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`

Author: bors

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();