Add a TracingMacros module providing the @Traced macro

[porglezomp]

This provides a @Traced macro which adds a tracing span around the body of the function. This macro allows customizing the operationName, the context, and the kind of the span, same as the withSpan function. It also exposes the span itself into the scope of the function, with a customizable name.

This doesn't attempt to support automatically adding parameters as attributes, because the scoping rules of attached macros aren't very amenable to controlling that. That could be added in the future if it's judged necessary.

Examples:

@Traced("preheat oven")
func preheatOven(temperature: Int) async throws -> Oven {
    span.attributes["oven.targetTemperature"] = temperature
    await sleep(for: .seconds(6))
    return Oven()
}

expands to:

func preheatOven(temperature: Int) async throws -> Oven {
    withSpan("preheat oven") { span in
        span.attributes["oven.targetTemperature"] = temperature
        await sleep(for: .seconds(6))
        return Oven()
    }
}

Notes:

This places the macros into a separate product so that users who don't want to pay the compile-time cost of macros don't have to use it, you opt-in to that cost by depending on the TracingMacros module.

This is an alternative to apple/swift-distributed-tracing#157 to avoid a semver major bump, since the minimum platform versions are already set here.

Resolves apple/swift-distributed-tracing#125

[ktoso]

Ok, awesome! Let's include this in the extras repo then rather the main then. As folks indicated bumping the required OS would have been technically breaking. Thanks @Lukasa for chiming in

[ktoso]

We want to give folks a moment to debate the swift-syntax issue before cutting a release.

But imho there's nothing we as a package can do to improve the situation; the package manager and how swift deals with macros needs to improve really to avoid these issues, unless we'd say we don't do any macros anywhere until then -- but that's not ok either tbh, we literarily developed a language feature for this specific macro, so let's have it available as opt in to folks.

[rauhul]

Will you get a warning in the expanded macro if the traced function is marked as throws but doesn't actually try anywhere?

[rauhul]

same with async/await

[porglezomp]

Nope—this is what motivated doing the extra work to copy over the effect specifier on the closure instead of leaving it inferred. See the commit message on 3951ba2 for a discussion of that.

[ktoso]

marking this as semver/none because we reverted the change.