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.

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)]
@@ -214,7 +222,8 @@ 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)
 }
 
@@ -222,7 +231,7 @@ 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>,
 {
@@ -230,6 +239,7 @@ where
 
     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,

compiler/rustc_passes/src/stability.rs

@@ -1,8 +1,10 @@
 //! A pass that annotates every item and method with its stability level,
 //! propagating default levels lexically from parent to children ast nodes.
 
-use attr::StabilityLevel;
+use rustc_attr::{
-use rustc_attr::{self as attr, ConstStability, Stability, Unstable, UnstableReason};
+    self as attr, ConstStability, DefaultBodyStability, Stability, StabilityLevel, Unstable,
+    UnstableReason,
+};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
@@ -89,6 +91,7 @@ struct Annotator<'a, 'tcx> {
     index: &'a mut Index,
     parent_stab: Option<Stability>,
     parent_const_stab: Option<ConstStability>,
+    parent_body_stab: Option<DefaultBodyStability>,
     parent_depr: Option<DeprecationEntry>,
     in_trait_impl: bool,
 }
@@ -156,12 +159,13 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
                 depr.map(|(d, _)| DeprecationEntry::local(d, def_id)),
                 None,
                 None,
+                None,
                 visit_children,
             );
             return;
         }
 
-        let (stab, const_stab) = attr::find_stability(&self.tcx.sess, attrs, item_sp);
+        let (stab, const_stab, body_stab) = attr::find_stability(&self.tcx.sess, attrs, item_sp);
         let mut const_span = None;
 
         let const_stab = const_stab.map(|(const_stab, const_span_node)| {
@@ -209,6 +213,15 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             }
         }
 
+        let body_stab = body_stab.map(|(body_stab, _span)| {
+            // FIXME: check that this item can have body stability
+
+            self.index.default_body_stab_map.insert(def_id, body_stab);
+            debug!(?self.index.default_body_stab_map);
+
+            body_stab
+        });
+
         let stab = stab.map(|(stab, span)| {
             // Error if prohibited, or can't inherit anything from a container.
             if kind == AnnotationKind::Prohibited
@@ -286,6 +299,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             depr.map(|(d, _)| DeprecationEntry::local(d, def_id)),
             stab,
             if inherit_const_stability.yes() { const_stab } else { None },
+            body_stab,
             visit_children,
         );
     }
@@ -295,12 +309,14 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         depr: Option<DeprecationEntry>,
         stab: Option<Stability>,
         const_stab: Option<ConstStability>,
+        body_stab: Option<DefaultBodyStability>,
         f: impl FnOnce(&mut Self),
     ) {
         // These will be `Some` if this item changes the corresponding stability attribute.
         let mut replaced_parent_depr = None;
         let mut replaced_parent_stab = None;
         let mut replaced_parent_const_stab = None;
+        let mut replaced_parent_body_stab = None;
 
         if let Some(depr) = depr {
             replaced_parent_depr = Some(replace(&mut self.parent_depr, Some(depr)));
@@ -312,6 +328,9 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             replaced_parent_const_stab =
                 Some(replace(&mut self.parent_const_stab, Some(const_stab)));
         }
+        if let Some(body_stab) = body_stab {
+            replaced_parent_body_stab = Some(self.parent_body_stab.replace(body_stab));
+        }
 
         f(self);
 
@@ -324,6 +343,9 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         if let Some(orig_parent_const_stab) = replaced_parent_const_stab {
             self.parent_const_stab = orig_parent_const_stab;
         }
+        if let Some(orig_parent_body_stab) = replaced_parent_body_stab {
+            self.parent_body_stab = orig_parent_body_stab;
+        }
     }
 }
 
@@ -613,6 +635,7 @@ fn stability_index(tcx: TyCtxt<'_>, (): ()) -> Index {
     let mut index = Index {
         stab_map: Default::default(),
         const_stab_map: Default::default(),
+        default_body_stab_map: Default::default(),
         depr_map: Default::default(),
         implications: Default::default(),
     };
@@ -623,6 +646,7 @@ fn stability_index(tcx: TyCtxt<'_>, (): ()) -> Index {
             index: &mut index,
             parent_stab: None,
             parent_const_stab: None,
+            parent_body_stab: None,
             parent_depr: None,
             in_trait_impl: false,
         };
@@ -673,6 +697,9 @@ pub(crate) fn provide(providers: &mut Providers) {
         stability_implications: |tcx, _| tcx.stability().implications.clone(),
         lookup_stability: |tcx, id| tcx.stability().local_stability(id.expect_local()),
         lookup_const_stability: |tcx, id| tcx.stability().local_const_stability(id.expect_local()),
+        lookup_default_body_stability: |tcx, id| {
+            tcx.stability().local_default_body_stability(id.expect_local())
+        },
         lookup_deprecation_entry: |tcx, id| {
             tcx.stability().local_deprecation_entry(id.expect_local())
         },
@@ -723,7 +750,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                 let features = self.tcx.features();
                 if features.staged_api {
                     let attrs = self.tcx.hir().attrs(item.hir_id());
-                    let (stab, const_stab) = attr::find_stability(&self.tcx.sess, attrs, item.span);
+                    let (stab, const_stab, _) =
+                        attr::find_stability(&self.tcx.sess, attrs, item.span);
 
                     // If this impl block has an #[unstable] attribute, give an
                     // error if all involved types and traits are stable, because

compiler/rustc_span/src/symbol.rs

@@ -1215,6 +1215,7 @@ symbols! {
         rustc_const_unstable,
         rustc_conversion_suggestion,
         rustc_def_path,
+        rustc_default_body_unstable,
         rustc_diagnostic_item,
         rustc_diagnostic_macros,
         rustc_dirty,

compiler/rustc_typeck/src/check/check.rs

@@ -18,6 +18,7 @@ use rustc_infer::infer::{DefiningAnchor, RegionVariableOrigin, TyCtxtInferExt};
 use rustc_infer::traits::Obligation;
 use rustc_lint::builtin::REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS;
 use rustc_middle::hir::nested_filter;
+use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
@@ -1104,12 +1105,31 @@ fn check_impl_items_against_trait<'tcx>(
                 missing_items.push(tcx.associated_item(trait_item_id));
             }
 
-            if let Some(required_items) = &must_implement_one_of {
             // true if this item is specifically implemented in this impl
             let is_implemented_here = ancestors
                 .leaf_def(tcx, trait_item_id)
                 .map_or(false, |node_item| !node_item.defining_node.is_from_trait());
 
+            if !is_implemented_here {
+                match tcx.eval_default_body_stability(trait_item_id, full_impl_span) {
+                    EvalResult::Deny { feature, reason, issue, is_soft, .. } => {
+                        default_body_is_unstable(
+                            tcx,
+                            full_impl_span,
+                            trait_item_id,
+                            feature,
+                            reason,
+                            issue,
+                            is_soft,
+                        )
+                    }
+
+                    // Unmarked default bodies are considered stable (at least for now).
+                    EvalResult::Allow | EvalResult::Unmarked => {}
+                }
+            }
+
+            if let Some(required_items) = &must_implement_one_of {
                 if is_implemented_here {
                     let trait_item = tcx.associated_item(trait_item_id);
                     if required_items.contains(&trait_item.ident(tcx)) {

compiler/rustc_typeck/src/check/mod.rs

@@ -99,6 +99,7 @@ pub use expectation::Expectation;
 pub use fn_ctxt::*;
 use hir::def::CtorOf;
 pub use inherited::{Inherited, InheritedBuilder};
+use rustc_middle::middle::stability::report_unstable;
 
 use crate::astconv::AstConv;
 use crate::check::gather_locals::GatherLocalsVisitor;
@@ -122,13 +123,14 @@ use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::source_map::DUMMY_SP;
 use rustc_span::symbol::{kw, Ident};
-use rustc_span::{self, BytePos, Span};
+use rustc_span::{self, BytePos, Span, Symbol};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::error_reporting::recursive_type_with_infinite_size_error;
 use rustc_trait_selection::traits::error_reporting::suggestions::ReturnsVisitor;
 use std::cell::RefCell;
+use std::num::NonZeroU32;
 
 use crate::require_c_abi_if_c_variadic;
 use crate::util::common::indenter;
@@ -662,6 +664,24 @@ fn missing_items_must_implement_one_of_err(
     err.emit();
 }
 
+fn default_body_is_unstable(
+    tcx: TyCtxt<'_>,
+    impl_span: Span,
+    _item_did: DefId,
+    feature: Symbol,
+    reason: Option<Symbol>,
+    issue: Option<NonZeroU32>,
+    is_soft: bool,
+) {
+    let soft_handler = |lint, span, msg: &_| {
+        tcx.struct_span_lint_hir(lint, hir::CRATE_HIR_ID, span, |lint| {
+            lint.build(msg).emit();
+        })
+    };
+
+    report_unstable(tcx.sess, feature, reason, issue, None, is_soft, impl_span, soft_handler)
+}
+
 /// Re-sugar `ty::GenericPredicates` in a way suitable to be used in structured suggestions.
 fn bounds_from_generic_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,