Replace mk_foo calls with infer_foo where possible.

There are several `mk_foo`/`intern_foo` pairs, where the former takes an
iterator and the latter takes a slice. (This naming convention is bad,
but that's a fix for another PR.)

This commit changes several `mk_foo` occurrences into `intern_foo`,
avoiding the need for some `.iter()`/`.into_iter()` calls. Affected
cases:
- mk_type_list
- mk_tup
- mk_substs
- mk_const_list

compiler/rustc_middle/src/mir/mod.rs

@@ -2525,14 +2525,12 @@ impl<'tcx> ConstantKind<'tcx> {
         }
 
         let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
-        let parent_substs = if let Some(parent_hir_id) = tcx.hir().opt_parent_id(hir_id) {
-            if let Some(parent_did) = parent_hir_id.as_owner() {
-                InternalSubsts::identity_for_item(tcx, parent_did.to_def_id())
-            } else {
-                tcx.mk_substs(Vec::<GenericArg<'tcx>>::new().into_iter())
-            }
+        let parent_substs = if let Some(parent_hir_id) = tcx.hir().opt_parent_id(hir_id)
+            && let Some(parent_did) = parent_hir_id.as_owner()
+        {
+            InternalSubsts::identity_for_item(tcx, parent_did.to_def_id())
         } else {
-            tcx.mk_substs(Vec::<GenericArg<'tcx>>::new().into_iter())
+            tcx.intern_substs(&[])
         };
         debug!(?parent_substs);
 

compiler/rustc_middle/src/ty/context.rs

@@ -1190,7 +1190,7 @@ impl<'tcx> TyCtxt<'tcx> {
         self.mk_imm_ref(
             self.lifetimes.re_static,
             self.type_of(self.require_lang_item(LangItem::PanicLocation, None))
-                .subst(self, self.mk_substs([self.lifetimes.re_static.into()].iter())),
+                .subst(self, self.intern_substs(&[self.lifetimes.re_static.into()])),
         )
     }
 

compiler/rustc_middle/src/ty/layout.rs

@@ -686,7 +686,7 @@ where
                         Increase this counter if you tried to implement this but
                         failed to do it without duplicating a lot of code from
                         other places in the compiler: 2
-                        tcx.mk_tup(&[
+                        tcx.intern_tup(&[
                             tcx.mk_array(tcx.types.usize, 3),
                             tcx.mk_array(Option<fn()>),
                         ])

compiler/rustc_middle/src/ty/relate.rs

@@ -673,7 +673,7 @@ pub fn super_relate_consts<'tcx, R: TypeRelation<'tcx>>(
                     for (a_arg, b_arg) in aa.iter().zip(ba.iter()) {
                         related_args.push(r.consts(a_arg, b_arg)?);
                     }
-                    let related_args = tcx.mk_const_list(related_args.iter());
+                    let related_args = tcx.intern_const_list(&related_args);
                     Expr::FunctionCall(func, related_args)
                 }
                 _ => return Err(TypeError::ConstMismatch(expected_found(r, a, b))),

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -418,7 +418,7 @@ impl<'tcx> ir::TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<ty::PolyExistential
 
 impl<'tcx> ir::TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<ty::Const<'tcx>> {
     fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.mk_const_list(v.iter()))
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_const_list(v))
     }
 }