split up #[rustc_deny_explicit_impl] attribute

This commit splits the `#[rustc_deny_explicit_impl(implement_via_object = ...)]` attribute
into two attributes `#[rustc_deny_explicit_impl]` and `#[rustc_do_not_implement_via_object]`.

This allows us to have special traits that can have user-defined impls but do not have the
automatic trait impl for trait objects (`impl Trait for dyn Trait`).

core/src/future/async_drop.rs

@@ -133,7 +133,9 @@ pub trait AsyncDrop {
 }
 
 #[lang = "async_destruct"]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 trait AsyncDestruct {
     type AsyncDestructor: Future<Output = ()>;
 }

core/src/marker.rs

@@ -141,7 +141,9 @@ unsafe impl<T: Sync + ?Sized> Send for &T {}
 )]
 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
 #[rustc_specialization_trait]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 #[rustc_coinductive]
 pub trait Sized {
     // Empty.
@@ -181,7 +183,9 @@ pub trait Sized {
 /// [^1]: Formerly known as *object safe*.
 #[unstable(feature = "unsize", issue = "18598")]
 #[lang = "unsize"]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait Unsize<T: ?Sized> {
     // Empty.
 }
@@ -815,7 +819,9 @@ impl<T: ?Sized> StructuralPartialEq for PhantomData<T> {}
     reason = "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
 )]
 #[lang = "discriminant_kind"]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait DiscriminantKind {
     /// The type of the discriminant, which must satisfy the trait
     /// bounds required by `mem::Discriminant`.
@@ -956,7 +962,9 @@ marker_impls! {
 #[unstable(feature = "const_destruct", issue = "133214")]
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 #[cfg_attr(not(bootstrap), const_trait)]
 pub trait Destruct {}
 
@@ -967,7 +975,9 @@ pub trait Destruct {}
 #[unstable(feature = "tuple_trait", issue = "none")]
 #[lang = "tuple_trait"]
 #[diagnostic::on_unimplemented(message = "`{Self}` is not a tuple")]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait Tuple {}
 
 /// A marker for pointer-like types.
@@ -1068,7 +1078,9 @@ marker_impls! {
     reason = "internal trait for implementing various traits for all function pointers"
 )]
 #[lang = "fn_ptr_trait"]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait FnPtr: Copy + Clone {
     /// Returns the address of the function pointer.
     #[lang = "fn_ptr_addr"]

core/src/mem/transmutability.rs

@@ -84,7 +84,9 @@ use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
 /// `usize` is stable, but not portable.
 #[unstable(feature = "transmutability", issue = "99571")]
 #[lang = "transmute_trait"]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 #[rustc_coinductive]
 pub unsafe trait TransmuteFrom<Src, const ASSUME: Assume = { Assume::NOTHING }>
 where

core/src/ptr/metadata.rs

@@ -53,7 +53,9 @@ use crate::ptr::NonNull;
 ///
 /// [`to_raw_parts`]: *const::to_raw_parts
 #[lang = "pointee_trait"]
-#[rustc_deny_explicit_impl(implement_via_object = false)]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait Pointee {
     /// The type for metadata in pointers and references to `Self`.
     #[lang = "metadata_type"]