perf: minimize identifier lookahead when parsing let

packages/babel-parser/benchmark/many-let-declarations/10-length-binding.mjs

@@ -0,0 +1,22 @@
+import Benchmark from "benchmark";
+import baseline from "@babel-baseline/parser";
+import current from "../../lib/index.js";
+import { report } from "../util.mjs";
+
+const suite = new Benchmark.Suite();
+function createInput(length) {
+  return "{ let foecnatsni };".repeat(length);
+}
+function benchCases(name, implementation, options) {
+  for (const length of [64, 128, 256, 512]) {
+    const input = createInput(length);
+    suite.add(`${name} ${length} let and length-10 binding identifiers`, () => {
+      implementation.parse(input, options);
+    });
+  }
+}
+
+benchCases("baseline", baseline);
+benchCases("current", current);
+
+suite.on("cycle", report).run();

packages/babel-parser/src/parser/statement.js

@@ -4,11 +4,7 @@ import * as N from "../types";
 import { types as tt, type TokenType } from "../tokenizer/types";
 import ExpressionParser from "./expression";
 import { Errors, SourceTypeModuleErrors } from "./error";
-import {
-  isIdentifierChar,
-  isIdentifierStart,
-  keywordRelationalOperator,
-} from "../util/identifier";
+import { isIdentifierChar, isIdentifierStart } from "../util/identifier";
 import { lineBreak } from "../util/whitespace";
 import * as charCodes from "charcodes";
 import {
@@ -49,6 +45,8 @@ const FUNC_NO_FLAGS = 0b000,
 
 const loneSurrogate = /[\uD800-\uDFFF]/u;
 
+const keywordRelationalOperator = /in(?:stanceof)?/y;
+
 /**
  * Convert tt.privateName to tt.hash + tt.name for backward Babel 7 compat.
  * For performance reasons this routine mutates `tokens`, it is okay
@@ -200,12 +198,18 @@ export default class StatementParser extends ExpressionParser {
     if (nextCh === charCodes.leftCurlyBrace) return true;
 
     if (isIdentifierStart(nextCh)) {
-      let pos = next + 1;
-      while (isIdentifierChar(this.input.charCodeAt(pos))) {
-        ++pos;
+      keywordRelationalOperator.lastIndex = next;
+      const matched = keywordRelationalOperator.exec(this.input);
+      if (
+        matched !== null &&
+        // We have seen `in` or `instanceof` so far, now check if the identfier
+        // ends here
+        !isIdentifierChar(this.input.charCodeAt(next + matched[0].length))
+      ) {
+        return false;
+      } else {
+        return true;
       }
-      const ident = this.input.slice(next, pos);
-      if (!keywordRelationalOperator.test(ident)) return true;
     }
     return false;
   }