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auto-impl: parser support #149335

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34 changes: 31 additions & 3 deletions compiler/rustc_ast/src/ast.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3562,6 +3562,7 @@ impl Item {
| ItemKind::DelegationMac(_)
| ItemKind::MacroDef(..) => None,
ItemKind::Static(_) => None,
ItemKind::AutoImpl(_) => None,
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@petrochenkov petrochenkov Dec 2, 2025

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There are generics in AutoImpl, but this returns None.

ItemKind::Const(i) => Some(&i.generics),
ItemKind::Fn(i) => Some(&i.generics),
ItemKind::TyAlias(i) => Some(&i.generics),
Expand Down Expand Up @@ -3705,6 +3706,14 @@ pub struct Impl {
pub items: ThinVec<Box<AssocItem>>,
}

#[derive(Clone, Encodable, Decodable, Debug)]
pub struct AutoImpl {
pub generics: Generics,
pub constness: Const,
pub of_trait: Box<TraitImplHeader>,
pub items: ThinVec<Box<AssocItem>>,
}

#[derive(Clone, Encodable, Decodable, Debug)]
pub struct TraitImplHeader {
pub defaultness: Defaultness,
Expand Down Expand Up @@ -3857,6 +3866,8 @@ pub enum ItemKind {
///
/// E.g., `impl<A> Foo<A> { .. }` or `impl<A> Trait for Foo<A> { .. }`.
Impl(Impl),
/// An `auto impl` implementation, such as a supertrait `auto impl`.
AutoImpl(Box<AutoImpl>),
/// A macro invocation.
///
/// E.g., `foo!(..)`.
Expand Down Expand Up @@ -3893,6 +3904,7 @@ impl ItemKind {
| ItemKind::ForeignMod(_)
| ItemKind::GlobalAsm(_)
| ItemKind::Impl(_)
| ItemKind::AutoImpl(_)
| ItemKind::MacCall(_)
| ItemKind::DelegationMac(_) => None,
}
Expand All @@ -3905,7 +3917,12 @@ impl ItemKind {
Use(..) | Static(..) | Const(..) | Fn(..) | Mod(..) | GlobalAsm(..) | TyAlias(..)
| Struct(..) | Union(..) | Trait(..) | TraitAlias(..) | MacroDef(..)
| Delegation(..) | DelegationMac(..) => "a",
ExternCrate(..) | ForeignMod(..) | MacCall(..) | Enum(..) | Impl { .. } => "an",
ExternCrate(..)
| ForeignMod(..)
| MacCall(..)
| Enum(..)
| Impl { .. }
| AutoImpl { .. } => "an",
}
}

Expand All @@ -3928,6 +3945,7 @@ impl ItemKind {
ItemKind::MacCall(..) => "item macro invocation",
ItemKind::MacroDef(..) => "macro definition",
ItemKind::Impl { .. } => "implementation",
ItemKind::AutoImpl { .. } => "`auto` implementation",
ItemKind::Delegation(..) => "delegated function",
ItemKind::DelegationMac(..) => "delegation",
}
Expand Down Expand Up @@ -3975,6 +3993,8 @@ pub enum AssocItemKind {
Delegation(Box<Delegation>),
/// An associated list or glob delegation item.
DelegationMac(Box<DelegationMac>),
/// An `auto impl` item
AutoImpl(Box<AutoImpl>),
}

impl AssocItemKind {
Expand All @@ -3985,15 +4005,21 @@ impl AssocItemKind {
| AssocItemKind::Type(box TyAlias { ident, .. })
| AssocItemKind::Delegation(box Delegation { ident, .. }) => Some(ident),

AssocItemKind::MacCall(_) | AssocItemKind::DelegationMac(_) => None,
AssocItemKind::MacCall(_)
| AssocItemKind::DelegationMac(_)
| AssocItemKind::AutoImpl(_) => None,
}
}

pub fn defaultness(&self) -> Defaultness {
match *self {
Self::Const(box ConstItem { defaultness, .. })
| Self::Fn(box Fn { defaultness, .. })
| Self::Type(box TyAlias { defaultness, .. }) => defaultness,
| Self::Type(box TyAlias { defaultness, .. })
| Self::AutoImpl(box AutoImpl {
of_trait: box TraitImplHeader { defaultness, .. },
..
}) => defaultness,
Self::MacCall(..) | Self::Delegation(..) | Self::DelegationMac(..) => {
Defaultness::Final
}
Expand All @@ -4010,6 +4036,7 @@ impl From<AssocItemKind> for ItemKind {
AssocItemKind::MacCall(a) => ItemKind::MacCall(a),
AssocItemKind::Delegation(delegation) => ItemKind::Delegation(delegation),
AssocItemKind::DelegationMac(delegation) => ItemKind::DelegationMac(delegation),
AssocItemKind::AutoImpl(ai) => ItemKind::AutoImpl(ai),
}
}
}
Expand All @@ -4025,6 +4052,7 @@ impl TryFrom<ItemKind> for AssocItemKind {
ItemKind::MacCall(a) => AssocItemKind::MacCall(a),
ItemKind::Delegation(d) => AssocItemKind::Delegation(d),
ItemKind::DelegationMac(d) => AssocItemKind::DelegationMac(d),
ItemKind::AutoImpl(ai) => AssocItemKind::AutoImpl(ai),
_ => return Err(item_kind),
})
}
Expand Down
16 changes: 16 additions & 0 deletions compiler/rustc_ast/src/visit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -551,6 +551,7 @@ macro_rules! common_visitor_and_walkers {
//fn visit_assoc_item(AssocItem, _ctxt: AssocCtxt);
fn visit_assoc_item_constraint(AssocItemConstraint);
fn visit_attribute(Attribute);
fn visit_auto_impl(AutoImpl);
fn visit_block(Block);
//fn visit_nested_use_tree((UseTree, NodeId));
fn visit_capture_by(CaptureBy);
Expand Down Expand Up @@ -836,6 +837,8 @@ macro_rules! common_visitor_and_walkers {
visit_visitable!($($mut)? vis, ident, generics, variant_data),
ItemKind::Impl(impl_) =>
visit_visitable!($($mut)? vis, impl_),
ItemKind::AutoImpl(auto_impl_) =>
visit_visitable!($($mut)? vis, auto_impl_),
ItemKind::Trait(trait_) =>
visit_visitable!($($mut)? vis, trait_),
ItemKind::TraitAlias(box TraitAlias { constness, ident, generics, bounds}) => {
Expand Down Expand Up @@ -881,6 +884,8 @@ macro_rules! common_visitor_and_walkers {
visit_visitable!($($mut)? vis, delegation),
AssocItemKind::DelegationMac(dm) =>
visit_visitable!($($mut)? vis, dm),
AssocItemKind::AutoImpl(ai) =>
visit_visitable!($($mut)? vis, ai),
}
V::Result::output()
}
Expand Down Expand Up @@ -945,6 +950,17 @@ macro_rules! common_visitor_and_walkers {
V::Result::output()
});

impl_walkable!(|&$($mut)? $($lt)? self: AutoImpl, vis: &mut V| {
let AutoImpl { generics, of_trait, items, constness } = self;
let TraitImplHeader { defaultness, safety, polarity, trait_ref } = &$($mut)? **of_trait;

try_visit!(vis.visit_generics(generics));
visit_visitable!($($mut)? vis, defaultness, safety, constness, polarity, trait_ref);

visit_visitable_with!($($mut)? vis, items, AssocCtxt::Impl { of_trait: true });
V::Result::output()
});

// Special case to call `visit_method_receiver_expr`.
impl_walkable!(|&$($mut)? $($lt)? self: MethodCall, vis: &mut V| {
let MethodCall { seg, receiver, args, span } = self;
Expand Down
45 changes: 45 additions & 0 deletions compiler/rustc_ast_lowering/src/item.rs
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Expand Up @@ -11,6 +11,7 @@ use rustc_hir::{
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::span_bug;
use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
use rustc_session::parse::feature_err;
use rustc_span::edit_distance::find_best_match_for_name;
use rustc_span::{DUMMY_SP, DesugaringKind, Ident, Span, Symbol, kw, sym};
use smallvec::{SmallVec, smallvec};
Expand Down Expand Up @@ -388,6 +389,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
constness,
})
}
ItemKind::AutoImpl(box AutoImpl { .. }) => todo!("we should implement lowering to HIR"),
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@petrochenkov petrochenkov Dec 3, 2025

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Could you report a regular error instead of an ICE here and for AssocItemKind::AutoImpl?
Upd: and in other crates too.

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@petrochenkov petrochenkov Dec 3, 2025

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Or report a fatal error in lowering and turn todo!s into unreachable!s in the later compilation stages.

ItemKind::Trait(box Trait {
constness,
is_auto,
Expand Down Expand Up @@ -887,6 +889,48 @@ impl<'hir> LoweringContext<'_, 'hir> {
true,
)
}
AssocItemKind::AutoImpl(ai) => {
let tcx = self.tcx;
if !tcx.features().supertrait_auto_impl() {
feature_err(
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@petrochenkov petrochenkov Dec 3, 2025

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Feature gating once in the parser should be enough.

&tcx.sess,
sym::supertrait_auto_impl,
i.span,
"feature is under construction",
)
.emit();
}
let generics = tcx.arena.alloc(hir::Generics {
has_where_clause_predicates: false,
params: &[],
predicates: &[],
where_clause_span: DUMMY_SP,
span: DUMMY_SP,
});
(
Ident::dummy(),
&*generics,
hir::TraitItemKind::AutoImpl(
tcx.arena.alloc(self.lower_poly_trait_ref_inner(
&ai.generics.params,
&TraitBoundModifiers {
constness: BoundConstness::Never,
asyncness: BoundAsyncness::Normal,
polarity: match ai.of_trait.polarity {
ImplPolarity::Positive => BoundPolarity::Positive,
ImplPolarity::Negative(span) => BoundPolarity::Negative(span),
},
},
&ai.of_trait.trait_ref,
ai.of_trait.trait_ref.path.span,
RelaxedBoundPolicy::Forbidden(RelaxedBoundForbiddenReason::SuperTrait),
ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
)),
&[],
),
false,
)
}
AssocItemKind::Type(box TyAlias {
ident,
generics,
Expand Down Expand Up @@ -1090,6 +1134,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
),
)
}
AssocItemKind::AutoImpl(_) => todo!(),
AssocItemKind::Delegation(box delegation) => {
let delegation_results = self.lower_delegation(delegation, i.id, is_in_trait_impl);
(
Expand Down
24 changes: 22 additions & 2 deletions compiler/rustc_ast_lowering/src/lib.rs
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Expand Up @@ -2074,21 +2074,41 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
PolyTraitRef { bound_generic_params, modifiers, trait_ref, span, parens: _ }: &PolyTraitRef,
rbp: RelaxedBoundPolicy<'_>,
itctx: ImplTraitContext,
) -> hir::PolyTraitRef<'hir> {
self.lower_poly_trait_ref_inner(
bound_generic_params,
modifiers,
trait_ref,
*span,
rbp,
itctx,
)
}

#[instrument(level = "debug", skip(self))]
fn lower_poly_trait_ref_inner(
&mut self,
bound_generic_params: &[GenericParam],
modifiers: &TraitBoundModifiers,
trait_ref: &TraitRef,
span: Span,
rbp: RelaxedBoundPolicy<'_>,
itctx: ImplTraitContext,
) -> hir::PolyTraitRef<'hir> {
let bound_generic_params =
self.lower_lifetime_binder(trait_ref.ref_id, bound_generic_params);
let trait_ref = self.lower_trait_ref(*modifiers, trait_ref, itctx);
let modifiers = self.lower_trait_bound_modifiers(*modifiers);

if let ast::BoundPolarity::Maybe(_) = modifiers.polarity {
self.validate_relaxed_bound(trait_ref, *span, rbp);
self.validate_relaxed_bound(trait_ref, span, rbp);
}

hir::PolyTraitRef {
bound_generic_params,
modifiers,
trait_ref,
span: self.lower_span(*span),
span: self.lower_span(span),
}
}

Expand Down
54 changes: 53 additions & 1 deletion compiler/rustc_ast_pretty/src/pprust/state/item.rs
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@@ -1,7 +1,7 @@
use ast::StaticItem;
use itertools::{Itertools, Position};
use rustc_ast::{self as ast, ModKind, TraitAlias};
use rustc_span::Ident;
use rustc_span::{Ident, Span};

use crate::pp::BoxMarker;
use crate::pp::Breaks::Inconsistent;
Expand Down Expand Up @@ -350,6 +350,32 @@ impl<'a> State<'a> {
let empty = item.attrs.is_empty() && items.is_empty();
self.bclose(item.span, empty, cb);
}
ast::ItemKind::AutoImpl(box ast::AutoImpl { generics, of_trait, items, constness }) => {
let (cb, ib) = self.head("");
self.print_visibility(&item.vis);

let &ast::TraitImplHeader { defaultness, safety, polarity, ref trait_ref } =
&**of_trait;
self.print_defaultness(defaultness);
self.print_safety(safety);
self.word("auto");
self.word("impl");
self.print_generic_params(&generics.params);
self.print_constness(*constness);
if let ast::ImplPolarity::Negative(_) = polarity {
self.word("!");
}
self.print_trait_ref(trait_ref);

self.space();
self.bopen(ib);
self.print_inner_attributes(&item.attrs);
for impl_item in items {
self.print_assoc_item(impl_item);
}
let empty = item.attrs.is_empty() && items.is_empty();
self.bclose(item.span, empty, cb);
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@petrochenkov petrochenkov Dec 2, 2025

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print_assoc_auto_impl was supposed to be reused here?

}
ast::ItemKind::Trait(box ast::Trait {
constness,
safety,
Expand Down Expand Up @@ -601,6 +627,9 @@ impl<'a> State<'a> {
self.word(";");
}
}
ast::AssocItemKind::AutoImpl(ai) => {
self.print_assoc_auto_impl(&item.attrs, ai, item.span)
}
ast::AssocItemKind::Delegation(deleg) => self.print_delegation(
&item.attrs,
vis,
Expand All @@ -621,6 +650,29 @@ impl<'a> State<'a> {
self.ann.post(self, AnnNode::SubItem(id))
}

fn print_assoc_auto_impl(&mut self, attrs: &[ast::Attribute], ai: &ast::AutoImpl, span: Span) {
let (cb, ib) = self.head("");
let &ast::TraitImplHeader { defaultness, safety, polarity, ref trait_ref } = &*ai.of_trait;
self.print_defaultness(defaultness);
self.print_safety(safety);
self.word("auto");
self.word("impl");
self.print_constness(ai.constness);
if let ast::ImplPolarity::Negative(_) = polarity {
self.word("!");
}
self.print_trait_ref(trait_ref);

self.space();
self.bopen(ib);
self.print_inner_attributes(&attrs);
for impl_item in &ai.items {
self.print_assoc_item(impl_item);
}
let empty = attrs.is_empty() && ai.items.is_empty();
self.bclose(span, empty, cb);
}

fn print_delegation(
&mut self,
attrs: &[ast::Attribute],
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_feature/src/unstable.rs
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Expand Up @@ -647,6 +647,8 @@ declare_features! (
(unstable, string_deref_patterns, "1.67.0", Some(87121)),
/// Allows `super let` statements.
(unstable, super_let, "1.88.0", Some(139076)),
/// Allows supertraits to be implemented with automatic defaults
(unstable, supertrait_auto_impl, "CURRENT_RUSTC_VERSION", Some(99999)),
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@petrochenkov petrochenkov Dec 2, 2025

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There's no tracking issue for the experiment?
Probably need a lang team approval and a tracking issue to start.

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@traviscross traviscross Dec 2, 2025 •
edited
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Thanks @petrochenkov for raising this. You're right; this should have a tracking issue documenting the lang experiment and its champion. Talked with @dingxiangfei2009, after he pinged me, and he's filing the tracking issue for this, which we'll nominate and discuss in a meeting.

/// Allows subtrait items to shadow supertrait items.
(unstable, supertrait_item_shadowing, "1.86.0", Some(89151)),
/// Allows the use of target_feature when a function is marked inline(always).
Expand Down
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