RFR: 8369564: Provide a MemorySegment API to read strings with known lengths [v8]

[Liam Miller-Cushon]

On Fri, 21 Nov 2025 12:55:13 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> I added more tests to cover regular and exception cases for the three new methods. I'm happy to take suggestions on additional test coverage, or if there's a better location for any of the tests.
>
> Thanks, these look great!
> 
> I think another test that tests the case where `srcIndex + numChars` overflows for `copy` and `allocateFrom`, with different char sets (one that takes the internal `bytesCompatible == true`, and one that takes the `bytesCompatible == false` route) would be good to have.

Thanks, I added coverage of `srcIndex + numChars` overflow for both `bytesCompatible` cases.

I think that in practice some of the cases were being caught later. I added ` Objects.checkFromIndexSize` assertions to `MemorySegment#copy` and `allocateFrom` to catch them immediately with a useful exception, instead of e.g. catching it in `String.substring`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549824444