[enums] Enhanced enums: state of the implementation

[Vicente Romero]

Hi all,

The initial support for enhanced enums have been recently pushed to the 
amber repo, see [1, 2, 3, 4]. The current implementation has been 
reviewed and compared with the current draft of the enhanced enums JEP 
[5], looking for synchronization and omission issues. Below there is a 
summary of the current state of the implementation,

Thanks,
Vicente

  * Implementation overview

In general terms the enhanced enum implementation is concise and pretty 
small (~500 lines of code). Almost all of the changes are in the 
front-end, and are mainly focused in providing for the additional 
expressibility of enhanced enums compared to legacy ones. There is one 
change that can't be considered a front-end one, it's related to the 
binary name of enhanced enum constants and will be mentioned below.

In the parsing area, the approach used for legacy enums is leveraged for 
enhanced enums. The basic idea is still that an enum constant is 
converted to a class creation expression that is assigned to a variable 
which type is the one of the enum constant. The main difference is that 
additional work is needed to parse the type arguments, in the case of 
generic enums.

The binary names of enhanced enum constants are different to that of 
legacy ones. For example give the following code snip:

|    enum Foo {|

|||A {},|
|||B {};|
|||}|

The binary names assigned by a javac compiler, with no knowledge of 
enhanced enums, to enum constants Foo.A and Foo.B are Foo$1 and Foo$2, 
whereas in the case of the current enhanced enums implementation they 
are Foo$$A and Foo$$B.


  * Sync issues with the current draft of the JEP 301


The naming convention proposed in the section above, and already 
implemented, is different to the one proposed in the JEP [5].


[1] http://mail.openjdk.java.net/pipermail/amber-dev/2017-March/000029.html
[2] http://mail.openjdk.java.net/pipermail/amber-dev/2017-March/000039.html
[3] http://mail.openjdk.java.net/pipermail/amber-dev/2017-March/000040.html
[4] http://mail.openjdk.java.net/pipermail/amber-dev/2017-March/000044.html
[5] http://openjdk.java.net/jeps/301