make relate's const ty assertion use semantic equality

compiler/rustc_trait_selection/src/traits/misc.rs

@@ -1,12 +1,14 @@
 //! Miscellaneous type-system utilities that are too small to deserve their own modules.
 
-use crate::traits::{self, ObligationCause};
+use crate::traits::{self, ObligationCause, ObligationCtxt};
 
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
+use rustc_infer::infer::canonical::Canonical;
 use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
 use rustc_infer::{infer::outlives::env::OutlivesEnvironment, traits::FulfillmentError};
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
+use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt, TypeVisitable};
+use rustc_span::DUMMY_SP;
 
 use super::outlives_bounds::InferCtxtExt;
 
@@ -131,3 +133,19 @@ pub fn type_allowed_to_implement_copy<'tcx>(
 
     Ok(())
 }
+
+pub fn check_const_param_definitely_unequal<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    canonical: Canonical<'tcx, (ParamEnv<'tcx>, Ty<'tcx>, Ty<'tcx>)>,
+) -> Result<(), ()> {
+    let (infcx, (param_env, ty_a, ty_b), _) =
+        tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical);
+    let ocx = ObligationCtxt::new(&infcx);
+
+    let result = ocx.eq(&ObligationCause::dummy(), param_env, ty_a, ty_b);
+    // use `select_where_possible` instead of `select_all_or_error` so that
+    // we don't get errors from obligations being ambiguous.
+    let errors = ocx.select_where_possible();
+
+    if errors.len() > 0 || result.is_err() { Err(()) } else { Ok(()) }
+}

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -554,6 +554,7 @@ pub fn provide(providers: &mut ty::query::Providers) {
         specialization_graph_of: specialize::specialization_graph_provider,
         specializes: specialize::specializes,
         subst_and_check_impossible_predicates,
+        check_const_param_definitely_unequal: misc::check_const_param_definitely_unequal,
         is_impossible_method,
         ..*providers
     };