Introduce lowering_arena to avoid creating AST nodes on the fly

Author: spastorino

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]);