JuliaLang / julia / #37610

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Standardize the entry-point for Julia execution (#50974)

This is a bit of a straw-man proposal (though I think mergable if people
agree) to standardize the execution entrypoint for Julia scripts. I
think there's at least four different ways that people might run a
script:

- As `julia main.jl`
- As a PkgCompiler sysimage, then calling the main entry point
- As a PkgCompiler "app", with the magic `julia_main` function
- As a StaticCompiler product with an explicit entrypoint specified on
the API.

The main problem I have with all of these variants is that they're all
different and it's kind of a pain to move between them. Here I propose
that we standardize on `Main.main(ARGS)` as the entrypoint for all
scripts. Downstream from that proposal, this PR then makes the following
changes:

1. If a system image has an existing `Main.main`, that is the entry
point for `julia -Jsysimage.so`.
2. If not, and the sysimage has a REPL, we call REPL.main (we could
handle this by defaulting `Main.main` to a weak import of `REPL.main`,
but for the purpose of this PR, it's an explicit fallback. That said, I
do want to emhpasize the direction of moving the REPL to be "just
another app".
3. If the REPL code is called and passed a script file, the REPL
executes any newly defined Main.main after loading the script file. As a
result, `julia` behaves the same as if we had generated a new system
image after loading `main.jl` and then running julia with that system
image.

The further downstream implication of this is that I'd like to get rid
of the distinction between PkgCompiler apps and system images. An app is
simply a system image with a `Main.main` function defined (note that
currently PkgCompiler uses `julia_main` instead).

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Co-authored-by: Martijn Visser <mgvisser@gmail.com>

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