[jdk17] RFR: 8268766: Desugaring of pattern matching enum switch should be improved [v6]

[Jan Lahoda]

> Currently, an enum switch with patterns is desugared in a very non-standard, and potentially slow, way. It would be better to use the standard `typeSwitch` bootstrap to classify the enum constants. The bootstrap needs to accept enum constants as labels in order to allow this. A complication is that if an enum constant is missing, that is not an incompatible change for the switch, and the switch should simply work as if the case for the missing constant didn't exist. So, the proposed solution is to have a new bootstrap `enumSwitch` that accepts `String`s in place of the enum constants, and will internally convert them to the appropriate enum constants, and then it will find the proper case similarly to `typeSwitch`.
> 
> How does this look?

Jan Lahoda has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Reflecting review comments.
 - Merge branch 'master' into JDK-8268766
 - Improving javadoc.
 - Updating javadoc, as suggested.
 - Updating javadoc, code and tests as suggested.
 - Creating a new bootstrap method for (pattern matching) enum switches, as suggested.
 - Adding and fixing test.
 - Merging master.
 - 8268766: Desugaring of pattern matching enum switch should be improved

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Changes: https://git.openjdk.java.net/jdk17/pull/81/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=81&range=05
  Stats: 416 lines in 6 files changed: 278 ins; 68 del; 70 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/81.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/81/head:pull/81

PR: https://git.openjdk.java.net/jdk17/pull/81