Rollup merge of rust-lang#94635 - jhpratt:merge-deprecated-attrs, r=davidtwco

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

Author: matthiaskrgr

core/src/lib.rs

@@ -167,6 +167,7 @@
 #![feature(const_refs_to_cell)]
 #![feature(decl_macro)]
 #![feature(derive_default_enum)]
+#![cfg_attr(not(bootstrap), feature(deprecated_suggestion))]
 #![feature(doc_cfg)]
 #![feature(doc_notable_trait)]
 #![feature(rustdoc_internals)]

std/src/lib.rs

@@ -263,6 +263,7 @@
 #![feature(doc_cfg)]
 #![feature(doc_cfg_hide)]
 #![feature(rustdoc_internals)]
+#![cfg_attr(not(bootstrap), feature(deprecated_suggestion))]
 #![feature(doc_masked)]
 #![feature(doc_notable_trait)]
 #![feature(dropck_eyepatch)]