[mypyc] Add hidden flag to skip the generation of C files

This can be useful when debugging mypyc issues. For example, you can
manually add some debug prints into the generated C and rerun mypyc
with `--skip-c-gen`. Now mypyc will build the code again, with your
manual changes included (this assumes everything else is the same as
during the previous run).

There are not plans currently to advertise this as an end-user feature.

Author: JukkaL

mypy/main.py

@@ -1128,6 +1128,11 @@ def add_invertible_flag(
     report_group.add_argument(
         "-a", dest="mypyc_annotation_file", type=str, default=None, help=argparse.SUPPRESS
     )
+    # Hidden mypyc feature: do not write any C files (keep existing ones and assume they exist).
+    # This can be useful when debugging mypyc bugs.
+    report_group.add_argument(
+        "--skip-c-gen", dest="mypyc_skip_c_generation", action="store_true", help=argparse.SUPPRESS
+    )
 
     other_group = parser.add_argument_group(title="Miscellaneous")
     other_group.add_argument("--quickstart-file", help=argparse.SUPPRESS)

mypy/options.py

@@ -408,6 +408,9 @@ def __init__(self) -> None:
 
         # Output html file for mypyc -a
         self.mypyc_annotation_file: str | None = None
+        # Skip writing C output files, but perform all other steps of a build (allows
+        # preserving manual tweaks to generated C file)
+        self.mypyc_skip_c_generation: bool = False
 
     def use_lowercase_names(self) -> bool:
         if self.python_version >= (3, 9):

mypyc/build.py

@@ -452,7 +452,8 @@ def mypyc_build(
         cfilenames = []
         for cfile, ctext in cfiles:
             cfile = os.path.join(compiler_options.target_dir, cfile)
-            write_file(cfile, ctext)
+            if not options.mypyc_skip_c_generation:
+                write_file(cfile, ctext)
             if os.path.splitext(cfile)[1] == ".c":
                 cfilenames.append(cfile)