[red-knot] Understand typing.Callable

[dhruvmanila]

Summary

Part of #15382

This PR implements a general callable type that wraps around a Signature and it uses that new type to represent typing.Callable.

It also implements Display support for Callable. The format is as:

([<arg name>][: <arg type>][ = <default type>], ...) -> <return type>

The / and * separators are added at the correct boundary for positional-only and keyword-only parameters. Now, as typing.Callable only has positional-only parameters, the rendered signature would be:

Callable[[int, str], None]
# (int, str, /) -> None

The / separator represents that all the arguments are positional-only.

The relationship methods that check assignability, subtype relationship, etc. are not yet implemented and will be done so as a follow-up.

Test Plan

Add test cases for display support for Signature and various mdtest for typing.Callable.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[carljm]

Very cool! Great work.

[dhruvmanila]

Post review changes

• Use (...) -> Unknown consistently
• Display ... for gradual form. This is done using an additional flag on Parameters to indicate the gradual form.
• Use Any for gradual form while Unknown for invalid forms
• Avoid special casing for param spec variable
• Don't error on bare Callable, infer it as (...) -> Unknown
• Remove various todos, add a couple more
• Improve display handling of Signature

Regarding the relation implementation, I think I'd prefer to add it in a follow-up PR to keep the changes separate and easier to review. I implemented is_single_valued and is_singleton but not member and static_member yet.

[carljm]

Looks good!

[AlexWaygood]

two tiny nits I spotted that probably don't warrant their own PR, but could maybe be addressed if you're doing further followup work on Callable types

[AlexWaygood]

nit

Suggested change

	pub(crate) fn is_keyword_only(&self) -> bool {
	matches!(self.kind, ParameterKind::KeywordOnly { .. })
	}
	pub(crate) fn is_positional_only(&self) -> bool {
	pub(crate) const fn is_keyword_only(&self) -> bool {
	matches!(self.kind, ParameterKind::KeywordOnly { .. })
	}
	pub(crate) const fn is_positional_only(&self) -> bool {

[dhruvmanila]

I might be wrong but I'm not sure if this is going to make any difference as we don't use any of these functions in the const context and I don't know we'll ever be able to (?) because parameters are always going to be dynamic i.e., we cannot statically determine during compile time whether a parameter is positional or not.