Make inductive cycles in coherence ambiguous always

Author: compiler-errors

compiler/rustc_lint/src/lib.rs

@@ -512,6 +512,11 @@ fn register_builtins(store: &mut LintStore) {
         "converted into hard error, see PR #104616 \
          <https://github.com/rust-lang/rust/pull/104616> for more information",
     );
+    store.register_removed(
+        "coinductive_overlap_in_coherence",
+        "converted into hard error, see PR #118649 \
+         <https://github.com/rust-lang/rust/pull/118649> for more information",
+    );
 }
 
 fn register_internals(store: &mut LintStore) {

compiler/rustc_lint_defs/src/builtin.rs

@@ -3366,7 +3366,6 @@ declare_lint_pass! {
         BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE,
         CENUM_IMPL_DROP_CAST,
         COHERENCE_LEAK_CHECK,
-        COINDUCTIVE_OVERLAP_IN_COHERENCE,
         CONFLICTING_REPR_HINTS,
         CONST_EVALUATABLE_UNCHECKED,
         CONST_ITEM_MUTATION,
@@ -4429,45 +4428,6 @@ declare_lint! {
     @feature_gate = sym::type_privacy_lints;
 }
 
-declare_lint! {
-    /// The `coinductive_overlap_in_coherence` lint detects impls which are currently
-    /// considered not overlapping, but may be considered to overlap if support for
-    /// coinduction is added to the trait solver.
-    ///
-    /// ### Example
-    ///
-    /// ```rust,compile_fail
-    /// #![deny(coinductive_overlap_in_coherence)]
-    ///
-    /// trait CyclicTrait {}
-    /// impl<T: CyclicTrait> CyclicTrait for T {}
-    ///
-    /// trait Trait {}
-    /// impl<T: CyclicTrait> Trait for T {}
-    /// // conflicting impl with the above
-    /// impl Trait for u8 {}
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// We have two choices for impl which satisfy `u8: Trait`: the blanket impl
-    /// for generic `T`, and the direct impl for `u8`. These two impls nominally
-    /// overlap, since we can infer `T = u8` in the former impl, but since the where
-    /// clause `u8: CyclicTrait` would end up resulting in a cycle (since it depends
-    /// on itself), the blanket impl is not considered to hold for `u8`. This will
-    /// change in a future release.
-    pub COINDUCTIVE_OVERLAP_IN_COHERENCE,
-    Deny,
-    "impls that are not considered to overlap may be considered to \
-    overlap in the future",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
-        reference: "issue #114040 <https://github.com/rust-lang/rust/issues/114040>",
-    };
-}
-
 declare_lint! {
     /// The `unknown_or_malformed_diagnostic_attributes` lint detects unrecognized or otherwise malformed
     /// diagnostic attributes.

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -10,7 +10,7 @@ use crate::solve::inspect;
 use crate::solve::inspect::{InspectGoal, ProofTreeInferCtxtExt, ProofTreeVisitor};
 use crate::traits::engine::TraitEngineExt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
-use crate::traits::select::{IntercrateAmbiguityCause, TreatInductiveCycleAs};
+use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::structural_normalize::StructurallyNormalizeExt;
 use crate::traits::NormalizeExt;
 use crate::traits::SkipLeakCheck;
@@ -31,7 +31,6 @@ use rustc_middle::traits::DefiningAnchor;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
-use rustc_session::lint::builtin::COINDUCTIVE_OVERLAP_IN_COHERENCE;
 use rustc_span::symbol::sym;
 use rustc_span::DUMMY_SP;
 use std::fmt::Debug;
@@ -197,7 +196,7 @@ fn overlap<'tcx>(
         .intercrate(true)
         .with_next_trait_solver(tcx.next_trait_solver_in_coherence())
         .build();
-    let selcx = &mut SelectionContext::new(&infcx);
+    let selcx = &mut SelectionContext::with_treat_inductive_cycle_as_ambig(&infcx);
     if track_ambiguity_causes.is_yes() {
         selcx.enable_tracking_intercrate_ambiguity_causes();
     }
@@ -224,62 +223,10 @@ fn overlap<'tcx>(
     );
 
     if overlap_mode.use_implicit_negative() {
-        for mode in [TreatInductiveCycleAs::Ambig, TreatInductiveCycleAs::Recur] {
-            if let Some(failing_obligation) = selcx.with_treat_inductive_cycle_as(mode, |selcx| {
-                impl_intersection_has_impossible_obligation(selcx, &obligations)
-            }) {
-                if matches!(mode, TreatInductiveCycleAs::Recur) {
-                    let first_local_impl = impl1_header
-                        .impl_def_id
-                        .as_local()
-                        .or(impl2_header.impl_def_id.as_local())
-                        .expect("expected one of the impls to be local");
-                    infcx.tcx.struct_span_lint_hir(
-                        COINDUCTIVE_OVERLAP_IN_COHERENCE,
-                        infcx.tcx.local_def_id_to_hir_id(first_local_impl),
-                        infcx.tcx.def_span(first_local_impl),
-                        format!(
-                            "implementations {} will conflict in the future",
-                            match impl1_header.trait_ref {
-                                Some(trait_ref) => {
-                                    let trait_ref = infcx.resolve_vars_if_possible(trait_ref);
-                                    format!(
-                                        "of `{}` for `{}`",
-                                        trait_ref.print_only_trait_path(),
-                                        trait_ref.self_ty()
-                                    )
-                                }
-                                None => format!(
-                                    "for `{}`",
-                                    infcx.resolve_vars_if_possible(impl1_header.self_ty)
-                                ),
-                            },
-                        ),
-                        |lint| {
-                            lint.note(
-                                "impls that are not considered to overlap may be considered to \
-                                overlap in the future",
-                            )
-                            .span_label(
-                                infcx.tcx.def_span(impl1_header.impl_def_id),
-                                "the first impl is here",
-                            )
-                            .span_label(
-                                infcx.tcx.def_span(impl2_header.impl_def_id),
-                                "the second impl is here",
-                            );
-                            lint.note(format!(
-                                "`{}` may be considered to hold in future releases, \
-                                    causing the impls to overlap",
-                                infcx.resolve_vars_if_possible(failing_obligation.predicate)
-                            ));
-                            lint
-                        },
-                    );
-                }
-
-                return None;
-            }
+        if let Some(_failing_obligation) =
+            impl_intersection_has_impossible_obligation(selcx, &obligations)
+        {
+            return None;
         }
     }
 

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

@@ -239,20 +239,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    // Sets the `TreatInductiveCycleAs` mode temporarily in the selection context
-    pub fn with_treat_inductive_cycle_as<T>(
-        &mut self,
-        treat_inductive_cycle: TreatInductiveCycleAs,
-        f: impl FnOnce(&mut Self) -> T,
-    ) -> T {
+    pub fn with_treat_inductive_cycle_as_ambig(
+        infcx: &'cx InferCtxt<'tcx>,
+    ) -> SelectionContext<'cx, 'tcx> {
         // Should be executed in a context where caching is disabled,
         // otherwise the cache is poisoned with the temporary result.
-        assert!(self.is_intercrate());
-        let treat_inductive_cycle =
-            std::mem::replace(&mut self.treat_inductive_cycle, treat_inductive_cycle);
-        let value = f(self);
-        self.treat_inductive_cycle = treat_inductive_cycle;
-        value
+        assert!(infcx.intercrate);
+        SelectionContext {
+            treat_inductive_cycle: TreatInductiveCycleAs::Ambig,
+            ..SelectionContext::new(infcx)
+        }
     }
 
     pub fn with_query_mode(

tests/ui/coherence/warn-when-cycle-is-error-in-coherence.rs

@@ -1,18 +1,15 @@
-#![deny(coinductive_overlap_in_coherence)]
-
 use std::borrow::Borrow;
 use std::cmp::Ordering;
 use std::marker::PhantomData;
 
 #[derive(PartialEq, Default)]
+//~^ ERROR conflicting implementations of trait `PartialEq<Interval<_>>` for type `Interval<_>`
 pub(crate) struct Interval<T>(PhantomData<T>);
 
 // This impl overlaps with the `derive` unless we reject the nested
 // `Interval<?1>: PartialOrd<Interval<?1>>` candidate which results
-// in a - currently inductive - cycle.
+// in a cycle.
 impl<T, Q> PartialEq<Q> for Interval<T>
-//~^ ERROR implementations of `PartialEq<Interval<_>>` for `Interval<_>` will conflict in the future
-//~| WARN this was previously accepted by the compiler but is being phased out
 where
     T: Borrow<Q>,
     Q: ?Sized + PartialOrd,

tests/ui/coherence/warn-when-cycle-is-error-in-coherence.stderr

@@ -1,51 +1,17 @@
-error: implementations of `PartialEq<Interval<_>>` for `Interval<_>` will conflict in the future
-  --> $DIR/warn-when-cycle-is-error-in-coherence.rs:13:1
+error[E0119]: conflicting implementations of trait `PartialEq<Interval<_>>` for type `Interval<_>`
+  --> $DIR/warn-when-cycle-is-error-in-coherence.rs:5:10
    |
 LL |   #[derive(PartialEq, Default)]
-   |            --------- the second impl is here
+   |            ^^^^^^^^^ conflicting implementation for `Interval<_>`
 ...
 LL | / impl<T, Q> PartialEq<Q> for Interval<T>
-LL | |
-LL | |
 LL | | where
 LL | |     T: Borrow<Q>,
 LL | |     Q: ?Sized + PartialOrd,
-   | |___________________________^ the first impl is here
+   | |___________________________- first implementation here
    |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #114040 <https://github.com/rust-lang/rust/issues/114040>
-   = note: impls that are not considered to overlap may be considered to overlap in the future
-   = note: `Interval<_>: PartialOrd` may be considered to hold in future releases, causing the impls to overlap
-note: the lint level is defined here
-  --> $DIR/warn-when-cycle-is-error-in-coherence.rs:1:9
-   |
-LL | #![deny(coinductive_overlap_in_coherence)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error
 
-Future incompatibility report: Future breakage diagnostic:
-error: implementations of `PartialEq<Interval<_>>` for `Interval<_>` will conflict in the future
-  --> $DIR/warn-when-cycle-is-error-in-coherence.rs:13:1
-   |
-LL |   #[derive(PartialEq, Default)]
-   |            --------- the second impl is here
-...
-LL | / impl<T, Q> PartialEq<Q> for Interval<T>
-LL | |
-LL | |
-LL | | where
-LL | |     T: Borrow<Q>,
-LL | |     Q: ?Sized + PartialOrd,
-   | |___________________________^ the first impl is here
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #114040 <https://github.com/rust-lang/rust/issues/114040>
-   = note: impls that are not considered to overlap may be considered to overlap in the future
-   = note: `Interval<_>: PartialOrd` may be considered to hold in future releases, causing the impls to overlap
-note: the lint level is defined here
-  --> $DIR/warn-when-cycle-is-error-in-coherence.rs:1:9
-   |
-LL | #![deny(coinductive_overlap_in_coherence)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
+For more information about this error, try `rustc --explain E0119`.