RFR: 8271623: Omit enclosing instance fields from inner classes that don't use it [v3]

[Florian Weimer]

On Mon, 11 Oct 2021 20:11:30 GMT, Liam Miller-Cushon <cushon at openjdk.org> wrote:

>> This change omits the synthetic `this$0` field from inner classes that do not access any enclosing instance state.
>
> Liam Miller-Cushon has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - Enable enclosing instance optimization for --release 18 and newer
>  - Add a test case with nested inner classes
>  - 8271623: Omit enclosing instance fields from inner classes that don't use it

Is it desirable to do this change for `Serializable` inner classes? If they do not define `serialVersionUID`, it won't be possible to deserialize existing data.

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PR: https://git.openjdk.java.net/jdk/pull/4966