Rule Request: Favor Key Path Expressions as Functions When Using Map & Filter

[72A12F4E]

As of Swift 5.2 you can now use a key path expression as a function when calling map/filter/etc.

I am proposing a rule that would identify & flag classic function calls that could be expressed using the new key path style.

Good

users.map(\.email)

users.filter(\.isAdmin)

Bad

users.map { $0.email }

users.filter { $0.isAdmin }

users.filter { user in
    return user.isAdmin 
}

I am not sure if this should be opt-in or not, but I would suggest that this be enabled by default.

• https://github.com/apple/swift-evolution/blob/master/proposals/0249-key-path-literal-function-expressions.md

[ZevEisenberg]

FYI, key paths can be quite slow currently, because they're not optimized. That might be good to add as a caveat in this rule's description. I just changed a bunch of { $0.foo } to (\.foo) in my app, only to revert the change when it choked while running tests. More info: https://bugs.swift.org/browse/SR-9323