deprecate DelayedInit outside of App

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(No description for SI-5479.)

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Imported From: https://issues.scala-lang.org/browse/SI-5479?orig=1
Reporter: @adriaanm

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@retronym said:
Marking as "fixed" because DelayedInit was shipped in 2.10.0.

I think it is unlikely we will spec its semantics post-factum; if anything we should deprecated its use outside of App as it is full of holes.

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@adriaanm said:
should we plan on supporting top-level defs to replace App in 2.12?

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@gkossakowski said:
What about just having compiler magic specifically for App and not try to come up with general mechanism? We could even spec the compiler magic.

I know that this is against Scala's spirit but let's face it: App is a very small feature that is mostly important to people learning and teaching Scala. Let's just deliver this feature in the most useful way to Scala users and move on to more important issues.

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@adriaanm said:
I agree that makes most sense.

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@adriaanm said:
scala/scala#3563