This keeps `reason` around for the time being. This is necessary to
avoid breakage during the bootstrap process. This change, as a whole,
brings `#[rustc_deprecated]` more in line with `#[deprecated]`.

To make this work, the `#[rustc_on_unimplemented]` data needs to be used to
report method resolution errors, which is most of what this commit does.

Fixes rust-lang#94581

Fix  rust-lang#94192.

…e_ptr_cast, r=petrochenkov

Improve suggestion when casting usize to (possibly) wide pointer

I thought rust-lang#92125 was a wonderful idea, so I went ahead and took a stab at it. Not sure if my approach is the best going forward, but I'm happy with the improvement in the error message.

Iwill definitely address any changes if people are more opinionated with the wordings or want more features.

Also, do I need to add a new error code?

(Fixes rust-lang#92125)

…avidtwco

Merge `#[deprecated]` and `#[rustc_deprecated]`

The first commit makes "reason" an alias for "note" in `#[rustc_deprecated]`, while still prohibiting it in `#[deprecated]`.

The second commit changes "suggestion" to not just be a feature of `#[rustc_deprecated]`. This is placed behind the new `deprecated_suggestion` feature. This needs a tracking issue; let me know if this PR will be approved and I can create one.

The third commit is what permits `#[deprecated]` to be used when `#![feature(staged_api)]` is enabled. This isn't yet used in stdlib (only tests), as it would require duplicating all deprecation attributes until a bootstrap occurs. I intend to submit a follow-up PR that replaces all uses and removes the remaining `#[rustc_deprecated]` code after the next bootstrap.

`@rustbot` label +T-libs-api +C-feature-request +A-attributes +S-waiting-on-review

Constify `Index{,Mut}` for `[T]`, `str`, and `[T; N]`

Several panic functions were rewired (via `const_eval_select`) to simpler implementations that do not require formatting for compile-time usage.

r? ```@oli-obk```

…-unimplemented, r=davidtwco

diagnostics: use rustc_on_unimplemented to recommend `[].iter()`

To make this work, the `#[rustc_on_unimplemented]` data needs to be used to
report method resolution errors, which is most of what this commit does.

Fixes rust-lang#94581

…ochenkov

Account for suggestions for complete removal of lines

Fix  rust-lang#94192.