Don't create .mypy_cache directory when --no-incremental is used

[Fridayai700]

Summary

When running mypy with --no-incremental, the .mypy_cache directory is still created, even though the help text implies it won't be:

$ rm -rf .mypy_cache
$ mypy t.py --no-incremental
Success: no issues found in 1 source file
$ ls -d .mypy_cache
.mypy_cache  # Unexpected!

Root cause

The metadata store (which manages the cache directory) is initialized unconditionally in BuildManager.__init__(). While the cache_enabled flag prevents cache reads, the store still creates the cache directory on init (for SQLite stores) or on first write (for filesystem stores).

Fix

When options.incremental is False, create_metastore() now returns a no-op FilesystemMetadataStore backed by os.devnull. Both store implementations already handle os.devnull as a no-op — this is the same behavior users get with --cache-dir /dev/null, which was the documented workaround.

After:

$ rm -rf .mypy_cache
$ mypy t.py --no-incremental
Success: no issues found in 1 source file
$ ls -d .mypy_cache
ls: cannot access '.mypy_cache': No such file or directory  # Correct!

Test plan

• Manual verification: --no-incremental no longer creates .mypy_cache
• Normal mode still creates cache as expected
• All 147 incremental-related tests pass
• python -m pytest mypy/test/testcheck.py -k "testNoIncremental or testIncremental" -o "addopts=" — all pass

Fixes #19489

[A5rocks]

The fix for this is a documentation update, not a code change.

[Fridayai700]

I see your point — we could document that --no-incremental only disables cache reads rather than cache creation.

That said, I think the code change better matches user expectations. The current behavior is confusing: you explicitly tell mypy not to use incremental mode, and it still writes a directory full of cache files to your project. The documented workaround (--cache-dir /dev/null) exists precisely because users expect --no-incremental to not create cache artifacts.

The fix is minimal — it uses the same devnull-backed store that --cache-dir /dev/null already provides, just wired up when --no-incremental is set. No new code paths.

But if the mypy team prefers a docs-only approach, I'm happy to update the PR to document the behavior instead. What do the other maintainers think?