RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v6]

[Per Minborg]

On Sun, 16 Mar 2025 00:34:45 GMT, John R Rose <jrose at openjdk.org> wrote:

> Comments on visual noise and side effects in `toString`.
> 
> `renderWrapped` is clever for a single stable value, but it makes for a very noisy display string, with confusing doubly-nested `[]`, for composite stable values. I'm talking about `StableFunction` mainly, I guess.
> 
> I suggest omitting the inner `[]` for such composites. A simple boolean on `renderWrapped` will do that trick. In that case, `renderWrapped` has the job of either presenting a fixed (recognizable) sentinel string, or else forwards, without further editorial comment, to the `toString` of the contained value.

The `toString()` for `StableValue` is inspired by `Optional` which works in the same way by adding `[ ]` around the contents. Any more thought in the reviewer community on how we should handle this?

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PR Comment: https://git.openjdk.org/jdk/pull/23972#issuecomment-2743849237