove(i);
             }

library/core/src/marker.rs

@@ -218,7 +218,7 @@ pub trait PointeeSized {
 /// - Structs `Foo<..., T1, ..., Tn, ...>` implement `Unsize<Foo<..., U1, ..., Un, ...>>`
 ///   where any number of (type and const) parameters may be changed if all of these conditions
 ///   are met:
-///   - Only the last field of `Foo` has a type involving the parameters `T1`, ..., `Tn`.
+///   - Other than `PhantomData<_>` fields, only the last field of `Foo` has a type involving the parameters `T1`, ..., `Tn`.
 ///   - All other parameters of the struct are equal.
 ///   - `Field<T1, ..., Tn>: Unsize<Field<U1, ..., Un>>`, where `Field<...>` stands for the actual
 ///     type of the struct's last field.

tests/ui/traits/dispatch-from-dyn-invalid-impls.rs

@@ -68,4 +68,16 @@ where
 {
 }
 
+struct Ptr<T: ?Sized>(Box<T>);
+
+impl<'a, T: ?Sized, U: ?Sized> DispatchFromDyn<&'a Ptr<U>> for &'a Ptr<T> {}
+//~^ ERROR conflicting implementations of trait `DispatchFromDyn<&Ptr<_>>` for type `&Ptr<_>`
+
+struct Inner<T: ?Sized>(T);
+#[repr(transparent)]
+struct Outer<T: ?Sized>(PhantomData<T>, Inner<T>);
+
+impl<'a, T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<&'a Outer<U>> for &'a Outer<T> {}
+//~^ ERROR  conflicting implementations of trait `DispatchFromDyn<&Outer<_>>` for type `&Outer<_>`
+
 fn main() {}

tests/ui/traits/dispatch-from-dyn-invalid-impls.stderr

@@ -1,3 +1,24 @@
+error[E0119]: conflicting implementations of trait `DispatchFromDyn<&Ptr<_>>` for type `&Ptr<_>`
+  --> $DIR/dispatch-from-dyn-invalid-impls.rs:73:1
+   |
+LL | impl<'a, T: ?Sized, U: ?Sized> DispatchFromDyn<&'a Ptr<U>> for &'a Ptr<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: conflicting implementation in crate `core`:
+           - impl<'a, T, U> DispatchFromDyn<&'a U> for &'a T
+             where T: Unsize<U>, T: ?Sized, U: ?Sized;
+   = note: downstream crates may implement trait `std::marker::Unsize<std::boxed::Box<_>>` for type `std::boxed::Box<_>`
+
+error[E0119]: conflicting implementations of trait `DispatchFromDyn<&Outer<_>>` for type `&Outer<_>`
+  --> $DIR/dispatch-from-dyn-invalid-impls.rs:80:1
+   |
+LL | impl<'a, T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<&'a Outer<U>> for &'a Outer<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: conflicting implementation in crate `core`:
+           - impl<'a, T, U> DispatchFromDyn<&'a U> for &'a T
+             where T: Unsize<U>, T: ?Sized, U: ?Sized;
+
 error[E0378]: the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields with 1 byte alignment that don't mention type/const generics, and nothing else
   --> $DIR/dispatch-from-dyn-invalid-impls.rs:19:1
    |
@@ -54,7 +75,7 @@ LL | |     T: Unsize<U>
    |
    = note: extra field `1` of type `OverAlignedZst` is not allowed
 
-error: aborting due to 5 previous errors
+error: aborting due to 7 previous errors
 
-Some errors have detailed explanations: E0374, E0375, E0378.
-For more information about an error, try `rustc --explain E0374`.
+Some errors have detailed explanations: E0119, E0374, E0375, E0378.
+For more information about an error, try `rustc --explain E0119`.