[docs] Include a real listing of the flags strict enables in the online documentation (#19062)

Fixes #19061

Currently, https://mypy.readthedocs.io/en/stable/command_line.html just
says

> You can see the list of flags enabled by strict mode in the full [mypy
--help](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-h)
output.

Which makes cross-referencing the documentation difficult. Instead,
there should be the same list there as appears when running `mypy
--help`: eg

> --warn-unused-configs, --disallow-any-generics,
--disallow-subclassing-any, --disallow-untyped-
                            calls, --disallow-untyped-defs, --disallow-
incomplete-defs, --check-untyped-defs, --disallow-
                            untyped-decorators, --warn-redundant-casts,
--warn-unused-ignores, --warn-return-any, --no-
implicit-reexport, --strict-equality, --extra-
                            checks

Ideally this section would be automatically generated from the code.

Author: wyattscarpenter

docs/source/command_line.rst

@@ -812,6 +812,14 @@ of the above sections.
     Note: the exact list of flags enabled by running :option:`--strict` may change
     over time.
 
+    .. include:: strict_list.rst
+    ..
+        The above file is autogenerated and included during html generation.
+        (That's an include directive, and this is a comment.)
+        It would be fine to generate it at some other time instead,
+        theoretically, but we already had a convenient hook during html gen.
+
+
 .. option:: --disable-error-code
 
     This flag allows disabling one or multiple error codes globally.

docs/source/html_builder.py

@@ -11,16 +11,37 @@
 from sphinx.builders.html import StandaloneHTMLBuilder
 from sphinx.environment import BuildEnvironment
 
+from mypy.main import define_options
+
 
 class MypyHTMLBuilder(StandaloneHTMLBuilder):
+    strict_file: Path
+
     def __init__(self, app: Sphinx, env: BuildEnvironment) -> None:
         super().__init__(app, env)
         self._ref_to_doc = {}
+        self.strict_file = Path(self.srcdir) / "strict_list.rst"
+        self._add_strict_list()
 
     def write_doc(self, docname: str, doctree: document) -> None:
         super().write_doc(docname, doctree)
         self._ref_to_doc.update({_id: docname for _id in doctree.ids})
 
+    def _add_strict_list(self) -> None:
+        strict_flags: list[str]
+        _, strict_flags, _ = define_options()
+        strict_part = ", ".join(f":option:`{s} <mypy {s}>`" for s in strict_flags)
+        if (
+            not strict_part
+            or strict_part.isspace()
+            or len(strict_part) < 20
+            or len(strict_part) > 2000
+        ):
+            raise ValueError(f"{strict_part=}, which doesn't look right (by a simple heuristic).")
+        self.strict_file.write_text(
+            "For this version of mypy, the list of flags enabled by strict is: " + strict_part
+        )
+
     def _verify_error_codes(self) -> None:
         from mypy.errorcodes import error_codes
 
@@ -55,6 +76,7 @@ def _write_ref_redirector(self) -> None:
     def finish(self) -> None:
         super().finish()
         self._write_ref_redirector()
+        self.strict_file.unlink()
 
 
 def setup(app: Sphinx) -> dict[str, Any]:

mypy/main.py

@@ -462,24 +462,18 @@ def __call__(
         parser.exit()
 
 
-def process_options(
-    args: list[str],
-    stdout: TextIO | None = None,
-    stderr: TextIO | None = None,
-    require_targets: bool = True,
-    server_options: bool = False,
-    fscache: FileSystemCache | None = None,
+def define_options(
     program: str = "mypy",
     header: str = HEADER,
-) -> tuple[list[BuildSource], Options]:
-    """Parse command line arguments.
-
-    If a FileSystemCache is passed in, and package_root options are given,
-    call fscache.set_package_root() to set the cache's package root.
-    """
-    stdout = stdout or sys.stdout
-    stderr = stderr or sys.stderr
-
+    stdout: TextIO = sys.stdout,
+    stderr: TextIO = sys.stderr,
+    server_options: bool = False,
+) -> tuple[CapturableArgumentParser, list[str], list[tuple[str, bool]]]:
+    """Define the options in the parser (by calling a bunch of methods that express/build our desired command-line flags).
+    Returns a tuple of:
+      a parser object, that can parse command line arguments to mypy (expected consumer: main's process_options),
+      a list of what flags are strict (expected consumer: docs' html_builder's _add_strict_list),
+      strict_flag_assignments (expected consumer: main's process_options)."""
     parser = CapturableArgumentParser(
         prog=program,
         usage=header,
@@ -1342,6 +1336,32 @@ def add_invertible_flag(
         dest="special-opts:files",
         help="Type-check given files or directories",
     )
+    return parser, strict_flag_names, strict_flag_assignments
+
+
+def process_options(
+    args: list[str],
+    stdout: TextIO | None = None,
+    stderr: TextIO | None = None,
+    require_targets: bool = True,
+    server_options: bool = False,
+    fscache: FileSystemCache | None = None,
+    program: str = "mypy",
+    header: str = HEADER,
+) -> tuple[list[BuildSource], Options]:
+    """Parse command line arguments.
+
+    If a FileSystemCache is passed in, and package_root options are given,
+    call fscache.set_package_root() to set the cache's package root.
+
+    Returns a tuple of: a list of source files, an Options collected from flags.
+    """
+    stdout = stdout if stdout is not None else sys.stdout
+    stderr = stderr if stderr is not None else sys.stderr
+
+    parser, _, strict_flag_assignments = define_options(
+        program, header, stdout, stderr, server_options
+    )
 
     # Parse arguments once into a dummy namespace so we can get the
     # filename for the config file and know if the user requested all strict options.
@@ -1526,11 +1546,9 @@ def set_strict_flags() -> None:
             targets.extend(p_targets)
         for m in special_opts.modules:
             targets.append(BuildSource(None, m, None))
-        return targets, options
     elif special_opts.command:
         options.build_type = BuildType.PROGRAM_TEXT
         targets = [BuildSource(None, None, "\n".join(special_opts.command))]
-        return targets, options
     else:
         try:
             targets = create_source_list(special_opts.files, options, fscache)
@@ -1539,7 +1557,7 @@ def set_strict_flags() -> None:
         # exceptions of different types.
         except InvalidSourceList as e2:
             fail(str(e2), stderr, options)
-        return targets, options
+    return targets, options
 
 
 def process_package_roots(