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`

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -419,7 +419,7 @@ 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))
     }
 }