From 07e946caf7a4b9e5781b15f75b927b9e695b4f7c Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 13 Oct 2019 23:27:18 +0200
Subject: [PATCH] lowering: connect to parser via function pointer instead

---
 src/librustc/hir/lowering.rs     | 18 ++++++++----------
 src/librustc_interface/passes.rs | 20 ++++----------------
 2 files changed, 12 insertions(+), 26 deletions(-)

diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index 062533f3d0972..747848edc7a8d 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -87,7 +87,10 @@ pub struct LoweringContext<'a> {
 
     resolver: &'a mut dyn Resolver,
 
-    parser: &'static dyn Parser,
+    /// HACK(Centril): there is a cyclic dependency between the parser and lowering
+    /// if we don't have this function pointer. To avoid that dependency so that
+    /// librustc is independent of the parser, we use dynamic dispatch here.
+    nt_to_tokenstream: NtToTokenstream,
 
     /// The items being lowered are collected here.
     items: BTreeMap<hir::HirId, hir::Item>,
@@ -183,12 +186,7 @@ pub trait Resolver {
     fn has_derives(&self, node_id: NodeId, derives: SpecialDerives) -> bool;
 }
 
-/// HACK(Centril): there is a cyclic dependency between the parser and lowering
-/// if we don't have this trait. To avoid that dependency so that librustc is
-/// independent of the parser, we use type erasure here.
-pub trait Parser {
-    fn nt_to_tokenstream(&self, nt: &Nonterminal, sess: &ParseSess, span: Span) -> TokenStream;
-}
+type NtToTokenstream = fn(&Nonterminal, &ParseSess, Span) -> TokenStream;
 
 /// Context of `impl Trait` in code, which determines whether it is allowed in an HIR subtree,
 /// and if so, what meaning it has.
@@ -246,7 +244,7 @@ pub fn lower_crate(
     dep_graph: &DepGraph,
     krate: &Crate,
     resolver: &mut dyn Resolver,
-    parser: &'static dyn Parser,
+    nt_to_tokenstream: NtToTokenstream,
 ) -> hir::Crate {
     // We're constructing the HIR here; we don't care what we will
     // read, since we haven't even constructed the *input* to
@@ -260,7 +258,7 @@ pub fn lower_crate(
         sess,
         cstore,
         resolver,
-        parser,
+        nt_to_tokenstream,
         items: BTreeMap::new(),
         trait_items: BTreeMap::new(),
         impl_items: BTreeMap::new(),
@@ -1034,7 +1032,7 @@ impl<'a> LoweringContext<'a> {
     fn lower_token(&mut self, token: Token) -> TokenStream {
         match token.kind {
             token::Interpolated(nt) => {
-                let tts = self.parser.nt_to_tokenstream(&nt, &self.sess.parse_sess, token.span);
+                let tts = (self.nt_to_tokenstream)(&nt, &self.sess.parse_sess, token.span);
                 self.lower_token_stream(tts)
             }
             _ => TokenTree::Token(token).into(),
diff --git a/src/librustc_interface/passes.rs b/src/librustc_interface/passes.rs
index 5078379e92053..328798862b864 100644
--- a/src/librustc_interface/passes.rs
+++ b/src/librustc_interface/passes.rs
@@ -5,7 +5,7 @@ use crate::proc_macro_decls;
 use log::{info, warn, log_enabled};
 use rustc::dep_graph::DepGraph;
 use rustc::hir;
-use rustc::hir::lowering::{lower_crate, Parser};
+use rustc::hir::lowering::lower_crate;
 use rustc::hir::def_id::{CrateNum, LOCAL_CRATE};
 use rustc::lint;
 use rustc::middle::{self, reachable, resolve_lifetime, stability};
@@ -38,12 +38,9 @@ use syntax::early_buffered_lints::BufferedEarlyLint;
 use syntax::ext::base::{NamedSyntaxExtension, ExtCtxt};
 use syntax::mut_visit::MutVisitor;
 use syntax::parse::{self, PResult};
-use syntax::parse::token::Nonterminal;
-use syntax::parse::ParseSess;
-use syntax::tokenstream::TokenStream;
 use syntax::util::node_count::NodeCounter;
 use syntax::symbol::Symbol;
-use syntax_pos::{FileName, Span};
+use syntax_pos::FileName;
 use syntax_ext;
 
 use rustc_serialize::json;
@@ -535,16 +532,6 @@ fn configure_and_expand_inner<'a>(
     Ok((krate, resolver))
 }
 
-fn parser() -> &'static dyn Parser {
-    struct Parse;
-    impl Parser for Parse {
-        fn nt_to_tokenstream(&self, nt: &Nonterminal, sess: &ParseSess, span: Span) -> TokenStream {
-            syntax::parse::nt_to_tokenstream(nt, sess, span)
-        }
-    }
-    &Parse
-}
-
 pub fn lower_to_hir(
     sess: &Session,
     cstore: &CStore,
@@ -554,7 +541,8 @@ pub fn lower_to_hir(
 ) -> Result<hir::map::Forest> {
     // Lower AST to HIR.
     let hir_forest = time(sess, "lowering AST -> HIR", || {
-        let hir_crate = lower_crate(sess, cstore, &dep_graph, &krate, resolver, parser());
+        let nt_to_tokenstream = syntax::parse::nt_to_tokenstream;
+        let hir_crate = lower_crate(sess, cstore, &dep_graph, &krate, resolver, nt_to_tokenstream);
 
         if sess.opts.debugging_opts.hir_stats {
             hir_stats::print_hir_stats(&hir_crate);