[Feature]: Support SPM package traits for plugin dependencies

[robingenz]

Description

Allow Capacitor plugin consumers to enable/disable SPM package traits on plugin dependencies

Platforms

• iOS
• Android
• Web

Request or proposed solution

Swift 6.1 introduced Package Traits (SE-0450) which allow package authors to define conditional dependencies that consumers can toggle via a traits parameter:

// Plugin's Package.swift
let package = Package(
    name: "CapacitorFirebaseAnalytics",
    traits: [
        .trait(name: "AdIdSupport", description: "Include IDFA collection support"),
    ],
    targets: [
        .target(
            name: "FirebaseAnalyticsPlugin",
            dependencies: [
                .product(name: "FirebaseAnalyticsCore", package: "firebase-ios-sdk"),
                .product(name: "FirebaseAnalyticsIdentitySupport", package: "firebase-ios-sdk",
                         condition: .when(traits: ["AdIdSupport"])),
            ]),
    ]
)

// Consumer's Package.swift
dependencies: [
    .package(url: "https://github.com/example/some-plugin.git", from: "1.0.0", traits: ["AdIdSupport"])
]

Since Capacitor manages SPM dependency resolution on behalf of the app developer, there is currently no way for a Capacitor app to pass traits to plugin dependencies. It would be great if Capacitor supported this, e.g. via capacitor.config.ts:

const config: CapacitorConfig = {
  ios: {
    packageTraits: {
      '@capawesome-team/capacitor-firebase-analytics': ['AdIdSupport'],
    },
  },
};

A concrete use case is capawesome-team/capacitor-firebase#942: the Firebase Analytics plugin needs to offer a choice between FirebaseAnalytics (with IDFA) and FirebaseAnalyticsCore (without IDFA). CocoaPods handles this via subspecs, but SPM has no equivalent mechanism — package traits are the intended solution.

Alternatives

No response

Additional Information

• SE-0450 proposal
• Package Traits documentation