Don't report functions that conform to a protocol as unused if removing them would break the build

[danwood]

Describe the bug

Periphery incorrectly marks methods as unused when they are required protocol conformance implementations. Removing these methods would break the build since the type would no longer conform to the protocol.

Reproduction

Given a protocol:

protocol DocumentRenamable {
    func canRename(in context: NSManagedObjectContext) -> Bool
    func currentName() -> String
    func applyRename(_ newName: String)
}

And a conforming extension:

extension Note: DocumentRenamable {
    func canRename(in context: NSManagedObjectContext) -> Bool { true }

    func currentName() -> String {
        // implementation
    }

    func applyRename(_ newName: String) {
        // implementation
    }
}

Periphery reports all three methods as unused, which is technically true!

Note+Renamable.swift:5:10: warning: Function 'canRename(in:)' is unused
Note+Renamable.swift:7:10: warning: Function 'currentName()' is unused
Note+Renamable.swift:12:10: warning: Function 'applyRename(_:)' is unused

The DocumentRenamable protocol is used elsewhere in the codebase, so that’s definitely in use. Following Periphery's suggestion to remove these methods would leave an empty extension Note: DocumentRenamable { } which fails to compile because Note no longer satisfies the protocol requirements.

Expected behavior

Periphery should check whether a method flagged as unused is required by a protocol the type conforms to. Methods that satisfy protocol requirements should not be marked as unused, since removing them would break the build.

Environment

3.4.0
swift-driver version: 1.127.14.1 Apple Swift version 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
Target: arm64-apple-macosx26.0
Xcode 26.2
Build version 17C52

[ileitch]

The functions should only be reported as unused if the protocol conformance is redundant, or the individual protocol members as never referenced on the existential type. If you're seeing different behavior, can you please provide a project that demonstrates the issue?

[JoeSzymanskiNoom]

I'm seeing this behavior also. This should be very simple to reproduce with the above example.

I believe the issue is that protocol conformance for a given function is required even if the function is never called on any instances (for example, future facing functionality, or functionality called by the external module itself).

I think there should be at least an option to capture this scenario, maybe something like "retain_unused_protocol_functions".

[ileitch]

This issue has been migrated to periphery-pro/issues#24 and is now tracked there.

Periphery is moving to a commercial model, and issues are now handled in that repository. You can read about what's changing, and what it means for existing users, in A New Chapter.