MacroSystem should add newline at the end of added members for an empty type

[bnbarham]

Description

Given a macro AddMembers that does as its name suggests, MacroSystem will currently expand the following:

@AddMembers struct Foo {}

To:

struct Foo {
  func addedMember() {
    ...
  }}

Note the lack of a newline after the added member. This usually makes sense as there would already be one, but when there isn't, we should add it.

rdar://116773975

[Matejkob]

Here is a possible test case for this bug:

func testAddMemberToEmptyDeclaration() {
  struct TestMacro: MemberMacro {
    static func expansion(
      of node: AttributeSyntax,
      providingMembersOf declaration: some DeclGroupSyntax,
      conformingTo protocols: [TypeSyntax],
      in context: some MacroExpansionContext
    ) throws -> [DeclSyntax] {
      return [DeclSyntax("var x = 0")]
    }
  }

  assertMacroExpansion(
    """
    @Test
    struct Foo {}
    """,
    expandedSource: """
      struct Foo {

        var x = 0
      }
      """,
    macros: [
      "Test": TestMacro.self
    ],
    indentationWidth: indentationWidth
  )
} 

I'm wondering if we should add an empty line before the new member. The current implementation of macro system in swift-syntax will do that. 🤔

[Matejkob]

What is interesting is that the case with a new line at the end of the declaration added by macro is neither works as expected:

func testAddMemberToEmptyDeclaration() {
  struct TestMacro: MemberMacro {
    static func expansion(
      of node: AttributeSyntax,
      providingMembersOf declaration: some DeclGroupSyntax,
      conformingTo protocols: [TypeSyntax],
      in context: some MacroExpansionContext
    ) throws -> [DeclSyntax] {
      return [DeclSyntax("var x = 0\n")]
    }
  }

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

The problem seems to be in the SyntaxExpressibleByStringInterpolation.init(stringLiteral:) since the following code:

(lldb) po MemberBlockItemListSyntax(stringLiteral: "\n\n var x = 0\n\n").description
"\n\n var x = 0"

is discarding trailing trivia 🤔

[ahoppen]

The problem seems to be in the SyntaxExpressibleByStringInterpolation.init(stringLiteral:) since the following code is discarding trailing trivia 🤔

Oh, that’s a bug as well. We should be adding the leading trivia of the endOfFile token to the trailing trivia of the parsed node. That’s probably best done in Parser.parseRemainder

[bnbarham]

I'm wondering if we should add an empty line before the new member. The current implementation of macro system in swift-syntax will do that. 🤔

Ah, nice catch. Yeah, ideally we wouldn't add the extra newline there.