Use tree-sitter for feature flag default rewriting (#56555)

Summary:
Pull Request resolved: #56555

Replaces the regex-based core of the Buck RN C++ feature flag override mechanism (D101484355) with a `tree-sitter` parser for sustainability.

Changelog: [Internal]

Reviewed By: robhogan

Differential Revision: D101629456

fbshipit-source-id: 1e891ba29db908da42e9c2657ed6152dc595d3fc

Author: motiz88

packages/react-native/ReactCommon/react/featureflags/rewrite_feature_flag_defaults.py

@@ -20,8 +20,32 @@
 import re
 import sys
 
+import tree_sitter_cpp
+from tree_sitter import Language, Parser, Query, QueryCursor
+
+
+_TARGET_CLASS = "ReactNativeFeatureFlagsDefaults"
+
+
+def _method_query(names: set[str]) -> str:
+    alternation = "|".join(re.escape(n) for n in sorted(names))
+    return f"""
+(class_specifier
+    name: (type_identifier) @class_name
+    body: (field_declaration_list
+        (function_definition
+            declarator: (function_declarator
+                declarator: (field_identifier) @method_name)
+            body: (compound_statement
+                (return_statement (_) @return_value)))
+    )
+    (#eq? @class_name "{_TARGET_CLASS}")
+    (#match? @method_name "^({alternation})$")
+)
+"""
+
 
-def cxx_literal(value: object) -> str:
+def cxx_literal(value: bool | int | float) -> str:
     if isinstance(value, bool):
         return "true" if value else "false"
     if isinstance(value, (int, float)):
@@ -33,33 +57,41 @@ def cxx_literal(value: object) -> str:
 
 
 def rewrite(source: bytes, overrides: dict[str, object]) -> bytes:
-    text = source.decode("utf-8")
-    for name, value in overrides.items():
-        cxx_type = "bool" if isinstance(value, bool) else "double"
-        pattern = rf"""
-            (                       # group 1: everything up to the value
-                {cxx_type} \s+      #   return type
-                {re.escape(name)}   #   method name
-                \s* \( \s* \)       #   parameter list
-                \s+ override        #   override specifier
-                \s* \{{             #   opening brace
-                [^}}]*?             #   body before the return (non-greedy, no nested braces)
-                return \s+          #   return keyword
+    lang = Language(tree_sitter_cpp.language())
+    tree = Parser(lang).parse(source)
+    matches = QueryCursor(Query(lang, _method_query(overrides.keys()))).matches(
+        tree.root_node
+    )
+
+    matched: set[str] = set()
+    replacements: list[tuple[int, int, bytes]] = []
+
+    for _, match in matches:
+        method_node = match["method_name"][0]
+        name = source[method_node.start_byte : method_node.end_byte].decode("utf-8")
+        rv_node = match["return_value"][0]
+        replacements.append(
+            (
+                rv_node.start_byte,
+                rv_node.end_byte,
+                cxx_literal(overrides[name]).encode("utf-8"),
             )
-            [^;]+                   # the value to replace
-            ( \s* ; )               # group 2: semicolon
-        """
-        text, n = re.subn(
-            pattern,
-            rf"\g<1>{cxx_literal(value)}\2",
-            text,
-            count=1,
-            flags=re.DOTALL | re.VERBOSE,
         )
-        if n != 1:
-            raise ValueError(f"{name} not matched")
+        matched.add(name)
+
+    unmatched = set(overrides.keys()) - matched
+    if unmatched:
+        raise ValueError(f"Unmatched flags: {', '.join(sorted(unmatched))}")
+
+    result = bytearray()
+    pos = 0
+    for start, end, replacement in replacements:
+        result.extend(source[pos:start])
+        result.extend(replacement)
+        pos = end
+    result.extend(source[pos:])
 
-    return text.encode("utf-8")
+    return bytes(result)
 
 
 def main() -> None: