Add capacity to PlaceholderExpander

diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index cfa7725..dc2f289 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs

@@ -508,6 +508,7 @@ pub fn fully_expand_fragment(&mut self, input_fragment: AstFragment) -> AstFragm
         // Unresolved macros produce dummy outputs as a recovery measure.
         invocations.reverse();
         let mut expanded_fragments = Vec::new();
+        let mut expanded_fragments_len = 0;
         let mut undetermined_invocations = Vec::new();
         let (mut progress, mut force) = (false, !self.monotonic);
         loop {
@@ -602,6 +603,7 @@ pub fn fully_expand_fragment(&mut self, input_fragment: AstFragment) -> AstFragm
                         expanded_fragments.push(Vec::new());
                     }
                     expanded_fragments[depth - 1].push((expn_id, expanded_fragment));
+                    expanded_fragments_len += 1;
                     invocations.extend(derive_invocations.into_iter().rev());
                 }
                 ExpandResult::Retry(invoc) => {
@@ -622,7 +624,7 @@ pub fn fully_expand_fragment(&mut self, input_fragment: AstFragment) -> AstFragm
         self.cx.force_mode = orig_force_mode;
 
         // Finally incorporate all the expanded macros into the input AST fragment.
-        let mut placeholder_expander = PlaceholderExpander::default();
+        let mut placeholder_expander = PlaceholderExpander::with_capacity(expanded_fragments_len);
         while let Some(expanded_fragments) = expanded_fragments.pop() {
             for (expn_id, expanded_fragment) in expanded_fragments.into_iter().rev() {
                 placeholder_expander

diff --git a/compiler/rustc_expand/src/placeholders.rs b/compiler/rustc_expand/src/placeholders.rs
index 05f9a5a..2db1842 100644
--- a/compiler/rustc_expand/src/placeholders.rs
+++ b/compiler/rustc_expand/src/placeholders.rs

@@ -218,12 +218,17 @@ fn mac_placeholder() -> Box<ast::MacCall> {
     }
 }
 
-#[derive(Default)]
 pub(crate) struct PlaceholderExpander {
     expanded_fragments: FxHashMap<ast::NodeId, AstFragment>,
 }
 
 impl PlaceholderExpander {
+    pub(crate) fn with_capacity(capacity: usize) -> Self {
+        PlaceholderExpander {
+            expanded_fragments: FxHashMap::with_capacity_and_hasher(capacity, Default::default()),
+        }
+    }
+
     pub(crate) fn add(&mut self, id: ast::NodeId, mut fragment: AstFragment) {
         fragment.mut_visit_with(self);
         self.expanded_fragments.insert(id, fragment);