fix few more

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_trait_selection\src\traits\project.rs:511:12
    |
511 |     selcx: &mut SelectionContext<'a, 'tcx>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&SelectionContext<'a, 'tcx>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_trait_selection\src\traits\specialize\specialization_graph.rs:201:28
    |
201 | fn iter_children(children: &mut Children) -> impl Iterator<Item = DefId> + '_ {
    |                            ^^^^^^^^^^^^^ help: consider changing to: `&Children`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

compiler/rustc_trait_selection/src/traits/project.rs

@@ -508,7 +508,7 @@ pub(super) fn opt_normalize_projection_type<'a, 'b, 'tcx>(
 /// because it contains `[type error]`. Yuck! (See issue #29857 for
 /// one case where this arose.)
 fn normalize_to_error<'a, 'tcx>(
-    selcx: &mut SelectionContext<'a, 'tcx>,
+    selcx: &SelectionContext<'a, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     projection_ty: ty::AliasTy<'tcx>,
     cause: ObligationCause<'tcx>,

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -239,10 +239,10 @@ fn fulfill_implication<'tcx>(
 
     let source_trait = ImplSubject::Trait(source_trait_ref);
 
-    let selcx = &mut SelectionContext::new(infcx);
+    let selcx = SelectionContext::new(infcx);
     let target_args = infcx.fresh_args_for_item(DUMMY_SP, target_impl);
     let (target_trait, obligations) =
-        util::impl_subject_and_oblig(selcx, param_env, target_impl, target_args, error_cause);
+        util::impl_subject_and_oblig(&selcx, param_env, target_impl, target_args, error_cause);
 
     // do the impls unify? If not, no specialization.
     let Ok(InferOk { obligations: more_obligations, .. }) = infcx

compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs

@@ -198,7 +198,7 @@ fn insert(
     }
 }
 
-fn iter_children(children: &mut Children) -> impl Iterator<Item = DefId> + '_ {
+fn iter_children(children: &Children) -> impl Iterator<Item = DefId> + '_ {
     let nonblanket = children.non_blanket_impls.iter().flat_map(|(_, v)| v.iter());
     children.blanket_impls.iter().chain(nonblanket).cloned()
 }

compiler/rustc_trait_selection/src/traits/util.rs

@@ -205,7 +205,7 @@ fn next(&mut self) -> Option<DefId> {
 /// returning the resulting subject and all obligations that arise.
 /// The obligations are closed under normalization.
 pub fn impl_subject_and_oblig<'a, 'tcx>(
-    selcx: &mut SelectionContext<'a, 'tcx>,
+    selcx: &SelectionContext<'a, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     impl_def_id: DefId,
     impl_args: GenericArgsRef<'tcx>,