derive(Error) with Backtrace causes errors on beta

[Heliozoa]

Hi,
Thanks for your work on thiserror.

I'm trying to use std::backtrace::Backtrace with thiserror now that std::backtrace is available on beta, but I'm getting a couple of errors.

https://play.rust-lang.org/?version=beta&mode=debug&edition=2021&gist=e66ea95c9e0f2b4bea73d1cb4b503716

Compiling playground v0.0.1 (/playground)
error[[E0432]](https://doc.rust-lang.org/beta/error-index.html#E0432): unresolved import `thiserror`
 --> src/lib.rs:5:10
  |
5 | #[derive(Error, Debug)]
  |          ^^^^^ no `ThiserrorProvide` in `__private`
  |
  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

error[[E0658]](https://doc.rust-lang.org/beta/error-index.html#E0658): use of unstable library feature 'error_generic_member_access'
 --> src/lib.rs:5:10
  |
5 | #[derive(Error, Debug)]
  |          ^^^^^
  |
  = note: [see issue #99301 <https://github.com/rust-lang/rust/issues/99301>](https://github.com/rust-lang/rust/issues/99301) for more information
  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

error[[E0599]](https://doc.rust-lang.org/beta/error-index.html#E0599): no method named `thiserror_provide` found for reference `&std::io::Error` in the current scope
  --> src/lib.rs:10:9
   |
10 |         source: io::Error,
   |         ^^^^^^ method not found in `&std::io::Error`

Some errors have detailed explanations: E0432, E0599, E0658.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `playground` due to 3 previous errors

I saw that there are other similar issues posted, but they seem to be related to rust-analyzer, and given that this is reproduceable on the playground I felt fairly confident that this is a new issue.

[dtolnay]

Rust 1.65 (current beta) only stabilizes the Backtrace type, not any of the functionality for using backtraces with the Error trait. You can't use backtraces in an error until the rest is stabilized as well.

[lvaroqui-gpfw]

Just my 2 cents, but I was really confused that all backtraces related features didn't work on stable (1.65 as of today). I think there should be a disclaimer in the docs until this hits stable.