Avoid eager file read in State.parse_file() when using native parser

When there was only one file in a parse batch, State.parse_file() was
called directly and invoked get_source() unconditionally, reading the
file eagerly in Python. This is unnecessary for on-disk files with the
native parser, which reads files itself in Rust. The fix skips
get_source() in that case, matching the behavior of the parallel path.

Fixes #21514

Author: Fix Bot

mypy/build.py

@@ -3211,7 +3211,11 @@ def parse_file(self, *, temporary: bool = False, raw_data: FileRawData | None =
             # The file was already parsed.
             return
 
-        if raw_data is None:
+        if raw_data is None and not (
+            self.options.native_parser
+            and self.source is None
+            and self.manager.fscache.exists(self.xpath, real_only=True)
+        ):
             source = self.get_source()
         else:
             source = ""