Enable the creation of syntax nodes from a string interpolation

[DougGregor]

Make the main categories of syntax nodes (DeclSyntax, ExprSyntax, StmtSyntax, TypeSyntax, and PatternSyntax) conform to ExpressiblyByStringInterpolation. The string literal portions are source text, and one can interpolate both additional source text and syntax nodes into that source text. The result will be parsed by the parser to produce a node of the given kind. For example, we can produce a TypeSyntax like this:

let tupleSyntax: TypeSyntax = "(Int, name: String)"

And then use that in another production, for example to create a function declaration:

let fnSyntax: DeclSyntax =
   "func \(name)(String) async throws -> \(tupleSyntax)"

This approach is intended to simplify code generation, by allowing code to generate Swift syntax trees in the most natural way possible---by writing the Swift code that generates them.

[DougGregor]

@swift-ci please test

[ktoso]

I'm loving this a lot 🥳

Managed to get out of the source generation business a while ago, but it'll be back -- very happy we'll have this at our disposal then 👍

[DougGregor]

Hmm, I think I should opt in SourceFileSyntax as well, because it's handy to generate whole source files like this.

[jpsim]

Looks great. Here's how it simplifies some rewriting logic in SwiftLint:

Before

let functionCall = FunctionCallExprSyntax(
    calledExpression: ExprSyntax(
        MemberAccessExprSyntax(
            base: node.first!.withoutTrivia(),
            dot: .periodToken(),
            name: .identifier("toggle"),
            declNameArguments: nil
        )
    ),
    leftParen: .leftParenToken(), argumentList: .init([]), rightParen: .rightParenToken(),
    trailingClosure: nil, additionalTrailingClosures: nil
)

let newNode = node
    .replacing(childAt: 0, with: ExprSyntax(functionCall))
    .removingLast()
    .removingLast()
    .withLeadingTrivia(node.leadingTrivia ?? .zero)
    .withTrailingTrivia(node.trailingTrivia ?? .zero)

After

let newNode = node
    .replacing(childAt: 0, with: "\(node.first!.withoutTrivia()).toggle()")
    .removingLast()
    .removingLast()
    .withLeadingTrivia(node.leadingTrivia ?? .zero)
    .withTrailingTrivia(node.trailingTrivia ?? .zero)

---

This is for the toggle_bool rule that rewrites someFlag = !someFlag to someFlag.toggle().

[DougGregor]

This is for the toggle_bool rule that rewrites someFlag = !someFlag to someFlag.toggle().

Beautiful!