port #[feature] to the new attribute parsing infrastructure

Author: jdonszelmann

compiler/rustc_attr_parsing/messages.ftl

@@ -259,3 +259,7 @@ attr_parsing_whole_archive_needs_static =
 attr_parsing_limit_invalid =
     `limit` must be a non-negative integer
     .label = {$error_str}
+
+attr_parsing_feature_single_word =
+    rust features are always a single identifier, not paths with multiple segments
+    .help = did you maybe mean `{$first_segment}`?

compiler/rustc_attr_parsing/src/attributes/crate_level.rs

@@ -1,4 +1,5 @@
 use super::prelude::*;
+use crate::session_diagnostics::{FeatureExpectedSingleWord, LimitInvalid};
 
 pub(crate) struct CrateNameParser;
 
@@ -147,3 +148,58 @@ impl<S: Stage> NoArgsAttributeParser<S> for RustcCoherenceIsCoreParser {
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::CrateLevel;
     const CREATE: fn(Span) -> AttributeKind = AttributeKind::RustcCoherenceIsCore;
 }
+
+pub(crate) struct FeatureParser;
+
+impl<S: Stage> CombineAttributeParser<S> for FeatureParser {
+    const PATH: &[Symbol] = &[sym::feature];
+    type Item = Ident;
+    const CONVERT: ConvertFn<Self::Item> = AttributeKind::Feature;
+
+    // FIXME: recursion limit is allowed on all targets and ignored,
+    //        even though it should only be valid on crates of course
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(ALL_TARGETS);
+    const TEMPLATE: AttributeTemplate = template!(List: &["feature1, feature2, ..."]);
+
+    fn extend<'c>(
+        cx: &'c mut AcceptContext<'_, '_, S>,
+        args: &'c ArgParser<'_>,
+    ) -> impl IntoIterator<Item = Self::Item> + 'c {
+        let ArgParser::List(list) = args else {
+            cx.expected_list(cx.attr_span);
+            return Vec::new();
+        };
+
+        if list.is_empty() {
+            cx.warn_empty_attribute(cx.attr_span);
+        }
+
+        let mut res = Vec::new();
+
+        for elem in list.mixed() {
+            let Some(elem) = elem.meta_item() else {
+                cx.expected_identifier(elem.span());
+                continue;
+            };
+            if let Err(arg_span) = elem.args().no_args() {
+                cx.expected_no_args(arg_span);
+                continue;
+            }
+
+            let path = elem.path();
+            let Some(ident) = path.word() else {
+                let first_segment = elem.path().segments().next().expect("at least one segment");
+                cx.emit_err(FeatureExpectedSingleWord {
+                    span: path.span(),
+                    first_segment_span: first_segment.span,
+                    first_segment: first_segment.name,
+                });
+                continue;
+            };
+
+            res.push(ident);
+        }
+
+        res
+    }
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -25,8 +25,9 @@ use crate::attributes::codegen_attrs::{
 };
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::crate_level::{
-    CrateNameParser, MoveSizeLimitParser, NoCoreParser, NoStdParser, PatternComplexityLimitParser,
-    RecursionLimitParser, RustcCoherenceIsCoreParser, TypeLengthLimitParser,
+    CrateNameParser, FeatureParser, MoveSizeLimitParser, NoCoreParser, NoStdParser,
+    PatternComplexityLimitParser, RecursionLimitParser, RustcCoherenceIsCoreParser,
+    TypeLengthLimitParser,
 };
 use crate::attributes::debugger::DebuggerViualizerParser;
 use crate::attributes::deprecation::DeprecationParser;
@@ -165,6 +166,7 @@ attribute_parsers!(
         Combine<AllowConstFnUnstableParser>,
         Combine<AllowInternalUnstableParser>,
         Combine<DebuggerViualizerParser>,
+        Combine<FeatureParser>,
         Combine<ForceTargetFeatureParser>,
         Combine<LinkParser>,
         Combine<ReprParser>,

compiler/rustc_attr_parsing/src/session_diagnostics.rs

@@ -968,3 +968,14 @@ pub(crate) struct LimitInvalid<'a> {
     pub value_span: Span,
     pub error_str: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(attr_parsing_feature_single_word)]
+pub(crate) struct FeatureExpectedSingleWord {
+    #[primary_span]
+    pub span: Span,
+
+    #[help]
+    pub first_segment_span: Span,
+    pub first_segment: Symbol,
+}

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -528,6 +528,9 @@ pub enum AttributeKind {
     /// Represents `#[export_stable]`.
     ExportStable,
 
+    /// Represents `#[feature(...)]`
+    Feature(ThinVec<Ident>, Span),
+
     /// Represents `#[ffi_const]`.
     FfiConst(Span),
 

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -45,6 +45,7 @@ impl AttributeKind {
             Dummy => No,
             ExportName { .. } => Yes,
             ExportStable => No,
+            Feature(..) => No,
             FfiConst(..) => No,
             FfiPure(..) => No,
             Fundamental { .. } => Yes,

compiler/rustc_passes/src/check_attr.rs

@@ -283,6 +283,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | AttributeKind::ObjcSelector { .. }
                     | AttributeKind::RustcCoherenceIsCore(..)
                     | AttributeKind::DebuggerVisualizer(..)
+                    | AttributeKind::Feature(..)
                 ) => { /* do nothing  */ }
                 Attribute::Unparsed(attr_item) => {
                     style = Some(attr_item.style);
@@ -1861,75 +1862,82 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     fn check_unused_attribute(&self, hir_id: HirId, attr: &Attribute, style: Option<AttrStyle>) {
         // Warn on useless empty attributes.
         // FIXME(jdonszelmann): this lint should be moved to attribute parsing, see `AcceptContext::warn_empty_attribute`
-        let note = if attr.has_any_name(&[
-            sym::allow,
-            sym::expect,
-            sym::warn,
-            sym::deny,
-            sym::forbid,
-            sym::feature,
-        ]) && attr.meta_item_list().is_some_and(|list| list.is_empty())
-        {
-            errors::UnusedNote::EmptyList { name: attr.name().unwrap() }
-        } else if attr.has_any_name(&[sym::allow, sym::warn, sym::deny, sym::forbid, sym::expect])
-            && let Some(meta) = attr.meta_item_list()
-            && let [meta] = meta.as_slice()
-            && let Some(item) = meta.meta_item()
-            && let MetaItemKind::NameValue(_) = &item.kind
-            && item.path == sym::reason
-        {
-            errors::UnusedNote::NoLints { name: attr.name().unwrap() }
-        } else if attr.has_any_name(&[sym::allow, sym::warn, sym::deny, sym::forbid, sym::expect])
-            && let Some(meta) = attr.meta_item_list()
-            && meta.iter().any(|meta| {
-                meta.meta_item().map_or(false, |item| item.path == sym::linker_messages)
-            })
-        {
-            if hir_id != CRATE_HIR_ID {
-                match style {
-                    Some(ast::AttrStyle::Outer) => {
-                        let attr_span = attr.span();
-                        let bang_position = self
-                            .tcx
-                            .sess
-                            .source_map()
-                            .span_until_char(attr_span, '[')
-                            .shrink_to_hi();
-
-                        self.tcx.emit_node_span_lint(
+        let note =
+            if attr.has_any_name(&[sym::allow, sym::expect, sym::warn, sym::deny, sym::forbid])
+                && attr.meta_item_list().is_some_and(|list| list.is_empty())
+            {
+                errors::UnusedNote::EmptyList { name: attr.name().unwrap() }
+            } else if attr.has_any_name(&[
+                sym::allow,
+                sym::warn,
+                sym::deny,
+                sym::forbid,
+                sym::expect,
+            ]) && let Some(meta) = attr.meta_item_list()
+                && let [meta] = meta.as_slice()
+                && let Some(item) = meta.meta_item()
+                && let MetaItemKind::NameValue(_) = &item.kind
+                && item.path == sym::reason
+            {
+                errors::UnusedNote::NoLints { name: attr.name().unwrap() }
+            } else if attr.has_any_name(&[
+                sym::allow,
+                sym::warn,
+                sym::deny,
+                sym::forbid,
+                sym::expect,
+            ]) && let Some(meta) = attr.meta_item_list()
+                && meta.iter().any(|meta| {
+                    meta.meta_item().map_or(false, |item| item.path == sym::linker_messages)
+                })
+            {
+                if hir_id != CRATE_HIR_ID {
+                    match style {
+                        Some(ast::AttrStyle::Outer) => {
+                            let attr_span = attr.span();
+                            let bang_position = self
+                                .tcx
+                                .sess
+                                .source_map()
+                                .span_until_char(attr_span, '[')
+                                .shrink_to_hi();
+
+                            self.tcx.emit_node_span_lint(
+                                UNUSED_ATTRIBUTES,
+                                hir_id,
+                                attr_span,
+                                errors::OuterCrateLevelAttr {
+                                    suggestion: errors::OuterCrateLevelAttrSuggestion {
+                                        bang_position,
+                                    },
+                                },
+                            )
+                        }
+                        Some(ast::AttrStyle::Inner) | None => self.tcx.emit_node_span_lint(
                             UNUSED_ATTRIBUTES,
                             hir_id,
-                            attr_span,
-                            errors::OuterCrateLevelAttr {
-                                suggestion: errors::OuterCrateLevelAttrSuggestion { bang_position },
-                            },
-                        )
-                    }
-                    Some(ast::AttrStyle::Inner) | None => self.tcx.emit_node_span_lint(
-                        UNUSED_ATTRIBUTES,
-                        hir_id,
-                        attr.span(),
-                        errors::InnerCrateLevelAttr,
-                    ),
-                };
-                return;
-            } else {
-                let never_needs_link = self
-                    .tcx
-                    .crate_types()
-                    .iter()
-                    .all(|kind| matches!(kind, CrateType::Rlib | CrateType::Staticlib));
-                if never_needs_link {
-                    errors::UnusedNote::LinkerMessagesBinaryCrateOnly
-                } else {
+                            attr.span(),
+                            errors::InnerCrateLevelAttr,
+                        ),
+                    };
                     return;
+                } else {
+                    let never_needs_link = self
+                        .tcx
+                        .crate_types()
+                        .iter()
+                        .all(|kind| matches!(kind, CrateType::Rlib | CrateType::Staticlib));
+                    if never_needs_link {
+                        errors::UnusedNote::LinkerMessagesBinaryCrateOnly
+                    } else {
+                        return;
+                    }
                 }
-            }
-        } else if attr.has_name(sym::default_method_body_is_const) {
-            errors::UnusedNote::DefaultMethodBodyConst
-        } else {
-            return;
-        };
+            } else if attr.has_name(sym::default_method_body_is_const) {
+                errors::UnusedNote::DefaultMethodBodyConst
+            } else {
+                return;
+            };
 
         self.tcx.emit_node_span_lint(
             UNUSED_ATTRIBUTES,