From 2d759046ba685f419b3de79367e5b973c1b84105 Mon Sep 17 00:00:00 2001
From: Jeffrey Seyfried <jeffrey.seyfried@gmail.com>
Date: Fri, 2 Sep 2016 03:35:59 +0000
Subject: [PATCH] Implement stackless placeholder expansion.

---
 src/libsyntax/ext/expand.rs       | 19 +++++++++++++++----
 src/libsyntax/ext/placeholders.rs | 11 +++++++----
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 68aca3ee32f..c5b637a9800 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -26,7 +26,6 @@ use ptr::P;
 use tokenstream::TokenTree;
 use util::small_vector::SmallVector;
 
-use std::collections::HashMap;
 use std::mem;
 use std::path::PathBuf;
 use std::rc::Rc;
@@ -182,10 +181,11 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
 
     // Fully expand all the invocations in `expansion`.
     fn expand(&mut self, expansion: Expansion) -> Expansion {
+        self.cx.recursion_count = 0;
         let (expansion, mut invocations) = self.collect_invocations(expansion);
         invocations.reverse();
 
-        let mut expansions = HashMap::new();
+        let mut expansions = vec![vec![(0, expansion)]];
         while let Some(invoc) = invocations.pop() {
             let Invocation { mark, module, depth, backtrace, .. } = invoc;
             self.cx.syntax_env.current_module = module;
@@ -198,13 +198,24 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             self.cx.recursion_count = depth + 1;
             let (expansion, new_invocations) = self.collect_invocations(expansion);
 
-            expansions.insert(mark.as_u32(), expansion);
+            if expansions.len() == depth {
+                expansions.push(Vec::new());
+            }
+            expansions[depth].push((mark.as_u32(), expansion));
             if !self.single_step {
                 invocations.extend(new_invocations.into_iter().rev());
             }
         }
 
-        expansion.fold_with(&mut PlaceholderExpander::new(expansions))
+        let mut placeholder_expander = PlaceholderExpander::new();
+        while let Some(expansions) = expansions.pop() {
+            for (mark, expansion) in expansions.into_iter().rev() {
+                let expansion = expansion.fold_with(&mut placeholder_expander);
+                placeholder_expander.add(mark, expansion);
+            }
+        }
+
+        placeholder_expander.remove(0)
     }
 
     fn collect_invocations(&mut self, expansion: Expansion) -> (Expansion, Vec<Invocation>) {
diff --git a/src/libsyntax/ext/placeholders.rs b/src/libsyntax/ext/placeholders.rs
index f1665bdde75..abadcf867b1 100644
--- a/src/libsyntax/ext/placeholders.rs
+++ b/src/libsyntax/ext/placeholders.rs
@@ -74,15 +74,18 @@ pub struct PlaceholderExpander {
 }
 
 impl PlaceholderExpander {
-    pub fn new(expansions: HashMap<ast::NodeId, Expansion>) -> Self {
+    pub fn new() -> Self {
         PlaceholderExpander {
-            expansions: expansions,
+            expansions: HashMap::new(),
         }
     }
 
+    pub fn add(&mut self, id: ast::NodeId, expansion: Expansion) {
+        self.expansions.insert(id, expansion);
+    }
+
     pub fn remove(&mut self, id: ast::NodeId) -> Expansion {
-        let expansion = self.expansions.remove(&id).unwrap();
-        expansion.fold_with(self)
+        self.expansions.remove(&id).unwrap()
     }
 }