…ter.

Add custom nth_back to Skip

Implementation of nth_back for Skip.
Part of rust-lang#54054

…=scottmcm

Implement nth_back for slice::{Iter, IterMut}

Part of rust-lang#54054.

I implemented `nth_back` as straightforwardly as I could, and then slightly changed `nth` to match `nth_back`. I believe I did so correctly, but please double-check 🙂

I also added the helper methods `zst_shrink`, `next_unchecked`, and `next_back_unchecked` to get rid of some duplicated code. These changes hopefully make this code easier to understand for new contributors like me.

I noticed the `is_empty!` and `len!` macros which sole purpose seems to be inlining, according to the comment right above them, but the `is_empty` and `len` methods are already marked with `#[inline(always)]`. Does that mean we could replace these macros with method calls, without affecting anything? I'd love to get rid of them.

…_syntax, r=estebank

suggest tuple struct syntax

refs rust-lang#57242

rustc: disallow cloning HIR nodes.

Besides being inefficient, cloning also risks creating broken HIR (without properly recreating all the IDs and whatnot, in which case you might as well reconstruct the entire node without ever `Clone`-ing anything).

We detect *some* detrimental situations (based on the occurrence of `HirId`s, I believe?), but it's better to statically disallow it, IMO.

One of the examples that is fixed by this PR is `tcx.hir().fn_decl{,_by_hir_id}`, which was cloning an entire `hir::FnDecl` *every single time it was called*.

r? @petrochenkov cc @rust-lang/compiler