[SR-2849] Add a warning for unused parameters

[swift-ci]

Previous ID	SR-2849
Radar	rdar://problem/31844597
Original Reporter	Alex (JIRA User)
Type	New Feature
Status	In Progress
Resolution

Additional Detail from JIRA

Votes	8
Component/s	Compiler
Labels	New Feature
Assignee	@theblixguy
Priority	Medium

md5: a0372c6b5db167e925ad73f87d7f903e

is duplicated by:

• SR-6393 Warn on unused arguments

Issue Description:

When _ GCC_WARN_UNUSED_PARAMETER_ is enabled unused parameters does not produce warning.

How to reproduce:

1. Set Yes to Build Settings/Apple LLVM 8.0 Warnings - All languages/Unused Parameters
2. Add methods or blocks with unused parameters e.g.

func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    // There is usually no need to use `tableView` or `section` parameters for simple tables
    return dataSource.count 
}

or

NotificationCenter.default.addObserver(forName: .UIApplicationWillTerminate, object: nil, queue: nil) { notification in
    // Prepare for app termination
}

Result:
No warnings.

Expected Result:
Xcode should warn about unused parameters: tableView, section and notification.
Also Xcode should suggest to replace these parameters with _ which is already possible to do:

func tableView(_: UITableView, numberOfRowsInSection _: Int) -> Int {
    return dataSource.count 
}

NotificationCenter.default.addObserver(forName: .UIApplicationWillTerminate, object: nil, queue: nil) { _ in
    // Prepare for app termination
}

This setting works as expected in Objective-C (requires __unused attribute to suppress the warning).

[belkadan]

Swift doesn't have a warning for this at all, but it's a reasonable request (other than Swift not having any toggleable warnings at all). And yes, once it's implemented it would probably make sense for it to follow the same Xcode build setting, though that's not part of the Swift project.

[belkadan]

Comment of mine from dup:

I think I prefer the "_ = y" idiom, since replacing the local name with _ means you can't document the parameter.

[jckarter]

We could conceivably allow either idiom. Forcing you to name the argument and ignore it in the function body feels excessive to me if there's no documentation to hang off of it.

[belkadan]

We already allow either idiom; this would just be about what the fix-it does.

[swift-ci]

Comment by Mathäus Zingerle (JIRA)

Upvoted.

Yet I would suggest to use an own setting for this, analogously to "treat warnings as errors":

GCC_TREAT_WARNINGS_AS_ERRORS = YES
SWIFT_TREAT_WARNINGS_AS_ERRORS = YES

=>

GCC_WARN_UNUSED_PARAMETER = YES
SWIFT_WARN_UNUSED_PARAMETER = YES

[theblixguy]

Here's a PR to enable this functionality (hidden behind a frontend flag): #28839

Let me know if you have any feedback!

[AnthonyLatsis]

@theblixguy Do you still view this as a compiler feature?