Auto merge of rust-lang#112320 - compiler-errors:do-not-impl-via-obj, r=lcnr

Add `implement_via_object` to `rustc_deny_explicit_impl` to control object candidate assembly

Some built-in traits are special, since they are used to prove facts about the program that are important for later phases of compilation such as codegen and CTFE. For example, the `Unsize` trait is used to assert to the compiler that we are able to unsize a type into another type. It doesn't have any methods because it doesn't actually *instruct* the compiler how to do this unsizing, but this is later used (alongside an exhaustive match of combinations of unsizeable types) during codegen to generate unsize coercion code.

Due to this, these built-in traits are incompatible with the type erasure provided by object types. For example, the existence of `dyn Unsize<T>` does not mean that the compiler is able to unsize `Box<dyn Unsize<T>>` into `Box<T>`, since `Unsize` is a *witness* to the fact that a type can be unsized, and it doesn't actually encode that unsizing operation in its vtable as mentioned above.

The old trait solver gets around this fact by having complex control flow that never considers object bounds for certain built-in traits:
https://github.com/rust-lang/rust/blob/2f896da247e0ee8f0bef7cd7c54cfbea255b9f68/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L61-L132

However, candidate assembly in the new solver is much more lovely, and I'd hate to add this list of opt-out cases into the new solver. Instead of maintaining this complex and hard-coded control flow, instead we can make this a property of the trait via a built-in attribute. We already have such a build attribute that's applied to every single trait that we care about: `rustc_deny_explicit_impl`. This PR adds `implement_via_object` as a meta-item to that attribute that allows us to opt a trait out of object-bound candidate assembly as well.

r? `@lcnr`

Author: bors

core/src/marker.rs

@@ -140,7 +140,8 @@ 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]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 #[rustc_coinductive]
 pub trait Sized {
     // Empty.
@@ -173,7 +174,8 @@ pub trait Sized {
 /// [nomicon-coerce]: ../../nomicon/coercions.html
 #[unstable(feature = "unsize", issue = "18598")]
 #[lang = "unsize"]
-#[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 pub trait Unsize<T: ?Sized> {
     // Empty.
 }
@@ -854,7 +856,8 @@ impl<T: ?Sized> StructuralEq for PhantomData<T> {}
     reason = "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
 )]
 #[lang = "discriminant_kind"]
-#[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 pub trait DiscriminantKind {
     /// The type of the discriminant, which must satisfy the trait
     /// bounds required by `mem::Discriminant`.
@@ -959,7 +962,8 @@ marker_impls! {
 #[unstable(feature = "const_trait_impl", issue = "67792")]
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
-#[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 #[const_trait]
 pub trait Destruct {}
 
@@ -970,7 +974,8 @@ pub trait Destruct {}
 #[unstable(feature = "tuple_trait", issue = "none")]
 #[lang = "tuple_trait"]
 #[rustc_on_unimplemented(message = "`{Self}` is not a tuple")]
-#[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 pub trait Tuple {}
 
 /// A marker for pointer-like types.
@@ -1025,7 +1030,8 @@ impl ConstParamTy for () {}
     reason = "internal trait for implementing various traits for all function pointers"
 )]
 #[lang = "fn_ptr_trait"]
-#[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 pub trait FnPtr: Copy + Clone {
     /// Returns the address of the function pointer.
     #[lang = "fn_ptr_addr"]

core/src/mem/transmutability.rs

@@ -7,6 +7,8 @@ use crate::marker::ConstParamTy;
 /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
 #[unstable(feature = "transmutability", issue = "99571")]
 #[lang = "transmute_trait"]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
 where
     Src: ?Sized,

core/src/ptr/metadata.rs

@@ -50,7 +50,8 @@ use crate::hash::{Hash, Hasher};
 ///
 /// [`to_raw_parts`]: *const::to_raw_parts
 #[lang = "pointee_trait"]
-#[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
+#[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
 pub trait Pointee {
     /// The type for metadata in pointers and references to `Self`.
     #[lang = "metadata_type"]