accessor macro expansion doesn't remove initializer from stored property

[mayoff]

Test code: the AutoEnvironmentKey branch of my fork of swift-macro-examples

Toolchain: the latest nightly, swift-DEVELOPMENT-SNAPSHOT-2023-02-15-a in Xcode 14.2 (14C18) on macOS 13.2.1 (22D68).

SE-0389, Attached Macros, says

The expansion of an accessor macro that does not specify one of willSet or didSet in its list of names must result in a computed property. A side effect of the expansion is to remove any initializer from the stored property itself; it is up to the implementation of the accessor macro to either diagnose the presence of the initializer (if it cannot be used) or incorporate it in the result.

The AutoEnvironmentKey macro in the linked repo is an accessor macro that does not specify either willSet or didSet, and indeed it returns get and set accessors in its expansion. But swiftc does not remove the initializer. The testAutoEnvironmentKey test case demonstrates this. It uses this input syntax:

extension EnvironmentValues {
  @AutoEnvironmentKey
  public var myValue: String = "don't abuse macros"
}

and expects this output syntax:

extension EnvironmentValues {
  public var myValue: String {
    get { self[EnvironmentKey_for_myValue.self] }
    set { self[EnvironmentKey_for_myValue.self] = newValue }
}
  private enum EnvironmentKey_for_myValue: EnvironmentKey {
    static var defaultValue: String { "don't abuse macros" }
  }
}

but the actual expanded syntax is this:

extension EnvironmentValues {
  public var myValue: String = "don't abuse macros" {
    get { self[EnvironmentKey_for_myValue.self] }
    set { self[EnvironmentKey_for_myValue.self] = newValue }
}
  private enum EnvironmentKey_for_myValue: EnvironmentKey {
    static var defaultValue: String { "don't abuse macros" }
  }
}

Note that the initializer has not been removed from the actual expanded syntax.

[trbernstein]

This error still exists on swiftlang-5.9.0.114.10. Is anyone already actively working on this problem?

[trbernstein]

I am now tackling this problem. I don't know how long it will take me to find a solution though, since I need to get into the code base first. I appreciate any help here.
As a starting point I have now taken the old commits regarding macros from @DougGregor and @rxwei.

However, if I imagine it correctly, removing a node from the AST should not be too complicated. Let's hope I'm not wrong.

[trbernstein]

During my research I came across two interesting facts:

1. swift seems to compile code like the one above (e.g. a property with an initial value and an accessor macro that generates a getter/setter) just fine, regardless of the resulting grammar error.

2. Writing the same code by hand leads to a compile time error, saying that a "variable with getter/setter cannot have an initial value.

Together this leads me to the conclusion, that compiler is removing the initial value after all, only at a later point. On the other hand, this seems very odd, as macros should not produce code, that if handwritten does not compile.

[DougGregor]

When you say "actual expanded syntax", what are you using to see that expanded syntax?

[trbernstein]

When you say "actual expanded syntax", what are you using to see that expanded syntax?

@DougGregor In my unit tests using assertMacroExpansion, which accepts an (expected) expandedSource argument, which will report any expansion mismatch. Unlike Xcode, which only shows the new accessors of the declaration of the attached macro, assertMacroExpansion runs all expansions in full, so that the macro attribute itself is no longer in the expanded source.

[jeudesprits]

This error still exists on swift-5.9-DEVELOPMENT-SNAPSHOT-2023-07-29-a

[bnbarham]

Transferred over to swift-syntax - the issue here is solely in MacroSystem, which assertMacroExpansion uses. The compiler itself is fine (ie. it does remove the initializer).

[ahoppen]

Tracked in Apple’s issue tracker as rdar://117442713