delegation: refactor AST -> HIR lowering

compiler/rustc_ast_lowering/src/delegation/attributes.rs

@@ -0,0 +1,86 @@
+use rustc_hir::attrs::{AttributeKind, InlineAttr};
+use rustc_hir::{self as hir};
+use rustc_span::Span;
+use rustc_span::def_id::DefId;
+
+use crate::LoweringContext;
+use crate::delegation::DelegationResolution;
+
+struct AdditionInfo {
+    pub equals: fn(&hir::Attribute) -> bool,
+    pub kind: AdditionKind,
+}
+
+enum AdditionKind {
+    Default { factory: fn(Span) -> hir::Attribute },
+    Inherit { factory: fn(Span, &hir::Attribute) -> hir::Attribute },
+}
+
+static ADDITIONS: &[AdditionInfo] = &[
+    AdditionInfo {
+        equals: |a| matches!(a, hir::Attribute::Parsed(AttributeKind::MustUse { .. })),
+        kind: AdditionKind::Inherit {
+            factory: |span, original_attr| {
+                let reason = match original_attr {
+                    hir::Attribute::Parsed(AttributeKind::MustUse { reason, .. }) => *reason,
+                    _ => None,
+                };
+
+                hir::Attribute::Parsed(AttributeKind::MustUse { span, reason })
+            },
+        },
+    },
+    AdditionInfo {
+        equals: |a| matches!(a, hir::Attribute::Parsed(AttributeKind::Inline(..))),
+        kind: AdditionKind::Default {
+            factory: |span| hir::Attribute::Parsed(AttributeKind::Inline(InlineAttr::Hint, span)),
+        },
+    },
+];
+
+impl<'hir> LoweringContext<'_, 'hir> {
+    pub(super) fn add_attrs_if_needed(&mut self, resolution: &DelegationResolution) {
+        let &DelegationResolution { span, sig_id, .. } = resolution;
+
+        const PARENT_ID: hir::ItemLocalId = hir::ItemLocalId::ZERO;
+        let new_attrs = self.create_new_attrs(span, sig_id, self.attrs.get(&PARENT_ID));
+
+        if !new_attrs.is_empty() {
+            let new_attrs = match self.attrs.get(&PARENT_ID) {
+                Some(existing_attrs) => self.arena.alloc_from_iter(
+                    existing_attrs.iter().map(|a| a.clone()).chain(new_attrs.into_iter()),
+                ),
+                None => self.arena.alloc_from_iter(new_attrs.into_iter()),
+            };
+
+            self.attrs.insert(PARENT_ID, new_attrs);
+        }
+    }
+
+    fn create_new_attrs(
+        &self,
+        span: Span,
+        sig_id: DefId,
+        existing: Option<&&[hir::Attribute]>,
+    ) -> Vec<hir::Attribute> {
+        ADDITIONS
+            .iter()
+            .filter_map(|addition| {
+                existing
+                    .is_none_or(|attrs| !attrs.iter().any(|a| (addition.equals)(a)))
+                    .then(|| match addition.kind {
+                        AdditionKind::Default { factory } => Some(factory(span)),
+                        AdditionKind::Inherit { factory, .. } =>
+                        {
+                            #[allow(deprecated)]
+                            self.tcx
+                                .get_all_attrs(sig_id)
+                                .iter()
+                                .find_map(|a| (addition.equals)(a).then(|| factory(span, a)))
+                        }
+                    })
+                    .flatten()
+            })
+            .collect::<Vec<_>>()
+    }
+}