What are package traits, and how do they differ from build configurations or separate packages for optional features?

Predict first

When is the Logging trait active?

// === run.sh ===
W=$(mktemp -d); cd $W; mkdir -p Lib/Sources/Lib Lib/Sources/App Consumer/Sources/Consumer
cat > Lib/Package.swift <<'E'
// swift-tools-version: 6.1
import PackageDescription
let package = Package(
    name: "Lib",
    products: [.library(name: "Lib", targets: ["Lib"]), .executable(name: "app", targets: ["App"])],
    traits: [.trait(name: "Logging", description: "Adds verbose logging"), .default(enabledTraits: [])],
    targets: [
        .target(name: "Lib", swiftSettings: [.define("LOGGING", .when(traits: ["Logging"]))]),
        .executableTarget(name: "App", dependencies: ["Lib"]),
    ]
)
E
cat > Lib/Sources/Lib/L.swift <<'E'
public func features() -> String {
    #if LOGGING
    "logging on"
    #else
    "logging off"
    #endif
}
E
printf 'import Lib\nprint(features())\n' > Lib/Sources/App/main.swift
(cd Lib; echo "default:           $(swift run app 2>&1 | tail -1)"
         echo "--traits Logging:  $(swift run --traits Logging app 2>&1 | tail -1)")
for t in '' ', traits: ["Logging"]'; do
cat > Consumer/Package.swift <<E
// swift-tools-version: 6.1
import PackageDescription
let package = Package(name: "Consumer", dependencies: [.package(path: "../Lib"$t)],
  targets: [.executableTarget(name: "Consumer", dependencies: [.product(name: "Lib", package: "Lib")])])
E
printf 'import Lib\nprint(features())\n' > Consumer/Sources/Consumer/main.swift
label="asks for nothing"; [ -n "$t" ] && label="asks for Logging"
echo "consumer $label: $(cd Consumer; swift run 2>&1 | tail -1)"
done

• off by default; on with --traits Logging; on for a consumer only when it names the trait
• always on once declared
• on for a consumer even when it does not name the trait