Formatting issue with placeholder

[HassanTaleb90]

Issue: Unable to format code when we have a placeholder.

Code example:
class <#name#>: <#super class#> {
<#code#>
}

diagnostic:
2:5: expected 'func' in function
2:13: expected parameter clause in function signature

example 2: (the placeholder "<#code#>" must be formatted)

[allevato]

Situations like this are tricky because we don't currently don't make any attempt to format invalid code, which that is.

The parse tree for your example looks like this:

SourceFileSyntax children=2
  0: CodeBlockItemListSyntax children=1
    0: CodeBlockItemSyntax children=1
      0: ClassDeclSyntax children=4
        0: classKeyword
        1: identifier("<#name#>")
        2: TypeInheritanceClauseSyntax children=2
          0: colon
          1: InheritedTypeListSyntax children=1
            0: InheritedTypeSyntax children=1
              0: SimpleTypeIdentifierSyntax children=1
                0: identifier("<#super class#>")
        3: MemberDeclBlockSyntax children=3
          0: leftBrace
          1: MemberDeclListSyntax children=1
            0: MemberDeclListItemSyntax children=1
              0: FunctionDeclSyntax children=3
                0: funcKeyword MISSING
                1: identifier("<#code#>")
                2: FunctionSignatureSyntax children=1
                  0: ParameterClauseSyntax children=3
                    0: leftParen MISSING
                    1: FunctionParameterListSyntax
                    2: rightParen MISSING
          2: rightBrace
  1: eof

It's actually really helpful that the name and super class placeholders come through just as identifiers, because we could treat those opaquely. The problematic node is code, since the parser tries to parse it as a function declaration with missing parts.

It looks like placeholders in expression positions degrade nicely to an EditorPlaceholderExprSyntax node, so something like for <#index#> in <#collection#> { <#stmt#> } comes all the way through without diagnostics. So, a placeholder in a MemberDeclList might uniquely be a problem that could be handled more gracefully.

@ahoppen do you think it would make sense to provide an EditorPlaceholderDeclSyntax that could be slotted in as the child of a MemberDeclListItemSyntax to support this specific case? Or maybe some other approach to make this produce a cleaner tree than trying to assume a broken function?

[ahoppen]

I think we could treat editor placeholders as unexpected tokens in member decl lists.

[ahoppen]

Here we go: swiftlang/swift-syntax#1188