Auto merge of #108112 - nnethercote:clarify-iterator-interners, r=oli-obk,compiler-errors

Clarify iterator interners

I found the iterator interners very confusing. This PR clarifies things.

r? `@compiler-errors`

5 files changed, 7 insertions, 9 deletions

diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index 0921fb5756d..d12e5f797fb 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 b6847b976be..3662463178f 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 a0c67f480d0..91b463800a8 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 c2388f9fa1b..9b3249e58e8 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 1309df18e15..e4a832e4728 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))