Detect and add a fix-it for using : instead of -> in a function

[j-f1]

I accidentally do this quite regularly when changing a computed property to a method. This change adds a fix-it (including adding a space before the inserted ->) to make it a little easier to fix.

[j-f1]

@swift-ci please test

[rintaro]

Thanks! This is a very nice diagnostic that was missing in the new parser.

[rintaro]

TokenDiagnostic is mainly for Lexer errors (and some white space errors).
For cases like this, we prefer to put : token to unexpectedBeforeArrow and create missing -> token, then handle that pattern in ParseDiagnosticsGenerator.

I.e.

    let (unexpectedBeforeArrow, arrow) = if let colon = self.consume(if: .colon) {
      (RawUnexpectedNodesSyntax([colon], arena: self.arena), self.missingToken(.arrow))
    } else {
      self.expect(.arrow)
    }

[j-f1]

I tried with that instead, but the fix-it produced this. How do you suggest I get it to output just the arrow?

-func foo() : [String] {}
+func foo() : -> [String] {}

[rintaro]

Sorry I accidentally removed the first part of the comment.
You'd need to handle that pattern in ParseDiagnosticsGenerator. visit(_: ReturnClauseSyntax) and generate appropriate diagnostics

[rintaro]

Instead of passing-in the unexpectedColon, let's consume it in this function:

Suggested change

	unexpectedColon: RawTokenSyntax? = nil
	acceptColon: Bool = false

So we can use this for subscript too. This is a pretty common mistake

subscript(idx: Index): Element { ... }