Fix preprocess_c_code() function to work under Closure optimization. (#26051)

Fix `preprocess_c_code()` JS library function to work under Closure
optimization.

---------

Co-authored-by: Alon Zakai <[email protected]>

Author: juj

src/lib/libc_preprocessor.js

@@ -81,6 +81,10 @@ addToLibrary({
 
     // Returns a tokenized array of the given string expression, i.e. "FOO > BAR && BAZ" -> ["FOO", ">", "BAR", "&&", "BAZ"]
     // Optionally keeps whitespace as tokens to be able to reconstruct the original input string.
+    /**
+     * @param {string} exprString
+     * @param {(number|boolean)=} keepWhitespace Optional, can be omitted. Defaults to false.
+    */
     function tokenize(exprString, keepWhitespace) {
       var out = [], len = exprString.length;
       for (var i = 0; i <= len; ++i) {
@@ -109,8 +113,12 @@ addToLibrary({
     }
 
     // Expands preprocessing macros on substring str[lineStart...lineEnd]
-    function expandMacros(str, lineStart, lineEnd) {
-      if (lineEnd === undefined) lineEnd = str.length;
+    /**
+    * @param {string} str
+    * @param {number} lineStart
+    * @param {number=} lineEnd Optional, may be omitted.
+    */
+    function expandMacros(str, lineStart, lineEnd=str.length) {
       var len = str.length;
       var out = '';
       for (var i = lineStart; i < lineEnd; ++i) {
@@ -168,7 +176,7 @@ addToLibrary({
 
           if (operatorAndPriority == 13 /* parens '(' */) {
             // Find the closing parens position
-            var j = find_closing_parens_index(tokens, i);
+            j = find_closing_parens_index(tokens, i);
             if (j) {
               tokens.splice(i, j+1-i, buildExprTree(tokens.slice(i+1, j)));
               return tokens;
@@ -224,7 +232,7 @@ addToLibrary({
       if (i < 0) i = len;
 
       // Find the first non-whitespace character on the line.
-      for (var j = lineStart; j < i && isWhitespace(code, j); ++j);
+      for (var j = lineStart; j < i && isWhitespace(code, j);) ++j;
 
       // Is this a non-preprocessor directive line?
       var thisLineIsInActivePreprocessingBlock = stack[stack.length-1];

test/test_other.py

@@ -10686,8 +10686,12 @@ def test_function_exports_are_small(self, args, opt, closure):
 
   # Tests the library_c_preprocessor.js functionality.
   @crossplatform
-  def test_c_preprocessor(self):
-    self.do_runf('test_c_preprocessor.c', cflags=['--js-library', path_from_root('src/lib/libc_preprocessor.js')])
+  @parameterized({
+    '': ([],),
+    'closure': (['--closure=1'],),
+  })
+  def test_c_preprocessor(self, args):
+    self.do_runf('test_c_preprocessor.c', cflags=['--js-library', path_from_root('src/lib/libc_preprocessor.js')] + args)
 
   # Test that legacy settings that have been fixed to a specific value and their value can no longer be changed,
   def test_legacy_settings_forbidden_to_change(self):