Consider match-case stmts for C901, PLR0912, and PLR0915

[blueraft]

Resolves #11421

Summary

Instead of counting match/case as one statement, consider each case as a conditional.

Test Plan

cargo test

[blueraft]

I'm not so sure about this, the equivalent else statement has the following check:

                   if clause.test.is_some() {
                        complexity += 1;
                    }

[dhruvmanila]

Can you clarify what you're not sure about? Is it that the catch-all case (_) acts as an else block of an if statement?

[blueraft]

Is it that the catch-all case (_) acts as an else block of an if statement?

Yep, complexity wise to me it's equivalent to the following if else statement.

if x == 30:
    print("foo")
else:
    print("bar")

[dhruvmanila]

Aren't they equivalent? We'd consider the above if else statement with complexity 2 and similarly for the following match statement:

match subject:
	case foo: ...
	case _: ...

[blueraft]

The if else statement has a complexity of 1, the &clause.test.is_some() evaluates to false for the else.

[crates/ruff_linter/src/rules/mccabe/rules/function_is_too_complex.rs:413:9] &source = "\nif x == 3:\n    print(\"foo\")\nelse:\n    print(\"bar\")\n "
[crates/ruff_linter/src/rules/mccabe/rules/function_is_too_complex.rs:79:21] &clause.test.is_some() = false
[crates/ruff_linter/src/rules/mccabe/rules/function_is_too_complex.rs:415:9] &get_complexity_number(&stmts) = 1

[dhruvmanila]

Oh I see. Hmm, that's interesting. Yeah, I think we could do that i.e., check if the last case pattern is either a _ or any variable as both act as a catch-all pattern.

[blueraft]

I think only _ acts as a wild card pattern if my understanding of the documentation is correct. In any case, I can make a follow up PR to handle this. :)

[dhruvmanila]

I think only _ acts as a wild card pattern if my understanding of the documentation is correct.

Yes, although a name pattern would also act in a similar way except that it also binds the value to the variable. So, in the following example foo will be same as subject. It's like doing foo = subject.

match subject:
	case 2: ...
	case foo: ...

You can even see that in CPython where it raises a syntax error similar to _ if the name pattern is not the last pattern:

match 1:
    case x:
        print(x)
    case 2:
        ...

Running python test.py:

$ python3.13 parser/_.py
  File "/Users/dhruv/playground/ruff/parser/_.py", line 2
    case x:
         ^
SyntaxError: name capture 'x' makes remaining patterns unreachable

In any case, I can make a follow up PR to handle this. :)

Sounds good.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[dhruvmanila]

Thank you for working on this!

[dhruvmanila]

Thank you!