Remove param env from relation altogether

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2653,10 +2653,6 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
         self.0.tcx
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        unreachable!("purely structural relation should not need a param-env")
-    }
-
     fn tag(&self) -> &'static str {
         "SameTypeModuloInfer"
     }

compiler/rustc_infer/src/infer/outlives/test_type_match.rs

@@ -144,10 +144,6 @@ impl<'tcx> TypeRelation<'tcx> for MatchAgainstHigherRankedOutlives<'tcx> {
         self.tcx
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        unreachable!("purely structural relation should not need a param-env")
-    }
-
     fn a_is_expected(&self) -> bool {
         true
     } // irrelevant

compiler/rustc_infer/src/infer/relate/combine.rs

@@ -563,6 +563,8 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 }
 
 pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx>;
+
     /// Register obligations that must hold in order for this relation to hold
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>);
 

compiler/rustc_infer/src/infer/relate/equate.rs

@@ -33,10 +33,6 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.fields.param_env
-    }
-
     fn a_is_expected(&self) -> bool {
         self.a_is_expected
     }
@@ -174,6 +170,10 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Equate<'_, '_, 'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.fields.param_env
+    }
+
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ty::ToPredicate<'tcx>>) {
         self.fields.register_predicates(obligations);
     }

compiler/rustc_infer/src/infer/relate/generalize.rs

@@ -182,10 +182,6 @@ where
         self.infcx.tcx
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.delegate.param_env()
-    }
-
     fn tag(&self) -> &'static str {
         "Generalizer"
     }

compiler/rustc_infer/src/infer/relate/glb.rs

@@ -32,10 +32,6 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.fields.param_env
-    }
-
     fn a_is_expected(&self) -> bool {
         self.a_is_expected
     }
@@ -138,6 +134,10 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Glb<'combine, 'infcx,
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Glb<'_, '_, 'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.fields.param_env
+    }
+
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ty::ToPredicate<'tcx>>) {
         self.fields.register_predicates(obligations);
     }

compiler/rustc_infer/src/infer/relate/lub.rs

@@ -32,10 +32,6 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.fields.param_env
-    }
-
     fn a_is_expected(&self) -> bool {
         self.a_is_expected
     }
@@ -138,6 +134,10 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx,
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Lub<'_, '_, 'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.fields.param_env
+    }
+
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ty::ToPredicate<'tcx>>) {
         self.fields.register_predicates(obligations);
     }

compiler/rustc_infer/src/infer/relate/nll.rs

@@ -431,10 +431,6 @@ where
         self.infcx.tcx
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.delegate.param_env()
-    }
-
     fn tag(&self) -> &'static str {
         "nll::subtype"
     }
@@ -670,6 +666,10 @@ impl<'tcx, D> ObligationEmittingRelation<'tcx> for TypeRelating<'_, 'tcx, D>
 where
     D: TypeRelatingDelegate<'tcx>,
 {
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.delegate.param_env()
+    }
+
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ty::ToPredicate<'tcx>>) {
         self.delegate.register_obligations(
             obligations

compiler/rustc_infer/src/infer/relate/sub.rs

@@ -39,10 +39,6 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
         self.fields.infcx.tcx
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.fields.param_env
-    }
-
     fn a_is_expected(&self) -> bool {
         self.a_is_expected
     }
@@ -203,6 +199,10 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Sub<'_, '_, 'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.fields.param_env
+    }
+
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ty::ToPredicate<'tcx>>) {
         self.fields.register_predicates(obligations);
     }

compiler/rustc_middle/src/ty/_match.rs

@@ -37,10 +37,6 @@ impl<'tcx> TypeRelation<'tcx> for MatchAgainstFreshVars<'tcx> {
         self.tcx
     }
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        panic!("relation should not need a param-env")
-    }
-
     fn a_is_expected(&self) -> bool {
         true
     } // irrelevant

compiler/rustc_middle/src/ty/relate.rs

@@ -23,8 +23,6 @@ pub enum Cause {
 pub trait TypeRelation<'tcx>: Sized {
     fn tcx(&self) -> TyCtxt<'tcx>;
 
-    fn param_env(&self) -> ty::ParamEnv<'tcx>;
-
     /// Returns a static string we can use for printouts.
     fn tag(&self) -> &'static str;