Introduce lowering_arena to avoid creating AST nodes on the fly

compiler/rustc_ast_lowering/src/item.rs

@@ -1,6 +1,6 @@
 use super::errors::{InvalidAbi, MisplacedRelaxTraitBound};
 use super::ResolverAstLoweringExt;
-use super::{AstOwner, ImplTraitContext, ImplTraitPosition};
+use super::{Arena, AstOwner, ImplTraitContext, ImplTraitPosition};
 use super::{FnDeclKind, LoweringContext, ParamMode};
 
 use rustc_ast::ptr::P;
@@ -25,6 +25,7 @@ use std::iter;
 pub(super) struct ItemLowerer<'a, 'hir> {
     pub(super) tcx: TyCtxt<'hir>,
     pub(super) resolver: &'a mut ResolverAstLowering,
+    pub(super) ast_arena: &'a Arena<'static>,
     pub(super) ast_index: &'a IndexVec<LocalDefId, AstOwner<'a>>,
     pub(super) owners: &'a mut IndexVec<LocalDefId, hir::MaybeOwner<&'hir hir::OwnerInfo<'hir>>>,
 }
@@ -60,6 +61,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             tcx: self.tcx,
             resolver: self.resolver,
             arena: self.tcx.hir_arena,
+            ast_arena: self.ast_arena,
 
             // HirId handling.
             bodies: Vec::new(),

compiler/rustc_ast_lowering/src/lib.rs

@@ -44,6 +44,7 @@ extern crate tracing;
 
 use crate::errors::{AssocTyParentheses, AssocTyParenthesesSub, MisplacedImplTrait};
 
+use rustc_arena::declare_arena;
 use rustc_ast::ptr::P;
 use rustc_ast::visit;
 use rustc_ast::{self as ast, *};
@@ -95,6 +96,13 @@ struct LoweringContext<'a, 'hir> {
     /// Used to allocate HIR nodes.
     arena: &'hir hir::Arena<'hir>,
 
+    /// Used to allocate temporary AST nodes for use during lowering.
+    /// This allows us to create "fake" AST -- these nodes can sometimes
+    /// be allocated on the stack, but other times we need them to live longer
+    /// than the current stack frame, so they can be collected into vectors
+    /// and things like that.
+    ast_arena: &'a Arena<'static>,
+
     /// Bodies inside the owner being lowered.
     bodies: Vec<(hir::ItemLocalId, &'hir hir::Body<'hir>)>,
     /// Attributes inside the owner being lowered.
@@ -140,6 +148,15 @@ struct LoweringContext<'a, 'hir> {
     generics_def_id_map: Vec<FxHashMap<LocalDefId, LocalDefId>>,
 }
 
+declare_arena!([
+    [] tys: rustc_ast::Ty,
+    [] aba: rustc_ast::AngleBracketedArgs,
+    [] ptr: rustc_ast::PolyTraitRef,
+    // This _marker field is needed because `declare_arena` creates `Arena<'tcx>` and we need to
+    // use `'tcx`. If we don't have this we get a compile error.
+    [] _marker: std::marker::PhantomData<&'tcx ()>,
+]);
+
 trait ResolverAstLoweringExt {
     fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>>;
     fn get_partial_res(&self, id: NodeId) -> Option<PartialRes>;
@@ -401,10 +418,13 @@ pub fn lower_to_hir<'hir>(tcx: TyCtxt<'hir>, (): ()) -> hir::Crate<'hir> {
         tcx.definitions_untracked().def_index_count(),
     );
 
+    let ast_arena = Arena::default();
+
     for def_id in ast_index.indices() {
         item::ItemLowerer {
             tcx,
             resolver: &mut resolver,
+            ast_arena: &ast_arena,
             ast_index: &ast_index,
             owners: &mut owners,
         }
@@ -964,12 +984,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 }
                 GenericArgs::Parenthesized(ref data) => {
                     self.emit_bad_parenthesized_trait_in_assoc_ty(data);
-                    self.lower_angle_bracketed_parameter_data(
-                        &data.as_angle_bracketed_args(),
-                        ParamMode::Explicit,
-                        itctx,
-                    )
-                    .0
+                    let aba = self.ast_arena.aba.alloc(data.as_angle_bracketed_args());
+                    self.lower_angle_bracketed_parameter_data(aba, ParamMode::Explicit, itctx).0
                 }
             };
             gen_args_ctor.into_generic_args(self)
@@ -1037,15 +1053,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                     self.with_dyn_type_scope(false, |this| {
                         let node_id = this.next_node_id();
-                        let ty = this.lower_ty(
-                            &Ty {
-                                id: node_id,
-                                kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
-                                span: this.lower_span(constraint.span),
-                                tokens: None,
-                            },
-                            itctx,
-                        );
+                        let ty = this.ast_arena.tys.alloc(Ty {
+                            id: node_id,
+                            kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
+                            span: this.lower_span(constraint.span),
+                            tokens: None,
+                        });
+                        let ty = this.lower_ty(ty, itctx);
 
                         hir::TypeBindingKind::Equality { term: ty.into() }
                     })
@@ -1181,12 +1195,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             && let Res::Def(DefKind::Trait | DefKind::TraitAlias, _) = partial_res.base_res()
         {
             let (bounds, lifetime_bound) = self.with_dyn_type_scope(true, |this| {
+                let poly_trait_ref = this.ast_arena.ptr.alloc(PolyTraitRef {
+                    bound_generic_params: vec![],
+                    trait_ref: TraitRef { path: path.clone(), ref_id: t.id },
+                    span: t.span
+                });
                 let bound = this.lower_poly_trait_ref(
-                    &PolyTraitRef {
-                        bound_generic_params: vec![],
-                        trait_ref: TraitRef { path: path.clone(), ref_id: t.id },
-                        span: t.span
-                    },
+                    poly_trait_ref,
                     itctx,
                 );
                 let bounds = this.arena.alloc_from_iter([bound]);