Fix inherent impls on negative coherence

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -300,12 +300,12 @@ fn negative_impl<'cx, 'tcx>(
     debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
     let tcx = selcx.infcx().tcx;
 
-    // create a parameter environment corresponding to a (placeholder) instantiation of impl1
-    let impl1_env = tcx.param_env(impl1_def_id);
-    let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id).unwrap();
-
     // Create an infcx, taking the predicates of impl1 as assumptions:
     tcx.infer_ctxt().enter(|infcx| {
+        // create a parameter environment corresponding to a (placeholder) instantiation of impl1
+        let impl1_env = tcx.param_env(impl1_def_id);
+
+        if let Some(impl1_trait_ref) = tcx.impl_trait_ref(impl1_def_id) {
             // Normalize the trait reference. The WF rules ought to ensure
             // that this always succeeds.
             let impl1_trait_ref = match traits::fully_normalize(
@@ -330,8 +330,7 @@ fn negative_impl<'cx, 'tcx>(
             // do the impls unify? If not, not disjoint.
             let Ok(InferOk { obligations: more_obligations, .. }) = infcx
             .at(&ObligationCause::dummy(), impl1_env)
-            .eq(impl1_trait_ref, impl2_trait_ref)
-        else {
+            .eq(impl1_trait_ref, impl2_trait_ref) else {
                 debug!(
                     "explicit_disjoint: {:?} does not unify with {:?}",
                     impl1_trait_ref, impl2_trait_ref
@@ -350,6 +349,31 @@ fn negative_impl<'cx, 'tcx>(
             } else {
                 false
             }
+        } else {
+            let ty1 = tcx.type_of(impl1_def_id);
+            let ty2 = tcx.type_of(impl2_def_id);
+
+            let Ok(InferOk { obligations, .. }) = infcx
+            .at(&ObligationCause::dummy(), impl1_env)
+            .eq(ty1, ty2) else {
+                debug!(
+                    "explicit_disjoint: {:?} does not unify with {:?}",
+                    ty1, ty2
+                );
+                return false;
+            };
+
+            let opt_failing_obligation = obligations
+                .into_iter()
+                .find(|o| negative_impl_exists(selcx, impl1_env, impl1_def_id, o));
+
+            if let Some(failing_obligation) = opt_failing_obligation {
+                debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
+                true
+            } else {
+                false
+            }
+        }
     })
 }
 

src/test/ui/coherence/coherence-negative-inherent.rs

@@ -0,0 +1,22 @@
+// check-pass
+
+#![feature(negative_impls)]
+#![feature(rustc_attrs)]
+#![feature(with_negative_coherence)]
+
+#[rustc_strict_coherence]
+trait Foo {}
+
+impl !Foo for u32 {}
+
+struct MyStruct<T>(T);
+
+impl<T: Foo> MyStruct<T> {
+    fn method(&self) {}
+}
+
+impl MyStruct<u32> {
+    fn method(&self) {}
+}
+
+fn main() {}