assertMacroExpansion should emit an error if macro is applied to node that is not a variable

[ahoppen]

When applying an accessor macro to e.g. a struct, assertMacroExpansion will currently happily swallow the attribute. It should, however, emit an error that accessor macros can’t be applied to structs. Ie. the following test case should emit an error.

func testAccessorOnStruct() {
  struct TestMacro: AccessorMacro {
    static func expansion(
      of node: AttributeSyntax,
      providingAccessorsOf declaration: some DeclSyntaxProtocol,
      in context: some MacroExpansionContext
    ) throws -> [AccessorDeclSyntax] {
      return []
    }
  }

  assertMacroExpansion(
    "@Test struct Foo {}",
    expandedSource: "struct Foo {}",
    macros: ["Test": TestMacro.self]
  )
}

rdar://115562563

[RayZhao1998]

I want to work on this. But I found in the testTypeWrapperTransform, CustomTypeWrapperMacro is also an AccessorMacro but it's applied to struct.

[RayZhao1998]

Also I've made a draft PR and I want to know whether my solution is correct. It only fails on this test testTypeWrapperTransform now.