Fix double execution in JSR223 Implementation

[rjolly]

The reason for this change is that currently, statements in the
JSR223 script engine are evaluated twice. Once when the "run" method
is called, and once when the value of the result is extracted.
The first one needs not be, it exists solely due to displayDefinitions
being called. If we put it behind a flag, the problem is solved.

[dottybot]

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[smarter]

I'm confused, what's the method that gets called twice exactly, and by whom ? Why is there a difference between scripting and non-scripting ?

[rjolly]

See in https://github.com/rjolly/dotty/blob/master/compiler/src/dotty/tools/repl/ScriptEngine.scala . In the eval method, first the method "run" is called, which at the end calls DisplayDefinitions which does a reflective method invocation (on something like rs$line$1.res0). Then, still in eval, I do a second reflective method invocation (and this is specific to the script engine) to extract the value, which is returned by the eval method.

[rjolly]

How to reproduce:

user@debian:~/dotty-0.17.0-RC1/lib$ jrunscript -J-Djava.class.path=dotty-compiler_0.17-0.17.0-RC1.jar:dotty-doc_0.17-0.17.0-RC1.jar:dotty-interfaces-0.17.0-RC1.jar:dotty-library_0.17-0.17.0-RC1.jar:scala-library-2.12.8.jar:scala-asm-6.0.0-scala-1.jar -l scala
scala> def fn = println("hello world")
def fn: Unit

scala> fn
hello world

hello world
()
scala>

[rjolly]

@smarter Is there something I can do to help this PR going forward ?

[smarter]

Very sorry about the delay, I'll try to review this properly this week!

[rjolly]

Is it alright to add a command line argument "repl-disable-display" ? The alternative is a constructor argument to ReplDriver, which is not perfect either.

[smarter]

Finally had time to look at this :). I think this is fine as is since it doesn't affect the code too much, but maybe in the future we could instead refactor ReplDriver slightly so you could override the method that run and print the definitions?

[rjolly]

Thanks ! Yes, override would be fine, but indeed it requires some refactoring. I'll think about it.