SE-0382: Expression Macros

[Douglas_Gregor]

@s-k

A result builder type and a closure to transform is passed to the macro and a transformed closure is returned. As I test it, however, the compiler gives me an error with the following output:

Your macro implementations looks good, thank you for working on it! I (very recently) fixed a compiler bug that caused the incorrect error messages, and with my locally-built compiler (and adding a bunch of @expression utterances), your result-builder macro works. My fix hasn't made it into a snapshot yet, but here's a one-off toolchain that contains it.

@s-k

Would expression macros support trailing closure syntax? I have the feeling that passing closures could be a major use-case.

Yes, they do! I updated your example macro to this:

let stringClosure = #apply(resultBuilder: StringAppender.self) {
    "This"
    "is"
    "a"
    "sentence."
}

and taught the macro implementation to look for a trailing closure as well:

        guard
            let resultBuilderSelfExpr = node.argumentList.first?.expression.as(MemberAccessExprSyntax.self),
            let resultBuilderName = resultBuilderSelfExpr.base?.withoutTrivia().description,
            let originalClosure = node.argumentList.dropFirst().first?.expression.as(ClosureExprSyntax.self) ??
              node.trailingClosure
        else {
            throw SomeError()
        }

and it works fine.

Doug