Auto merge of #113591 - mdibaiee:genericargs-cleanup, r=oli-obk

refactor(rustc_middle): Substs -> GenericArg

resolves #110793

- [x] rename `SubstsRef` and `InternalSubsts` to `GenericArgsRef<'tcx>` and `GenericArgs<'tcx>`.
- [x] rename variables and fields currently using `substs` to `args`.
- [x] update the module name of `ty::subst` to `ty::generic_args` or sth. Make that module private and publicly reexport its content in the ty module.
- [x] rename `EarlyBinder::subst(_identity)` to `EarlyBinder::instantiate(_identity)`.
- [x] types called `[a-zA-Z]+Substs` renamed to `XArgs`.
- [x] functions containing `substs` now use `args` or `generic_args` (mostly the former).

However, the verb of "substituting" is still being used here and there, mostly in comments. I think that can be a separate PR as part of rust-lang/rust#110254 to change the verb to `replace_generics` or something similar.

src/eval.rs

@@ -380,7 +380,7 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
                 tcx,
                 ty::ParamEnv::reveal_all(),
                 start_id,
-                tcx.mk_substs(&[ty::subst::GenericArg::from(main_ret_ty)]),
+                tcx.mk_args(&[ty::GenericArg::from(main_ret_ty)]),
             )
             .unwrap()
             .unwrap();

src/machine.rs

@@ -972,7 +972,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
                 panic!("extern_statics cannot contain wildcards")
             };
             let (shim_size, shim_align, _kind) = ecx.get_alloc_info(alloc_id);
-            let def_ty = ecx.tcx.type_of(def_id).subst_identity();
+            let def_ty = ecx.tcx.type_of(def_id).instantiate_identity();
             let extern_decl_layout = ecx.tcx.layout_of(ty::ParamEnv::empty().and(def_ty)).unwrap();
             if extern_decl_layout.size != shim_size || extern_decl_layout.align.abi != shim_align {
                 throw_unsup_format!(