Auto merge of #85481 - lcnr:const-equate, r=matthewjasper

deal with `const_evaluatable_checked` in `ConstEquate`

Failing to evaluate two constants which do not contain inference variables should not result in ambiguity.

2 files changed, 34 insertions, 3 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index fc9739f70d4..120680092ba 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -6,7 +6,7 @@ use rustc_errors::ErrorReported;
 use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
 use rustc_middle::mir::abstract_const::NotConstEvaluatable;
 use rustc_middle::mir::interpret::ErrorHandled;
-use rustc_middle::ty::error::ExpectedFound;
+use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::ToPredicate;
 use rustc_middle::ty::{self, Binder, Const, Ty, TypeFoldable};
@@ -591,7 +591,16 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                             )
                         }
                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
-                            ProcessResult::Unchanged
+                            if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
+                                ProcessResult::Unchanged
+                            } else {
+                                // Two different constants using generic parameters ~> error.
+                                let expected_found = ExpectedFound::new(true, c1, c2);
+                                ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
+                                    expected_found,
+                                    TypeError::ConstMismatch(expected_found),
+                                ))
+                            }
                         }
                     }
                 }
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index a292de148a6..ea5eb2b6866 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -557,6 +557,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::ConstEquate(c1, c2) => {
                     debug!(?c1, ?c2, "evaluate_predicate_recursively: equating consts");
 
+                    if self.tcx().features().const_evaluatable_checked {
+                        // FIXME: we probably should only try to unify abstract constants
+                        // if the constants depend on generic parameters.
+                        //
+                        // Let's just see where this breaks :shrug:
+                        if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
+                            (c1.val, c2.val)
+                        {
+                            if self
+                                .tcx()
+                                .try_unify_abstract_consts(((a.def, a.substs), (b.def, b.substs)))
+                            {
+                                return Ok(EvaluatedToOk);
+                            }
+                        }
+                    }
+
                     let evaluate = |c: &'tcx ty::Const<'tcx>| {
                         if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
                             self.infcx
@@ -591,7 +608,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             )
                         }
                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
-                            Ok(EvaluatedToAmbig)
+                            if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
+                                Ok(EvaluatedToAmbig)
+                            } else {
+                                // Two different constants using generic parameters ~> error.
+                                Ok(EvaluatedToErr)
+                            }
                         }
                     }
                 }