Replace #[rustc_do_not_implement_via_object] with #[rustc_dyn_incompatible_trait], which makes the marked trait dyn-incompatible.

Removes the attribute from `MetaSized` and `PointeeSized`, with a special case in the trait solvers for `MetaSized`.

`dyn MetaSized` is a perfectly cromulent type, and seems to only have had #[rustc_do_not_implement_via_object] so the builtin object
candidate does not overlap with the builtin MetaSized impl that all `dyn` types get.
Resolves this with a special case by checking `is_sizedness_trait` where the trait solvers previously checked `implement_via_object`.

`dyn PointeeSized` alone is rejected for other reasons (since `dyn PointeeSized` is considered to have no principal trait because `PointeeSized`
is removed at an earlier stage of the compiler), but `(dyn PointeeSized + Send)` is valid and equivalent to `dyn Send`.

Author: zachs18

compiler/rustc_attr_parsing/src/attributes/traits.rs

@@ -94,12 +94,12 @@ impl<S: Stage> NoArgsAttributeParser<S> for DenyExplicitImplParser {
     const CREATE: fn(Span) -> AttributeKind = AttributeKind::DenyExplicitImpl;
 }
 
-pub(crate) struct DoNotImplementViaObjectParser;
-impl<S: Stage> NoArgsAttributeParser<S> for DoNotImplementViaObjectParser {
-    const PATH: &[Symbol] = &[sym::rustc_do_not_implement_via_object];
+pub(crate) struct DynIncompatibleTraitParser;
+impl<S: Stage> NoArgsAttributeParser<S> for DynIncompatibleTraitParser {
+    const PATH: &[Symbol] = &[sym::rustc_dyn_incompatible_trait];
     const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::Trait)]);
-    const CREATE: fn(Span) -> AttributeKind = AttributeKind::DoNotImplementViaObject;
+    const CREATE: fn(Span) -> AttributeKind = AttributeKind::DynIncompatibleTrait;
 }
 
 // Specialization

compiler/rustc_attr_parsing/src/context.rs

@@ -78,8 +78,8 @@ use crate::attributes::stability::{
 use crate::attributes::test_attrs::{IgnoreParser, ShouldPanicParser};
 use crate::attributes::traits::{
     AllowIncoherentImplParser, CoinductiveParser, DenyExplicitImplParser,
-    DoNotImplementViaObjectParser, FundamentalParser, MarkerParser, ParenSugarParser,
-    PointeeParser, SkipDuringMethodDispatchParser, SpecializationTraitParser, TypeConstParser,
+    DynIncompatibleTraitParser, FundamentalParser, MarkerParser, ParenSugarParser, PointeeParser,
+    SkipDuringMethodDispatchParser, SpecializationTraitParser, TypeConstParser,
     UnsafeSpecializationMarkerParser,
 };
 use crate::attributes::transparency::TransparencyParser;
@@ -242,7 +242,7 @@ attribute_parsers!(
         Single<WithoutArgs<ConstStabilityIndirectParser>>,
         Single<WithoutArgs<CoroutineParser>>,
         Single<WithoutArgs<DenyExplicitImplParser>>,
-        Single<WithoutArgs<DoNotImplementViaObjectParser>>,
+        Single<WithoutArgs<DynIncompatibleTraitParser>>,
         Single<WithoutArgs<EiiForeignItemParser>>,
         Single<WithoutArgs<ExportStableParser>>,
         Single<WithoutArgs<FfiConstParser>>,

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1321,13 +1321,13 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "`#[rustc_deny_explicit_impl]` enforces that a trait can have no user-provided impls"
     ),
     rustc_attr!(
-        rustc_do_not_implement_via_object,
+        rustc_dyn_incompatible_trait,
         AttributeType::Normal,
         template!(Word),
         ErrorFollowing,
         EncodeCrossCrate::No,
-        "`#[rustc_do_not_implement_via_object]` opts out of the automatic trait impl for trait objects \
-        (`impl Trait for dyn Trait`)"
+        "`#[rustc_dyn_incompatible_trait]` marks a trait as dyn-incompatible, \
+        even if it otherwise satisfies the requirements to be dyn-compatible."
     ),
     rustc_attr!(
         rustc_has_incoherent_inherent_impls, AttributeType::Normal, template!(Word),

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -728,9 +728,6 @@ pub enum AttributeKind {
     /// Represents [`#[deprecated]`](https://doc.rust-lang.org/stable/reference/attributes/diagnostics.html#the-deprecated-attribute).
     Deprecation { deprecation: Deprecation, span: Span },
 
-    /// Represents `#[rustc_do_not_implement_via_object]`.
-    DoNotImplementViaObject(Span),
-
     /// Represents [`#[doc]`](https://doc.rust-lang.org/stable/rustdoc/write-documentation/the-doc-attribute.html).
     /// Represents all other uses of the [`#[doc]`](https://doc.rust-lang.org/stable/rustdoc/write-documentation/the-doc-attribute.html)
     /// attribute.
@@ -743,6 +740,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_dummy]`.
     Dummy,
 
+    /// Represents `#[rustc_dyn_incompatible_trait]`.
+    DynIncompatibleTrait(Span),
+
     /// Implementation detail of `#[eii]`
     EiiDeclaration(EiiDecl),
 

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -43,10 +43,10 @@ impl AttributeKind {
             DebuggerVisualizer(..) => No,
             DenyExplicitImpl(..) => No,
             Deprecation { .. } => Yes,
-            DoNotImplementViaObject(..) => No,
             Doc(_) => Yes,
             DocComment { .. } => Yes,
             Dummy => No,
+            DynIncompatibleTrait(..) => No,
             EiiDeclaration(_) => Yes,
             EiiForeignItem => No,
             EiiImpls(..) => No,

compiler/rustc_hir_analysis/src/coherence/mod.rs

@@ -211,9 +211,7 @@ fn check_object_overlap<'tcx>(
                 // This is a WF error tested by `coherence-impl-trait-for-trait-dyn-compatible.rs`.
             } else {
                 let mut supertrait_def_ids = elaborate::supertrait_def_ids(tcx, component_def_id);
-                if supertrait_def_ids
-                    .any(|d| d == trait_def_id && tcx.trait_def(d).implement_via_object)
-                {
+                if supertrait_def_ids.any(|d| d == trait_def_id) {
                     let span = tcx.def_span(impl_def_id);
                     return Err(struct_span_code_err!(
                         tcx.dcx(),

compiler/rustc_hir_analysis/src/collect.rs

@@ -926,7 +926,8 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
     );
 
     let deny_explicit_impl = find_attr!(attrs, AttributeKind::DenyExplicitImpl(_));
-    let implement_via_object = !find_attr!(attrs, AttributeKind::DoNotImplementViaObject(_));
+    let force_dyn_incompatible =
+        find_attr!(attrs, AttributeKind::DynIncompatibleTrait(span) => *span);
 
     ty::TraitDef {
         def_id: def_id.to_def_id(),
@@ -941,7 +942,7 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
         skip_boxed_slice_during_method_dispatch,
         specialization_kind,
         must_implement_one_of,
-        implement_via_object,
+        force_dyn_incompatible,
         deny_explicit_impl,
     }
 }

compiler/rustc_middle/src/traits/mod.rs

@@ -764,6 +764,9 @@ pub enum DynCompatibilityViolation {
     /// `Self: Sized` declared on the trait.
     SizedSelf(SmallVec<[Span; 1]>),
 
+    /// Trait is marked `#[rustc_dyn_incompatible_trait]`.
+    ExplicitlyDynIncompatible(SmallVec<[Span; 1]>),
+
     /// Supertrait reference references `Self` an in illegal location
     /// (e.g., `trait Foo : Bar<Self>`).
     SupertraitSelf(SmallVec<[Span; 1]>),
@@ -788,6 +791,9 @@ impl DynCompatibilityViolation {
     pub fn error_msg(&self) -> Cow<'static, str> {
         match self {
             DynCompatibilityViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
+            DynCompatibilityViolation::ExplicitlyDynIncompatible(_) => {
+                "it opted out of dyn-compatibility".into()
+            }
             DynCompatibilityViolation::SupertraitSelf(spans) => {
                 if spans.iter().any(|sp| *sp != DUMMY_SP) {
                     "it uses `Self` as a type parameter".into()
@@ -861,6 +867,7 @@ impl DynCompatibilityViolation {
     pub fn solution(&self) -> DynCompatibilityViolationSolution {
         match self {
             DynCompatibilityViolation::SizedSelf(_)
+            | DynCompatibilityViolation::ExplicitlyDynIncompatible(_)
             | DynCompatibilityViolation::SupertraitSelf(_)
             | DynCompatibilityViolation::SupertraitNonLifetimeBinder(..)
             | DynCompatibilityViolation::SupertraitConst(_) => {
@@ -894,6 +901,7 @@ impl DynCompatibilityViolation {
         match self {
             DynCompatibilityViolation::SupertraitSelf(spans)
             | DynCompatibilityViolation::SizedSelf(spans)
+            | DynCompatibilityViolation::ExplicitlyDynIncompatible(spans)
             | DynCompatibilityViolation::SupertraitNonLifetimeBinder(spans)
             | DynCompatibilityViolation::SupertraitConst(spans) => spans.clone(),
             DynCompatibilityViolation::AssocConst(_, span)

compiler/rustc_middle/src/ty/context.rs

@@ -710,10 +710,6 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.trait_def(def_id).is_fundamental
     }
 
-    fn trait_may_be_implemented_via_object(self, trait_def_id: DefId) -> bool {
-        self.trait_def(trait_def_id).implement_via_object
-    }
-
     fn trait_is_unsafe(self, trait_def_id: Self::DefId) -> bool {
         self.trait_def(trait_def_id).safety.is_unsafe()
     }

compiler/rustc_middle/src/ty/trait_def.rs

@@ -7,6 +7,7 @@ use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_hir::{self as hir, find_attr};
 use rustc_macros::{Decodable, Encodable, HashStable};
+use rustc_span::Span;
 use tracing::debug;
 
 use crate::query::LocalCrate;
@@ -69,10 +70,9 @@ pub struct TraitDef {
     /// must be implemented.
     pub must_implement_one_of: Option<Box<[Ident]>>,
 
-    /// Whether to add a builtin `dyn Trait: Trait` implementation.
-    /// This is enabled for all traits except ones marked with
-    /// `#[rustc_do_not_implement_via_object]`.
-    pub implement_via_object: bool,
+    /// Whether the trait should be considered dyn-incompatible, even if it otherwise
+    /// satisfies the requirements to be dyn-compatible.
+    pub force_dyn_incompatible: Option<Span>,
 
     /// Whether a trait is fully built-in, and any implementation is disallowed.
     /// This only applies to built-in traits, and is marked via