RFR: 8294943: Implement record patterns in enhanced for [v6]

[Maurizio Cimadamore]

On Mon, 7 Nov 2022 09:56:45 GMT, Aggelos Biboudis <abimpoudis at openjdk.org> wrote:

>> This PR enables the ability to use record patterns in the enhanced-for, initializing the record components accordingly: 
>> 
>> 
>> record Complex(double real, double img) {}
>> 
>> List<Complex> list = ...;
>> 
>> for (Complex(var real, var img) : list) {
>>     // can use “real” and “img” directly
>> }
>> 
>> 
>> This PR proposes an implementation for the "Record Patterns in Enhanced For" [subtask](https://bugs.openjdk.org/browse/JDK-8294943) regarding the following [CSR](https://bugs.openjdk.org/browse/JDK-8294944) (note the different JBS entries), summarised by the following:
>> 
>> - It enhances the grammar for the `EnhancedForStatement` to support record patterns too, alongside `LocalVariableDeclarations`.
>> - Any pattern variables introduced by the record pattern in the header of the pattern are definitely matched in the statement block of the enhanced for. 
>> - The record patterns are only permitted when the pattern is exhaustive over the enhanced for's expression.
>> - In the case that the element of the iteration is `null`, the switch raises a `MatchException` wrapping the `NullPointerException`.
>> - The enhanced for, supports record patterns for both arrays and reference types.
>> 
>> Currently, the precise meaning of the enhanced for statement is given by translation into a basic for statement. By introducing record patterns in the pattern header, the new meaning is defined by the new translation which incorporates a switch whose selector expression is the enhanced for's expression, and whose singleton case has the given record pattern as a sole label would be exhaustive. Note, that in cases where the imaginary switch would reach the default clause and end abruptly, the enhanced for each will end abruptly for the same reason.
>> 
>> For more information on the changes please see:
>> 
>> - the JEP: [JEP 432](https://openjdk.org/jeps/432) 
>> - the CSR: [JEP 432 - JDK-8294944](https://bugs.openjdk.org/browse/JDK-8294944) 
>> - the current [specification draft](https://cr.openjdk.java.net/~gbierman/jep432%2b433/jep432+433-20221018/specs/patterns-switch-record-patterns-jls.html#jls-14.14.2) 
>> 
>> Looking forward for your review.
>
> Aggelos Biboudis has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review

Looks good - only doubt is on translation strategy - but that could also be improved upon with separate PR.

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/TransPatterns.java line 736:

> 734:                  * <pre>{@code
> 735:                  *     for (<type-of-coll-item> N$temp : coll) {
> 736:                  *     switch (N$temp) {

This is obviously a correct translation (I like that it delegates to what we already know how to do). I wonder if it would be better to use a pattern with `if/instanceof` (as the switch machinery is a tad expensive, and has an invokedynamic, which in this case could be avoided, I think?).

-------------

Marked as reviewed by mcimadamore (Reviewer).

PR: https://git.openjdk.org/jdk/pull/10798