Address nits

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -378,6 +378,7 @@ pub fn normalize_param_env_or_error<'tcx>(
     )
 }
 
+/// Normalize a type and process all resulting obligations, returning any errors
 pub fn fully_normalize<'a, 'tcx, T>(
     infcx: &InferCtxt<'a, 'tcx>,
     cause: ObligationCause<'tcx>,
@@ -412,6 +413,8 @@ where
     Ok(resolved_value)
 }
 
+/// Process an obligation (and any nested obligations that come from it) to
+/// completion, returning any errors
 pub fn fully_solve_obligation<'a, 'tcx>(
     infcx: &InferCtxt<'a, 'tcx>,
     obligation: PredicateObligation<'tcx>,
@@ -421,6 +424,8 @@ pub fn fully_solve_obligation<'a, 'tcx>(
     engine.select_all_or_error(infcx)
 }
 
+/// Process a set of obligations (and any nested obligations that come from them)
+/// to completion
 pub fn fully_solve_obligations<'a, 'tcx>(
     infcx: &InferCtxt<'a, 'tcx>,
     obligations: impl IntoIterator<Item = PredicateObligation<'tcx>>,
@@ -430,6 +435,9 @@ pub fn fully_solve_obligations<'a, 'tcx>(
     engine.select_all_or_error(infcx)
 }
 
+/// Process a bound (and any nested obligations that come from it) to completion.
+/// This is a convenience function for traits that have no generic arguments, such
+/// as auto traits, and builtin traits like Copy or Sized.
 pub fn fully_solve_bound<'a, 'tcx>(
     infcx: &InferCtxt<'a, 'tcx>,
     cause: ObligationCause<'tcx>,

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

@@ -707,6 +707,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         ty: Ty<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
+        cause: &ObligationCause<'tcx>,
     ) -> Option<(Ty<'tcx>, DefId)> {
         let tcx = self.tcx();
         if tcx.features().trait_upcasting {
@@ -720,7 +721,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         let obligation = traits::Obligation::new(
-            ObligationCause::dummy(),
+            cause.clone(),
             param_env,
             ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx),
         );
@@ -735,7 +736,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 item_def_id: tcx.lang_items().deref_target()?,
                 substs: trait_ref.substs,
             },
-            ObligationCause::dummy(),
+            cause.clone(),
             0,
             // We're *intentionally* throwing these away,
             // since we don't actually use them.
@@ -809,7 +810,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         let target_trait_did = principal_def_id_b.unwrap();
                         let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
                         if let Some((deref_output_ty, deref_output_trait_did)) = self
-                            .need_migrate_deref_output_trait_object(source, obligation.param_env)
+                            .need_migrate_deref_output_trait_object(
+                                source,
+                                obligation.param_env,
+                                &obligation.cause,
+                            )
                         {
                             if deref_output_trait_did == target_trait_did {
                                 self.tcx().struct_span_lint_hir(