Port #[may_dangle] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -191,6 +191,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_macro_transparency]`.
     MacroTransparency(Transparency),
 
+    /// Represents [`#[may_dangle]`](https://std-dev-guide.rust-lang.org/tricky/may-dangle.html).
+    MayDangle(Span),
+
     /// Represents [`#[repr]`](https://doc.rust-lang.org/stable/reference/type-layout.html#representations).
     Repr(ThinVec<(ReprAttr, Span)>),
 

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -30,6 +30,7 @@ pub(crate) mod cfg;
 pub(crate) mod confusables;
 pub(crate) mod deprecation;
 pub(crate) mod repr;
+pub(crate) mod semantics;
 pub(crate) mod stability;
 pub(crate) mod transparency;
 pub(crate) mod util;

compiler/rustc_attr_parsing/src/attributes/semantics.rs

@@ -0,0 +1,20 @@
+use rustc_attr_data_structures::AttributeKind;
+use rustc_span::{Symbol, sym};
+
+use crate::attributes::{AttributeOrder, OnDuplicate, SingleAttributeParser};
+use crate::context::{AcceptContext, Stage};
+use crate::parser::ArgParser;
+
+pub(crate) struct MayDangleParser;
+impl<S: Stage> SingleAttributeParser<S> for MayDangleParser {
+    const PATH: &[Symbol] = &[sym::may_dangle];
+
+    const ATTRIBUTE_ORDER: AttributeOrder = AttributeOrder::KeepFirst;
+
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Warn;
+
+    fn convert(cx: &mut AcceptContext<'_, '_, S>, _args: &ArgParser<'_>) -> Option<AttributeKind> {
+        // FIXME: check that there's no args (this is currently checked elsewhere)
+        Some(AttributeKind::MayDangle(cx.attr_span))
+    }
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -19,6 +19,7 @@ use crate::attributes::allow_unstable::{AllowConstFnUnstableParser, AllowInterna
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::deprecation::DeprecationParser;
 use crate::attributes::repr::ReprParser;
+use crate::attributes::semantics::MayDangleParser;
 use crate::attributes::stability::{
     BodyStabilityParser, ConstStabilityIndirectParser, ConstStabilityParser, StabilityParser,
 };
@@ -104,6 +105,7 @@ attribute_parsers!(
         // tidy-alphabetical-start
         Single<ConstStabilityIndirectParser>,
         Single<DeprecationParser>,
+        Single<MayDangleParser>,
         Single<TransparencyParser>,
         // tidy-alphabetical-end
     ];

compiler/rustc_hir/src/hir.rs

@@ -1302,6 +1302,7 @@ impl AttributeExt for Attribute {
             // FIXME: should not be needed anymore when all attrs are parsed
             Attribute::Parsed(AttributeKind::Deprecation { span, .. }) => *span,
             Attribute::Parsed(AttributeKind::DocComment { span, .. }) => *span,
+            Attribute::Parsed(AttributeKind::MayDangle(span)) => *span,
             a => panic!("can't get the span of an arbitrary parsed attribute: {a:?}"),
         }
     }

compiler/rustc_passes/src/check_attr.rs

@@ -147,6 +147,27 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | AttributeKind::ConstStabilityIndirect
                     | AttributeKind::MacroTransparency(_),
                 ) => { /* do nothing  */ }
+                &Attribute::Parsed(AttributeKind::MayDangle(attr_span)) => {
+                    // Check if `#[may_dangle]` is applied to a lifetime or type generic parameter in `Drop` impl.
+                    if let hir::Node::GenericParam(param) = self.tcx.hir_node(hir_id)
+                        && matches!(
+                            param.kind,
+                            hir::GenericParamKind::Lifetime { .. }
+                                | hir::GenericParamKind::Type { .. }
+                        )
+                        && matches!(param.source, hir::GenericParamSource::Generics)
+                        && let parent_hir_id = self.tcx.parent_hir_id(hir_id)
+                        && let hir::Node::Item(item) = self.tcx.hir_node(parent_hir_id)
+                        && let hir::ItemKind::Impl(impl_) = item.kind
+                        && let Some(trait_) = impl_.of_trait
+                        && let Some(def_id) = trait_.trait_def_id()
+                        && self.tcx.is_lang_item(def_id, hir::LangItem::Drop)
+                    {
+                        // OK
+                    } else {
+                        self.dcx().emit_err(errors::InvalidMayDangle { attr_span });
+                    }
+                }
                 Attribute::Unparsed(_) => {
                     match attr.path().as_slice() {
                         [sym::diagnostic, sym::do_not_recommend, ..] => {
@@ -225,7 +246,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                         [sym::collapse_debuginfo, ..] => self.check_collapse_debuginfo(attr, span, target),
                         [sym::must_not_suspend, ..] => self.check_must_not_suspend(attr, span, target),
                         [sym::must_use, ..] => self.check_must_use(hir_id, attr, target),
-                        [sym::may_dangle, ..] => self.check_may_dangle(hir_id, attr),
                         [sym::rustc_pass_by_value, ..] => self.check_pass_by_value(attr, span, target),
                         [sym::rustc_allow_incoherent_impl, ..] => {
                             self.check_allow_incoherent_impl(attr, span, target)
@@ -1589,27 +1609,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
-    /// Checks if `#[may_dangle]` is applied to a lifetime or type generic parameter in `Drop` impl.
-    fn check_may_dangle(&self, hir_id: HirId, attr: &Attribute) {
-        if let hir::Node::GenericParam(param) = self.tcx.hir_node(hir_id)
-            && matches!(
-                param.kind,
-                hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. }
-            )
-            && matches!(param.source, hir::GenericParamSource::Generics)
-            && let parent_hir_id = self.tcx.parent_hir_id(hir_id)
-            && let hir::Node::Item(item) = self.tcx.hir_node(parent_hir_id)
-            && let hir::ItemKind::Impl(impl_) = item.kind
-            && let Some(trait_) = impl_.of_trait
-            && let Some(def_id) = trait_.trait_def_id()
-            && self.tcx.is_lang_item(def_id, hir::LangItem::Drop)
-        {
-            return;
-        }
-
-        self.dcx().emit_err(errors::InvalidMayDangle { attr_span: attr.span() });
-    }
-
     /// Checks if `#[cold]` is applied to a non-function.
     fn check_cold(&self, hir_id: HirId, attr: &Attribute, span: Span, target: Target) {
         match target {