diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 1ec1f0cb11052..004b945eada11 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -2589,7 +2589,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             DefKind::InlineConst => substs.as_inline_const().parent_substs(),
             other => bug!("unexpected item {:?}", other),
         };
-        let parent_substs = tcx.mk_substs(parent_substs.iter());
+        let parent_substs = tcx.intern_substs(parent_substs);
 
         assert_eq!(typeck_root_substs.len(), parent_substs.len());
         if let Err(_) = self.eq_substs(
diff --git a/compiler/rustc_codegen_cranelift/src/codegen_i128.rs b/compiler/rustc_codegen_cranelift/src/codegen_i128.rs
index 638b2d573b5dd..b4a2537b5ea93 100644
--- a/compiler/rustc_codegen_cranelift/src/codegen_i128.rs
+++ b/compiler/rustc_codegen_cranelift/src/codegen_i128.rs
@@ -56,7 +56,7 @@ pub(crate) fn maybe_codegen<'tcx>(
                     Some(fx.easy_call("__multi3", &[lhs, rhs], val_ty))
                 }
             } else {
-                let out_ty = fx.tcx.mk_tup([lhs.layout().ty, fx.tcx.types.bool].iter());
+                let out_ty = fx.tcx.intern_tup(&[lhs.layout().ty, fx.tcx.types.bool]);
                 let oflow = CPlace::new_stack_slot(fx, fx.layout_of(fx.tcx.types.i32));
                 let lhs = lhs.load_scalar(fx);
                 let rhs = rhs.load_scalar(fx);
@@ -78,7 +78,7 @@ pub(crate) fn maybe_codegen<'tcx>(
         }
         BinOp::Add | BinOp::Sub | BinOp::Mul => {
             assert!(checked);
-            let out_ty = fx.tcx.mk_tup([lhs.layout().ty, fx.tcx.types.bool].iter());
+            let out_ty = fx.tcx.intern_tup(&[lhs.layout().ty, fx.tcx.types.bool]);
             let out_place = CPlace::new_stack_slot(fx, fx.layout_of(out_ty));
             let (param_types, args) = if fx.tcx.sess.target.is_like_windows {
                 let (lhs_ptr, lhs_extra) = lhs.force_stack(fx);
diff --git a/compiler/rustc_codegen_cranelift/src/intrinsics/llvm_x86.rs b/compiler/rustc_codegen_cranelift/src/intrinsics/llvm_x86.rs
index d2ae6978ca2a8..cbac2e667652b 100644
--- a/compiler/rustc_codegen_cranelift/src/intrinsics/llvm_x86.rs
+++ b/compiler/rustc_codegen_cranelift/src/intrinsics/llvm_x86.rs
@@ -191,7 +191,7 @@ fn llvm_add_sub<'tcx>(
     // carry0 | carry1 -> carry or borrow respectively
     let cb_out = fx.bcx.ins().bor(cb0, cb1);
 
-    let layout = fx.layout_of(fx.tcx.mk_tup([fx.tcx.types.u8, fx.tcx.types.u64].iter()));
+    let layout = fx.layout_of(fx.tcx.intern_tup(&[fx.tcx.types.u8, fx.tcx.types.u64]));
     let val = CValue::by_val_pair(cb_out, c, layout);
     ret.write_cvalue(fx, val);
 }
diff --git a/compiler/rustc_codegen_cranelift/src/main_shim.rs b/compiler/rustc_codegen_cranelift/src/main_shim.rs
index 3e3b685713427..26327107df4c5 100644
--- a/compiler/rustc_codegen_cranelift/src/main_shim.rs
+++ b/compiler/rustc_codegen_cranelift/src/main_shim.rs
@@ -119,7 +119,7 @@ pub(crate) fn maybe_create_entry_wrapper(
                     tcx,
                     ParamEnv::reveal_all(),
                     report.def_id,
-                    tcx.mk_substs([GenericArg::from(main_ret_ty)].iter()),
+                    tcx.intern_substs(&[GenericArg::from(main_ret_ty)]),
                 )
                 .unwrap()
                 .unwrap()
diff --git a/compiler/rustc_codegen_cranelift/src/num.rs b/compiler/rustc_codegen_cranelift/src/num.rs
index afacbec644582..05905a7bcdf30 100644
--- a/compiler/rustc_codegen_cranelift/src/num.rs
+++ b/compiler/rustc_codegen_cranelift/src/num.rs
@@ -289,7 +289,7 @@ pub(crate) fn codegen_checked_int_binop<'tcx>(
         _ => bug!("binop {:?} on checked int/uint lhs: {:?} rhs: {:?}", bin_op, in_lhs, in_rhs),
     };
 
-    let out_layout = fx.layout_of(fx.tcx.mk_tup([in_lhs.layout().ty, fx.tcx.types.bool].iter()));
+    let out_layout = fx.layout_of(fx.tcx.intern_tup(&[in_lhs.layout().ty, fx.tcx.types.bool]));
     CValue::by_val_pair(res, has_overflow, out_layout)
 }
 
diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
index ace15cfb02477..3dc0ac03312e9 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
@@ -27,8 +27,6 @@ use rustc_middle::ty::Instance;
 use std::cell::RefCell;
 use std::ffi::CString;
 
-use std::iter;
-
 pub mod mapgen;
 
 const UNUSED_FUNCTION_COUNTER_ID: CounterValueReference = CounterValueReference::START;
@@ -201,7 +199,7 @@ fn declare_unused_fn<'tcx>(cx: &CodegenCx<'_, 'tcx>, def_id: DefId) -> Instance<
         tcx.symbol_name(instance).name,
         cx.fn_abi_of_fn_ptr(
             ty::Binder::dummy(tcx.mk_fn_sig(
-                iter::once(tcx.mk_unit()),
+                [tcx.mk_unit()],
                 tcx.mk_unit(),
                 false,
                 hir::Unsafety::Unsafe,
diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index b0295481ca5a3..39afb4af6f68e 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -22,7 +22,6 @@ use rustc_target::abi::{self, Align, HasDataLayout, Primitive};
 use rustc_target::spec::{HasTargetSpec, PanicStrategy};
 
 use std::cmp::Ordering;
-use std::iter;
 
 fn get_simple_intrinsic<'ll>(
     cx: &CodegenCx<'ll, '_>,
@@ -798,7 +797,7 @@ fn get_rust_try_fn<'ll, 'tcx>(
     let i8p = tcx.mk_mut_ptr(tcx.types.i8);
     // `unsafe fn(*mut i8) -> ()`
     let try_fn_ty = tcx.mk_fn_ptr(ty::Binder::dummy(tcx.mk_fn_sig(
-        iter::once(i8p),
+        [i8p],
         tcx.mk_unit(),
         false,
         hir::Unsafety::Unsafe,
@@ -806,7 +805,7 @@ fn get_rust_try_fn<'ll, 'tcx>(
     )));
     // `unsafe fn(*mut i8, *mut i8) -> ()`
     let catch_fn_ty = tcx.mk_fn_ptr(ty::Binder::dummy(tcx.mk_fn_sig(
-        [i8p, i8p].iter().cloned(),
+        [i8p, i8p],
         tcx.mk_unit(),
         false,
         hir::Unsafety::Unsafe,
@@ -814,7 +813,7 @@ fn get_rust_try_fn<'ll, 'tcx>(
     )));
     // `unsafe fn(unsafe fn(*mut i8) -> (), *mut i8, unsafe fn(*mut i8, *mut i8) -> ()) -> i32`
     let rust_fn_sig = ty::Binder::dummy(cx.tcx.mk_fn_sig(
-        [try_fn_ty, i8p, catch_fn_ty].into_iter(),
+        [try_fn_ty, i8p, catch_fn_ty],
         tcx.types.i32,
         false,
         hir::Unsafety::Unsafe,
diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs b/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
index ec5707505c89f..f6a3937870edd 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
@@ -96,7 +96,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let loc_ty = self
             .tcx
             .type_of(self.tcx.require_lang_item(LangItem::PanicLocation, None))
-            .subst(*self.tcx, self.tcx.mk_substs([self.tcx.lifetimes.re_erased.into()].iter()));
+            .subst(*self.tcx, self.tcx.intern_substs(&[self.tcx.lifetimes.re_erased.into()]));
         let loc_layout = self.layout_of(loc_ty).unwrap();
         let location = self.allocate(loc_layout, MemoryKind::CallerLocation).unwrap();
 
diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index 5bb78d172531f..5e77cfa39f6df 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -1608,7 +1608,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             .collect::<SmallVec<[_; 8]>>();
         v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
         v.dedup();
-        let existential_predicates = tcx.mk_poly_existential_predicates(v.into_iter());
+        let existential_predicates = tcx.intern_poly_existential_predicates(&v);
 
         // Use explicitly-specified region bound.
         let region_bound = if !lifetime.is_elided() {
@@ -3109,7 +3109,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         debug!(?output_ty);
 
-        let fn_ty = tcx.mk_fn_sig(input_tys.into_iter(), output_ty, decl.c_variadic, unsafety, abi);
+        let fn_ty = tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, unsafety, abi);
         let bare_fn_ty = ty::Binder::bind_with_vars(fn_ty, bound_vars);
 
         if !self.allow_ty_infer() && !(visitor.0.is_empty() && infer_replacements.is_empty()) {
diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index 4fd06c23e1025..5e25a4554ed41 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -1936,7 +1936,7 @@ pub(super) fn check_type_bounds<'tcx>(
             .into()
         }
     });
-    let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
+    let bound_vars = tcx.intern_bound_variable_kinds(&bound_vars);
     let impl_ty_substs = tcx.intern_substs(&substs);
     let container_id = impl_ty.container_id(tcx);
 
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index 82562ac75e1a3..4720fea8ef4ab 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -15,8 +15,6 @@ use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_target::spec::abi::Abi;
 
-use std::iter;
-
 fn equate_intrinsic_type<'tcx>(
     tcx: TyCtxt<'tcx>,
     it: &hir::ForeignItem<'_>,
@@ -139,14 +137,10 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
     let intrinsic_name = tcx.item_name(intrinsic_id);
     let name_str = intrinsic_name.as_str();
 
-    let bound_vars = tcx.mk_bound_variable_kinds(
-        [
-            ty::BoundVariableKind::Region(ty::BrAnon(0, None)),
-            ty::BoundVariableKind::Region(ty::BrEnv),
-        ]
-        .iter()
-        .copied(),
-    );
+    let bound_vars = tcx.intern_bound_variable_kinds(&[
+        ty::BoundVariableKind::Region(ty::BrAnon(0, None)),
+        ty::BoundVariableKind::Region(ty::BrEnv),
+    ]);
     let mk_va_list_ty = |mutbl| {
         tcx.lang_items().va_list().map(|did| {
             let region = tcx.mk_re_late_bound(
@@ -378,21 +372,21 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
                 (
                     1,
                     vec![tcx.mk_imm_ref(tcx.mk_re_late_bound(ty::INNERMOST, br), param(0))],
-                    tcx.mk_projection(discriminant_def_id, tcx.mk_substs([param(0).into()].iter())),
+                    tcx.mk_projection(discriminant_def_id, tcx.intern_substs(&[param(0).into()])),
                 )
             }
 
             kw::Try => {
                 let mut_u8 = tcx.mk_mut_ptr(tcx.types.u8);
                 let try_fn_ty = ty::Binder::dummy(tcx.mk_fn_sig(
-                    iter::once(mut_u8),
+                    [mut_u8],
                     tcx.mk_unit(),
                     false,
                     hir::Unsafety::Normal,
                     Abi::Rust,
                 ));
                 let catch_fn_ty = ty::Binder::dummy(tcx.mk_fn_sig(
-                    [mut_u8, mut_u8].iter().cloned(),
+                    [mut_u8, mut_u8],
                     tcx.mk_unit(),
                     false,
                     hir::Unsafety::Normal,
@@ -447,7 +441,7 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
         };
         (n_tps, 0, inputs, output, unsafety)
     };
-    let sig = tcx.mk_fn_sig(inputs.into_iter(), output, false, unsafety, Abi::RustIntrinsic);
+    let sig = tcx.mk_fn_sig(inputs, output, false, unsafety, Abi::RustIntrinsic);
     let sig = ty::Binder::bind_with_vars(sig, bound_vars);
     equate_intrinsic_type(tcx, it, n_tps, n_lts, sig)
 }
@@ -545,13 +539,7 @@ pub fn check_platform_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>)
         }
     };
 
-    let sig = tcx.mk_fn_sig(
-        inputs.into_iter(),
-        output,
-        false,
-        hir::Unsafety::Unsafe,
-        Abi::PlatformIntrinsic,
-    );
+    let sig = tcx.mk_fn_sig(inputs, output, false, hir::Unsafety::Unsafe, Abi::PlatformIntrinsic);
     let sig = ty::Binder::dummy(sig);
     equate_intrinsic_type(tcx, it, n_tps, 0, sig)
 }
diff --git a/compiler/rustc_hir_analysis/src/lib.rs b/compiler/rustc_hir_analysis/src/lib.rs
index 6111046a51956..11240cf22e4b1 100644
--- a/compiler/rustc_hir_analysis/src/lib.rs
+++ b/compiler/rustc_hir_analysis/src/lib.rs
@@ -113,7 +113,6 @@ use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
 
-use std::iter;
 use std::ops::Not;
 
 use astconv::AstConv;
@@ -348,7 +347,7 @@ fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
     }
 
     let se_ty = tcx.mk_fn_ptr(expected_return_type.map_bound(|expected_return_type| {
-        tcx.mk_fn_sig(iter::empty(), expected_return_type, false, hir::Unsafety::Normal, Abi::Rust)
+        tcx.mk_fn_sig([], expected_return_type, false, hir::Unsafety::Normal, Abi::Rust)
     }));
 
     require_same_types(
@@ -434,7 +433,7 @@ fn check_start_fn_ty(tcx: TyCtxt<'_>, start_def_id: DefId) {
             }
 
             let se_ty = tcx.mk_fn_ptr(ty::Binder::dummy(tcx.mk_fn_sig(
-                [tcx.types.isize, tcx.mk_imm_ptr(tcx.mk_imm_ptr(tcx.types.u8))].iter().cloned(),
+                [tcx.types.isize, tcx.mk_imm_ptr(tcx.mk_imm_ptr(tcx.types.u8))],
                 tcx.types.isize,
                 false,
                 hir::Unsafety::Normal,
diff --git a/compiler/rustc_hir_typeck/src/check.rs b/compiler/rustc_hir_typeck/src/check.rs
index 3feb23d3481e6..bf8259ff70fa9 100644
--- a/compiler/rustc_hir_typeck/src/check.rs
+++ b/compiler/rustc_hir_typeck/src/check.rs
@@ -264,7 +264,7 @@ fn check_lang_start_fn<'tcx>(
         let fn_generic = generics.param_at(0, tcx);
         let generic_ty = tcx.mk_ty_param(fn_generic.index, fn_generic.name);
         let expected_fn_sig =
-            tcx.mk_fn_sig([].iter(), &generic_ty, false, hir::Unsafety::Normal, Abi::Rust);
+            tcx.mk_fn_sig([], generic_ty, false, hir::Unsafety::Normal, Abi::Rust);
         let expected_ty = tcx.mk_fn_ptr(Binder::dummy(expected_fn_sig));
 
         // we emit the same error to suggest changing the arg no matter what's wrong with the arg
diff --git a/compiler/rustc_hir_typeck/src/closure.rs b/compiler/rustc_hir_typeck/src/closure.rs
index b1268c5f7923e..cf296a7bf6530 100644
--- a/compiler/rustc_hir_typeck/src/closure.rs
+++ b/compiler/rustc_hir_typeck/src/closure.rs
@@ -126,7 +126,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // the `closures` table.
         let sig = bound_sig.map_bound(|sig| {
             self.tcx.mk_fn_sig(
-                iter::once(self.tcx.intern_tup(sig.inputs())),
+                [self.tcx.intern_tup(sig.inputs())],
                 sig.output(),
                 sig.c_variadic,
                 sig.unsafety,
@@ -326,7 +326,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         debug!(?ret_param_ty);
 
         let sig = projection.rebind(self.tcx.mk_fn_sig(
-            input_tys.iter(),
+            input_tys,
             ret_param_ty,
             false,
             hir::Unsafety::Normal,
diff --git a/compiler/rustc_hir_typeck/src/generator_interior/mod.rs b/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
index 3f61a1a83e5ae..14e3ba83b1037 100644
--- a/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
+++ b/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
@@ -312,7 +312,7 @@ pub fn resolve_interior<'a, 'tcx>(
 
     // Extract type components to build the witness type.
     let type_list = fcx.tcx.mk_type_list(type_causes.iter().map(|cause| cause.ty));
-    let bound_vars = fcx.tcx.mk_bound_variable_kinds(bound_vars.iter());
+    let bound_vars = fcx.tcx.intern_bound_variable_kinds(&bound_vars);
     let witness =
         fcx.tcx.mk_generator_witness(ty::Binder::bind_with_vars(type_list, bound_vars.clone()));
 
diff --git a/compiler/rustc_hir_typeck/src/upvar.rs b/compiler/rustc_hir_typeck/src/upvar.rs
index e12a575d5acde..7c8abb4186f11 100644
--- a/compiler/rustc_hir_typeck/src/upvar.rs
+++ b/compiler/rustc_hir_typeck/src/upvar.rs
@@ -301,7 +301,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Build a tuple (U0..Un) of the final upvar types U0..Un
         // and unify the upvar tuple type in the closure with it:
-        let final_tupled_upvars_type = self.tcx.mk_tup(final_upvar_tys.iter());
+        let final_tupled_upvars_type = self.tcx.intern_tup(&final_upvar_tys);
         self.demand_suptype(span, substs.tupled_upvars_ty(), final_tupled_upvars_type);
 
         let fake_reads = delegate
@@ -315,8 +315,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.typeck_results.borrow_mut().closure_size_eval.insert(
                 closure_def_id,
                 ClosureSizeProfileData {
-                    before_feature_tys: self.tcx.mk_tup(before_feature_tys.into_iter()),
-                    after_feature_tys: self.tcx.mk_tup(after_feature_tys.into_iter()),
+                    before_feature_tys: self.tcx.intern_tup(&before_feature_tys),
+                    after_feature_tys: self.tcx.intern_tup(&after_feature_tys),
                 },
             );
         }
diff --git a/compiler/rustc_metadata/src/native_libs.rs b/compiler/rustc_metadata/src/native_libs.rs
index ffb52d167b177..d823989bb02b8 100644
--- a/compiler/rustc_metadata/src/native_libs.rs
+++ b/compiler/rustc_metadata/src/native_libs.rs
@@ -502,7 +502,7 @@ impl<'tcx> Collector<'tcx> {
                 .subst_identity()
                 .fn_sig(self.tcx)
                 .inputs()
-                .map_bound(|slice| self.tcx.mk_type_list(slice.iter())),
+                .map_bound(|slice| self.tcx.intern_type_list(slice)),
         );
 
         argument_types
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index f955b312b238a..46184cddd51f5 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/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);
 
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 117f3decd1a6e..e2f32cdca3ca3 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -67,7 +67,7 @@ use rustc_target::abi::{Layout, LayoutS, TargetDataLayout, VariantIdx};
 use rustc_target::spec::abi;
 use rustc_type_ir::sty::TyKind::*;
 use rustc_type_ir::WithCachedTypeInfo;
-use rustc_type_ir::{DynKind, InternAs, InternIteratorElement, Interner, TypeFlags};
+use rustc_type_ir::{CollectAndApply, DynKind, Interner, TypeFlags};
 
 use std::any::Any;
 use std::borrow::Borrow;
@@ -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()])),
         )
     }
 
@@ -1660,11 +1660,11 @@ impl<'tcx> TyCtxt<'tcx> {
         unsafety: hir::Unsafety,
     ) -> PolyFnSig<'tcx> {
         sig.map_bound(|s| {
-            let params_iter = match s.inputs()[0].kind() {
-                ty::Tuple(params) => params.into_iter(),
+            let params = match s.inputs()[0].kind() {
+                ty::Tuple(params) => *params,
                 _ => bug!(),
             };
-            self.mk_fn_sig(params_iter, s.output(), s.c_variadic, unsafety, abi::Abi::Rust)
+            self.mk_fn_sig(params, s.output(), s.c_variadic, unsafety, abi::Abi::Rust)
         })
     }
 
@@ -1835,8 +1835,12 @@ impl<'tcx> TyCtxt<'tcx> {
         if ts.is_empty() { self.types.unit } else { self.mk_ty(Tuple(self.intern_type_list(&ts))) }
     }
 
-    pub fn mk_tup<I: InternAs<Ty<'tcx>, Ty<'tcx>>>(self, iter: I) -> I::Output {
-        iter.intern_with(|ts| self.intern_tup(ts))
+    pub fn mk_tup<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<Ty<'tcx>, Ty<'tcx>>,
+    {
+        T::collect_and_apply(iter, |ts| self.intern_tup(ts))
     }
 
     #[inline]
@@ -2157,11 +2161,12 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
-    pub fn mk_const_list<I: InternAs<ty::Const<'tcx>, &'tcx List<ty::Const<'tcx>>>>(
-        self,
-        iter: I,
-    ) -> I::Output {
-        iter.intern_with(|xs| self.intern_const_list(xs))
+    pub fn mk_const_list<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<ty::Const<'tcx>, &'tcx List<ty::Const<'tcx>>>,
+    {
+        T::collect_and_apply(iter, |xs| self.intern_const_list(xs))
     }
 
     pub fn intern_const_list(self, cs: &[ty::Const<'tcx>]) -> &'tcx List<ty::Const<'tcx>> {
@@ -2210,18 +2215,24 @@ impl<'tcx> TyCtxt<'tcx> {
         if ts.is_empty() { List::empty() } else { self._intern_bound_variable_kinds(ts) }
     }
 
-    pub fn mk_fn_sig<I>(
+    // Unlike various other `mk_*` functions, this one uses `I: IntoIterator`
+    // instead of `I: Iterator`. Unlike those other functions, this one doesn't
+    // have a `intern_fn_sig` variant that can be used for cases where `I` is
+    // something like a `Vec`. That's because of the need to combine `inputs`
+    // and `output`.
+    pub fn mk_fn_sig<I, T>(
         self,
         inputs: I,
         output: I::Item,
         c_variadic: bool,
         unsafety: hir::Unsafety,
         abi: abi::Abi,
-    ) -> <I::Item as InternIteratorElement<Ty<'tcx>, ty::FnSig<'tcx>>>::Output
+    ) -> T::Output
     where
-        I: Iterator<Item: InternIteratorElement<Ty<'tcx>, ty::FnSig<'tcx>>>,
+        I: IntoIterator<Item = T>,
+        T: CollectAndApply<Ty<'tcx>, ty::FnSig<'tcx>>,
     {
-        inputs.chain(iter::once(output)).intern_with(|xs| ty::FnSig {
+        T::collect_and_apply(inputs.into_iter().chain(iter::once(output)), |xs| ty::FnSig {
             inputs_and_output: self.intern_type_list(xs),
             c_variadic,
             unsafety,
@@ -2229,38 +2240,47 @@ impl<'tcx> TyCtxt<'tcx> {
         })
     }
 
-    pub fn mk_poly_existential_predicates<
-        I: InternAs<PolyExistentialPredicate<'tcx>, &'tcx List<PolyExistentialPredicate<'tcx>>>,
-    >(
-        self,
-        iter: I,
-    ) -> I::Output {
-        iter.intern_with(|xs| self.intern_poly_existential_predicates(xs))
+    pub fn mk_poly_existential_predicates<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<
+                PolyExistentialPredicate<'tcx>,
+                &'tcx List<PolyExistentialPredicate<'tcx>>,
+            >,
+    {
+        T::collect_and_apply(iter, |xs| self.intern_poly_existential_predicates(xs))
     }
 
-    pub fn mk_predicates<I: InternAs<Predicate<'tcx>, &'tcx List<Predicate<'tcx>>>>(
-        self,
-        iter: I,
-    ) -> I::Output {
-        iter.intern_with(|xs| self.intern_predicates(xs))
+    pub fn mk_predicates<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<Predicate<'tcx>, &'tcx List<Predicate<'tcx>>>,
+    {
+        T::collect_and_apply(iter, |xs| self.intern_predicates(xs))
     }
 
-    pub fn mk_type_list<I: InternAs<Ty<'tcx>, &'tcx List<Ty<'tcx>>>>(self, iter: I) -> I::Output {
-        iter.intern_with(|xs| self.intern_type_list(xs))
+    pub fn mk_type_list<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<Ty<'tcx>, &'tcx List<Ty<'tcx>>>,
+    {
+        T::collect_and_apply(iter, |xs| self.intern_type_list(xs))
     }
 
-    pub fn mk_substs<I: InternAs<GenericArg<'tcx>, &'tcx List<GenericArg<'tcx>>>>(
-        self,
-        iter: I,
-    ) -> I::Output {
-        iter.intern_with(|xs| self.intern_substs(xs))
+    pub fn mk_substs<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<GenericArg<'tcx>, &'tcx List<GenericArg<'tcx>>>,
+    {
+        T::collect_and_apply(iter, |xs| self.intern_substs(xs))
     }
 
-    pub fn mk_place_elems<I: InternAs<PlaceElem<'tcx>, &'tcx List<PlaceElem<'tcx>>>>(
-        self,
-        iter: I,
-    ) -> I::Output {
-        iter.intern_with(|xs| self.intern_place_elems(xs))
+    pub fn mk_place_elems<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<PlaceElem<'tcx>, &'tcx List<PlaceElem<'tcx>>>,
+    {
+        T::collect_and_apply(iter, |xs| self.intern_place_elems(xs))
     }
 
     pub fn mk_substs_trait(
@@ -2289,13 +2309,12 @@ impl<'tcx> TyCtxt<'tcx> {
         ty::AliasTy { def_id, substs, _use_mk_alias_ty_instead: () }
     }
 
-    pub fn mk_bound_variable_kinds<
-        I: InternAs<ty::BoundVariableKind, &'tcx List<ty::BoundVariableKind>>,
-    >(
-        self,
-        iter: I,
-    ) -> I::Output {
-        iter.intern_with(|xs| self.intern_bound_variable_kinds(xs))
+    pub fn mk_bound_variable_kinds<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<ty::BoundVariableKind, &'tcx List<ty::BoundVariableKind>>,
+    {
+        T::collect_and_apply(iter, |xs| self.intern_bound_variable_kinds(xs))
     }
 
     /// Emit a lint at `span` from a lint struct (some type that implements `DecorateLint`,
@@ -2380,13 +2399,13 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     pub fn late_bound_vars(self, id: HirId) -> &'tcx List<ty::BoundVariableKind> {
-        self.mk_bound_variable_kinds(
-            self.late_bound_vars_map(id.owner)
+        self.intern_bound_variable_kinds(
+            &self
+                .late_bound_vars_map(id.owner)
                 .and_then(|map| map.get(&id.local_id).cloned())
                 .unwrap_or_else(|| {
                     bug!("No bound vars found for {}", self.hir().node_to_string(id))
-                })
-                .iter(),
+                }),
         )
     }
 
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index 1d76f435e26d9..3d0f9a5053cb3 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/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()>),
                         ])
diff --git a/compiler/rustc_middle/src/ty/relate.rs b/compiler/rustc_middle/src/ty/relate.rs
index 322afeb2d340c..2ba25e8bfadc9 100644
--- a/compiler/rustc_middle/src/ty/relate.rs
+++ b/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))),
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index 97ee2b1fc5dd1..9c1a6f716d021 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ 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))
     }
 }
 
diff --git a/compiler/rustc_mir_build/src/thir/cx/mod.rs b/compiler/rustc_mir_build/src/thir/cx/mod.rs
index 3b7ceb3adccb0..74c35ef0fc241 100644
--- a/compiler/rustc_mir_build/src/thir/cx/mod.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/mod.rs
@@ -133,9 +133,9 @@ impl<'tcx> Cx<'tcx> {
                     bug!("closure expr does not have closure type: {:?}", closure_ty);
                 };
 
-                let bound_vars = self.tcx.mk_bound_variable_kinds(std::iter::once(
-                    ty::BoundVariableKind::Region(ty::BrEnv),
-                ));
+                let bound_vars = self
+                    .tcx
+                    .intern_bound_variable_kinds(&[ty::BoundVariableKind::Region(ty::BrEnv)]);
                 let br = ty::BoundRegion {
                     var: ty::BoundVar::from_usize(bound_vars.len() - 1),
                     kind: ty::BrEnv,
diff --git a/compiler/rustc_mir_transform/src/shim.rs b/compiler/rustc_mir_transform/src/shim.rs
index 7f388b065ade4..682ad081f5cf3 100644
--- a/compiler/rustc_mir_transform/src/shim.rs
+++ b/compiler/rustc_mir_transform/src/shim.rs
@@ -597,7 +597,7 @@ fn build_call_shim<'tcx>(
         let untuple_args = sig.inputs();
 
         // Create substitutions for the `Self` and `Args` generic parameters of the shim body.
-        let arg_tup = tcx.mk_tup(untuple_args.iter());
+        let arg_tup = tcx.intern_tup(untuple_args);
 
         (Some([ty.into(), arg_tup.into()]), Some(untuple_args))
     } else {
diff --git a/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs b/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs
index 949ef04dd4d06..59a2227cd3646 100644
--- a/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs
+++ b/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs
@@ -781,8 +781,8 @@ fn transform_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, options: TransformTyOptio
                 let output = transform_ty(tcx, fn_sig.skip_binder().output(), options);
                 ty = tcx.mk_fn_ptr(ty::Binder::bind_with_vars(
                     tcx.mk_fn_sig(
-                        parameters.iter(),
-                        &output,
+                        parameters,
+                        output,
                         fn_sig.c_variadic(),
                         fn_sig.unsafety(),
                         fn_sig.abi(),
@@ -813,21 +813,18 @@ fn transform_substs<'tcx>(
     substs: SubstsRef<'tcx>,
     options: TransformTyOptions,
 ) -> SubstsRef<'tcx> {
-    let substs: Vec<GenericArg<'tcx>> = substs
-        .iter()
-        .map(|subst| {
-            if let GenericArgKind::Type(ty) = subst.unpack() {
-                if is_c_void_ty(tcx, ty) {
-                    tcx.mk_unit().into()
-                } else {
-                    transform_ty(tcx, ty, options).into()
-                }
+    let substs = substs.iter().map(|subst| {
+        if let GenericArgKind::Type(ty) = subst.unpack() {
+            if is_c_void_ty(tcx, ty) {
+                tcx.mk_unit().into()
             } else {
-                subst
+                transform_ty(tcx, ty, options).into()
             }
-        })
-        .collect();
-    tcx.mk_substs(substs.iter())
+        } else {
+            subst
+        }
+    });
+    tcx.mk_substs(substs)
 }
 
 /// Returns a type metadata identifier for the specified FnAbi using the Itanium C++ ABI with vendor
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index 0921fb5756d66..d12e5f797fb94 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -359,7 +359,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                     let b_last_ty = b_tys.last().unwrap();
 
                     // Substitute just the tail field of B., and require that they're equal.
-                    let unsized_a_ty = tcx.mk_tup(a_rest_tys.iter().chain([b_last_ty]));
+                    let unsized_a_ty = tcx.mk_tup(a_rest_tys.iter().chain([b_last_ty]).copied());
                     let mut nested_goals = ecx.infcx.eq(goal.param_env, unsized_a_ty, b_ty)?;
 
                     // Similar to ADTs, require that the rest of the fields are equal.
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs b/compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs
index b6847b976be6e..3662463178f85 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs
@@ -191,10 +191,10 @@ pub(crate) fn extract_tupled_inputs_and_output_from_callable<'tcx>(
         ty::FnDef(def_id, substs) => Ok(Some(
             tcx.fn_sig(def_id)
                 .subst(tcx, substs)
-                .map_bound(|sig| (tcx.mk_tup(sig.inputs().iter()), sig.output())),
+                .map_bound(|sig| (tcx.intern_tup(sig.inputs()), sig.output())),
         )),
         ty::FnPtr(sig) => {
-            Ok(Some(sig.map_bound(|sig| (tcx.mk_tup(sig.inputs().iter()), sig.output()))))
+            Ok(Some(sig.map_bound(|sig| (tcx.intern_tup(sig.inputs()), sig.output()))))
         }
         ty::Closure(_, substs) => {
             let closure_substs = substs.as_closure();
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index a0c67f480d024..91b463800a814 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -2012,7 +2012,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             let sig = match inputs.kind() {
                 ty::Tuple(inputs) if infcx.tcx.is_fn_trait(trait_ref.def_id()) => {
                     infcx.tcx.mk_fn_sig(
-                        inputs.iter(),
+                        *inputs,
                         infcx.next_ty_var(TypeVariableOrigin {
                             span: DUMMY_SP,
                             kind: TypeVariableOriginKind::MiscVariable,
@@ -2023,7 +2023,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     )
                 }
                 _ => infcx.tcx.mk_fn_sig(
-                    std::iter::once(inputs),
+                    [inputs],
                     infcx.next_ty_var(TypeVariableOrigin {
                         span: DUMMY_SP,
                         kind: TypeVariableOriginKind::MiscVariable,
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 011fe742a69cd..9143d28b8eede 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -1923,7 +1923,7 @@ fn confirm_builtin_candidate<'cx, 'tcx>(
 ) -> Progress<'tcx> {
     let tcx = selcx.tcx();
     let self_ty = obligation.predicate.self_ty();
-    let substs = tcx.mk_substs([self_ty.into()].iter());
+    let substs = tcx.intern_substs(&[self_ty.into()]);
     let lang_items = tcx.lang_items();
     let item_def_id = obligation.predicate.def_id;
     let trait_def_id = tcx.trait_of_item(item_def_id).unwrap();
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 1309df18e15b0..e4a832e472813 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -564,10 +564,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                 .into()
                             }
                         });
-                        let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
+                        let bound_vars = tcx.intern_bound_variable_kinds(&bound_vars);
                         let assoc_ty_substs = tcx.intern_substs(&substs);
-
-                        let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
                         let bound =
                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs
index 8b32c0119e0e3..41924dc2a6d93 100644
--- a/compiler/rustc_ty_utils/src/abi.rs
+++ b/compiler/rustc_ty_utils/src/abi.rs
@@ -141,8 +141,8 @@ fn fn_sig_for_fn_abi<'tcx>(
 
             ty::Binder::bind_with_vars(
                 tcx.mk_fn_sig(
-                    [env_ty, resume_ty].iter(),
-                    &ret_ty,
+                    [env_ty, resume_ty],
+                    ret_ty,
                     false,
                     hir::Unsafety::Normal,
                     rustc_target::spec::abi::Abi::Rust,
diff --git a/compiler/rustc_ty_utils/src/consts.rs b/compiler/rustc_ty_utils/src/consts.rs
index a9fbad55dac55..852156c24f42a 100644
--- a/compiler/rustc_ty_utils/src/consts.rs
+++ b/compiler/rustc_ty_utils/src/consts.rs
@@ -144,7 +144,7 @@ fn recurse_build<'tcx>(
             for &id in args.iter() {
                 new_args.push(recurse_build(tcx, body, id, root_span)?);
             }
-            let new_args = tcx.mk_const_list(new_args.iter());
+            let new_args = tcx.intern_const_list(&new_args);
             tcx.mk_const(Expr::FunctionCall(fun, new_args), node.ty)
         }
         &ExprKind::Binary { op, lhs, rhs } if check_binop(op) => {
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index cb28731681ba0..5a991e03dee52 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -69,38 +69,37 @@ pub trait Interner: Sized {
     type PlaceholderRegion: Clone + Debug + Hash + Ord;
 }
 
-pub trait InternAs<T: ?Sized, R> {
+/// Imagine you have a function `F: FnOnce(&[T]) -> R`, plus an iterator `iter`
+/// that produces `T` items. You could combine them with
+/// `f(&iter.collect::<Vec<_>>())`, but this requires allocating memory for the
+/// `Vec`.
+///
+/// This trait allows for faster implementations, intended for cases where the
+/// number of items produced by the iterator is small. There is a blanket impl
+/// for `T` items, but there is also a fallible impl for `Result<T, E>` items.
+pub trait CollectAndApply<T, R>: Sized {
     type Output;
-    fn intern_with<F>(self, f: F) -> Self::Output
+
+    /// Produce a result of type `Self::Output` from `iter`. The result will
+    /// typically be produced by applying `f` on the elements produced by
+    /// `iter`, though this may not happen in some impls, e.g. if an error
+    /// occured during iteration.
+    fn collect_and_apply<I, F>(iter: I, f: F) -> Self::Output
     where
+        I: Iterator<Item = Self>,
         F: FnOnce(&[T]) -> R;
 }
 
-impl<I, T, R, E> InternAs<T, R> for I
-where
-    E: InternIteratorElement<T, R>,
-    I: Iterator<Item = E>,
-{
-    type Output = E::Output;
-    fn intern_with<F>(self, f: F) -> Self::Output
+/// The blanket impl that always collects all elements and applies `f`.
+impl<T, R> CollectAndApply<T, R> for T {
+    type Output = R;
+
+    /// Equivalent to `f(&iter.collect::<Vec<_>>())`.
+    fn collect_and_apply<I, F>(mut iter: I, f: F) -> R
     where
+        I: Iterator<Item = T>,
         F: FnOnce(&[T]) -> R,
     {
-        E::intern_with(self, f)
-    }
-}
-
-pub trait InternIteratorElement<T, R>: Sized {
-    type Output;
-    fn intern_with<I: Iterator<Item = Self>, F: FnOnce(&[T]) -> R>(iter: I, f: F) -> Self::Output;
-}
-
-impl<T, R> InternIteratorElement<T, R> for T {
-    type Output = R;
-    fn intern_with<I: Iterator<Item = Self>, F: FnOnce(&[T]) -> R>(
-        mut iter: I,
-        f: F,
-    ) -> Self::Output {
         // This code is hot enough that it's worth specializing for the most
         // common length lists, to avoid the overhead of `SmallVec` creation.
         // Lengths 0, 1, and 2 typically account for ~95% of cases. If
@@ -127,23 +126,17 @@ impl<T, R> InternIteratorElement<T, R> for T {
     }
 }
 
-impl<'a, T, R> InternIteratorElement<T, R> for &'a T
-where
-    T: Clone + 'a,
-{
-    type Output = R;
-    fn intern_with<I: Iterator<Item = Self>, F: FnOnce(&[T]) -> R>(iter: I, f: F) -> Self::Output {
-        // This code isn't hot.
-        f(&iter.cloned().collect::<SmallVec<[_; 8]>>())
-    }
-}
-
-impl<T, R, E> InternIteratorElement<T, R> for Result<T, E> {
+/// A fallible impl that will fail, without calling `f`, if there are any
+/// errors during collection.
+impl<T, R, E> CollectAndApply<T, R> for Result<T, E> {
     type Output = Result<R, E>;
-    fn intern_with<I: Iterator<Item = Self>, F: FnOnce(&[T]) -> R>(
-        mut iter: I,
-        f: F,
-    ) -> Self::Output {
+
+    /// Equivalent to `Ok(f(&iter.collect::<Result<Vec<_>>>()?))`.
+    fn collect_and_apply<I, F>(mut iter: I, f: F) -> Result<R, E>
+    where
+        I: Iterator<Item = Result<T, E>>,
+        F: FnOnce(&[T]) -> R,
+    {
         // This code is hot enough that it's worth specializing for the most
         // common length lists, to avoid the overhead of `SmallVec` creation.
         // Lengths 0, 1, and 2 typically account for ~95% of cases. If
diff --git a/src/tools/clippy/clippy_lints/src/methods/needless_collect.rs b/src/tools/clippy/clippy_lints/src/methods/needless_collect.rs
index 82d3b830d4f39..8ddbacc3d7ad4 100644
--- a/src/tools/clippy/clippy_lints/src/methods/needless_collect.rs
+++ b/src/tools/clippy/clippy_lints/src/methods/needless_collect.rs
@@ -173,7 +173,7 @@ fn is_contains_sig(cx: &LateContext<'_>, call_id: HirId, iter_expr: &Expr<'_>) -
         && let Some(iter_item) = cx.tcx
             .associated_items(iter_trait)
             .find_by_name_and_kind(cx.tcx, Ident::with_dummy_span(Symbol::intern("Item")), AssocKind::Type, iter_trait)
-        && let substs = cx.tcx.mk_substs([GenericArg::from(typeck.expr_ty_adjusted(iter_expr))].into_iter())
+        && let substs = cx.tcx.intern_substs(&[GenericArg::from(typeck.expr_ty_adjusted(iter_expr))])
         && let proj_ty = cx.tcx.mk_projection(iter_item.def_id, substs)
         && let Ok(item_ty) = cx.tcx.try_normalize_erasing_regions(cx.param_env, proj_ty)
     {
diff --git a/src/tools/clippy/clippy_lints/src/redundant_slicing.rs b/src/tools/clippy/clippy_lints/src/redundant_slicing.rs
index 245a02ea26e61..398329e455bfa 100644
--- a/src/tools/clippy/clippy_lints/src/redundant_slicing.rs
+++ b/src/tools/clippy/clippy_lints/src/redundant_slicing.rs
@@ -11,8 +11,6 @@ use rustc_middle::ty::adjustment::{Adjust, AutoBorrow, AutoBorrowMutability};
 use rustc_middle::ty::subst::GenericArg;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
-use std::iter;
-
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for redundant slicing expressions which use the full range, and
@@ -136,7 +134,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantSlicing {
                 } else if let Some(target_id) = cx.tcx.lang_items().deref_target() {
                     if let Ok(deref_ty) = cx.tcx.try_normalize_erasing_regions(
                         cx.param_env,
-                        cx.tcx.mk_projection(target_id, cx.tcx.mk_substs(iter::once(GenericArg::from(indexed_ty)))),
+                        cx.tcx.mk_projection(target_id, cx.tcx.intern_substs(&[GenericArg::from(indexed_ty)])),
                     ) {
                         if deref_ty == expr_ty {
                             let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;
diff --git a/src/tools/miri/src/eval.rs b/src/tools/miri/src/eval.rs
index d61e17cbf9a4a..ebb71b57ae395 100644
--- a/src/tools/miri/src/eval.rs
+++ b/src/tools/miri/src/eval.rs
@@ -363,7 +363,7 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
                 tcx,
                 ty::ParamEnv::reveal_all(),
                 start_id,
-                tcx.mk_substs(::std::iter::once(ty::subst::GenericArg::from(main_ret_ty))),
+                tcx.intern_substs(&[ty::subst::GenericArg::from(main_ret_ty)]),
             )
             .unwrap()
             .unwrap();