Allow enabling individual experimental features

[ilevkivskyi]

Ref #13685

[ilevkivskyi]

Hm it looks like this causes a crash, I will try to fix.

[ilevkivskyi]

Hm, the second crash shows that we can't probably say Unpack support is complete yet. We should probably at least support *args: Unpack[tuple[int, str, int]] (supporting unions will be tricky, and we don't support unions of TypedDicts anyway, and unions are not specified by the PEP).

[ilevkivskyi]

Btw a simple repro for the crash is

class E(Generic[T]):
    def __init__(self, *args: Unpack[tuple[int, str]]) -> None: ...
E()

Also while playing with this I have found two bugs in the **kwds support for Unpack. First, this crashes:

class C:
    def __init__(self, **kwds: Unpack) -> None: ...

Instead we should give an error about missing type arg. Second, this doesn't substitute implicit Any for a generic TypedDict, causing weird error:

T = TypeVar("T")
class TD(TypedDict, Generic[T]):
    x: T
    y: int

def foo(**kwds: Unpack[TD]) -> None: ...  # Should be same as `TD[Any]`
foo(x="yes", y=42)

[sobolevn]

Looks cool!

Question: can we somehow bind feature ids to error codes?
So users won't have to memorize two separate things and their mappings?

[ilevkivskyi]

can we somehow bind feature ids to error codes?

No, not really. Many features (in particular those that are relevant here like Unpack and TypeVarTuple) don't have any errors related to them. This PR is specifically for such cases.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

sublime_debugger (https://github.com/daveleroy/sublime_debugger)
- modules/core/event.py:5: error: "TypeVarTuple" is not supported by mypy yet  [misc]
+ modules/core/event.py:5: error: "TypeVarTuple" support is experimental, use --enable-incomplete-feature=TypeVarTuple to enable  [misc]
- modules/core/event.py:27: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/event.py:27: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/event.py:37: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/event.py:37: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/event.py:59: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/event.py:59: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/event.py:69: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/event.py:69: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/event.py:75: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/event.py:75: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/core.py:12: error: "TypeVarTuple" is not supported by mypy yet  [misc]
+ modules/core/core.py:12: error: "TypeVarTuple" support is experimental, use --enable-incomplete-feature=TypeVarTuple to enable  [misc]
- modules/core/core.py:30: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/core.py:30: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/core.py:33: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/core.py:33: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/core.py:36: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/core.py:36: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/core.py:42: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/core.py:42: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]
- modules/core/core.py:51: error: "Unpack" is not supported yet, use --enable-incomplete-features  [misc]
+ modules/core/core.py:51: error: "Unpack" support is experimental, use --enable-incomplete-feature=Unpack to enable  [misc]