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Add gated const item paths support for selected builtin attributes. #154708

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74951ce
rustc_attr_parsing: allow const item paths in selected builtin attrs
BarronKane Apr 2, 2026
c214df9
fix: tidy use sorting, rustc version, and issue attribution
BarronKane Apr 2, 2026
4176d12
fix: const-attr-paths need .stderr issue tracking
BarronKane Apr 2, 2026
0a02f2d
fix: const-attr-paths hir pretty printing
BarronKane Apr 2, 2026
59783c3
refactor: more general terminology and use existing attr parse machinery
BarronKane Apr 3, 2026
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23 changes: 22 additions & 1 deletion compiler/rustc_ast_lowering/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -42,12 +42,13 @@ use rustc_ast::node_id::NodeMap;
use rustc_ast::{self as ast, *};
use rustc_attr_parsing::{AttributeParser, Late, OmitDoc};
use rustc_data_structures::fingerprint::Fingerprint;
use rustc_data_structures::fx::FxIndexSet;
use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
use rustc_data_structures::sorted_map::SortedMap;
use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_data_structures::steal::Steal;
use rustc_data_structures::tagged_ptr::TaggedRef;
use rustc_errors::{DiagArgFromDisplay, DiagCtxtHandle};
use rustc_hir::attrs::AttrResolution;
use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
use rustc_hir::def_id::{CRATE_DEF_ID, LOCAL_CRATE, LocalDefId};
use rustc_hir::definitions::{DefPathData, DisambiguatorState};
Expand Down Expand Up @@ -155,6 +156,7 @@ struct LoweringContext<'a, 'hir, R> {
impl<'a, 'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'a, 'hir, R> {
fn new(tcx: TyCtxt<'hir>, resolver: &'a mut R) -> Self {
let registered_tools = tcx.registered_tools(());
let attr_res_map = resolver.all_attr_resolutions();
Self {
tcx,
resolver,
Expand Down Expand Up @@ -209,6 +211,7 @@ impl<'a, 'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'a, 'hir, R> {
tcx.sess,
tcx.features(),
registered_tools,
attr_res_map,
Late,
),
delayed_lints: Vec::new(),
Expand Down Expand Up @@ -239,6 +242,7 @@ impl SpanLowerer {
struct ResolverDelayedAstLowering<'a, 'tcx> {
node_id_to_def_id: NodeMap<LocalDefId>,
partial_res_map: NodeMap<PartialRes>,
attr_res_map: FxIndexMap<rustc_span::AttrId, Vec<AttrResolution<ast::NodeId>>>,
next_node_id: NodeId,
base: &'a ResolverAstLowering<'tcx>,
}
Expand All @@ -253,6 +257,16 @@ impl<'a, 'tcx> ResolverAstLoweringExt<'tcx> for ResolverDelayedAstLowering<'a, '
self.partial_res_map.get(&id).copied().or_else(|| self.base.get_partial_res(id))
}

fn all_attr_resolutions(
&self,
) -> FxIndexMap<rustc_span::AttrId, Vec<AttrResolution<ast::NodeId>>> {
let mut map = self.base.all_attr_resolutions();
for (attr_id, resolutions) in &self.attr_res_map {
map.entry(*attr_id).or_default().extend(resolutions.iter().copied());
}
map
}

fn get_import_res(&self, id: NodeId) -> PerNS<Option<Res<NodeId>>> {
self.base.get_import_res(id)
}
Expand Down Expand Up @@ -345,6 +359,12 @@ impl<'tcx> ResolverAstLowering<'tcx> {
self.partial_res_map.get(&id).copied()
}

fn all_attr_resolutions(
&self,
) -> FxIndexMap<rustc_span::AttrId, Vec<AttrResolution<ast::NodeId>>> {
self.attr_res_map.clone()
}

/// Obtains per-namespace resolutions for `use` statement with the given `NodeId`.
fn get_import_res(&self, id: NodeId) -> PerNS<Option<Res<NodeId>>> {
self.import_res_map.get(&id).copied().unwrap_or_default()
Expand Down Expand Up @@ -663,6 +683,7 @@ pub fn lower_delayed_owner(tcx: TyCtxt<'_>, def_id: LocalDefId) {
let mut resolver = ResolverDelayedAstLowering {
next_node_id: resolver.next_node_id,
partial_res_map: Default::default(),
attr_res_map: Default::default(),
node_id_to_def_id: Default::default(),
base: resolver,
};
Expand Down
148 changes: 104 additions & 44 deletions compiler/rustc_attr_parsing/src/attributes/repr.rs
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Original file line number Diff line number Diff line change
@@ -1,9 +1,14 @@
use rustc_abi::{Align, Size};
use rustc_ast::{IntTy, LitIntType, LitKind, UintTy};
use rustc_hir::attrs::{IntType, ReprAttr};
use rustc_hir::attrs::{AttrIntValue, AttrResolutionKind, AttrResolved, IntType, ReprAttr};
use rustc_hir::def::{DefKind, Res};
use rustc_session::parse::feature_err;

use super::prelude::*;
use crate::session_diagnostics::{self, IncorrectReprFormatGenericCause};
use crate::ShouldEmit;
use crate::session_diagnostics::{
self, AttrConstGenericNotSupported, AttrConstPathNotConst, IncorrectReprFormatGenericCause,
};

/// Parse #[repr(...)] forms.
///
Expand Down Expand Up @@ -100,7 +105,10 @@ fn int_type_of_word(s: Symbol) -> Option<IntType> {
}
}

fn parse_repr<S: Stage>(cx: &AcceptContext<'_, '_, S>, param: &MetaItemParser) -> Option<ReprAttr> {
fn parse_repr<S: Stage>(
cx: &mut AcceptContext<'_, '_, S>,
param: &MetaItemParser,
) -> Option<ReprAttr> {
use ReprAttr::*;

// FIXME(jdonszelmann): invert the parsing here to match on the word first and then the
Expand All @@ -122,7 +130,7 @@ fn parse_repr<S: Stage>(cx: &AcceptContext<'_, '_, S>, param: &MetaItemParser) -
parse_repr_align(cx, l, param.span(), AlignKind::Align)
}

(Some(sym::packed), ArgParser::NoArgs) => Some(ReprPacked(Align::ONE)),
(Some(sym::packed), ArgParser::NoArgs) => Some(ReprPacked(AttrIntValue::Lit(1))),
(Some(sym::packed), ArgParser::List(l)) => {
parse_repr_align(cx, l, param.span(), AlignKind::Packed)
}
Expand Down Expand Up @@ -189,8 +197,13 @@ enum AlignKind {
Align,
}

enum AlignmentParseError {
Message(String),
AlreadyErrored,
}

fn parse_repr_align<S: Stage>(
cx: &AcceptContext<'_, '_, S>,
cx: &mut AcceptContext<'_, '_, S>,
list: &MetaItemListParser,
param_span: Span,
align_kind: AlignKind,
Expand All @@ -214,31 +227,21 @@ fn parse_repr_align<S: Stage>(
return None;
};

let Some(lit) = align.lit() else {
match parse_alignment_or_const_path(
cx,
align,
match align_kind {
Packed => {
cx.emit_err(session_diagnostics::IncorrectReprFormatPackedExpectInteger {
span: align.span(),
});
}
Align => {
cx.emit_err(session_diagnostics::IncorrectReprFormatExpectInteger {
span: align.span(),
});
}
}

return None;
};

match parse_alignment(&lit.kind, cx) {
Ok(literal) => Some(match align_kind {
AlignKind::Packed => ReprAttr::ReprPacked(literal),
AlignKind::Align => ReprAttr::ReprAlign(literal),
Packed => "repr(packed)",
Align => "repr(align)",
},
) {
Ok(value) => Some(match align_kind {
AlignKind::Packed => ReprAttr::ReprPacked(value),
AlignKind::Align => ReprAttr::ReprAlign(value),
}),
Err(message) => {
Err(AlignmentParseError::Message(message)) => {
cx.emit_err(session_diagnostics::InvalidReprGeneric {
span: lit.span,
span: align.span(),
repr_arg: match align_kind {
Packed => "packed".to_string(),
Align => "align".to_string(),
Expand All @@ -247,6 +250,7 @@ fn parse_repr_align<S: Stage>(
});
None
}
Err(AlignmentParseError::AlreadyErrored) => None,
}
}

Expand Down Expand Up @@ -281,9 +285,74 @@ fn parse_alignment<S: Stage>(
Ok(align)
}

fn parse_alignment_or_const_path<S: Stage>(
cx: &mut AcceptContext<'_, '_, S>,
arg: &MetaItemOrLitParser,
attr_name: &'static str,
) -> Result<AttrIntValue, AlignmentParseError> {
if let Some(lit) = arg.lit() {
return parse_alignment(&lit.kind, cx)
.map(|align| AttrIntValue::Lit(u128::from(align.bytes())))
.map_err(AlignmentParseError::Message);
}

let Some(meta) = arg.meta_item() else {
return Err(AlignmentParseError::Message("not an unsuffixed integer".to_string()));
};

if !matches!(meta.args(), ArgParser::NoArgs) {
return Err(AlignmentParseError::Message("not an unsuffixed integer".to_string()));
}

let path_span = meta.path().span();
let feature_enabled = cx.features_option().is_some_and(|features| features.const_attr_paths())
|| path_span.allows_unstable(sym::const_attr_paths);

if !feature_enabled {
if matches!(cx.stage.should_emit(), ShouldEmit::Nothing) {
return Ok(AttrIntValue::Lit(1));
}

feature_err(
cx.sess(),
sym::const_attr_paths,
meta.span(),
"const item paths in builtin attributes are experimental",
)
.emit();
return Err(AlignmentParseError::AlreadyErrored);
}

cx.record_attr_resolution_request(AttrResolutionKind::Const, meta.path().0.clone());

let Some(resolution) = cx.attr_resolution(AttrResolutionKind::Const, path_span) else {
// `parse_limited(sym::repr)` runs before lowering for callers that only care whether
// `repr(packed(...))` exists at all.
if matches!(cx.stage.should_emit(), ShouldEmit::Nothing) {
return Ok(AttrIntValue::Lit(1));
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@BarronKane BarronKane Apr 7, 2026

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For who does end up reviewing this PR, I'm taking another look at things, and I want to note that in fn parse_alignment_or_const_path<S: Stage>, I'm returning 1 on ShouldEmit::Nothing. This was a deliberate decision because repr(packed) itself returns 1 by default elsewhere, and is effectively no-op otherwise, but part of not being obtrusive to the codebase I shimmed it in there in order to avoid a heavier refactor of repr.rs to make it typed. Categorically it is fake data, but I think it's defensible from this stance. If the lang and compiler teams want to go this direction of resolutions within attributes, this should be pulled out for a better solution so future work doesn't trip over it. Deferred state should be represented properly, so I wanted to note this now.

}
return Err(AlignmentParseError::Message("not an unsuffixed integer".to_string()));
};

match resolution {
AttrResolved::Resolved(Res::Def(DefKind::Const { .. }, def_id)) => {
Ok(AttrIntValue::Const { def_id, span: path_span })
}
AttrResolved::Resolved(Res::Def(DefKind::ConstParam, _)) => {
cx.emit_err(AttrConstGenericNotSupported { span: path_span, attr_name });
Err(AlignmentParseError::AlreadyErrored)
}
AttrResolved::Resolved(res) => {
cx.emit_err(AttrConstPathNotConst { span: path_span, attr_name, thing: res.descr() });
Err(AlignmentParseError::AlreadyErrored)
}
AttrResolved::Error => Err(AlignmentParseError::AlreadyErrored),
}
}

/// Parse #[align(N)].
#[derive(Default)]
pub(crate) struct RustcAlignParser(Option<(Align, Span)>);
pub(crate) struct RustcAlignParser(ThinVec<(AttrIntValue, Span)>);

impl RustcAlignParser {
const PATH: &[Symbol] = &[sym::rustc_align];
Expand All @@ -301,22 +370,15 @@ impl RustcAlignParser {
return;
};

let Some(lit) = align.lit() else {
cx.emit_err(session_diagnostics::IncorrectReprFormatExpectInteger {
span: align.span(),
});

return;
};

match parse_alignment(&lit.kind, cx) {
Ok(literal) => self.0 = Ord::max(self.0, Some((literal, cx.attr_span))),
Err(message) => {
match parse_alignment_or_const_path(cx, align, "rustc_align") {
Ok(literal) => self.0.push((literal, cx.attr_span)),
Err(AlignmentParseError::Message(message)) => {
cx.emit_err(session_diagnostics::InvalidAlignmentValue {
span: lit.span,
span: align.span(),
error_part: message,
});
}
Err(AlignmentParseError::AlreadyErrored) => {}
}
}
}
Expand All @@ -335,8 +397,7 @@ impl<S: Stage> AttributeParser<S> for RustcAlignParser {
]);

fn finalize(self, _cx: &FinalizeContext<'_, '_, S>) -> Option<AttributeKind> {
let (align, span) = self.0?;
Some(AttributeKind::RustcAlign { align, span })
(!self.0.is_empty()).then_some(AttributeKind::RustcAlign { aligns: self.0 })
}
}

Expand All @@ -358,7 +419,6 @@ impl<S: Stage> AttributeParser<S> for RustcAlignStaticParser {
AllowedTargets::AllowList(&[Allow(Target::Static), Allow(Target::ForeignStatic)]);

fn finalize(self, _cx: &FinalizeContext<'_, '_, S>) -> Option<AttributeKind> {
let (align, span) = self.0.0?;
Some(AttributeKind::RustcAlign { align, span })
(!self.0.0.is_empty()).then_some(AttributeKind::RustcAlign { aligns: self.0.0 })
}
}
21 changes: 20 additions & 1 deletion compiler/rustc_attr_parsing/src/context.rs
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Expand Up @@ -9,7 +9,7 @@ use private::Sealed;
use rustc_ast::{AttrStyle, MetaItemLit, NodeId};
use rustc_errors::{Diag, Diagnostic, Level};
use rustc_feature::{AttrSuggestionStyle, AttributeTemplate};
use rustc_hir::attrs::AttributeKind;
use rustc_hir::attrs::{AttrResolutionKind, AttributeKind};
use rustc_hir::lints::AttributeLintKind;
use rustc_hir::{AttrPath, HirId};
use rustc_parse::parser::Recovery;
Expand Down Expand Up @@ -502,6 +502,25 @@ impl<'f, 'sess: 'f, S: Stage> AcceptContext<'f, 'sess, S> {
pub(crate) fn adcx(&mut self) -> AttributeDiagnosticContext<'_, 'f, 'sess, S> {
AttributeDiagnosticContext { ctx: self, custom_suggestions: Vec::new() }
}

pub(crate) fn attr_resolution(
&self,
kind: AttrResolutionKind,
path_span: Span,
) -> Option<rustc_hir::attrs::AttrResolved<NodeId>> {
self.shared.cx.attr_resolution(self.attr_id, kind, path_span)
}

pub(crate) fn record_attr_resolution_request(
&mut self,
kind: AttrResolutionKind,
path: rustc_ast::Path,
) {
self.shared.cx.record_attr_resolution_request(
self.attr_id,
crate::interface::AttrResolutionRequest { kind, path },
);
}
}

impl<'f, 'sess, S: Stage> Deref for AcceptContext<'f, 'sess, S> {
Expand Down
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