add eval (-e) expression evaluation to command line

[philwalk]

Implements a command line expression evaluator.

bin/scala -e 'println("Hello World!")'
Hello World!

bin/scala -e 'println(33*56)'
1848

# bin/scala -e 'val x = 100f ; println(x)'
100.0

A temporary script file is created and executed, with a prefix inserted:

@main def main(args: String *): Unit =
  <expression-goes-here>

The -save option is disabled, to avoid uselessly creating a jar.

[som-snytt]

I would omit the newline character as a needless complication.

I don't remember whether scala 2 has to write a file, but I bet it doesn't. Is it necessary to write a file?

Tired recently of deleting scala temp files from my WSL /tmp due to running scala and dotty tests.

[som-snytt]

oh, possibly newline and indent matters for braceless syntax.

[philwalk]

I would omit the newline character as a needless complication.

Sounds good.

Tired recently of deleting scala temp files from my WSL /tmp due to running scala and dotty tests.

Totally agree. I reviewed the scala2 code, and it doesn't create a file, although it's has different Runners available.
I will poke around a bi more and see if I can find something.

[philwalk]

A new class dotty.tools.scripting.StringDriver provides a way to compile and run source code from a String, rather than from a file.

The code it shares with dotty.tools.scripting.ScriptingDriver, was refactored to new file dotty.tools.scripting.Util.

[som-snytt]

There was a previous effort to avoid dependencies on packages that might become modules in 2.13, which did not happen. I'm conflicted about sys.props because OT1H it's just a Map interface for System.getProperty or ies, but OTOH it is not so trivial that it didn't harbor a couple of bugs.

I guess the convenience here is java.io.File.pathSeparator. No big deal, I just happened to have recently seen the Scala 2 commit about not using sys.props.

[philwalk]

I wasn't aware of the discussion around packages that might become modules, but java.io.File.pathSeparator is a good alternative in any case.

It turns out that pathsep is defined identically in dotty.tools.scripting.Main, so the definition maybe belongs in dotty.tools.scripting.Util.

[BarkingBad]

LGTM