Auto merge of #110496 - WaffleLapkin:🏳️‍⚧️sound, r=compiler-errors

Don't transmute `&List<GenericArg>` <-> `&List<Ty>`

In #93505 we allowed safely transmuting between `&List<GenericArg<'_>>` and `&List<Ty<'_>>`. This was possible because `GenericArg` is a tagged pointer and the tag for types is `0b00`, such that a `GenericArg` with a type inside has the same layout as `Ty`.

While this was meant as an optimization, it doesn't look like it was actually any perf or max-rss win (see https://github.com/rust-lang/rust/pull/94799#issuecomment-1064340003, https://github.com/rust-lang/rust/pull/94841, https://github.com/rust-lang/rust/pull/110496#issuecomment-1513799140).

Additionally the way it was done is quite fragile — `unsafe` code was not properly documented or contained in a module, types were not marked as `repr(C)` (making the transmutes possibly unsound). All of this makes the code maintenance harder and blocks other possible optimizations (as an example I've found out about these `transmutes` when my change caused them to sigsegv compiler).

Thus, I think we can safely (pun intended) remove those transmutes, making maintenance easier, optimizations possible, code less cursed, etc.

r? `@compiler-errors`

clippy_utils/src/ty.rs

@@ -975,7 +975,7 @@ pub fn approx_ty_size<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> u64 {
     }
     match (cx.layout_of(ty).map(|layout| layout.size.bytes()), ty.kind()) {
         (Ok(size), _) => size,
-        (Err(_), ty::Tuple(list)) => list.as_substs().types().map(|t| approx_ty_size(cx, t)).sum(),
+        (Err(_), ty::Tuple(list)) => list.iter().map(|t| approx_ty_size(cx, t)).sum(),
         (Err(_), ty::Array(t, n)) => {
             n.try_eval_target_usize(cx.tcx, cx.param_env).unwrap_or_default() * approx_ty_size(cx, *t)
         },