The strange carets say that the unused member is x_=.

10 |    var x: Int = 42
   |        ^^^
   |        unused private member

Not sure why it gets a no-op setter, but "using" it, of course, silences the warning.

Moreover, the warning "should" say that the var is not set. Adding x_=(27), as simple assignment is simple assignment. It's charming that it is printed as an infix operator!

          final class $anon() extends Object(), Foo {
            var x: Int = 42
            def x_=(x$1: Int): Unit = ()
            {
              final class $anon() extends Object(), Bar {
                println($anon.this.x)
              }
              new $anon():Bar
            }:Bar @unchecked match
              {
                case _ => ()
              }
            this.x = 27
            this x_= 27
          }
          new $anon():Foo

For comparison

def f: Unit =
  var x: Int = 42
  println(x)

warns

17 |  var x: Int = 42
   |      ^
   |      unset local variable, consider using an immutable val instead

As a footnote, scastie is on 3.7.0-RC1, which is so to speak quite aged.

It's not only about the setter. If you reassign the variable inside Bar, you still get a warning.
https://scastie.scala-lang.org/0WRV7uO7Tmuy8Z4DKHYUVw

In fact, this is a minimized version of some code in the project I'm working on, where we do mutate that variable, and we use 3.7.0, not 3.7.0-RC1

@mberndt123 the setter is also not used from inside Bar. To answer my other question, the setter is expanded later.

My fix tweaks the conditions for warning so it reports "unset var" (when that is true).