rustc_attr_parsing: allow const item paths in selected builtin attrs

Add gated support for const item paths in selected builtin attributes,
including `repr(align)`, `repr(packed)`, `rustc_align`, and
`rustc_align_static`.

Resolve attribute const paths during late resolution, carry them through
HIR as attr int values, and evaluate them at the layout/codegen use
sites. This keeps the feature scoped to const item paths without adding
general expression support in attributes.

Also adds UI/codegen/incremental coverage and fixes the gated-syntax
diagnostics so unresolved names do not leak before the feature gate.

Author: BarronKane

compiler/rustc_ast_lowering/src/lib.rs

@@ -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::AttrConstResolution;
 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};
@@ -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(()).iter().map(|x| x.name).collect();
+        let attr_const_res_map = resolver.all_attr_const_resolutions();
         Self {
             tcx,
             resolver,
@@ -209,6 +211,7 @@ impl<'a, 'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'a, 'hir, R> {
                 tcx.sess,
                 tcx.features(),
                 registered_tools,
+                attr_const_res_map,
                 Late,
             ),
             delayed_lints: Vec::new(),
@@ -239,6 +242,7 @@ impl SpanLowerer {
 struct ResolverDelayedAstLowering<'a, 'tcx> {
     node_id_to_def_id: NodeMap<LocalDefId>,
     partial_res_map: NodeMap<PartialRes>,
+    attr_const_res_map: FxIndexMap<rustc_span::AttrId, Vec<AttrConstResolution<ast::NodeId>>>,
     next_node_id: NodeId,
     base: &'a ResolverAstLowering<'tcx>,
 }
@@ -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_const_resolutions(
+        &self,
+    ) -> FxIndexMap<rustc_span::AttrId, Vec<AttrConstResolution<ast::NodeId>>> {
+        let mut map = self.base.all_attr_const_resolutions();
+        for (attr_id, resolutions) in &self.attr_const_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)
     }
@@ -345,6 +359,12 @@ impl<'tcx> ResolverAstLowering<'tcx> {
         self.partial_res_map.get(&id).copied()
     }
 
+    fn all_attr_const_resolutions(
+        &self,
+    ) -> FxIndexMap<rustc_span::AttrId, Vec<AttrConstResolution<ast::NodeId>>> {
+        self.attr_const_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()
@@ -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_const_res_map: Default::default(),
         node_id_to_def_id: Default::default(),
         base: resolver,
     };

compiler/rustc_attr_parsing/src/attributes/repr.rs

@@ -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::{AttrConstResolved, AttrIntValue, 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.
 ///
@@ -122,7 +127,7 @@ 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)
         }
@@ -189,6 +194,11 @@ enum AlignKind {
     Align,
 }
 
+enum AlignmentParseError {
+    Message(String),
+    AlreadyErrored,
+}
+
 fn parse_repr_align<S: Stage>(
     cx: &AcceptContext<'_, '_, S>,
     list: &MetaItemListParser,
@@ -214,31 +224,21 @@ 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(),
@@ -247,6 +247,7 @@ fn parse_repr_align<S: Stage>(
             });
             None
         }
+        Err(AlignmentParseError::AlreadyErrored) => None,
     }
 }
 
@@ -281,9 +282,71 @@ fn parse_alignment<S: Stage>(
     Ok(align)
 }
 
+fn parse_alignment_or_const_path<S: Stage>(
+    cx: &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()));
+    }
+
+    if let Some(features) = cx.features_option()
+        && !features.const_attr_paths()
+        && !meta.span().allows_unstable(sym::const_attr_paths)
+    {
+        feature_err(
+            cx.sess(),
+            sym::const_attr_paths,
+            meta.span(),
+            "const item paths in builtin attributes are experimental",
+        )
+        .emit();
+        return Err(AlignmentParseError::AlreadyErrored);
+    }
+
+    let Some(resolution) = cx.attr_const_resolution(meta.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));
+        }
+        return Err(AlignmentParseError::Message("not an unsuffixed integer".to_string()));
+    };
+
+    match resolution {
+        AttrConstResolved::Resolved(Res::Def(DefKind::Const { .. }, def_id)) => {
+            Ok(AttrIntValue::Const { def_id, span: meta.path().span() })
+        }
+        AttrConstResolved::Resolved(Res::Def(DefKind::ConstParam, _)) => {
+            cx.emit_err(AttrConstGenericNotSupported { span: meta.path().span(), attr_name });
+            Err(AlignmentParseError::AlreadyErrored)
+        }
+        AttrConstResolved::Resolved(res) => {
+            cx.emit_err(AttrConstPathNotConst {
+                span: meta.path().span(),
+                attr_name,
+                thing: res.descr(),
+            });
+            Err(AlignmentParseError::AlreadyErrored)
+        }
+        AttrConstResolved::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];
@@ -301,22 +364,15 @@ 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) => {}
                 }
             }
         }
@@ -335,8 +391,7 @@ 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 })
     }
 }
 
@@ -358,7 +413,6 @@ 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 })
     }
 }

compiler/rustc_attr_parsing/src/context.rs

@@ -15,7 +15,7 @@ use rustc_hir::{AttrPath, HirId};
 use rustc_parse::parser::Recovery;
 use rustc_session::Session;
 use rustc_session::lint::{Lint, LintId};
-use rustc_span::{ErrorGuaranteed, Span, Symbol};
+use rustc_span::{AttrId, ErrorGuaranteed, Span, Symbol};
 
 use crate::AttributeParser;
 // Glob imports to avoid big, bitrotty import lists
@@ -414,6 +414,7 @@ pub struct Late;
 /// Gives [`AttributeParser`]s enough information to create errors, for example.
 pub struct AcceptContext<'f, 'sess, S: Stage> {
     pub(crate) shared: SharedContext<'f, 'sess, S>,
+    pub(crate) attr_id: Option<AttrId>,
 
     /// The outer span of the attribute currently being parsed
     ///
@@ -497,6 +498,13 @@ impl<'f, 'sess: 'f, S: Stage> SharedContext<'f, 'sess, S> {
 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_const_resolution(
+        &self,
+        path_span: Span,
+    ) -> Option<rustc_hir::attrs::AttrConstResolved<NodeId>> {
+        self.attr_id.and_then(|attr_id| self.shared.cx.attr_const_resolution(attr_id, path_span))
+    }
     }
 }