Support JEP 512: Compact Source Files and Instance Main Methods

[vorburger]

In enola-dev/enola#974 for enola-dev/enola#971 with enola-dev/enola#970, I am using "Java's fancy modern main style", AKA the JEP 445 & 463 & 477 & 495 stuff:

• https://openjdk.org/jeps/445
• https://openjdk.org/jeps/463
• https://openjdk.org/jeps/477
• https://openjdk.org/jeps/495

So I have e.g. a hello.java which looks e.g. like this, which is actually valid (with --enable-preview) Java 21 syntax:

import static java.lang.System.out;

void main() {
    out.println("Hello World: " + dev.enola.common.Version.get());
}

google-java-format chokes on this with:

    stderr: learn/jbang/hello.java:26:6: error: did not generate token "void"
void main() {

This is on v1.25.1, which is the 2nd most recent release today; looking at the 1.25.2 Release Notes, I have no reason to believe that this (just) got fixed there; ergo reporting this here.

Stating that google-java-format does not support --enable-preview would be valid. But from what little I figured (so far) this feature, contrary to e.g. those preview String Templates which then got removed again, this syntax appears to be here to stay, so probably should be supported, sooner or later.

[cushon]

These are still preview features, so we may want to wait until the feature is finalized. We've added support for preview features before, and may do that again depending on the feature. But it's wasted effort if the preview feature changes or is removed, so it's probably a case-by-case thing.

[sormuras]

It's integrated by JEP 512: Compact Source Files and Instance Main Methods now via openjdk/jdk@d29700c and soon it will also be available for downloading/testing at https://jdk.java.net/25/ — presumably starting with "Build 22 (2025/5/8|9)".

[cushon]

Initial support for instance main methods has been released in https://github.com/google/google-java-format/releases/tag/v1.34.0