init

compiler/rustc_feature/src/unstable.rs

@@ -197,6 +197,8 @@ declare_features! (
     (internal, compiler_builtins, "1.13.0", None),
     /// Allows writing custom MIR
     (internal, custom_mir, "1.65.0", None),
+    /// Allows using bunch of default auto traits
+    (unstable, default_auto_traits, "CURRENT_RUSTC_VERSION", None),
     /// Outputs useful `assert!` messages
     (unstable, generic_assert, "1.63.0", None),
     /// Allows using the `rust-intrinsic`'s "ABI".

compiler/rustc_hir/src/lang_items.rs

@@ -47,6 +47,13 @@ impl LanguageItems {
             .enumerate()
             .filter_map(|(i, id)| id.map(|id| (LangItem::from_u32(i as u32).unwrap(), id)))
     }
+
+    pub fn is_default_trait(&self, def_id: DefId) -> bool {
+        DEFAULT_TRAITS
+            .iter()
+            .find(|&&default_trait| self.get(default_trait) == Some(def_id))
+            .is_some()
+    }
 }
 
 // The actual lang items defined come at the end of this file in one handy table.
@@ -139,6 +146,8 @@ pub fn extract(attrs: &[ast::Attribute]) -> Option<(Symbol, Span)> {
 
 language_item_table! {
 //  Variant name,            Name,                     Getter method name,         Target                  Generic requirements;
+    SyncDrop,                sym::sync_drop,           sync_drop_trait,            Target::Trait,          GenericRequirement::None;
+    Leak,                    sym::leak,                leak_trait,                 Target::Trait,          GenericRequirement::None;
     Sized,                   sym::sized,               sized_trait,                Target::Trait,          GenericRequirement::Exact(0);
     Unsize,                  sym::unsize,              unsize_trait,               Target::Trait,          GenericRequirement::Minimum(1);
     /// Trait injected by `#[derive(PartialEq)]`, (i.e. "Partial EQ").
@@ -349,6 +358,9 @@ pub enum GenericRequirement {
     Exact(usize),
 }
 
+pub static DEFAULT_TRAITS: &'static [LangItem] =
+    &[LangItem::Sized, LangItem::Leak, LangItem::SyncDrop];
+
 pub static FN_TRAITS: &'static [LangItem] = &[LangItem::Fn, LangItem::FnMut, LangItem::FnOnce];
 
 pub static OPERATORS: &'static [LangItem] = &[

compiler/rustc_hir/src/weak_lang_items.rs

@@ -24,6 +24,8 @@ macro_rules! weak_lang_items {
 }
 
 weak_lang_items! {
+    SyncDrop,           sync_drop;
+    Leak,               leak;
     PanicImpl,          rust_begin_unwind;
     EhPersonality,      rust_eh_personality;
     EhCatchTypeinfo,    rust_eh_catch_typeinfo;

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -16,19 +16,60 @@ use crate::bounds::Bounds;
 use crate::errors;
 
 impl<'tcx> dyn AstConv<'tcx> + '_ {
-    /// Sets `implicitly_sized` to true on `Bounds` if necessary
-    pub(crate) fn add_implicitly_sized(
+    /// Sets Sized or default_auto_traits to true on `Bounds` if necessary
+    pub(crate) fn add_implicit_traits(
         &self,
         bounds: &mut Bounds<'tcx>,
         self_ty: Ty<'tcx>,
         ast_bounds: &'tcx [hir::GenericBound<'tcx>],
         self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
         span: Span,
+    ) {
+        hir::lang_items::DEFAULT_TRAITS.iter().for_each(|default_trait| {
+            self.add_implicit_trait(
+                *default_trait,
+                bounds,
+                self_ty,
+                ast_bounds,
+                self_ty_where_predicates,
+                span,
+            );
+        });
+    }
+
+    pub(crate) fn add_implicit_trait(
+        &self,
+        trait_: hir::LangItem,
+        bounds: &mut Bounds<'tcx>,
+        self_ty: Ty<'tcx>,
+        ast_bounds: &'tcx [hir::GenericBound<'tcx>],
+        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
+        span: Span,
     ) {
         let tcx = self.tcx();
-        let sized_def_id = tcx.lang_items().sized_trait();
-        let mut seen_negative_sized_bound = false;
-        let mut seen_positive_sized_bound = false;
+        let trait_id = tcx.lang_items().get(trait_);
+
+        if self.check_for_implicit_trait(trait_id, ast_bounds, self_ty_where_predicates)
+            && (trait_ == hir::LangItem::Sized || tcx.features().default_auto_traits)
+        {
+            // There was no `?Trait` or `!Trait` bound;
+            // add `Trait` if it's available.
+            bounds.push_lang_item_trait(tcx, self_ty, trait_, span);
+        }
+    }
+
+    fn check_for_implicit_trait(
+        &self,
+        trait_def_id: Option<DefId>,
+        ast_bounds: &'tcx [hir::GenericBound<'tcx>],
+        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
+    ) -> bool {
+        let Some(trait_def_id) = trait_def_id else {
+            return false;
+        };
+        let tcx = self.tcx();
+        let mut seen_negative_bound = false;
+        let mut seen_positive_bound = false;
 
         // Try to find an unbound in bounds.
         let mut unbounds: SmallVec<[_; 1]> = SmallVec::new();
@@ -40,17 +81,13 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                 match modifier {
                     hir::TraitBoundModifier::Maybe => unbounds.push(ptr),
                     hir::TraitBoundModifier::Negative => {
-                        if let Some(sized_def_id) = sized_def_id
-                            && ptr.trait_ref.path.res == Res::Def(DefKind::Trait, sized_def_id)
-                        {
-                            seen_negative_sized_bound = true;
+                        if ptr.trait_ref.path.res == Res::Def(DefKind::Trait, trait_def_id) {
+                            seen_negative_bound = true;
                         }
                     }
                     hir::TraitBoundModifier::None => {
-                        if let Some(sized_def_id) = sized_def_id
-                            && ptr.trait_ref.path.res == Res::Def(DefKind::Trait, sized_def_id)
-                        {
-                            seen_positive_sized_bound = true;
+                        if ptr.trait_ref.path.res == Res::Def(DefKind::Trait, trait_def_id) {
+                            seen_positive_bound = true;
                         }
                     }
                     _ => {}
@@ -68,36 +105,29 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
             }
         }
 
-        if unbounds.len() > 1 {
-            tcx.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
-                spans: unbounds.iter().map(|ptr| ptr.span).collect(),
-            });
-        }
-
-        let mut seen_sized_unbound = false;
+        let mut seen_unbound = false;
         for unbound in unbounds {
-            if let Some(sized_def_id) = sized_def_id
-                && unbound.trait_ref.path.res == Res::Def(DefKind::Trait, sized_def_id)
-            {
-                seen_sized_unbound = true;
-                continue;
+            let unbound_def_id = unbound.trait_ref.trait_def_id();
+            if unbound_def_id == Some(trait_def_id) {
+                seen_unbound = true;
             }
-            // There was a `?Trait` bound, but it was not `?Sized`; warn.
-            tcx.dcx().span_warn(
-                unbound.span,
-                "relaxing a default bound only does something for `?Sized`; \
-                all other traits are not bound by default",
-            );
-        }
 
-        if seen_sized_unbound || seen_negative_sized_bound || seen_positive_sized_bound {
-            // There was in fact a `?Sized`, `!Sized` or explicit `Sized` bound;
-            // we don't need to do anything.
-        } else if sized_def_id.is_some() {
-            // There was no `?Sized`, `!Sized` or explicit `Sized` bound;
-            // add `Sized` if it's available.
-            bounds.push_sized(tcx, self_ty, span);
+            let emit_relax_err = || {
+                tcx.dcx().span_warn(
+                    unbound.span,
+                    "relaxing a default bound only does something for `?Sized` and `default_auto_traits`; \
+                    all other traits are not bound by default",
+                );
+            };
+
+            match unbound_def_id {
+                Some(def_id) if !tcx.lang_items().is_default_trait(def_id) => emit_relax_err(),
+                None => emit_relax_err(),
+                _ => {}
+            }
         }
+
+        !(seen_unbound || seen_negative_bound || seen_positive_bound)
     }
 
     /// This helper takes a *converted* parameter type (`param_ty`)

compiler/rustc_hir_analysis/src/astconv/object_safety.rs

@@ -53,6 +53,14 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 potential_assoc_types.extend(cur_potential_assoc_types);
             }
         }
+        // FIXME: support polarity for trait objects
+        if tcx.features().default_auto_traits {
+            hir::lang_items::DEFAULT_TRAITS.iter().for_each(|default_trait| {
+                if *default_trait != hir::LangItem::Sized {
+                    bounds.push_lang_item_trait(tcx, dummy_self, *default_trait, span);
+                }
+            });
+        }
 
         let mut trait_bounds = vec![];
         let mut projection_bounds = vec![];

compiler/rustc_hir_analysis/src/bounds.rs

@@ -76,11 +76,24 @@ impl<'tcx> Bounds<'tcx> {
         ));
     }
 
-    pub fn push_sized(&mut self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) {
-        let sized_def_id = tcx.require_lang_item(LangItem::Sized, Some(span));
-        let trait_ref = ty::TraitRef::new(tcx, sized_def_id, [ty]);
-        // Preferable to put this obligation first, since we report better errors for sized ambiguity.
-        self.clauses.insert(0, (trait_ref.to_predicate(tcx), span));
+    pub fn push_lang_item_trait(
+        &mut self,
+        tcx: TyCtxt<'tcx>,
+        ty: Ty<'tcx>,
+        lang_item: LangItem,
+        span: Span,
+    ) {
+        assert_eq!(lang_item.target(), rustc_hir::Target::Trait);
+        let Some(trait_def_id) = tcx.lang_items().get(lang_item) else {
+            return;
+        };
+        let trait_ref = ty::TraitRef::new(tcx, trait_def_id, [ty]);
+        if lang_item == LangItem::Sized {
+            // Preferable to put this obligation first, since we report better errors for sized ambiguity.
+            self.clauses.insert(0, (trait_ref.to_predicate(tcx), span));
+        } else {
+            self.clauses.push((trait_ref.to_predicate(tcx), span));
+        }
     }
 
     pub fn clauses(&self) -> impl Iterator<Item = (ty::Clause<'tcx>, Span)> + '_ {

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -316,8 +316,8 @@ fn bounds_from_generic_predicates<'tcx>(
             ty::ClauseKind::Trait(trait_predicate) => {
                 let entry = types.entry(trait_predicate.self_ty()).or_default();
                 let def_id = trait_predicate.def_id();
-                if Some(def_id) != tcx.lang_items().sized_trait() {
-                    // Type params are `Sized` by default, do not add that restriction to the list
+                if !tcx.lang_items().is_default_trait(def_id) {
+                    // Do not add that restriction to the list
                     // if it is a positive requirement.
                     entry.push(trait_predicate.def_id());
                 }

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -29,7 +29,7 @@ fn associated_type_bounds<'tcx>(
     let icx = ItemCtxt::new(tcx, assoc_item_def_id);
     let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, PredicateFilter::All);
     // Associated types are implicitly sized unless a `?Sized` bound is found
-    icx.astconv().add_implicitly_sized(&mut bounds, item_ty, ast_bounds, None, span);
+    icx.astconv().add_implicit_traits(&mut bounds, item_ty, ast_bounds, None, span);
 
     let trait_def_id = tcx.local_parent(assoc_item_def_id);
     let trait_predicates = tcx.trait_explicit_predicates_and_bounds(trait_def_id);
@@ -68,7 +68,7 @@ fn opaque_type_bounds<'tcx>(
         let icx = ItemCtxt::new(tcx, opaque_def_id);
         let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, PredicateFilter::All);
         // Opaque types are implicitly sized unless a `?Sized` bound is found
-        icx.astconv().add_implicitly_sized(&mut bounds, item_ty, ast_bounds, None, span);
+        icx.astconv().add_implicit_traits(&mut bounds, item_ty, ast_bounds, None, span);
         debug!(?bounds);
 
         tcx.arena.alloc_from_iter(bounds.clauses())

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -146,7 +146,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
 
             ItemKind::Trait(_, _, generics, self_bounds, ..)
             | ItemKind::TraitAlias(generics, self_bounds) => {
-                is_trait = Some(self_bounds);
+                is_trait = Some((self_bounds, item.span));
                 generics
             }
             ItemKind::OpaqueTy(OpaqueTy { generics, .. }) => generics,
@@ -169,11 +169,27 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
     // on a trait we must also consider the bounds that follow the trait's name,
     // like `trait Foo: A + B + C`.
     if let Some(self_bounds) = is_trait {
-        predicates.extend(
-            icx.astconv()
-                .compute_bounds(tcx.types.self_param, self_bounds, PredicateFilter::All)
-                .clauses(),
-        );
+        let mut bounds =
+            icx.astconv().compute_bounds(tcx.types.self_param, self_bounds.0, PredicateFilter::All);
+
+        let sized_trait = tcx.lang_items().sized_trait();
+        let trait_def_id = def_id.to_def_id();
+        hir::lang_items::DEFAULT_TRAITS.iter().for_each(|default_trait| {
+            if *default_trait != hir::LangItem::Sized
+                && (Some(trait_def_id) == sized_trait
+                    || !tcx.lang_items().is_default_trait(def_id.to_def_id()))
+            {
+                icx.astconv().add_implicit_trait(
+                    *default_trait,
+                    &mut bounds,
+                    tcx.types.self_param,
+                    self_bounds.0,
+                    None,
+                    self_bounds.1,
+                );
+            }
+        });
+        predicates.extend(bounds.clauses());
     }
 
     // In default impls, we can assume that the self type implements
@@ -199,7 +215,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
                 let param_ty = icx.astconv().hir_id_to_bound_ty(param.hir_id);
                 let mut bounds = Bounds::default();
                 // Params are implicitly sized unless a `?Sized` bound is found
-                icx.astconv().add_implicitly_sized(
+                icx.astconv().add_implicit_traits(
                     &mut bounds,
                     param_ty,
                     &[],