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_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -207,6 +207,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
     def_ident_span => { table }
     lookup_stability => { table }
     lookup_const_stability => { table }
+    lookup_default_body_stability => { table }
     lookup_deprecation_entry => { table }
     visibility => { table }
     unused_generic_params => { table }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1029,6 +1029,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             if should_encode_stability(def_kind) {
                 self.encode_stability(def_id);
                 self.encode_const_stability(def_id);
+                self.encode_default_body_stability(def_id);
                 self.encode_deprecation(def_id);
             }
             if should_encode_variances(def_kind) {
@@ -1385,6 +1386,18 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         }
     }
 
+    fn encode_default_body_stability(&mut self, def_id: DefId) {
+        debug!("EncodeContext::encode_default_body_stability({:?})", def_id);
+
+        // The query lookup can take a measurable amount of time in crates with many items. Check if
+        // the stability attributes are even enabled before using their queries.
+        if self.feat.staged_api || self.tcx.sess.opts.unstable_opts.force_unstable_if_unmarked {
+            if let Some(stab) = self.tcx.lookup_default_body_stability(def_id) {
+                record!(self.tables.lookup_default_body_stability[def_id] <- stab)
+            }
+        }
+    }
+
     fn encode_deprecation(&mut self, def_id: DefId) {
         debug!("EncodeContext::encode_deprecation({:?})", def_id);
         if let Some(depr) = self.tcx.lookup_deprecation(def_id) {

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -343,6 +343,7 @@ define_tables! {
     def_ident_span: Table<DefIndex, LazyValue<Span>>,
     lookup_stability: Table<DefIndex, LazyValue<attr::Stability>>,
     lookup_const_stability: Table<DefIndex, LazyValue<attr::ConstStability>>,
+    lookup_default_body_stability: Table<DefIndex, LazyValue<attr::DefaultBodyStability>>,
     lookup_deprecation_entry: Table<DefIndex, LazyValue<attr::Deprecation>>,
     // As an optimization, a missing entry indicates an empty `&[]`.
     explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Predicate<'static>, Span)>>,