Auto merge of #79519 - cjgillot:noattr, r=wesleywiser

Store HIR attributes in a side table

Same idea as #72015 but for attributes.
The objective is to reduce incr-comp invalidations due to modified attributes.
Notably, those due to modified doc comments.

Implementation:
- collect attributes during AST->HIR lowering, in `LocalDefId -> ItemLocalId -> &[Attributes]` nested tables;
- access the attributes through a `hir_owner_attrs` query;
- local refactorings to use this access;
- remove `attrs` from HIR data structures one-by-one.

Change in behaviour:
- the HIR visitor traverses all attributes at once instead of parent-by-parent;
- attribute arrays are sometimes duplicated: for statements and variant constructors;
- as a consequence, attributes are marked as used after unused-attribute lint emission to avoid duplicate lints.

~~Current bug: the lint level is not correctly applied in `std::backtrace_rs`, triggering an unused attribute warning on `#![no_std]`. I welcome suggestions.~~

compiler/rustc_ast_lowering/src/expr.rs

@@ -258,9 +258,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         ex.span = e.span;
                     }
                     // Merge attributes into the inner expression.
-                    let mut attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
-                    attrs.extend::<Vec<_>>(ex.attrs.into());
-                    ex.attrs = attrs.into();
+                    if !e.attrs.is_empty() {
+                        let old_attrs = self.attrs.get(&ex.hir_id).map(|la| *la).unwrap_or(&[]);
+                        self.attrs.insert(
+                            ex.hir_id,
+                            &*self.arena.alloc_from_iter(
+                                e.attrs
+                                    .iter()
+                                    .map(|a| self.lower_attr(a))
+                                    .chain(old_attrs.iter().cloned()),
+                            ),
+                        );
+                    }
                     return ex;
                 }
 
@@ -272,12 +281,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
             };
 
-            hir::Expr {
-                hir_id: self.lower_node_id(e.id),
-                kind,
-                span: e.span,
-                attrs: e.attrs.iter().map(|a| self.lower_attr(a)).collect::<Vec<_>>().into(),
-            }
+            let hir_id = self.lower_node_id(e.id);
+            self.lower_attrs(hir_id, &e.attrs);
+            hir::Expr { hir_id, kind, span: e.span }
         })
     }
 
@@ -618,14 +624,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 hir::Guard::If(self.lower_expr(cond))
             }
         });
-        hir::Arm {
-            hir_id: self.next_id(),
-            attrs: self.lower_attrs(&arm.attrs),
-            pat,
-            guard,
-            body: self.lower_expr(&arm.body),
-            span: arm.span,
-        }
+        let hir_id = self.next_id();
+        self.lower_attrs(hir_id, &arm.attrs);
+        hir::Arm { hir_id, pat, guard, body: self.lower_expr(&arm.body), span: arm.span }
     }
 
     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
@@ -669,7 +670,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             Ident::with_dummy_span(sym::_task_context),
             hir::BindingAnnotation::Mutable,
         );
-        let param = hir::Param { attrs: &[], hir_id: self.next_id(), pat, ty_span: span, span };
+        let param = hir::Param { hir_id: self.next_id(), pat, ty_span: span, span };
         let params = arena_vec![self; param];
 
         let body_id = self.lower_body(move |this| {
@@ -690,12 +691,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
             span,
             Some(hir::Movability::Static),
         );
-        let generator = hir::Expr {
-            hir_id: self.lower_node_id(closure_node_id),
-            kind: generator_kind,
-            span,
-            attrs: ThinVec::new(),
-        };
+        let generator =
+            hir::Expr { hir_id: self.lower_node_id(closure_node_id), kind: generator_kind, span };
 
         // `future::from_generator`:
         let unstable_span =
@@ -849,7 +846,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
             hir_id: loop_hir_id,
             kind: hir::ExprKind::Loop(loop_block, None, hir::LoopSource::Loop, span),
             span,
-            attrs: ThinVec::new(),
         });
 
         // mut pinned => loop { ... }
@@ -1026,7 +1022,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
         let destructure_let = self.stmt_let_pat(
-            ThinVec::new(),
+            None,
             whole_span,
             Some(rhs),
             pat,
@@ -1785,7 +1781,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         // `let mut __next`
         let next_let = self.stmt_let_pat(
-            ThinVec::new(),
+            None,
             desugared_span,
             None,
             next_pat,
@@ -1795,7 +1791,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // `let <pat> = __next`
         let pat = self.lower_pat(pat);
         let pat_let = self.stmt_let_pat(
-            ThinVec::new(),
+            None,
             desugared_span,
             Some(next_expr),
             pat,
@@ -1819,12 +1815,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
             hir::LoopSource::ForLoop,
             e.span.with_hi(orig_head_span.hi()),
         );
-        let loop_expr = self.arena.alloc(hir::Expr {
-            hir_id: self.lower_node_id(e.id),
-            kind,
-            span: e.span,
-            attrs: ThinVec::new(),
-        });
+        let loop_expr =
+            self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: e.span });
 
         // `mut iter => { ... }`
         let iter_arm = self.arm(iter_pat, loop_expr);
@@ -2159,21 +2151,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
         kind: hir::ExprKind<'hir>,
         attrs: AttrVec,
     ) -> hir::Expr<'hir> {
-        hir::Expr { hir_id: self.next_id(), kind, span, attrs }
+        let hir_id = self.next_id();
+        self.lower_attrs(hir_id, &attrs);
+        hir::Expr { hir_id, kind, span }
     }
 
     fn field(&mut self, ident: Ident, expr: &'hir hir::Expr<'hir>, span: Span) -> hir::Field<'hir> {
         hir::Field { hir_id: self.next_id(), ident, span, expr, is_shorthand: false }
     }
 
     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
-        hir::Arm {
-            hir_id: self.next_id(),
-            attrs: &[],
-            pat,
-            guard: None,
-            span: expr.span,
-            body: expr,
-        }
+        hir::Arm { hir_id: self.next_id(), pat, guard: None, span: expr.span, body: expr }
     }
 }