Switch library to Rust 2021

[m-ou-se]

This is blocked on the beta/bootstrap compiler being updated to 1.56.

[m-ou-se]

This is blocked on an issue with assert and debug_assert using the edition of libccore instead of the one who invoked it. (For assert only on Rust 2015 and 2018, for debug_assert on all editions.)

https://rust-lang.zulipchat.com/#narrow/stream/268952-edition-2021/topic/libcore.202021.20bug/near/252504325

[camelid]

(Note that this is blocked on #88638 (comment). The bootstrap compiler is now 1.56, so that's not a blocker.)

[rukai]

I tried setting core to 2021 and got this error:

Adding rustc_const_stable to panic_fmt resolved the issue but its strange that this issue didnt appear until the edition was set to 2021.

Is this another blocker? Or are we supposed to just add a rustc_const_stable?

[m-ou-se]

strange that this issue didnt appear until the edition was set to 2021.

That's expected. panic!("just a string") didn't use panic_fmt in Rust 2018, but does in Rust 2021. See https://doc.rust-lang.org/edition-guide/rust-2021/panic-macro-consistency.html

[m-ou-se]

@rust-lang/wg-const-eval: Looks like the "const-stable functions can only call other const-stable functions" check doesn't play well with #[rustc_do_not_const_check].