Swiftlint: Functions w/ Only Multiple Trailing Closures Trigger "No Space in Method Call" (Swift 5.3)

[giladronat]

New Issue Checklist

• [x] Updated SwiftLint to the latest version
• [x] I searched for existing GitHub issues

Describe the bug

When using Swift 5.3's multiple trailing closure syntax (disabling the rule for multiple_closures_with_trailing_closure), a function call with only multiple trailing closures as arguments will trigger "No Space in Method Call Violation."

Common examples of such functions are:

1. SwiftUI Button initializer:

Button { // No Space in Method Call Violation
    print("Button tapped")
} label: {
    Text("Button")
}

1. Combine sink Operator:

Just(1)
    .sink { completion in // No Space in Method Call Violation
        print(completion)
    } receiveValue: { value in
        print(value)
    }
    .store(in: &disposeBag)

Function calls that begin with non-closure parameters followed by multiple trailing closures work just fine. For example:

UIView.animate(withDuration: 0.5) { // All clear, no violation
    print("animating")
} completion: { _ in
    print("done")
}

Removing the space properly triggers "Opening Brace Spacing Violation" (see swiftlint autocorrect output below).

Complete output when running SwiftLint, including the stack trace and command used

$ swiftlint lint
Loading configuration from '.swiftlint.yml'
Linting Swift files at paths
Linting 'ContentView.swift' (1/2)
Linting 'LintTestApp.swift' (2/2)
.../LintTest/ContentView.swift:17:15: warning: No Space in Method Call Violation: Don't add a space between the method name and the parentheses. (no_space_in_method_call)
Done linting! Found 1 violations, 0 serious in 2 files.

$ swiftlint autocorrect
Loading configuration from '.swiftlint.yml'
Correcting Swift files at paths
Correcting 'AnimationView.swift' (1/4)
Correcting 'LintViewModel.swift' (2/4)
Correcting 'ContentView.swift' (3/4)
Correcting 'LintTestApp.swift' (4/4)
.../LintTest/LintTest/ContentView.swift:17:15 Corrected No Space in Method Call
.../LintTest/LintTest/ContentView.swift:17:15 Corrected Opening Brace Spacing
.../LintTest/LintTest/LintViewModel.swift:20:18 Corrected No Space in Method Call
.../LintTest/LintTest/LintViewModel.swift:20:18 Corrected Opening Brace Spacing
Done correcting 4 files!

Environment

• SwiftLint version (run swiftlint version to be sure)?
  0.39.2
• Installation method used (Homebrew, CocoaPods, building from source, etc)?
  Homebrew
• Paste your configuration file:

disabled_rules:
  - multiple_closures_with_trailing_closure

# Implicitly relevant:
# - no_space_in_method_call
# - opening_brace

• Are you using nested configurations?
  No.
• Which Xcode version are you using (check xcodebuild -version)?
  Xcode 12.0
  Build version 12A6159
  (Beta 1)
• Do you have a sample that shows the issue?

Button { // No Space in Method Call Violation
    print("Button tapped")
} label: {
    Text("Button")
}

Additional Comments

Looking into fixing this by adjusting NoSpaceInMethodCallRule.swift. Once the issue is resolved, it may be worth considering disabling the multiple_closures_with_trailing_closure rule by default. While the new syntax isn't universally loved, it mitigates the call site ambiguity that motivated the rule's creation.

[ohayon]

I am seeing this same issue, and getting two warnings:

Button { // No Space in Method Call Violation
    print("Button tapped")
} label: { // Multiple Closures with Trailing Closure Violation
    Text("Button")
}

[giladronat]

@ohayon I assumed the "Multiple Closures with Trailing Closure" to be working correctly in detecting multiple closures when a trailing closure is used, and that it could be disabled now. However, it could be modified to detect _multiple trailing closures_ instead.

This way, it won't trigger on:

Button {
    print("Tapped")
} label: {
    Text("Button")
}

But will trigger on the current violation:

Button(action: {
    print("Tapped")
}) {
    Text("Button")
}

And trigger on a three-closure parameters function not using three trailing closure:

myMethod(param1: { "closure1" }) {
    "closure 2"
} param3: {
    "closure 3"
}

I'm playing around with the SourceKit params for structures to understand how some of the current rules around these are implemented, then learn how to determine when a closure is a proper trailing closure.

[marcelofabri]

@giladronat Can you open another issue to discuss multiple_closures_with_trailing_closure? Let's focus on no_space_in_method_call here 🙏

[giladronat]

Wow fantastic work @marcelofabri, thank you. Yes, will gladly open another issue to discuss multiple_closures_with_trailing_closure.

Regarding #3280, if you have a moment, could you explain how the initial closure's body offset matching the body offset of the function call determines it's the beginning of trailing closures (this line)?

I'll admit I spent far too long trying to understand what differentiates a trailing closure from non-trailing closures and unnamed closure arguments in order to PR a fix I knew accounted for this case without breaking others. For example, looking at TrailingClosureRule and MultipleClosuresWithTrailingClosures gave me the impression the solution would be more involved.

[giladronat]

Opened #3295 to discuss multiple_closures_with_trailing_closure in Swift 5.3.