`_const` parameter modifier (and other underscored parameter modifiers)

MrAlirezaa · 2023-10-15T20:25:00.347Z

Hello all,

I noticed some unfamiliar syntax in the swift-testing codebase and was hoping someone could explain it. Specifically, in the Suite macro definition, the first parameter displayName has a _const modifier:

@attached(member) @attached(peer) public macro Suite(
  _ displayName: _const String? = nil,
  _ traits: any SuiteTrait...
) = #externalMacro(module: "TestingMacros", type: "SuiteDeclarationMacro")

(see it on github)

I'm not familiar with the _const parameter modifier. Could someone please explain what it does and how it affects the argument passed to this function?

I'm also interested to learn more about these underscored parameter modifiers in general. I see there's a UnderscoredAttributes.md reference in the Swift repo that covers the underscored attributes. I wonder if there is a similar documentation or note available for these kind of parameter modifiers. If anyone has pointers to documentation or examples for these modifiers, I'd appreciate it! Just trying to level up on some of the deeper Swift syntax.

hooman · 2023-10-15T21:28:29.254Z

It is this:

[Returned for revision] SE-0359: Build-Time Constant Values Announcements

Hello Swift community, The review for SE-0359 "Build-Time Constant Values" ended on June 6th. The proposal is being returned for revision. The review brought up a number of concerns about the proposal, but the overarching theme is that it isn't clear how this proposal fits into the overall picture for build-time constant evaluation in Swift, and with that it does not stand well on its own. This manifests in a few different ways: Concerns about the virality of @const: Noted in several places…

MrAlirezaa · 2023-10-15T21:52:23.161Z

Oh, Thanks @hooman