Auto merge of #125380 - compiler-errors:wc-obj-safety, r=oli-obk

Make `WHERE_CLAUSES_OBJECT_SAFETY` a regular object safety violation

#### The issue

In #50781, we have known about unsound `where` clauses in function arguments:

```rust
trait Impossible {}

trait Foo {
    fn impossible(&self)
    where
        Self: Impossible;
}

impl Foo for &() {
    fn impossible(&self)
    where
        Self: Impossible,
    {}
}

// `where` clause satisfied for the object, meaning that the function now *looks* callable.
impl Impossible for dyn Foo {}

fn main() {
    let x: &dyn Foo = &&();
    x.impossible();
}
```

... which currently segfaults at runtime because we try to call a method in the vtable that doesn't exist. :(

#### What did u change

This PR removes the `WHERE_CLAUSES_OBJECT_SAFETY` lint and instead makes it a regular object safety violation. I choose to make this into a hard error immediately rather than a `deny` because of the time that has passed since this lint was authored, and the single (1) regression (see below).

That means that it's OK to mention `where Self: Trait` where clauses in your trait, but making such a trait into a `dyn Trait` object will report an object safety violation just like `where Self: Sized`, etc.

```rust
trait Impossible {}

trait Foo {
    fn impossible(&self)
    where
        Self: Impossible; // <~ This definition is valid, just not object-safe.
}

impl Foo for &() {
    fn impossible(&self)
    where
        Self: Impossible,
    {}
}

fn main() {
    let x: &dyn Foo = &&(); // <~ THIS is where we emit an error.
}
```

#### Regressions

From a recent crater run, there's only one crate that relies on this behavior: #124305 (comment). The crate looks unmaintained and there seems to be no dependents.

#### Further

We may later choose to relax this (e.g. when the where clause is implied by the supertraits of the trait or something), but this is not something I propose to do in this FCP.

For example, given:

```
trait Tr {
  fn f(&self) where Self: Blanket;
}

impl<T: ?Sized> Blanket for T {}
```

Proving that some placeholder `S` implements `S: Blanket` would be sufficient to prove that the same (blanket) impl applies for both `Concerete: Blanket` and `dyn Trait: Blanket`.

Repeating here that I don't think we need to implement this behavior right now.

----

r? lcnr

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -881,7 +881,7 @@ fn check_object_unsafe_self_trait_by_name(tcx: TyCtxt<'_>, item: &hir::TraitItem
         _ => {}
     }
     if !trait_should_be_self.is_empty() {
-        if tcx.check_is_object_safe(trait_def_id) {
+        if tcx.is_object_safe(trait_def_id) {
             return;
         }
         let sugg = trait_should_be_self.iter().map(|span| (*span, "Self".to_string())).collect();

compiler/rustc_hir_analysis/src/coherence/mod.rs

@@ -191,7 +191,7 @@ fn check_object_overlap<'tcx>(
         });
 
         for component_def_id in component_def_ids {
-            if !tcx.check_is_object_safe(component_def_id) {
+            if !tcx.is_object_safe(component_def_id) {
                 // Without the 'object_safe_for_dispatch' feature this is an error
                 // which will be reported by wfcheck. Ignore it here.
                 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -182,7 +182,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             // For recursive traits, don't downgrade the error. (#119652)
                             is_downgradable = false;
                         }
-                        tcx.check_is_object_safe(id)
+                        tcx.is_object_safe(id)
                     }
                     _ => false,
                 })

compiler/rustc_interface/src/passes.rs

@@ -832,7 +832,7 @@ fn analysis(tcx: TyCtxt<'_>, (): ()) -> Result<()> {
         let traits = tcx.traits(LOCAL_CRATE);
 
         for &tr in traits {
-            if !tcx.check_is_object_safe(tr) {
+            if !tcx.is_object_safe(tr) {
                 continue;
             }
 

compiler/rustc_lint/src/multiple_supertrait_upcastable.rs

@@ -38,10 +38,10 @@ declare_lint_pass!(MultipleSupertraitUpcastable => [MULTIPLE_SUPERTRAIT_UPCASTAB
 impl<'tcx> LateLintPass<'tcx> for MultipleSupertraitUpcastable {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
         let def_id = item.owner_id.to_def_id();
-        // NOTE(nbdd0121): use `object_safety_violations` instead of `check_is_object_safe` because
+        // NOTE(nbdd0121): use `object_safety_violations` instead of `is_object_safe` because
         // the latter will report `where_clause_object_safety` lint.
         if let hir::ItemKind::Trait(_, _, _, _, _) = item.kind
-            && cx.tcx.object_safety_violations(def_id).is_empty()
+            && cx.tcx.is_object_safe(def_id)
         {
             let direct_super_traits_iter = cx
                 .tcx

compiler/rustc_lint_defs/src/builtin.rs

@@ -136,7 +136,6 @@ declare_lint_pass! {
         USELESS_DEPRECATED,
         WARNINGS,
         WASM_C_ABI,
-        WHERE_CLAUSES_OBJECT_SAFETY,
         WRITES_THROUGH_IMMUTABLE_POINTER,
         // tidy-alphabetical-end
     ]
@@ -2093,47 +2092,6 @@ declare_lint! {
     "detects labels that are never used"
 }
 
-declare_lint! {
-    /// The `where_clauses_object_safety` lint detects for [object safety] of
-    /// [where clauses].
-    ///
-    /// [object safety]: https://doc.rust-lang.org/reference/items/traits.html#object-safety
-    /// [where clauses]: https://doc.rust-lang.org/reference/items/generics.html#where-clauses
-    ///
-    /// ### Example
-    ///
-    /// ```rust,no_run
-    /// trait Trait {}
-    ///
-    /// trait X { fn foo(&self) where Self: Trait; }
-    ///
-    /// impl X for () { fn foo(&self) {} }
-    ///
-    /// impl Trait for dyn X {}
-    ///
-    /// // Segfault at opt-level 0, SIGILL otherwise.
-    /// pub fn main() { <dyn X as X>::foo(&()); }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// The compiler previously allowed these object-unsafe bounds, which was
-    /// incorrect. This is a [future-incompatible] lint to transition this to
-    /// a hard error in the future. See [issue #51443] for more details.
-    ///
-    /// [issue #51443]: https://github.com/rust-lang/rust/issues/51443
-    /// [future-incompatible]: ../index.md#future-incompatible-lints
-    pub WHERE_CLAUSES_OBJECT_SAFETY,
-    Warn,
-    "checks the object safety of where clauses",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
-        reference: "issue #51443 <https://github.com/rust-lang/rust/issues/51443>",
-    };
-}
-
 declare_lint! {
     /// The `proc_macro_derive_resolution_fallback` lint detects proc macro
     /// derives using inaccessible names from parent modules.

compiler/rustc_middle/src/query/mod.rs

@@ -1309,7 +1309,7 @@ rustc_queries! {
     query object_safety_violations(trait_id: DefId) -> &'tcx [ObjectSafetyViolation] {
         desc { |tcx| "determining object safety of trait `{}`", tcx.def_path_str(trait_id) }
     }
-    query check_is_object_safe(trait_id: DefId) -> bool {
+    query is_object_safe(trait_id: DefId) -> bool {
         desc { |tcx| "checking if trait `{}` is object safe", tcx.def_path_str(trait_id) }
     }
 

compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/transform.rs

@@ -367,7 +367,7 @@ pub fn transform_instance<'tcx>(
             let trait_method = tcx.associated_item(method_id);
             let trait_id = trait_ref.skip_binder().def_id;
             if traits::is_vtable_safe_method(tcx, trait_id, trait_method)
-                && tcx.object_safety_violations(trait_id).is_empty()
+                && tcx.is_object_safe(trait_id)
             {
                 // Trait methods will have a Self polymorphic parameter, where the concreteized
                 // implementatation will not. We need to walk back to the more general trait method

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -714,7 +714,7 @@ impl<'tcx> EvalCtxt<'_, InferCtxt<'tcx>> {
         };
 
         // Do not consider built-in object impls for non-object-safe types.
-        if bounds.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
+        if bounds.principal_def_id().is_some_and(|def_id| !tcx.is_object_safe(def_id)) {
             return;
         }
 

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -133,7 +133,7 @@ impl<'a, 'tcx> EvalCtxt<'a, InferCtxt<'tcx>> {
     }
 
     fn compute_object_safe_goal(&mut self, trait_def_id: DefId) -> QueryResult<'tcx> {
-        if self.interner().check_is_object_safe(trait_def_id) {
+        if self.interner().is_object_safe(trait_def_id) {
             self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         } else {
             Err(NoSolution)

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -789,7 +789,7 @@ impl<'tcx> EvalCtxt<'_, InferCtxt<'tcx>> {
         let Goal { predicate: (a_ty, _), .. } = goal;
 
         // Can only unsize to an object-safe trait.
-        if b_data.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
+        if b_data.principal_def_id().is_some_and(|def_id| !tcx.is_object_safe(def_id)) {
             return Err(NoSolution);
         }
 

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -421,7 +421,7 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 }
 
                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
-                    if !self.selcx.tcx().check_is_object_safe(trait_def_id) {
+                    if !self.selcx.tcx().is_object_safe(trait_def_id) {
                         ProcessResult::Error(FulfillmentErrorCode::Select(Unimplemented))
                     } else {
                         ProcessResult::Changed(vec![])

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -13,7 +13,7 @@ use super::elaborate;
 use crate::infer::TyCtxtInferExt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{self, Obligation, ObligationCause};
-use rustc_errors::{FatalError, MultiSpan};
+use rustc_errors::FatalError;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::query::Providers;
@@ -23,7 +23,6 @@ use rustc_middle::ty::{
 };
 use rustc_middle::ty::{GenericArg, GenericArgs};
 use rustc_middle::ty::{TypeVisitableExt, Upcast};
-use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 use rustc_target::abi::Abi;
@@ -65,45 +64,14 @@ fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [Object
     )
 }
 
-fn check_is_object_safe(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
-    let violations = tcx.object_safety_violations(trait_def_id);
-
-    if violations.is_empty() {
-        return true;
-    }
-
-    // If the trait contains any other violations, then let the error reporting path
-    // report it instead of emitting a warning here.
-    if violations.iter().all(|violation| {
-        matches!(
-            violation,
-            ObjectSafetyViolation::Method(_, MethodViolationCode::WhereClauseReferencesSelf, _)
-        )
-    }) {
-        for violation in violations {
-            if let ObjectSafetyViolation::Method(
-                _,
-                MethodViolationCode::WhereClauseReferencesSelf,
-                span,
-            ) = violation
-            {
-                lint_object_unsafe_trait(tcx, *span, trait_def_id, violation);
-            }
-        }
-        return true;
-    }
-
-    false
+fn is_object_safe(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
+    tcx.object_safety_violations(trait_def_id).is_empty()
 }
 
 /// We say a method is *vtable safe* if it can be invoked on a trait
 /// object. Note that object-safe traits can have some
 /// non-vtable-safe methods, so long as they require `Self: Sized` or
 /// otherwise ensure that they cannot be used when `Self = Trait`.
-///
-/// [`MethodViolationCode::WhereClauseReferencesSelf`] is considered object safe due to backwards
-/// compatibility, see <https://github.com/rust-lang/rust/issues/51443> and
-/// [`WHERE_CLAUSES_OBJECT_SAFETY`].
 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: ty::AssocItem) -> bool {
     debug_assert!(tcx.generics_of(trait_def_id).has_self);
     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
@@ -112,9 +80,7 @@ pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: ty::A
         return false;
     }
 
-    virtual_call_violations_for_method(tcx, trait_def_id, method)
-        .iter()
-        .all(|v| matches!(v, MethodViolationCode::WhereClauseReferencesSelf))
+    virtual_call_violations_for_method(tcx, trait_def_id, method).is_empty()
 }
 
 fn object_safety_violations_for_trait(
@@ -163,47 +129,6 @@ fn object_safety_violations_for_trait(
     violations
 }
 
-/// Lint object-unsafe trait.
-fn lint_object_unsafe_trait(
-    tcx: TyCtxt<'_>,
-    span: Span,
-    trait_def_id: DefId,
-    violation: &ObjectSafetyViolation,
-) {
-    // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
-    // It's also hard to get a use site span, so we use the method definition span.
-    tcx.node_span_lint(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |err| {
-        err.primary_message(format!(
-            "the trait `{}` cannot be made into an object",
-            tcx.def_path_str(trait_def_id)
-        ));
-        let node = tcx.hir().get_if_local(trait_def_id);
-        let mut spans = MultiSpan::from_span(span);
-        if let Some(hir::Node::Item(item)) = node {
-            spans.push_span_label(item.ident.span, "this trait cannot be made into an object...");
-            spans.push_span_label(span, format!("...because {}", violation.error_msg()));
-        } else {
-            spans.push_span_label(
-                span,
-                format!(
-                    "the trait cannot be made into an object because {}",
-                    violation.error_msg()
-                ),
-            );
-        };
-        err.span_note(
-            spans,
-            "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
-                call to be resolvable dynamically; for more information visit \
-                <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
-        );
-        if node.is_some() {
-            // Only provide the help if its a local trait, otherwise it's not
-            violation.solution().add_to(err);
-        }
-    });
-}
-
 fn sized_trait_bound_spans<'tcx>(
     tcx: TyCtxt<'tcx>,
     bounds: hir::GenericBounds<'tcx>,
@@ -929,7 +854,7 @@ pub fn contains_illegal_impl_trait_in_trait<'tcx>(
 pub fn provide(providers: &mut Providers) {
     *providers = Providers {
         object_safety_violations,
-        check_is_object_safe,
+        is_object_safe,
         generics_require_sized_self,
         ..*providers
     };

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -870,7 +870,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         if let Some(principal) = data.principal() {
                             if !self.infcx.tcx.features().object_safe_for_dispatch {
                                 principal.with_self_ty(self.tcx(), self_ty)
-                            } else if self.tcx().check_is_object_safe(principal.def_id()) {
+                            } else if self.tcx().is_object_safe(principal.def_id()) {
                                 principal.with_self_ty(self.tcx(), self_ty)
                             } else {
                                 return;

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1222,7 +1222,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             // `T` -> `Trait`
             (_, &ty::Dynamic(data, r, ty::Dyn)) => {
                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
-                if let Some(did) = object_dids.find(|did| !tcx.check_is_object_safe(*did)) {
+                if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
                     return Err(TraitNotObjectSafe(did));
                 }
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -798,7 +798,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
 
                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
-                    if self.tcx().check_is_object_safe(trait_def_id) {
+                    if self.tcx().is_object_safe(trait_def_id) {
                         Ok(EvaluatedToOk)
                     } else {
                         Ok(EvaluatedToErr)

src/librustdoc/clean/types.rs

@@ -1451,7 +1451,7 @@ impl Trait {
         tcx.trait_def(self.def_id).safety
     }
     pub(crate) fn is_object_safe(&self, tcx: TyCtxt<'_>) -> bool {
-        tcx.check_is_object_safe(self.def_id)
+        tcx.is_object_safe(self.def_id)
     }
 }
 

tests/ui/const-generics/generic_const_exprs/object-safety-err-where-bounds.rs

@@ -1,22 +1,21 @@
 #![feature(generic_const_exprs)]
 #![allow(incomplete_features)]
-#![deny(where_clauses_object_safety)]
 
 
 const fn bar<T: ?Sized>() -> usize { 7 }
 
 trait Foo {
     fn test(&self) where [u8; bar::<Self>()]: Sized;
-    //~^ ERROR the trait `Foo` cannot be made into an object
-    //~| WARN this was previously accepted by the compiler but is being phased out
 }
 
 impl Foo for () {
     fn test(&self) where [u8; bar::<Self>()]: Sized {}
 }
 
 fn use_dyn(v: &dyn Foo) {
+    //~^ ERROR the trait `Foo` cannot be made into an object
     v.test();
+    //~^ ERROR the trait `Foo` cannot be made into an object
 }
 
 fn main() {}