How to determine if a lambda closes over state in a macro?

[armanbilge]

I want to determine at compile-time if a lambda (closure) closes over state. Is there way to do this with a macro?

I know how to do this in a compiler plugin h/t Scala Native.

case fn @ Closure(env, target, _) =>
  if env.nonEmpty then
    report.error(
      s"Closing over local state of ${env.map(_.symbol.show).mkString(", ")} in function transformed to CFuncPtr results in undefined behaviour.",
      fn.srcPos
    )

https://github.com/scala-native/scala-native/blob/e90d39ead9c79b72d2df1d14ede3b8a8ffac0d94/nscplugin/src/main/scala-3/scala/scalanative/nscplugin/NirGenExpr.scala#L2677-L2682

However, the definition of Closure in the compiler AST differs from the Quotes AST, specifically the lack of env.

https://github.com/lampepfl/dotty/blob/938d405f05e3b47eb18183a6d6330b6324505cdf/compiler/src/dotty/tools/dotc/ast/Trees.scala#L594

https://github.com/lampepfl/dotty/blob/938d405f05e3b47eb18183a6d6330b6324505cdf/library/src/scala/quoted/Quotes.scala#L1390

[nicolasstucki]

The closure environment is not yet computed when we expand macros. This is something we compute in a later phase.

It might be hard or inconsistent if we compute the closure environment before or during the expansion of a macro. Macros can change the captured sets of closures outside their scope.

It might be simpler to provide a method that lists all local variables that are used but not defined in a specific tree.