Rollup merge of #96478 - WaffleLapkin:rustc_default_body_unstable, r=Aaron1011

Implement `#[rustc_default_body_unstable]`

This PR implements a new stability attribute — `#[rustc_default_body_unstable]`.

`#[rustc_default_body_unstable]` controls the stability of default bodies in traits.
For example:
```rust
pub trait Trait {
    #[rustc_default_body_unstable(feature = "feat", isssue = "none")]
    fn item() {}
}
```
In order to implement `Trait` user needs to either
- implement `item` (even though it has a default implementation)
- enable `#![feature(feat)]`

This is useful in conjunction with [`#[rustc_must_implement_one_of]`](https://github.com/rust-lang/rust/pull/92164), we may want to relax requirements for a trait, for example allowing implementing either of `PartialEq::{eq, ne}`, but do so in a safe way — making implementation of only `PartialEq::ne` unstable.

r? `@Aaron1011`
cc `@nrc` (iirc you were interested in this wrt `read_buf`), `@danielhenrymantilla` (you were interested in the related `#[rustc_must_implement_one_of]`)
P.S. This is my first time working with stability attributes, so I'm not sure if I did everything right 😅

compiler/rustc_expand/src/base.rs

@@ -772,7 +772,7 @@ impl SyntaxExtension {
                 )
             })
             .unwrap_or_else(|| (None, helper_attrs));
-        let (stability, const_stability) = attr::find_stability(&sess, attrs, span);
+        let (stability, const_stability, body_stability) = attr::find_stability(&sess, attrs, span);
         if let Some((_, sp)) = const_stability {
             sess.parse_sess
                 .span_diagnostic
@@ -784,6 +784,17 @@ impl SyntaxExtension {
                 )
                 .emit();
         }
+        if let Some((_, sp)) = body_stability {
+            sess.parse_sess
+                .span_diagnostic
+                .struct_span_err(sp, "macros cannot have body stability attributes")
+                .span_label(sp, "invalid body stability attribute")
+                .span_label(
+                    sess.source_map().guess_head_span(span),
+                    "body stability attribute affects this macro",
+                )
+                .emit();
+        }
 
         SyntaxExtension {
             kind,