improve diagnostic for unnamed closure parameters

[AppAppWorks]

fix #2340

[AppAppWorks]

@bnbarham would you mind reviewing this PR too? :)

[AppAppWorks]

Updated, please have a look.

[ahoppen]

Thank you @AppAppWorks. Looks great!

[ahoppen]

@swift-ci Please test

[AppAppWorks]

@ahoppen I've found that some of my code comments are misleading and the test case for missing both identifier and type is missing.

I've rewritten the comments and added a new test case as follows, if you also think they're necessary and good, I'll force-commit again.

let names = self.parseParameterNames()
var colon = self.consume(if: .colon)
// try to parse the type regardless of the presence of the preceding colon
// to tackle any unnamed parameter or missing colon
// e.g. [X], (:[X]) or (x [X])
let canParseType = withLookahead { $0.canParseType() }
let type: RawTypeSyntax?
if canParseType {
  type = self.parseType(misplacedSpecifiers: misplacedSpecifiers)
  if colon == nil {
    // mark the preceding colon as missing if the type is present
    // e.g. [X] or (x [X])
    colon = missingToken(.colon)
  }
} else if colon != nil {
  // mark the type as missing if the preceding colon is present
  // e.g. (:) or (_:)
  type = RawTypeSyntax(RawMissingTypeSyntax(arena: self.arena))
} else {
  type = nil
}

assertParse(
  """
  test { (1️⃣:2️⃣) in }
  """,
  diagnostics: [
    DiagnosticSpec(
      locationMarker: "1️⃣",
      message: "expected identifier in parameter",
      fixIts: ["insert identifier"]
    ),
    DiagnosticSpec(
      locationMarker: "2️⃣",
      message: "expected type in parameter",
      fixIts: ["insert type"]
    ),
  ],
  fixedSource: """
    test { (<#identifier#>: <#type#>) in }
    """
)

[ahoppen]

Force push your improved changes and I’ll review them again.

[AppAppWorks]

Pushed, please check.

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test