Implement #[rustc_default_body_unstable]

This attribute allows to mark default body of a trait function as
unstable. This means that implementing the trait without implementing
the function will require enabling unstable feature.

This is useful in conjunction with `#[rustc_must_implement_one_of]`,
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.

Author: WaffleLapkin

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -417,6 +417,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 || attr.has_name(sym::stable)
                 || attr.has_name(sym::rustc_const_unstable)
                 || attr.has_name(sym::rustc_const_stable)
+                || attr.has_name(sym::rustc_default_body_unstable)
             {
                 struct_span_err!(
                     self.sess,

compiler/rustc_attr/src/builtin.rs

@@ -131,6 +131,14 @@ impl ConstStability {
     }
 }
 
+/// Represents the `#[rustc_default_body_unstable]` attribute.
+#[derive(Encodable, Decodable, Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(HashStable_Generic)]
+pub struct DefaultBodyStability {
+    pub level: StabilityLevel,
+    pub feature: Symbol,
+}
+
 /// The available stability levels.
 #[derive(Encodable, Decodable, PartialEq, Copy, Clone, Debug, Eq, Hash)]
 #[derive(HashStable_Generic)]
@@ -214,22 +222,24 @@ pub fn find_stability(
     sess: &Session,
     attrs: &[Attribute],
     item_sp: Span,
-) -> (Option<(Stability, Span)>, Option<(ConstStability, Span)>) {
+) -> (Option<(Stability, Span)>, Option<(ConstStability, Span)>, Option<(DefaultBodyStability, Span)>)
+{
     find_stability_generic(sess, attrs.iter(), item_sp)
 }
 
 fn find_stability_generic<'a, I>(
     sess: &Session,
     attrs_iter: I,
     item_sp: Span,
-) -> (Option<(Stability, Span)>, Option<(ConstStability, Span)>)
+) -> (Option<(Stability, Span)>, Option<(ConstStability, Span)>, Option<(DefaultBodyStability, Span)>)
 where
     I: Iterator<Item = &'a Attribute>,
 {
     use StabilityLevel::*;
 
     let mut stab: Option<(Stability, Span)> = None;
     let mut const_stab: Option<(ConstStability, Span)> = None;
+    let mut body_stab: Option<(DefaultBodyStability, Span)> = None;
     let mut promotable = false;
     let mut allowed_through_unstable_modules = false;
 
@@ -243,6 +253,7 @@ where
             sym::stable,
             sym::rustc_promotable,
             sym::rustc_allowed_through_unstable_modules,
+            sym::rustc_default_body_unstable,
         ]
         .iter()
         .any(|&s| attr.has_name(s))
@@ -280,7 +291,7 @@ where
 
             let meta_name = meta.name_or_empty();
             match meta_name {
-                sym::rustc_const_unstable | sym::unstable => {
+                sym::rustc_const_unstable | sym::rustc_default_body_unstable | sym::unstable => {
                     if meta_name == sym::unstable && stab.is_some() {
                         handle_errors(
                             &sess.parse_sess,
@@ -295,6 +306,13 @@ where
                             AttrError::MultipleStabilityLevels,
                         );
                         break;
+                    } else if meta_name == sym::rustc_default_body_unstable && body_stab.is_some() {
+                        handle_errors(
+                            &sess.parse_sess,
+                            attr.span,
+                            AttrError::MultipleStabilityLevels,
+                        );
+                        break;
                     }
 
                     let mut feature = None;
@@ -405,11 +423,14 @@ where
                             };
                             if sym::unstable == meta_name {
                                 stab = Some((Stability { level, feature }, attr.span));
-                            } else {
+                            } else if sym::rustc_const_unstable == meta_name {
                                 const_stab = Some((
                                     ConstStability { level, feature, promotable: false },
                                     attr.span,
                                 ));
+                            } else {
+                                body_stab =
+                                    Some((DefaultBodyStability { level, feature }, attr.span));
                             }
                         }
                         (None, _, _) => {
@@ -542,7 +563,7 @@ where
         }
     }
 
-    (stab, const_stab)
+    (stab, const_stab, body_stab)
 }
 
 pub fn find_crate_name(sess: &Session, attrs: &[Attribute]) -> Option<Symbol> {

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,

compiler/rustc_feature/src/builtin_attrs.rs

@@ -499,6 +499,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     ungated!(rustc_const_unstable, Normal, template!(List: r#"feature = "name""#), DuplicatesOk),
     ungated!(rustc_const_stable, Normal, template!(List: r#"feature = "name""#), DuplicatesOk),
+    ungated!(
+        rustc_default_body_unstable, Normal,
+        template!(List: r#"feature = "name", reason = "...", issue = "N""#), DuplicatesOk
+    ),
     gated!(
         allow_internal_unstable, Normal, template!(Word, List: "feat1, feat2, ..."), DuplicatesOk,
         "allow_internal_unstable side-steps feature gating and stability checks",

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) {
@@ -1397,6 +1398,18 @@ 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)>>,

compiler/rustc_middle/src/middle/stability.rs

@@ -5,7 +5,7 @@ pub use self::StabilityLevel::*;
 
 use crate::ty::{self, DefIdTree, TyCtxt};
 use rustc_ast::NodeId;
-use rustc_attr::{self as attr, ConstStability, Deprecation, Stability};
+use rustc_attr::{self as attr, ConstStability, DefaultBodyStability, Deprecation, Stability};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{Applicability, Diagnostic};
 use rustc_feature::GateIssue;
@@ -61,6 +61,7 @@ pub struct Index {
     /// are filled by the annotator.
     pub stab_map: FxHashMap<LocalDefId, Stability>,
     pub const_stab_map: FxHashMap<LocalDefId, ConstStability>,
+    pub default_body_stab_map: FxHashMap<LocalDefId, DefaultBodyStability>,
     pub depr_map: FxHashMap<LocalDefId, DeprecationEntry>,
     /// Mapping from feature name to feature name based on the `implied_by` field of `#[unstable]`
     /// attributes. If a `#[unstable(feature = "implier", implied_by = "impliee")]` attribute
@@ -86,6 +87,10 @@ impl Index {
         self.const_stab_map.get(&def_id).copied()
     }
 
+    pub fn local_default_body_stability(&self, def_id: LocalDefId) -> Option<DefaultBodyStability> {
+        self.default_body_stab_map.get(&def_id).copied()
+    }
+
     pub fn local_deprecation_entry(&self, def_id: LocalDefId) -> Option<DeprecationEntry> {
         self.depr_map.get(&def_id).cloned()
     }
@@ -492,6 +497,63 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
+    /// Evaluates the default-impl stability of an item.
+    ///
+    /// Returns `EvalResult::Allow` if the item's default implementation is stable, or unstable but the corresponding
+    /// `#![feature]` has been provided. Returns `EvalResult::Deny` which describes the offending
+    /// unstable feature otherwise.
+    pub fn eval_default_body_stability(self, def_id: DefId, span: Span) -> EvalResult {
+        let is_staged_api = self.lookup_stability(def_id.krate.as_def_id()).is_some();
+        if !is_staged_api {
+            return EvalResult::Allow;
+        }
+
+        let stability = self.lookup_default_body_stability(def_id);
+        debug!(
+            "body stability: inspecting def_id={def_id:?} span={span:?} of stability={stability:?}"
+        );
+
+        // Only the cross-crate scenario matters when checking unstable APIs
+        let cross_crate = !def_id.is_local();
+        if !cross_crate {
+            return EvalResult::Allow;
+        }
+
+        // Issue #38412: private items lack stability markers.
+        if skip_stability_check_due_to_privacy(self, def_id) {
+            return EvalResult::Allow;
+        }
+
+        match stability {
+            Some(DefaultBodyStability {
+                level: attr::Unstable { reason, issue, is_soft, .. },
+                feature,
+            }) => {
+                if span.allows_unstable(feature) {
+                    debug!("body stability: skipping span={:?} since it is internal", span);
+                    return EvalResult::Allow;
+                }
+                if self.features().active(feature) {
+                    return EvalResult::Allow;
+                }
+
+                //let suggestion = suggestion_for_allocator_api(self, def_id, span, feature);
+                EvalResult::Deny {
+                    feature,
+                    reason: reason.to_opt_reason(),
+                    issue,
+                    suggestion: None,
+                    is_soft,
+                }
+            }
+            Some(_) => {
+                // Stable APIs are always ok to call
+                EvalResult::Allow
+            }
+            None => EvalResult::Unmarked,
+        }
+    }
+
     /// Checks if an item is stable or error out.
     ///
     /// If the item defined by `def_id` is unstable and the corresponding `#![feature]` does not

compiler/rustc_middle/src/query/mod.rs

@@ -1094,6 +1094,11 @@ rustc_queries! {
         separate_provide_extern
     }
 
+    query lookup_default_body_stability(def_id: DefId) -> Option<attr::DefaultBodyStability> {
+        desc { |tcx| "looking up default body stability of `{}`", tcx.def_path_str(def_id) }
+        separate_provide_extern
+    }
+
     query should_inherit_track_caller(def_id: DefId) -> bool {
         desc { |tcx| "computing should_inherit_track_caller of `{}`", tcx.def_path_str(def_id) }
     }

compiler/rustc_middle/src/ty/parameterized.rs

@@ -64,6 +64,7 @@ trivially_parameterized_over_tcx! {
     rustc_ast::Attribute,
     rustc_ast::MacArgs,
     rustc_attr::ConstStability,
+    rustc_attr::DefaultBodyStability,
     rustc_attr::Deprecation,
     rustc_attr::Stability,
     rustc_hir::Constness,