perf(ast/estree): pre-allocate CodeBuffer for JSON output (#9340)

`ESTree` serialization pre-allocate capacity in `CodeBuffer` for JSON, based on an heuristic ratio of source text length to JSON length, to avoid the buffer having to grow during serialization.

crates/oxc_ast/src/serialize.rs

@@ -16,31 +16,54 @@ use crate::ast::*;
 ///
 /// Most consumers (and Oxc crates) will use only 1 of these methods, so we don't want to needlessly
 /// compile all 4 serializers when only 1 is used.
+///
+/// Initial capacity for serializer's buffer is an estimate based on our benchmark fixtures
+/// of ratio of source text size to JSON size.
+///
+/// | File                       | Compact TS | Compact JS | Pretty TS | Pretty JS |
+/// |----------------------------|------------|------------|-----------|-----------|
+/// | antd.js                    |         10 |          9 |        76 |        72 |
+/// | cal.com.tsx                |         10 |          9 |        40 |        37 |
+/// | checker.ts                 |          7 |          6 |        27 |        24 |
+/// | pdf.mjs                    |         13 |         12 |        71 |        67 |
+/// | RadixUIAdoptionSection.jsx |         10 |          9 |        45 |        44 |
+/// |----------------------------|------------|------------|-----------|-----------|
+/// | Maximum                    |         13 |         12 |        76 |        72 |
+///
+/// It's better to over-estimate than under-estimate, as having to grow the buffer is expensive,
+/// so have gone on the generous side.
+const JSON_CAPACITY_RATIO_COMPACT: usize = 16;
+const JSON_CAPACITY_RATIO_PRETTY: usize = 80;
+
 impl Program<'_> {
     /// Serialize AST to ESTree JSON, including TypeScript fields.
     pub fn to_estree_ts_json(&self) -> String {
-        let mut serializer = CompactTSSerializer::new();
+        let capacity = self.source_text.len() * JSON_CAPACITY_RATIO_COMPACT;
+        let mut serializer = CompactTSSerializer::with_capacity(capacity);
         self.serialize(&mut serializer);
         serializer.into_string()
     }
 
     /// Serialize AST to ESTree JSON, without TypeScript fields.
     pub fn to_estree_js_json(&self) -> String {
-        let mut serializer = CompactJSSerializer::new();
+        let capacity = self.source_text.len() * JSON_CAPACITY_RATIO_COMPACT;
+        let mut serializer = CompactJSSerializer::with_capacity(capacity);
         self.serialize(&mut serializer);
         serializer.into_string()
     }
 
     /// Serialize AST to pretty-printed ESTree JSON, including TypeScript fields.
     pub fn to_pretty_estree_ts_json(&self) -> String {
-        let mut serializer = PrettyTSSerializer::new();
+        let capacity = self.source_text.len() * JSON_CAPACITY_RATIO_PRETTY;
+        let mut serializer = PrettyTSSerializer::with_capacity(capacity);
         self.serialize(&mut serializer);
         serializer.into_string()
     }
 
     /// Serialize AST to pretty-printed ESTree JSON, without TypeScript fields.
     pub fn to_pretty_estree_js_json(&self) -> String {
-        let mut serializer = PrettyJSSerializer::new();
+        let capacity = self.source_text.len() * JSON_CAPACITY_RATIO_PRETTY;
+        let mut serializer = PrettyJSSerializer::with_capacity(capacity);
         self.serialize(&mut serializer);
         serializer.into_string()
     }

crates/oxc_estree/src/serialize/mod.rs

@@ -79,6 +79,11 @@ impl<C: Config, F: Formatter> ESTreeSerializer<C, F> {
         Self { buffer: CodeBuffer::new(), formatter: F::new(), config: C::new() }
     }
 
+    /// Create new [`ESTreeSerializer`] with specified buffer capacity.
+    pub fn with_capacity(capacity: usize) -> Self {
+        Self { buffer: CodeBuffer::with_capacity(capacity), formatter: F::new(), config: C::new() }
+    }
+
     /// Consume this [`ESTreeSerializer`] and convert buffer to string.
     pub fn into_string(self) -> String {
         self.buffer.into_string()